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Geosynthetics and Geosystems in Hydraulic and Coastal Engineering KRYSTIANW PILARCZVK

Rijkswaterstaat, Delft, Netherlands

A.A.BALKEMA/ROTTERDAM/BROOKFIELD/2000

Auteursrec1tel 1k beschen'ld mater1aal

Th£publication ofthis book was sponsored by: RUkswaterstaat, the Road and Hydraulic Engineering Division Van der Burghweg I, P.O. Box 5044, 2600 GA Delft, Netherlands Tel: +31-15-2518518, fax: +31-15-2518555 This book was prepared by Krystian W. Pilarczyk from the Road and Hydraulic Engineering Division of the Rijkswatcrstaat (Dutch Public Works Department) in the Netherlands. The views, analyses, and conclusions in this book are those of the author and do not necessarily represent the view of the RijkswaterstaaL Neither the author or the Rijkswaterstaat assume any liabilities with respect to the use of, or for damage resulting from the use of, any information disclosed in this boo. k . Furthermore, the citation of trade names, and other proprietary marks does not constitute an endorsement or approval of the use of such commercial products or services, or of the companies that provide them. Authorization to photocopy items for internal or personal use, or the internal or personal use of specific clients, is granted by A.A.Balkema. Rotterdam, provided that the base fee of USS1.50 per copy, plus USS0.10 per page is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA O1923, USA. For those organizations that have been granted a photocopy license by CCC, a separate system of payment has been arranged. The fee code for users of the Transactional Reporting Service is: 90 5809 302 6/00 USS 1.50 + USS0.10. Published by A.A.Balkema, P.O.Box 1675, 3000 BR Rotterdam, Netherlands Fax: +31.10.4135947; E-mail: [email protected]; Internet site: http://www.balkema.nl A.A. Balkema Publishers, Old Post Road, Brookfield, VT 05036-9704, USA Fax: 802.276.3837; E-mail: [email protected] ISBN 90 5809 302 6

e 2000 A..A.Balkema, Rotterdam Printed in the Netherlands

Auteursroc•1teh1k beschermd matena,,I

Brief contents

lJNITS AND CONVERSION FACTORS GLOSSARYOFGEOSYNTHE'.I)C TERMS PREFACE ACKNOWI .EDGEMENTS

CHAPTERS

t 2 3 4

5

6 7 8 9 10 11 12

Introduction General design methodology Geosynthetics; properties and functions Revetments and bed protections Fill-containing geosystems (bags, mattresses and geotubes) Geocontainers Geotextile forms for sand structures Screens and curtains Inflatable dams Geosynthetics in dams, dikes, banks and dune reinforcement Erosion-control systems Remaining questions and closing remarks;

durability, execution and damage, and quality control

1 11 .41 69

211

421 578 619 725

769 820

825

PROFESSIONAi.HEIP AND lJSEFlTL ADDRESSES

.211

INDEX

VI Auteursr0clitel1jk bescherrid matenaal

Table of contents

UNITS AND CONVERSION FACTORS

XY

xvn

GLOSSARY OF GEOSYNTHETIC TERMS ACKNOWI .EDGEMENTS

xx

PREFACE

XXI

I CNTRODUCTION I.I General

1 I 3 8

1.2 A short overview of geos):'.stems and applications References

2 GENERAL DESIGN METHODOLOGY

11 lI 13 15

2. 1 2.2 2.3 2.4

Design process and methodolozy Design models (tools) Final design Interactions and structural responses 2.4.1 Wave - structure interactions 2.4.2 The load - strength concept 2.5 Principal failure modes and fault tree analysis References

18

19 31 35

44

3 GEOSYNTHETICS; PROPERTIES AND FUNCTIONS 3. l Overview of materials and products 3. l . l Basic materials, processing and end products 3.1.2 Functional applications and properties of end produCJs 3.1.3 Long-term behaviour and durability 3. 1.4 Remaining aspects 3.2 Specifications and test methods References

47 47 48 53 56 61 63 67

4 REVETMENTS AND BED PROTECTIONS 4.1 Introduction 4.2 General approach 4.2.1 Slope and bed protection 4.2.1.1 General considerations 4.2.1.2 General overview on stability criteria for wave attack 4.2.1.3 Optimization ofslope stability under wave attack

69 69 71 71 71 75 80

vu Auteursr0clitel1jk bescherrid matenaal

4.2.1.4 Stability criteria for cu"enJ anack

81 84 87

4.2.I.S Scour and toe protection 4.2.1.6 Protection against ovenopping . Joints and transitions 4.2.17 4.2.1.8 Construction (execution) 4.2.2 Geotechnical aspects 4.2.2.1 General 4.2.2.2 Geotechnical limits

4.3 The design of revetments incorporating geotextiles 4.3.1 General 4.3.2 Design methodology and considerations 4.3.3 Design process 4. 4 Introduction to load - strength concept 4.4.1 General criteria 4.4.2 Basic prindples for geotexJile design 4.4.3 lnJeraction of slope protection components 4.4.4 Example of the stability of a block revetment 4.4. 5 Sliding and geotechnical instability 4.5 Design and stability criteria for geotextiles 4.5.1 Hydraulic loadings 4.5.2 Subsoil and soil tightness 4.5.2.1 Subsoil 4.5.2.2 Determination of soil tightness 4.5.2.3 Geometrically tight geotexJiles 4.5.2.4 Geometrically open geotextiles 4.5.2.5 Additional notes and remarlcs 4.5.3 Requirements concerning water permeability 4.5.3.1 General 4.5.3.2 Water permeability normal to the interface 4.5.3.3 Water permeability parallel to the interface 4. 6 Strength aspects of design of geotextiles 4.6.1 Choice of basic material and type of geotextile 4.6.2 Determination of strength

89

90 92 92 92 94 94

95

101 107 107

108

109 110 114

118

118 130

130

132

136

141 147 149

149

151

158 159

4.6.3 Resistance of geotexJiles to falling stones 4.6.4 Resistance of geotextiles to penetration of stones under wave attack

4. 7 Placement, transition structures and other design aspects 4.7. I Placement and contact with subsoil 4.7.2 Transitions 4.7.2.1 Seams and overlaps 4.7.2.2 Transition structures 4.7.2.3 Toe protection 4.7.2.4 CoMections to structural elements 4.7.3 Deformation of a slope due to internal migration 4.8 Reinforced vegetation and the penetrability of geotextiles 4.8.1 Reinforced vegetation

4.8.2 Penetration through geotextiles t,y reed

4.9 Verification of the design 4.10 Examples of calculation 4.10.1 Example I: Slope protection 4.10.2 Example II: Bed protection 4.10.3 Example Ill: Breakwater List of main symbols References

160 162 164

lfil ill

ill l12

ill ill

�

1M lfil 1.82 189 192

126. 1.26.

126 200 202

203

205 VDI

Auteursr0clitel1jk bescherrid matenaal

Appendix 4.1: Overview of the existing geotextile filter criteria

213

5 FILL-CONTAINING GEOSYSTEMS (bags, mattresses and georubes) 5.1 Introduction 5.2 Applications and specification of the systems 5.2.1 Sa n d -and mortar-filled bags 5.2.2 Sand-filled mattresses 5.2.3 Mortar-filled mattresses 5.2.4 Mexican systems 5.2.5 Some design considerations for mortar-filled geosystems acc. to Silvester (1990) 5.2.6 Longard Tubes 5.2.7 Georubes 5.2.8 Container systems 5.3 Examples of application. Worldwide experience 5.3. l A review of sand-and mortar-filled fabric bags and rubes (HYDRAULICS RESEARCH LID., 1984) 5.3.2 Experience with Longard rubes in Italy 5.3.3 1he large geotextile groyne al North Ki"a Beach, Gold Coast, Australia 5.3 .4 Additional information 5.3.5 Concluding remarks 5.4 A review of selected literature 5.4.1 Sandbag stability and wave run-up on bench slopes (Jacobs and Kobayashi, 1983, 1985) 5.4.2 Large-scale model studies of arctic island protection (Tekmarine, 1982) 5.4.3 A laboratory study of the stability of sand-filled nylon bag breakwater structures (Ray, 1977) 5.4.4 aosure of estuarine channels in tidal regions; behaviour of dumped material when exposed to currents and wave action (Venis, 1967) 5.4.5 Sandbag slope protection: design, construction, and performance (Gadd, 1988) 5.4.6 Mortar-filled containers, lab and ocean experiences (Mexico), (Po"az, 19'19) 5.4.7 Breakwater of concrete-filled hoses (DEUT HYDRAULICS, 1973) 5.4.8 The stability of shore protection with sand sausages on a circular island (DELFT HYDRAUUCS, 1975) 5.4.9 Resistance of gecbags and mattresses against cu"ent attack 5.4. lO Foreshore protection mattresses (Pilarczyk, 1995) 5.4.11 1he stability of georubes and geocontainers (DELFT HYDRAUUCSINICOWN, 1994) 5.4.12 Experimental study on georexlile rube (Miki et al., 1996) 5.4.13 Theoretical and experimental considerations of impact forces by Liu (1981) 5.4.14 Orientation of rubes and other design aspectS acc. to Silvester (1990) 5.4.15 Conclusions 5. 5 Recommended design criteria for geobags and geomattresses on slopes 5.5 .1 General 5.S.2 General stability criteria S.S.2.1 Wave-wad stability S.S.2.2 Flow-food stability 5.5.2.3 Seil-mechanical stability S.S.3 Stability criteria and design case/or geobags S.5.3.l Characteristic dimensions and resistance to wave anack (Wouters, 1998) 5.5.3.2 Design rules with regard to flow load 5.5.3.3 Scil-mechmzical stability of sandbags and sand mattresses 5.S.3.4 Design case/or sandbags under wave attack 5.5.4 Sand Mattresses 5.5.4.1 Design rules with regard to wave load

217 217 219. 219. 2ll

m 211

2ll

232 ill � 250.

1N

� 257 259 26J. 261 261 2fil.

262

21l

2ll

lli 212

286 289 290 292 296 298 300 30 I 302 302 303 303 305 :JS 316 316 320 320 321 322 322

IX

Aut�ursr1chtqhjk beschermd materiaal

5.5.4.2 Design rules with regard to flow load 5.5.4. 3 Design rules with regard to soil-mechanical stability

5.5 .5 Stability criteria and design case for concrete mattresses 5.5.5. l Permeability 5.5.5.2 Design rules with regard to wave load 5.5.5.3 Design rules with regard to flow load 5.5.5.4 Design rules with regard to soil-mechanical stability 5.5.5.5 Design case/or concrete mattress under wave attack 5.5.5.6 Other design considerations (see also Sedion 5.2.3) 5.6 Recommended design criteria for geotubes 5.6. l The computation of the shape of the filled bags or tubes 5.6. l. l lntrodw:tion to shape calculation 5.6. l.2 Design procedure accordingly to Liu, Goh &: Silvester (Silvester, 1990) 5.6.1.3 Recommended design metlwd 5.6.2 Stability of geotubes 5.6.2.l Theoretical derivation of the stability of geotubes 5.6.2.2 Conclusions concerning stability aspects for wave attack 5.6.2.3 Conclusions on stability aspects for current attack 5.6.3 Design aspects of sand-filled geotubes 5.6.3.1 General design aspects 5.6.3.2 Durability/UV-protection 5.6.3.3 Wave transmission 5.6.3.4 Longitudinal reef system and use of geosystems 5.6.3.5 General construction aspects 5.7 Conclusions and recommendations References and bibliography Appendix 5.1: Geosynthetic tubes for confining pressurized slurry (Leshchinsky et al., 1995, 1996) Appendix 5.2: Simple analysis of deformation of sand-sausages (K. Kazimierowicz) Appendix 5.3: Two-dimensional analysis of geosynthetic tubes (R.H. Plaut) Appendix 5.4: Dewatering sewage sludge with geotextile tubes (J. Fowler, R.M. Bagby and E. Trainer) 6 GEOCONTAINERS 6.1 General 6. l. l lntroduction 6. l.2 Container systm 6.1.3 Overview of the dumping process and some design considerations 6.2 Opening of the barge 6.2.l Introduction 6.2.2 Required circumference of the geotextile 6.2.3 Tension in the geotextile when the barge stans to open 6.2.3. l Containerfilled with slurry 6.2.3.2 Container filled with sand 6.2.4 Tension in geotextile during the fall of a geocontainer through the opening 6.2.4. l literature 6.2.4.2 Possible improvements 6.2.4.3 Failure mechanism 6.2.4.4 Lower pan of geocontainer passing through opening (stage Ill) 6.2.4.5 Failure of sand (stan of stage IV) 6.2.5 Tension just before the container leaves the barge 6.2.6 Analysis of Fowler's article 6.2.7 Influence of the length of the circumference

323 323 323 323 324 330 330 331 333 336 336 336

340

341 342 342 344

346

349 349 352 353 355 359

364

366

372 392 399 414 421 421 421 422 424 426 426 427 429 429 430 432 432 433 434 435 437 438 440 442

X Auteursroc11teh1k beschermd matena,,J

6.2.8 OJnclusions 6.3 Loading during impact 6.3.1 Dumping vdocity 6.3.l .1 Theory 6.3.1.2 ACZ aperiments 6.3.2 Stress and strain in the gt.otextile during impact geoconlainer filled wilh slurry 6.3.2.l Palmerton's calculalions 6.3.2.2 Analytical calculalions 6.3.2.3 The effect of pleasts and seams (Adel, 1996) 6.3.3 Example calculalions of impact 6.3.4 DefonnaJion during impact of geocontainer filled with sand 6.3.5 Influence of air 6.3.5. l Air in the fill during dumping 6.3.5.2 lnfl/U!11ll of air in fill on defonnaJion of container during impact 6.3.5.3 Stress in geoteJCJile during dumping due to air 6.3.6 Impact not parallel to the sea bottom 6.3.7 Influence of subsoil 6.3.7.l Soft subsoil 6.3.7.2 Stress and strain in geotextile due to bumps in the sea bed 6.4 Shape geocolllainer after dumping (Leshchioslcy approach) 6.4.l Timoshenko's method 6.4.2 Extensions 6.4.2.l Elastic foundalion 6.4.3 Slwrtcomings 6.4.4 Mechanismsfor dry sand 6.4.5 Mechanism/or wet sand 6.4.6 OJlculations compared with measuremenis 6.4.6.1 Dry sand 6.4.6.2 Wet sand 6.5 Defonnations due to lateral forces and wave attack 6.5.l DefonnaJions in container due to lateral forces 6.5.2 Stability under wave attack 6.6 Scaling rules 6.6.1 Geometrical scaling 6.6.2 Velocity, time, stresses and forces 6.6.3 Different scaling rules? 6. 7 Overall design and applications 6.7.l Design concepts 6. 7 .2 Summary of the dumping process and comment on the design rules 6.7.3 Structural components and design aspects 6.7.4 Past experience and applications 6.8 Conclusions and recommendations List of main symbols References and bibliography Appendix 6.1: Friction and tensile forces in the geotextiles during the release of the geocontainer (K. Pilarczyk) Appendix 6.2: Overview of geocontainer projects in the United States (J. Fowler and E. Trainer) Appendix 6.3: A 4000cy geotextile container filled with maintenance dredged material, Port Authority of New York & New Jersy,(J. Fowler and D. Toups) Appendix 6.4: The migration of fines from contaminated sediment through geosynthetic fabric containers utiliz.ed in dredging operations (H.K.. Moo-Young and Cb. E. Ochola)

445 446 446 446 449 452 452 454 457 459 459 461 461 463 465

466

467 467 468 470 470 471 471 474 474 475 475 475 476 479 479 482 483 483 483 484 487 487 495 500 504 507 51O 513 516 520 530 571

XI Aut�ursr1chtqhjk beschermd materiaal

578 7 GEOTEXTILE FORMS FOR SAND STRUCTURES 578 7.1 Introduction 7.2 Construction of steep slopes of sand under water using synthetic fiber screens 579 (Voskamp, 1983) 7.2.I Introduction 579 7.2.2 The ENKA solution 581 582 7.2.3 Execution of the tests 584 7.2.4 Design for application in an artificial island 7.2.5 Installation method 586 1.2.5.1 lnstallation method of the sand island using a fabric 586 7.2.5.2 Installation of the fabric 586 588 1.2.6 Economic evaluation 588 7.2. 7 Conclusions 7.3 Hydrostatically supponed sand structures in offshore engineering (Dowse et al., 1979) 589 589 1.3.1 Introduction 591 7.3.2 Geotechnical principles 7.3.3 C-Onstruction procedure 592 7.3.4 Model and prototype testing 593 595 7.3.5 The Sandisle geotechnical theory reviewed 7.3. 6 Island applications 600 7.3.7 The load-carrying capacity of Sandisle structures 607 7.3.8 Conclusions 608 7.4 Other large bags of special design 608 610 References Appendix 7. I: Hydraulically-filled geomembrane bags for land reclamation 612 (Bridle et al., 1998) 8 SCREENS AND CURTAINS 8. l Introduction 8.2 Floating screens (cunains) 8.2.1 Constructional aspects 8.2.2 Calculation method 8.2.2. I lntroduetion 8.2.2.2 Screen in a steady flow 8.2.2.3 Cables in a steady flow 8.2.2.4 Anchorforces 8.2.2.5 Impulsive force due to unsteadyflow 8.2.2.6 Screen in wave 8.2.2.7 Oscillations and oscillating forces 8.2.2.8 Wind force and wind set-up 8.2.2.9 Density differences andfloating debris 8.2.2.10 Proposed calculation method 8.2.2.11 Example 8.2.3 References for floating screens 8.3 Bottom screens 8.3.1 Constructional aspects 8.3.2 Calculation method 8.3.2.1 Introduction 8.3.2.2 Screen in steady flow 8.3.2.3 Screen in waves 8.3.2.4 Inclined (horizontal) bottom screens 8.3.3 References for bottom screens

619 619 620 620 623 623 624 629 637 638 641 647 647 648 649 655 661 662 662 664 664 665 669 670 676

XII Auteursrec1telqk beschermd matenaal

8.4 Examples of the application of and experience with screens and curtains 8.4.1 A silt screen in the Rotterdam harbour (The Netherlands) 8.4.2 GESEP bottom screens (Belgium) 8.4.3 The Silt Proteaor and Oil Sweeper (Japan) 8.4.4 The ro-boom silt amain (Denmark) 8.4.5 USA examples of screens and cunains 8.4.6 Bottom screens in the Meghna river (Bangladesh) 8.4.7 Field tests with BEROSIN horizontal cunains (Vlieland, Netherlands) 8.4.8 The installation of geotextiles under current conditions (Germany) 8.5 BEROSIN curtains for erosion control 8.6 Anificial seaweed for erosion control and scour prevention 8.6.1 Seaweedfrom a historical perspective 8.6.2 New developments (Cegrass and Seabed Scour Control Systems) 8.7 Floating flexible breakwaters 8.8 Concl.usions List of main symbols References and bibliography

677 677 679 681 685 686 688 689 691 693 697 697 700 708 713 714 716

9 INFLATABLE DAMS 9.1 Introduction 9.2 Principles and definitions 9. 3 Calculation methods 9.3.1 lntrodudion 9.3.2 Analytical solutions 9.3.3 Numerical solutions 9.3.4 Example of an analytical approximation 9.4 Construction aspects and past experience 9.4.1 lntroducJion 9.4.2 lmprovedfabric 9.4.3 Higer dams 9.4.4 A permanent struaure 9.4.5 Operation and maintenance 9.4.6 Design life and durability 9.4.7 Installation 9.5 Flapped (collapsible) weirs 9.5.1 Two-sides retaining weir (double flaps) 9.5.2 One-side retaining weir (singular flap) 9.6 Inflatable offshore sills and breakwaters 9.6.1 Backgrounds of developments 9.6.2 Example ofapplication 9.7 Applications and special features of inflatable systems 9.7. I Applications 9.7.2 Special features and charaaeristics of inflatable systems List of main symbols References and bibliography Appendix 9.1: The large-scale inflatable dam at Ramspol (The Netherlands)

725 725 727 729 729 730 733 735 738 138 738 739 740 74 2 743 743 744 744 749 752 752 155 757 757 758 759 759 764

10 GEOSYNTHETICS IN DAMS, DIKES, BANKS AND DUNE REINFORCEMENT 10.1 lntrod.uction 1O. 2 Geosynthetics in dams and dikes 10.2.1 General 10.2.2 General information on the use andfunctions of geosynthetics

769 769 770 770 771

XIII Auteursrec1tel Jk beschen'ld mater1aal

10.2.3 Geosynthetics and dike improvemmJ 10.2.3.1 Applications 10.2.3.2 Design aspeas 10.2.4 Basic calculaJion method/or geotextile-stabilizedfolllldaJions 10.2.5 Steep slope reinforcemmJ 10.2.6 Case Study by (CUR/RWSINGO, 1996) 10.3 Geosystems in dike construction and emergency measures 10.4 Alternative dune a nd bank reinforcement References Appendix 10.1: Stabilization of coastal slopes by anchored geo.cyntbctic system (Ghiassian et al., 1997) 11 EROSION-CONTROL SYSTEMS 11.1 Introduction 11.2 Grass covers and reinforcement measures 11.2.1 General 11.2.2 Maintenance 11.2.3 Design 11.2.4 Constructive aspectS 11.3 Short overview of erosion conttol materials and systems 11.3.1 General 11.3.2 Geotextiles as tilter and reinforcement of vegetation 11.3.3 Composite mats for reinforcei11CJ1t of soil and/or vegetation or as protective blankets 11.3.4 Open-cell systems 11.3.4.1 Introduction 11.3.4.2 Design aspectS 11.3.5 Open block mats and sand- or conc.rete-ftlled mattresses 11.3.6 Erosion-control design software (Sprague. 1997) 11.4 Geotextile silt fences 11.5 Conclusions References Appendix 11.1: The reduction of soil erosion by pre-formed systems (P.R. Rankilor) Appendix 11.2: Armater; product information and calculation guide (AKZO/W. Gevers) Appendix 11.3: Three dimensional synthetic mats in dike and bank protection (J.A. van Herpen)

115 115

m

118 783 786 789 794 799 801 820 820 821 821 823 827 830 832 832 833 838 840 840 842 849 853 855 856 857 860 872 882

12 REMAINING ASPECTS AND CLOSING REMARKS; durability, execution and damage, and quality control 12.1 Durability 12.2 Execution and damage 12.3 Quality assurance 12.3.1 Cutijication 12.3.2 Quality assessment 12.3.3 Practical and pe,:formance tests 12.3.4 Termsqfreferencelbtdldingspecifications 12.4 Closing remarb References

895 895 896 897 897 898 899 899 900 901

PROFESSIONAL HELP AND USEFUL ADDRESSES

903

INDEX

911 XIV

Aut,,ursr'Chtel jk bescr.ermd 111ater1a I

Units and Conversion Factors English Customary to Metric Units

Multiply

by

To obtain

25,4 25.4 0.3048 0.0929 0.0283 0.9144 0.836 0.7646 3.785 4.546 l.6093 259.0 0.4047 l.8532 knots foot-pounds l.3558 millibars l.0197•10-3 28.35 ounces pounds (lb) 0.4536 pound-force/ft 14,6 pli, pound-force/inch 175 psi, pound-force/incb2 6.894 pound-mass/ft3 16.018 degrees (angle) 0.1745 (5/9\

2:

Q. = 0.2 exp(-2.3 R,,)

(2.14)

with Q0 and R,, as defined in Equations 2.10 and 2.11.

Wave overtopping per wave. The average overtopping discharge does not provide information on the amount of water of a given overtopping wave passing the crest. The overtopping volumes of individual waves deviate considerably from the average dischar­ ge. By means of the average overtopping discharge the probability distribution function of the overtopping discharges can be computed. To give an impression of the relation between the average overtopping discharge q and the expected value of the maximum vol­ ume i n the largest overtopping wave Vmu• this relation is given for two situations in Figure 2.14. 100.000

lfflI

�

-t C.

•

0

.s"'

10.000

0

.s

H. s2.5m

w"

-

0

1.000

.,

E E

.,

� �

100

-

.

,,,

H, =lm

-

ton a - 1/4 = 0.04 SOIi during 1 hour

E 10

0.1

11

100

I[

1000

overage overlapping discharge q (1/s per m)

Figure 2.14 The relation between the average overtopping discharge and the 111Mimum volume of the highest overtopping wave

29 Auteursrec1telqk beschermd matenaal

Conditions are for a storm with a duration of 1 hour, a slope gradient of 1:4 and a wave steepness of s.,,, = 0.04 with a T/Tm ratio of 1. 15. Relations have been drawn for a wave height of H, = I m and 2.5 m. For small average overtopping discharges the ratio Vm,,/q is of the order of 1000 and for large average overtopping discharges of the order of 100. To get an indication of the instantaneous discharge during the passage of one wave, the maximum volume in an overtopping wave should be divided by an effective fraction of the wave period. This can be roughly approximated by (0.3 to 0.4) T and provides an average value of a maximum discharge. This figure can be applied as an input into the stability criteria for the protection of the splash area and the inner slope (i.e. applying the criterion by Knauss, 1979). More information on wave overtopping volumes per wave can be found i n (Pilarczyk, 1990 and Van der Meer and Janssen, 1994). There are no generally valid recommendations for acceptable levels of overtopping for seawalls and/or dikes. In standard Dutch practice a safe value of about 0.002 m3/s for a grassed crest and rear slope is recommended. Recent experience indicates that this value can be increased to 0.005 or even to 0.01 m3/s for a 'good' quality grassmat on a clay sublayer. Information on a proper clay specification for a grassmat can be found i n the guidelines (CUR/TAW, 1991). Fukuda, et al. (1974) suggest the following figures, based on field observations, on allowable overtopping related to inconvenience for persons or vehicles located 3 m behind the breakwater:

= 4. I0-65 m33/mls: inconvenience for pedestrians, and q = 3. 10· m /mls: danger for people and traffic. q

More information can be found in CUR/CIRIA (1991) and CUR/RWS (1995). The design aspects on protection against overtopping are discussed in Chapter 4. c) Wave transmission

Structures such as breakwaters constructed with low crest levels will transmit wave energy into the area behind the breakwater. The transmission performance of low-crested breakwaters is dependent upon the geometry of the structure, principally the crest freeboard, R,,, the crest width, Be and the water depth, h, but also the pe.nneability, P, and on the wave conditions, principally the wave period (commonly contained in the surf similarity parameter, t). Various hydraulic model test results measured for rock structures have been re­ analyzed to provide a single prediction method. This relates the transmission coefficient C, to the relative crest freeboard, RjH.. The data used is plotted in Figure 2.15. The prediction equations describing the data may be summarized as:

Range of validily -2.00 < R/H, < -1.13 -1.13 < R/H, < 1.2 1.2 < R/H, < 2.0

Equation

ct = o.so C,

= 0.46 - 0.3 RjH,

C, = 0.10

(2.15a) (2.15b) (2.15c)

30 Auteursroc•1teh1k beschermd matena,,I

1.0

-

0.8

..

·o 0.6 :i: 0

.E�

• 3• • .v

•• • --

••

A

0

0

.�

•

'

"·,.

..

•

C

.fl!

--"

• 0 0 �

0.4

A

' " .,·• · ••

/lo A

• • A

�

i"

0.2

'\.

• •• • �•

� 6 I>

0

0

0

.

•

A

•

0

'\.o o

• •

•

0

--''-----'------''-----'--�'-----'--__,J 0 L- --'0 -1.0 1.0 2.0 -2.0

relative crest height R/Hmo or R/H,

Figure 2. IS Wave transmission over and through low-crested structures The equations give a very simplistic description of the data available, but will often be sufficient for a preliminary estimate of performance. A few remarks can be made about Figure 2.15. The points with R/H,>l and C, > 0.15 are caused by a low wave height relative to the stone diameter {H/Dn50 "" 1). The low wave travels simply through the crest which consists of armour stones. Transmission coefficients of 0.5 can be found in such cases. However, a structure under design conditions (with regard to stability) with R/H, > 1 will always show transmission coefficients smaller than 0.1. Furthermore, it should be noted that physical limits are C,= 1 and C,=0, for freeboards R/H. < >2 respectively, although some transmission may remain even for R/H.>2 due to transmission through structures with a sufficiently permeable core.

2.4.2 The load - strength concept Once the hydraulic design conditions have been established, actual design loads have to be formulated. For a given structure many different modes of failure may be distinguished, each with a different critical loading condition. For a structure as a whole, instability may occur due to failure of the subsoil, or the front or rear slope. Each of these failure modes may be induced by geotechnical or hydrodynamical phenomena. The present section is restricted to the stability of the front slope. Moreover, only instability as a result of hydrodynamical processes has been taken into account. Starting with the hydraulic input (waves, water levels) and the description of the structure, external pressures on the seaward slope are determined. Together with the internal characteristics of the structure (porosity of the revetment and secondary layers) 31 Aut�ursr1chtqhjk beschermd materiaal

these pressures result in an internal flow field with corresponding internal pressures. The resultant load on the revetment has to be compared with the structural strength, which can be mobilized to resist these loads. If this strength is inadequate, the revetment will deform and may ultimately fail (Figure 2.16).

UI

I

�P.,(y.1)

1\nt (.)', 1)

Uydr•ulic CoDditiom

Ovu,aU

Hydraulic

Co•d.itioa.s lls,U

O.d/Slope Geometry

Trusfa-

Fuoctioo I

...

11

Ex1enal

Surfxe

l'u (y, I) Hydraulic

l'ropcrti .. ol Structure

_, Soil

--

l-lydnulic

Tn.nsfu

Fum:tio■ II

...

...

Coodilioos

al loluoal Surface Pim (y, t)

Mechanieol Propcrtiu

olSlruc"""

...

Tnnd'er Fuoclioa

Ill

...

Rupoose ol

s1rue:.,,.

-

Figure 2.16 System approach. Transfer functions.

The phenomena which may be relevant can be divided roughly according to the three components of the system: water, soil and structure. The interaction between these com.ponents can be described using three Transfer Functions (see Figure 2.16): I. The Transfer Function from the overall hydraulic conditions, e.g. wave height H or mean current velocity U, to the hydraulic conditions along the external surface, i.e. the boundary between free water and the protection or soil, e.g. external pressure P. II. The Transfer Function from the hydraulic conditions along the externa.1 surface to those along the internal surface, i.e. the boundary between protection and soil. The hydraulic conditions along the internal surface can be described as the internal pressure P.

ill. The structural response of the protection to the loads along both surfaces. Information about these functions can be obtained by means of measurements in nature and (scale) model tests. If quantitative knowledge of the physical phenomena involved is available, or if there is enough to hand experience, then mathematical models or empirical formulae containing information are formulated and referred to as "models". All three Transfer Functions can be described in one model, or individually in three separate models, depending on the type of structure and the loading. The distinction between the three functions here mainly serves as a framework to describe the different phenomena that are important for the modelling. 32 A I ursr,,cntel jk bescherrnd materiaal

In many cases, the various processes cannot be described as yet. Therefore a "black box• approach is followed i n which the relation between critical strength parameters, structural characteristics and hydraulic parameters are obtained empirically. P�ibilities to control the hydraulic loads and strength. The primary function of any slope protection, flexible or not, is to protect the edge of the land and water against hydraulic loads by waves, tides and currents. The detennination of the hydraulic design conditions is the result of a quantification of the local conditions in combination with a certain level of safety. This way, the design conditions are defined and presented in the fonn of a water level, a wave height and a wave period, and are usually completed with some expectation regarding the fonn of the energy density spectrum and sometimes even completed with an estimate for the duration of the selected design cond.ition. However, the fact that the design conditions are fixed does not at all mean that the loads on the slope revetment structure are also fixed. Within certain limits, of course, it is possible for the designer to influence and consequently to choose the size, the sort and the place of attack of the hydraulic loads, by a proper selection of the geometry, layout and materials for the structure. In Chapter 4 the mathematical fonnulae for the calculation of external and internal hydraulic loads will be discussed. The parameters in the formulae can be manipulated by the designer to control the performance and effectiveness of structures. For example, by changing the slope steepness the breaker type of waves can be influenced. The value of the breaker index (Eq. 2.1): tan a

�=- - ­½ (Hof¼)

is not only decisive for the type of breaker but also for the levels of run-up and ru n - ­ down, and the stability of the protective units. For a given value of the wave steepness HJ'-o the value of � increases with increasing slope steepness. The type of breaker itself determines the way a breaker exerts loads on a slope and thus on a slope revetment. This can be with huge wave impact or, on the contrary, with large masses of water running up and down the slope. Because the levels of wave run-up and run-down are also influenced by the value of �, the slope steepness detennines the required crest elevation and the level where the maximum wave impact takes place and the level where other damage mechan­ isms endanger the structure's stability. It is therefore essential to realize that it is possible to choose the critical damage mechanism by manipulating the slope steepness and by applying benns in the slope. In Chapter 4 these phenomena will be presented in a number of mathematical fonnulae. Especially for rubble slopes, or other types of randomly placed slope prote­ ctions, the influence of the combination of slope steepness and wave steepness is difficult to establish on the basis of a physical description of the phenomena, because for these types of structures it is the combination of parallel and perpendicular flow on and in the cover layer that determines the cover layer stability (Figure 2.17). The stability of these types of structures must therefore be determined by empirical formulae or model experiments. For instance, for block revetments and geomattresses, by varying the cover layer permeability it is possible to focus the hydraulic loads on certain parts of the revetment structure and to relieve other parts. So the design can be optimized for the locally availa­ ble construction materials and techniques. For example, a more impermeable cover layer leads to fairly limited pressure variations in the sublayers, even during large pressure 33 Auteursrec1tel Jk beschen'ld mater1aal

variations on the outside of the revetment. Consequently, the internal stability can be secured easily but the stability of the cover layer is severely jeopardized in this situation by the resulting upward loads on the cover layer from the filter layers at the moment of maximum wave run-down. With a very permeable(= open) cover layer, however, the hydraulic gradients across the cover layer will always be small, even under severe wave attack, but in this situation the loads on the sublayers are large because hardly any damping occurs through the cover layer. In this situation a real danger also exists as to the erosion of filter material through the cover layer. erosion filling and filter material extemal gradients

andpr=u=

intemal st!.bility interfaces (with influence of geotextile)

cover layer

filter

I I /

I

)

sand or day

I..._

- - - ---

.,,.. .,,.. .,,..

/

Figure 2.17 Definition sketch for the components of loads and strocture

Hence the designer should pay special attention to the design of filter layers and internal interfaces. In Chapter 4 the formulae for the calculation of the underlying physical processes are presented. It appears that the ratio of the cover layer and sublayer p e r ­ meabilities k/k, and the geometry of the structure determine the value of the leakage factor A, which in turn controls the pressure diffe.rence over the cover layer b and the internal hydraulic gradients i. Application of a very thin granular filter layer underneath the cover layer reduces the upward hydraulic gradients over the cover layer and consequently leads to a reduction of the required weight of the cover layer. However, the loads on the base material (sand or clay) will increase. The examples above, which all refer to placed block revetments and geomattresses, are merely presented to illustrate the fact that no strict procedure can be given for the determination of the external or internal geometry of the structure to cover all practical situations. During the process of designing a slope revetment, numerous, more or less 34 Aut�ursr1chtqhjk beschermd materiaal

subjective, choices must be made upon adequate consideration of the price of construction materials, locally applicable construction techniques, technical restrictions, functional requirements and personal preferences. The external hydraulic loading can also be diminished or spread by special (sub­ merged) guiding structures, the construction of artificial reefs/bars in front of the main structure, by applying berms/trapezoidal shapes o f structures, etc. It will be clear now that the possibilities to control the external and internal hydraulic loads by variations in the geometry of the structure are numerous. It seems somewhat illogical, but the possibilities for a designer to control the strength of a structure (i.e. revetment) are by far more Limited than those for manipulating the loads. On the other hand, it should be mentioned that the change of a construction detail often influences both strength and loads. With respect to the sublayers, the choice o f the granular materials should be such that the material itself and the internal interfaces are sufficiently stable under design conditions and will remain stable during the design lifetime of the structure. A geotextile sublayer should be selected for the purpose it must serve and should be (and stay) sufficiently strong to resist the tearing and punching forces during construction and operational use. &pecially the risk of possible clogging of the geotextile should be avoided. For the cover layer there are in fact only two possibilities to increase the strength: • by increasing weight of the element (thickness, specific density), and • by increasing the capabilities of cooperation between individual elements in such a way that perpendicular forces and moments can be absorbed (interlocking, cables, clamping). By manipulating the strength in one of these ways the designer should be aware that improvement o f only the strength of a cover layer without taking the rest of the structure into consideration will usually solve half of the problem only; another constituent of the structure, for example geotechnical/internal stability, may now be the weakest. A more detailed description of possible failure modes is given in the next section.

2.5. PRINCIPAL FAILURE MODES AND FAULT TREE ANALYSIS Failures can occur during both construction and operation. Typical loadings and responses for loose protective units are wave height and displacement relative to the original placement position. Both loading and response are functions of time. The response is determined by the rock system characteristics such as weight and shape. Also, the loading may, to a certain extent, be affected by the system, for example through permeability. The loadings are mainly determined by the (hydraulic and geotechnic) boundary condi­ tions. The description of the physical boundary conditions and their use as loading descriptors in the design formulae for various types of structures (interactions and responses) is treated extensively in (CUR/TAW, 1991, CUR/RWS, 1995).

General reasons for failure. Before proceeding with the various "physical" failure modes

to be discussed below, some overall failure m.odes are mentioned. One should realize that, no matter which failure mode is considered, the design as well as the construction may be based upon wrong data. If so, the reason for that can usually be traced back to:

35

Aut�ursr1chtqhjk beschermd materiaal

-

Insufficient data (quantity, choice of period of measurement); Errors in measurements (instruments, calibration, reference values); Improper data handling and processing (analyses of extrem.e s and correlations); Modelling (wrong input data and/or boundary conditions, unqualified users, selection of an unsuitable model, insufficient calibration or verification).

Another source of failure is wrong construction. This can also occur wi. th a properly designed structure and is mainly due to either one or a combination of unqualified contractors, or the lack of or insufficient implementation of quality systems. Failure occurs when the response exceeds a value, which relates to the functional requirements of the structure. In practice, failure thus corresponds to unacceptable displacements and/or deformations associated with a certain defined loading: the failure loading. In general, failure mechanisms (or failure modes) are named after their resulting displacements or movements and the common characteristic i s a relatively large increase of response (e.g. stone transport) due to a minor increase i n loading (e.g. wave height) . An overview of generally applicable principal failure m.echanisms with the corresponding loadings is given in Figure 2.18 (the marin e t-ype concepts are only used for the purpose of illustration). Unfortunately, only few of the failure modes given i n Figure 2.18 can, at the present state of the art, be properly described in terms of well-defined limit states in terms of loads and responses. The failure modes which can at present be modelled to a certain extent will be described below. For each mode the characteristic loadings, the principal and secondary load.ing parameters, the governing system characteristics and the resulting responses are given first (CUR/RWS, 1995). This is followed by a brief description of the failure mechanisms. A summary of the failure mechanisms and their characteristic parameters is given in Table 2.3.

overtopplng

erosion outer slope

wave overtopping

slip circle inner slope

�quetaction

micro instabmty

driftlng ice

�

'pip ing'

ship collision

Figure 2.18 Typical failure modes of a hydraulic structure

36 A I ursr,,cntel jk bescherrnd materiaal

Table 2.3 Failure mechanisms and characteristic parameters Mechanism

Loading

(Principal)

loading

System

Response

parameters

characteristics

characteristics

Settlement

Weight

Specific density of materlaLr: saturation degree; pore water pressure; time

Soil compressibility: soil permeability; layer thicknesses

Crest lowering Horizontal deformations

Movement of rock cover

Waves Currenl

Wave height: wave period; angle of incidence; time velocities: turbulence strength; velocities; thickness

Stone diameter and density; permeabi-

Rocking; sliding lifting; rolling

Jee

lily

Migration o f sublayers and/ or filters

(Tidal) waves; shipinduced water movements; other dropping water levels

Hydraulic gradients: internal flow velocities

Layer permeabilities and thicknesses: grain sizes

Internal material tran.,;port rate

Piping

Hydraulic gradient

Internal channel flow velocities

Pipe (internal channel) lengdi; hydraulic resistance; grain size

Internal material transport rate

Sliding of structure

Weight of structure or suucture elements

Weight of

Friction angle, cohesion and permeability of soil/ core and cover layer(s)

Sliding of (a significant pan of) die structure; collapse

Scour

Waves; currents

Orbital arul current velocities; turbulence intensity

Sediment grain size; structure slope; permeability of structure

Degradation of seabed in front of structure

Liquefaction

Waves; earthquakes

Wave height and period; pore-water pressures; (relative) shear stress, ampli-

Permeability; compaction; thickness of layers; friction angles

Serious deformation of strucrure·• collapse

con.,t;truction

materials; pore-water pressures (influenced by wave heighl/period); slope angle

tude;

acceleration; frequency; number of loading cycles

a. settlement (and heave)

Loading(s): weight. Loading parameters (principal): specific density of materials. Loading parameters (secondary): pore(water) pressures, time. System characteristics: soil compressibility and penneability, thickness of compressible layers. Response(s): crest lowering and horizontal deformations. 37 Auteursrec1telqk beschermd matenaal

The weight of a structure causes an extra load on the subsoil, which may then be compacted or squeezed, either instantaneously or (for low-permeability compressible layers) retarded. In addition, the structure itself may become densified during construction or during the first stages of its operation. As a consequence of all the above processes, the crest level settles and the structure's capability to limit ovenopping under conditions of high water levels and wave attack is reduced. Differential settlements lead to uneven surfaces which make some rocks more susceptible to being washed out and also to undermining of the suppon for crest struc­ tures. For submerged structures, however, settlement often leads to increases in armour stability. A deformation mechanism opposite to settlement is heave, which may be caused by the expansion of ice crystals within the soil mass (frost heave) or by soils that swell because of taking up water after the confining pressures have been re.moved.

b. movement of cover layer elements Loading(s): waves, currents. Loading parameters (principal): wave height and period, velocity. Loading parameters (secondary): time, angle of incidence. System characteristics: stone diameter and density, permeability, slope angle. Response(s): rocking, sliding (see mechanism e.), lifting, rolling. Waves and currents determine the lift and drag forces acting on the stones of the cover layer. The inenial forces are also determined by the stone characteristics. The stone weight, but also forces due to friction and interlocking with other stones, are the stabilizing forces. The dynamic (loss ot) balance of all these forces may result in a great variety of the above-mentioned stone movements. These responses may be allowed for in the design, but care should be taken to avoid responses large enough to initiate other failure modes such as damage of the filter layer.

migration of the sublayer or the core material Loading(s): hydraulic gradients, internal flow. Loading parameters (principal): water pressures and velocities. Loading parameters (secondary): System characteristics: layer permeabilities and thicknesses, grain size. Response(s): material transpon out of structure. c.

'

Due to a difference in water !eve.I or due to local (gene.ration ot) excess pore water pressures, an internal flow may be established. When a certain critical hydraulic gradient and the corresponding flow velocities occur, the finer grains are transponed out of the inner layers through the coarser material of the upper layers. Often these finer grains can thus easily also pass through the cover layer, resulting in a loss of mate.rial from the filter layer and/or from the core. The results are settlement and/or deformation (here settlement is not an initiating (sub.a) but a secondary mechanism). d. piping

Loading(s): hydraulic gradient. Loading parameters (principal): head difference, water velocities and pressures. System characteristics: grain size, seepage length or length of "pipe". Response(s): material transpon out of structure.

38 Auteursrec1telqk beschermd matenaal

Piping refers to the formation of stable open channels with a concentrated flow in a granular skeleton. This phenomenon occurs preferentially at structural interfaces, such as at boundaries between permeable (e.g. sand) and less permeable (e.g. clay) materials or where loosely packed and densely packed granular materials adjoin one another. S u sce p ­ tible also can be (local) parts of the soil which are relatively permeable or subject to a particularly high hydraulic gradient. In either case, a channel may be formed and a concentrated seepage flow is established. Depending on its grain size, the erodible soil (e.g. sand) may migrate through the channel. The channel may become longer due to erosion at its ends, eventually finding a way out at the low head side of the structure. In the case of lateral erosion, the width (or the 'diameter of the pipe') also increases. The eroded material (e.g. sand) can be transported out of the system. Progressive internal erosion (mechanism c.) may occur if the flow resistance decreases as the pipe diameter increases. The eventual consequences of the loss of volume are settlement (mechanism a.) or even the collapse of the structure.

e. sliding of (parts ot) the structure

Loading(s): weight of structure; waves Loading parameters (principal): density of building materials. Loading parameters (secondary): pore-water pressures (influenced by wave height/period); hydraulic gradients (e.g. due to internal set-up); slope angle. System characteristics: soil or interfacial friction angle and cohesion. Response(s): sliding of (a significant part of) the structure, collapse. The stability of a rock slope is determined by the slope angle, specific weight, pore pressures and by internal friction and cohesion (interlocking). Horizontal accelerations are also important during earthquakes or wave shock loading. Sliding can occur anywhere along (local) failure planes in the structure and/or subsoil where the effective shear resistance is not sufficient, but preferentially along interfaces between different materials (e.g. annour and undedayer/geotextile) because here the local friction is reduced. Sliding of an entire structure (river bank, breakwater), often including subsoil, is referred to as overall stability. The subsoil also takes part in supporting the structure and in the generation of excess pore pressures and liquefaction in any fine layers beneath rock structures, which may be important for toe stability and slope support. Excess pore pressures can be caused by dynamic loading or by rapid fall of the external water level. Crest structures may also move (slide) under wave loading when the friction between the structure and the underly­ ing rock is not sufficient. Local sliding of a toe structure but also overall sliding can be initiated by excessive scour development (see f . below).

f. scour and erosion

Loading(s): waves, currents. Loading parameters (principal): orbital and current velocity, turbulence intensity. Loading parameters (secondary): wave period, angle of incidence, time. System characteristics: sediment grain size, structure slope, stone size, Response(s): degradation of seabed or riverbed adjacent to the structure. Waves and currents cause water movements near the seabed, which may generate a sediment transport. Interactions with the structure (wave reflection, currents, generation of turbulence) may affect the natural sediment transport of bed or beach materials.

39 Aut,,ursr'Chtel jk bescr.ermd 111ater1a I

Compared to the natural sediments, most structures can be considered to be rigid and non-erodable, although some may be permeable to sediment and thus impose a physical boundary cond.ition on the transport processes. Local scour may lead to slopes, which will, provided they are sufficiently steep, initiate sliding (see under e. above). Erosion of unprotected under-layers and/or core material due to the malfunctioning of the cover layer is a m.echanism which may result from damage to the cover layer. g. liquefaction Loading(s): pore water pressures, waves, earthquakes. Loading parameters (principal): pore water pressure, wave height and period, (relative) shear amplitude, acceleration, frequency. Loading parameters (secondary): number of waves/loading cycles, degree of compaction. System characteristics: thickness of compressible layers. Response(s): serious deformations of structure, collapse. Cyclic loadings generate excess pore water pressures when the deformations resulting from the loading cause compaction at the same time as the drainage capacity for dissipa­ tion of the resulting increases in pore pressure is low. This implies that loosely packed granul.ar soils are most susceptible. Liquefaction refers to a situation in (fme) granular materials where excess pore pressures are generated to such a degree that intergranular contact is lost. The whole medium then loses all its shear strength and behaves like a thick flu.id. Under these circumstances any shear loading causes sliding or stability fa.ilure. Earthquakes are a known dynamic cause of Liquefaction. b. ship collision Loading(s): impact force of sailing ship; Loading parameters: sailing velocity and ship characteristics; System characteristics and response(s): see mechanism under b. Collisions in general may be regarded as special events (discussed in the following section) causing local initial damage to the cover layer. i. material deterioration Loading(s): temperature variations, radiation (UV). Loading parameters (principal): range and rate of temperature fluctuations, solar intensity. Loading parameters (secondary): number of cycles, duration of exposure. System characteristics: material characteristics and thickness. Response(s): breakage, reduction of stone size, change of layer permeability. Material deterioration is mainly a reduction of strength. Vermin attack may be regarded as a special case of deterioration. Such types of strength reduction or fatigue should typically be subjected to an inspection and maintenance scheme. j. other mechanisms Besides the principal mechanisms discussed, a variety of other mechanisms can be distinguished. Some only differ from the principal mechanisms by the specific location on the structure, for instance at transitions. Vandalism or theft are other mechanisms which may have to be considered. 40 A I ursr,,cntel jk bescherrnd materiaal

rmal remark

A failure mechanism may be accepted, depending on: - the possibility of (emergency) repair; - whether the mechanism is destructive or only leads to the loss of functions. In general, rapid failure mechanisms, such as sliding, do not allow any repair or migitative measures. Scour, wave-induced stone movements and vandalism usually show a more gradual failure process. On the other band, the time available for measures may be limited to, for example, the period between two tides, storms (sea) or flood waves (rivers).

Fault or event tree. A structure is generalJy planned to fulfil its .functions for a certain prescribed period of time normally called the lifetime of the structure. When its principal functions can no longer be fulfilled, the structure can be said to have failed. Such failures may result from structure degradation (alJowed for in the design}, increased or excessive loadings, or a design fault. Failure is certainly not restricted to co.mpleted structures, but may also be defined during construction. Also, interruptions, damage, delays or cost overruns during design and/or construction stages may to a certain extent be defined as "failures". Failure is ultimately caused by events associated with a failure 1nechanism. Referring to the above and to the description of damage given in the next section, an event in the context of failure of a structure takes place during a storm, a flood wave or even a whole season. The acceptance of such events is determined by the associated risks (expressed either monetarily or otherwise). It may be practical to distinguish "special" events. Special events represent a large risk but with respect to the "loading" when they occur there exists only limited uncertainty, which is reflected by a narrow (spike-type) probabil­ ity density curve. Non-special events (which might be distinguished from the above as "normal" events) are characterized by a wider probability density function, often approximated with a normal distribution, which facilitates the use of probabilistic methods. These events comprise the common pure technical mechanisms attributed to hydraulic and geotecbnical loadings. Special events are often related to human factors (errors), equipment (failure), or organisational, political and sociological (strikes) conditions. Examples are environmental or political problems, which can cause interruptions during the planning or design stage but also during construction. Other examples are changes i n the design, especially when the design is not finished, but construction has already started; these may cause consider ­ able delays. In order to find alJ possible relevant mechanisms that might lead to the failure of a structure, a fault tree analysis must be carried out (Figure 2.19). A fault or failure tree is a logical diagram showing all the (partial) failure mechanisms as separate branches (or roots) of the tree that might either cause or contribute to the failure of the structure (the trunk of the tree). Similarly, event trees are used to show in a logic diagram how a structure may function (with complementary probabilities with respect to the correspon­ ding failure tree). Failure does not necessarily imply a total collapse or destruction of the structure. Some reduced level of functioning and/or a certain residual strength may remain after failure.

41 Auteursr0clitel1jk bescherrid matenaal

b.uman

..

... · ts of God"

explosion sabottte

I failure dile i section 1 �J • I

'

ac

I ovuflow I

,

' I

/

/

'

inundation

r ...,_ .

I

failure dike on2

erOSiOD

I inner slo&M II !slide DianeI I I

nood dile > kvel be�

I ovuto

wave > Slope run .up stabilit)

r

-- � -

•

failure dike sect.ion N '

generally: failure I lload > sire .

UOSIOD.

ootcr slooe revcuueo, failure

I

wave > rcvdmc. nt altacl strength

I

inle11111J erosion I p,pmg ' '

etc.

slope water > pressure stabilily

Figure 2 .19 Example of a fault tree

Definition of damage. Damage must be propedy defined before the design process can proceed and allow, for instance, for different structural concepts to be compared. In general, damage is defined as a certain change of the state of the structure. The state of a structure is reflected by the following three characteristics: I) the external boundaries or contours of the structure; 2) typical cross-sections of the structure and their configuration; 3) the integrity of constituent elements (e.g. rocks, crown wall). Changes of the types 1) and 2) often correspond to a certain physical loss from or displacement of the material of the structure, resulting in a loss of functions. Often such damage can easily be observed or measured by setting up an efficient monitoring programme. In practice, a gradual loss of functions will be observed when damage increases. Failure can be defined as corresponding to an ultimate degree of damage which relates to an unacceptable loss of functions. This point is usually reached after a period of time, depending on the evolution of the damage. At this point functional requirements (e.g. wave transmission, scour protection) can no longer be met and the structure is said to have failed. For example, the degradation of a breakwater crest may lead to the exceeding of a critical value of wave disturbance in the harbour basin behind. The resu.lting downtime in ship handling may reduce the 'economy' of the harbour to below the projected level, so the structure fails.

42 Auteursroc11teh1k beschermd matena,,J

STRUC T structure

i­

z UJ

systems response

:£ ;z

soil

water

0

�

GE.0 general approach

s1Rucr

1-

z UJ � z 0 Cl:

llydraullc loldlno

(ocavn Ii currental

GE protection systems Figure 2.20 Principles of the integrated approach

43 Aut'3ursr"chtel Jk 'Jeschermd matena"I

By making clear definitions of required functions and damage , failure is related to a certain damage level. The latter can, for example, be practically expressed in terms o f displacement of armour stone, in deformation or in settlement. Eventually, the physical relationship between damage and loss of functions and -on the other hand- loading, can be used to link failure to a certain loading level, for example, expressed as wave height or wave-induced pressure. Practical dimensionless damage and loading parameters for revetment structures are described in Chapter 4. As for hydraulic stability, a measure for damage contains a number or a volume of displaced units, while the loadings appear as a wave height or head (H) or as a current velocity (U). Rock under wave attack, for example, can be designed using the damage volume (SJ and the loading parameter (H/.6.D�. When a {sudden) progressive increase of damage as a function of the loading level cannot be observed, the point of failure is, for practical reasons, supposed to be reached at one particular degree of damage (CUR/CIRIA, 1991, CUR/RWS, 1995). Failure can occur to parts of a structure and to an entire structure (e.g. partial failure of an armour layer and total failure of a breakwater due to liquefaction of the subsoil). Partial failure as such is generally regarded as less serious than total failure. Some failure mechanisms can be allowed to occur repetitively up to a certain limit (e.g. the displacement of an armour stone in a dynamically stable rock slope). For other mechanisms, not even a single occurrence can be accepted (e.g. liquefaction of the subsoil under a breakwater). Repetitive occurrences of one mechanism will lead to increasing damage and the frequency of repetition will determine the rate of damage development. Consequently, the damage will not only increase with the loading level but also with time. An important question therefore is whether (at a certain constant loading level) the rate of damage will decrease or increase in time. Note: Failure modes have been discussed very extensively due to the fact that the geosystems (or structures containing geosystems), such as new engineering systems, must/should, for a proper comparison with the more conventional systems, also be analyzed in this integrated way (Figure 2.20). Only by recognizing the strong and weak points of the new systems one can ensure the avoiding of unexpected failures and getting the 'bad product name', the latter which is generally very difficult to restore. REFERENCES CUR/CIRIA, 1991, "Manual on use of rock in coastal engineering". Centre for Civil Engineering Research and Codes (CUR), Gouda, The Netherlands. CUR/TAW, 1991, "Guidelines on design of river dikes", Technical Advisory Committee on Water Defences (TA�. Published by the Centre for Civil Engineering Research and Codes (CUR), Gouda, The Netherlands. CUR/RWS, 1995, Manual on the use of Rock in Hydraulic Engineering, A.A. Balkema Publisher, Rotterdam. Fakuda,N., Uno, T. and Irie, J., 1974, "Field observations of wave overtopping of wave absorbing revetment", Coastal Engineering in Japan, Vol. 17; 117-128. Knauss, J. 1979, "Computation of maximum discharge at overflow rock-fill dams". J3th Congress des Grands Ba"ages, New Delhi, Q50, R.9. PIANC 1987, "Guidelines for the design and construction of flexible revetments incorpor ating geotextiles for inland waterways", PIANC, Suppl. to Bulletin 57, Brussels. 44 Auteursr0clitel1jk bescherrid matenaal

PIANC 1992, "Guidelines for the design and construction of flexible revetments incorpor ating geotextiles in marine environment", PIANC, Suppl. to Bulletin 78/79, Brussels. Pilarczyk, K.W. ed., 1990, "Coastal Protection", Balkema Pub/., Rotterdam. TAW, 1974, " Wave run-up and overtopping", Technical Advisory Committee on Water Defences in The Netherlands, Government Publishing Office, The Hague, The Nether­ lands.

Van der Meer, J.W. and J.P.F.M. Janssen, 1994, Wave run-up and wave ovenopping at dikes and revetments, Delft Hydraulics, Publication no. 485.

45 Auteursrec11teh1k beschermd matena,,J

Manufacturing or geotextlles

46 Auteursrec itehJk beschermd matenc.al

CHAPTER 3

Geosynthetics and geosystems; properties and functions

3.1 OVERVIEW OF MATERIALS AND PRODUCTS The purpose of thi.s Section is to provide a quick orientation and introduction to the application possibilities of geosynthetics. The term geotextiles has already become common in civil engineering. Geotextiles are now incorporated in many engineering designs, and new applications are continuously growing. On the other hand, due to the relatively recent technological developments in the chemistry and textile industry, new synthetic materials and related products have appeared with a number of applications in civil engineering. These new products contain, for instance, such products as geotextiles (woven and nonwoven), geomembranes, geonets, geogrids, geomats, geocells and other geocomposites, and are collectively called geosynthetics. Woven fabrics, nonwovens, geome.mbranes, grids, mattings and composites can be produced from several types of basic materials. Each end product has at least one main functional property; strength., soil tightness, permeability or impermeability. For applicati­ ons in civil engineering projects the most important functions are reinforcement, filtering or separation and screen. Woven fabrics can perform reinforcing functions as well as filter functions. Non­ wovens have a filter and separation function. Geomembranes can form an impermeable screen. Sometimes it becomes evident that geosynthetics, chosen on the basis of a 1nain function, do not meet the requirements for a secondary function. lo this case one can choose another type of geosynthetic which does fulfil the requirements for the main and secondary functions, or one can choose several types of geosynthetics which, each in part and together, meet all requirements. The last choice is called "the separation of func­ tions". In the course of tim.e the properties of geosynthetics may change by ageing, creep, hydrolysis, mechanical damage and chemical and biological attack. Moreover, geosynthe­ tics have finite (limited) dimensions, therefore seams, connections and overlaps are weak spots in applications and may form a limitation to the applicability of the product. Also, negligent execution, scattering of remains and leaching of toxic elements from geosynthe­ tics may have a harmful effect on the environment. Geosynthetics can be grouped according to product and technology or according to functions and applications. The classification systems shown in Figure 3.1 and in Table 3.1 will serve as an introductory guide. The field of geosynthetics and related products is still rapidly developing and provid.iog new alternatives for solving many engineering problems. For a more detailed review of these materials, the reader is referred to specialired textbooks such as those written by Koerner and Welsh (1980), Giroud (1987), (Koerner (1990), and NGO Handbook (Van Santvoort, 1994). 47 A I ursr,,cntel jk bescherrnd materiaal

3. I. I Basic materials, processing and end products Geosynthetics are used for several purposes in civil engineering, especially as a reinforce­ ment, as a filter or separation layer, or as a screen. Depending on the functions to be p e r ­ formed, the properties of geosynthetics must meet different requirements. These proper­ ties depend on the type of basic material and processing technology. Table 3. I Classification of geosyn1be1ics based on their function FUNCTION

PURPOSE

Filtration Drainage Separation Protection (screen) Waterproofing .Erosion control Soil reinforcement Soil stabilization Asphalt reinforcement Deep consolidation

PRODUCTS

Soil particles retention Fluids transpon Avoiding migration and contamination of fine panicles Avoiding construction and longterm damages Pluid barrier Organic soil retention Soil strengthening Base reinforcement Avoiding reflective cracking Speeding up soil consolidation through fast drainage

Geotextiles, geocomposites Geonets, geocomposi1es Geotextiles, geocomposites Geotextiles, geonets, geocomposites Geome1nbranes, geocomposites Geoma1s. biornats. geocells Geogrids, woven geo1extiles Geogrids, woven geote.,uiles Geogrids, geotextiles Venical strip drains

geotextiles and geotextile-related products

one dimensional

I

straps

I

I

cables

geomats

woven

I

yams

mono

filament

tubes

extruded

tapes

chemical bonded

geonets

I

mattresses

geogrids

I

I

thermo bonded

I

bags &

knitted geotextiles

geotextiles

I

I

I

nonwoven

geotextiles

multifilament

three d imenslonal

two d imensional

woven

I

bonded

needle punched

Figure 3.1 Classification of geosynthetics based on technology

48 Aut�ursr1chtqhjk beschermd materiaal

BASIC MATERIALS.

Five main polymers are used in the manufacturing of geosynthetics:

Polyester (PET); Polyp.ropylene (PP); Polyethylene (PE), with the species HDPE ('high density') and LOPE {'low density');

"' "' polymer if 100 years

> 100 years

> 100 years > 100 years

> 100 years

capability against erosion

good

good

good

good

good

strength (kN/m)

?

PET: till 1500 PA: till 800 PP: till 250

till 250

till 800

PET: till 40 others: till 20

strain at break (%)

?

PET: 10-20 PA : 20-30 p p : 10 - 20

10 • 20

20 - 30

PET: 20 - 40 others: 25-75

environmental damage (removal)

good

moderate to good

good

moderate moderate to good

possible up to PP: 130 °C PE: 90 °C

·c

pp

PA and PET: 100 °c PP: 80 •c PE: 60 °C

I) The UV-resistance as mentioned above is sufficient for the installation of geotextiles which in permanent situations will be covered by soil or placed underwater. For geotextiles permanently exposed 10 U V ­ radiation special testing in this respect is recommended (i.e. Xeno-tests). 2) These temperatures can b e regarded as an acceptable (shon-term) increase of the surrounding tempera­ ture at a certain location. These values can also be used as a warning for situations where the duration of such temperatures can be longer than I year. 3) The lifespan of the most geotextiles can be at least 100 years when special additives are added. The lifespan without additives will be much shorter, i.e. 15 to 30 years for polypropylene. Note: Because of the great variation of geotextiles available on the market these values should be treated as an indication only.

51 A I ursr,,cntel jk bescherrnd materiaal

I1 I I I I I I • II I •I • --l . I I

II

I

l--

I [-

.�

a) example of monoftlament woven geotextile structure

b) example of woven geotextile structure of the · tape· type

c) example of geotextile structure of the DOS type c·d1rect1onally oriented structure} with warp knitting with weft Insertion

d) example of a mono-directional extruded geogrid structure

• e) example of a geonet structure

fl example of a woven geogrid structure

g} exarll)le showing the structure of a geocomposlte for drainage

h) example of a geocell w ith honey-comb structure

Figure 3.3 Overview of geosynthetic products

52 AutursrP.cht'lllJk beschermd materiaal

An overview of some end products is presented in Figure 3.3. A summary of the general properties and specifications of geotextiles related to possible design requirements in civil engineering are listed in Table 3.3. More detailed information can be found in various Product Catalogues, and in Koerner (1990). More current information appears in the international journal Geotextiles and Geomembranes, proceedings of the International Geosynthetics Society (IGS), and in Geotechnical Fabrics Repon published by Industrial Fabrics Association International (IFAI) in USA.

3. I .2 Functional applications and propenies of end products When designing civil engineering constructions, the functions to be performed have to be analyzed first; after that, the suitable materials and products can be selected. When geosynthetics are provided, their design and performance, irrespective of their compositi­ on or type, can be determined by identifying the main functions the geosynthetics are required to perform in a given structure. In civil engineering constructions, geotextiles/geosynthetics perform five essential functions either individually or in combination, depending on the applications. These are:

separation, drainage, filtration, reinforcement, and protection.

The first step in evaluating geosynthetic design and performance is to identify the key functions relative to the applications. Table 3.4 identifies these functions for a variety of typical applications. The next step is to identify the factors that will influence or affect geotextile/geosyn­ thetic performance and determine the properties of the geosynthetic required t o withstand these influences (Tables 3.5a,b). Finally, a concise specification on the required functional properties of the geotexti­ le/ geosynthetic and installation and storage procedure is essential to ensure a correct delivery and installation of quality geotextile/geosynthetic on site. Properties such as strength and elongation are derived from the basic materials (polymers) and from the shape of the product (permeability and soil tightness). For instance, strength and stiffness are the two distinctive properties of soil reinforcement with geosynthetics, therefore, it requests a strong, relatively stiff and preferably water­ permeable material. Moreover, the changes in temperature, alteration, creep and damage may have a great effect on the admissible stress. Then a woven fabric of polyester is a logical choice. Hence, only polyester woven fabrics and polyester grids with a high E ­ modulus are suitable as reinforcing material. For a filter or separation function the material has to be flexible, water-permeable and soil-tight. A nonwoven or a lightweight woven fabric of polyethylene is the material to be chosen. A screen or liner function requests a watertight geosyntbetic like a geomembrane of polyethylene. It bas to be noted that in most applications a geosynthetic fulfils a main function and a minor function. For instance, a geosynthetic with a filter function often bas to absorb tensile stresses. The relation between the properties and functional applications of geosynthetics is presented in Tables 3.5.

53 Auteursrec1telqk beschermd matenaal

Table 3.4 Typical fuoclions vs application area

function

" "'

0

"'"' a. "

:.::

typical appllcatlon area

�

C:

·;a �

• • •

unpaved roads, storage yards pave roads. parking areas embankments reinforced soil walls, slopes

"C

0 0 0

•

stone gabion filters earth dam filters coastal, river revetment filters hydraulic fill, redamation works waste landfill dosures

0

watse landfill containments synthetic liner containments tunnel waterproofing railtrack maintenance sport fields, recreational parks geocomposite product systems

0

"' ....

Figure 3.8 Critical fall-height for stones

59 Aut�ursr1chtqhjk beschermd materiaal

When designing the geosynthetic the effect of possible damage bas to be taken into account. Attention has to be paid to possible damages by lorry traffic. Sometimes, depen­ ding on the location, there is a risk of vandalism. Operation restrictions and execution conditions are discussed in individual chapters.

AGEING Raised temperature and ultraviolet radiation have a negative effect on geosynthetics because they stimulate oxidation by which the molecular chains are cut off. Once this process has started, the molecular chains degrade continuously and the original molecular structu.re changes. It involves a substantial reduction of the mechanical resistance. The geosynthetic becomes brittle. This phenomenon is called 'ageing'. Some basic materials are more sensitive to ageing than others; see Table 3.8. To reduce the se.nsitivity t o ageing, anti-oxidising agents and UV-stabilizers are added during the production process. A well-known anti-oxidising agent is carbon black. Some stabilizers, including carbon black, have a negative effect on the mechanical properties of a geosynthetic. ln each phase of the production process of a geosynthetic the temperature is raised for the processing of basic materials and half-finished products. It may result in the start of the I ageing process. Therefore, quality control has to be performed on the end products. By means of standard testing methods, geosynthetics can be compared with each other, j but it is difficult to use the results for a realistic life-cycle calculation. Hence only a qualitative comparison of the most used basic materials is presented in Table 3. 8. Table 3.8 Resistanee of geosynthetics against affection

I

basic 1naterial

PET

pp

LDPE

HDPE

PA

time of exposure

short long

short long

short long

short long

short long

Dilute acids Concentrated acids Dilute alkali Concentrated alkali

++

++ ++ ++ ++ ++ + ++ 0 ++ ++ ++ ++ ++ ++ ++ ++

0

+

++ 0 0

Salt Mineral oil Glycol M.icro organisms

++ ++ ++ ++ ++ + ++ ++ 0 ++ ++ ++

UV-light UV-light (stabilized) Heat, dry (up to 100°c) Steam (up to 100°C) Hydrolysis Detergents

+ 0 ++ + ++ ++

++ + ++ +

++ ++ ++ ++

++ ++

0

0

0

++

0

++ ++

++ ++

++ + ++ ++ 0

0

++ 0

++ ++

++ + ++ ++

Short = during execution; Long = during usage Degree of resistance: - = non-resistant; 0 = moderate; +

I

++ ++ + 0 ++ ++ 0 ++ ++ ++ + ++ ++ 0 ++ + ++ ++ 0

++ + + ++ ++ ++ ++

++

0

++ ++ + 0 0

++ ++

++ ++ + ++

++ ++ 0

+

0 + ++ + ++ + ++ + ++ ++ ++ ++

= passable; ++ = good

60 Aut 'c sr�c'1t01 j� beschermd materiaal

The following aspects have to be taken into account when geosynthetics are applied: - the temperature which may occur during application and the time of exposure; - exposure to sunlight, the duration and intensity; - the possibility of leaching of anti-oxidising agents and UV- stabilizers, resulting m subsoil pollution; - the possibility of the presence of metals in the surroundings of the geosynthetic, which can act as catalysts in an ageing process. It is not possible to calculate and to express in a figure the decay of the properties of a geosynthetic (for instance the strength). During the design period these phenomena have to be considered and taken into account when selecting the type of the basic material.

HYDROLYSIS

Some geosynthetics like nylon (polyamide) and, to a lesser extent, polyester are sensitive to hydrolysis under wet conditions (reaction with water). At moderate temperatures a Joss of strength of 5 % has to be calculated. A rapid decline in strength occurs at temperatures above 80 °c.

CHEMICAL AND BIOLOGICAL A1TACK

Geosynthetics have to be resistant to the chemicals and the micro-organisms present in the surrounding soil. Under some conditions the strength of a woven fabric and the watertightness of a geomembrane can be affected substantially. For instance, reinforcing materials made of polyester are strongly attacked under high-alkaline conditions. Polypropylene can be attacked by some fungi in such a way that threads, fibres or membranes split (fibrillation). Table 3.8 presents a general view of the resistance of geosynthetics against harmful effects. 3.1.4 Remaining aspects In addition to functional properties, mentioned in 3.1.2, and the change in properties m.entioned in 3. I . 3, some other limiting conditions are of importance for the choice of a geosynthetic and the method of installation. In this paragraph the most important limiting conditions are discussed.

A SEPARATION OF FUNCTIONS

Usually, a geosyntbetic is chosen on the basis of requirements for a main function, for instance a nonwoven for a filter function. But it may occur that the geosynthetic chosen to meet the requirements of the main function does not match certain secondary loadings, for instance mechanical loading. Sometimes the geosynthetic cannot meet the require­ ments of a secondary function like the bearing of tensile strain. ln such a case the so­ called "separation of functions" can be necessary. This may lead to the choice of another type of geosynthetic (for instance a composite), which meets the requirements for both functions. Of course, the effect of the second geosynthetic on the main function of the first one has to be taken into account. Another possibility is the application of two different types of geosynthetics which together can perform the required functions.

FINITE DIMENSIONS

Geosynthetics are finite and therefore it is necessary to make connections or overlaps. Seams and overlaps are weak spots in a construction and they are vulnerable. Therefore, 61 Auteursrec1telqk beschermd matenaal

they have to be limited as much as possible. If geosynthetics are applied as reinforcement, seams at right angles to the direction of the leading force are unacceptable. Also the termination of a geosynthetic and the connection to another pa.rt of the construction asks for special attention. Jointing systems which are without strain can be made with a loose overlap of circa 1 meter or with a simple staple o r overlap seam. Jointing systems under stress have to b e avoided as much as possible. They are always weaker than the original, non-connected geosynthetic. In particular cases in which heavy forces occur in the main direction as well as at a right angle to the main direction, it is usual to apply two layers, one in each direction. If it is not possible to avoid a joint in an assembly under strain, a loose overlap is out of question, unless the overlap is as large as the total required anchoring length. Woven fabrics can be sewed. The most common seams with their limitations are mentioned in Table 3.9. Staple seams can be made i n situ using a specially adapted sewing machine. Overlap seams cannot be realized in situ because for this purpose sewing machines with a long free arm and several needles are required. These sewing machines are very vulnerable in situ. A non-selvage side should be provided with a hem by a sewing machine to prevent ravelling. Table 3.9 The most frequently applied seams

The most frequently applied seams staple seams description shape

strength of the seam in % of the strength of the woven fabric soil tighmess

single wrapped

overlap seams

folded

single wrapped

folded

g;

It

25-50

30-60

60-80

60-80

doubtful in fine graded soil

guaranteed

doubtful

guaranteed

DAMAGE TO THE ENVIRONMENT Geosynthetics may burden the environment. On the one hand, with a careless installation remaining parts may get scattered into the surroundings so that, for instance, animals get entangled o r the screw propellers of ships jammed. On the other hand, toxic additives like softeners, anti-oxidants, UV-stabilizers and compounds with chloride may leach and pollute the environment. In cuRINGO (1995) the cautiousness which has to be practised during the removal, r e u- se or incineration of geosynthetics is discussed. Separate collection prevents that geosynthetics are regarded as chemical waste. QUALITY CONTROL To verify whether the geosynthetics meet the prescribed requirements, quality control has to be performed. In most cases the presentation of a certificate is sufficient.

62 Auteursrec1telqk beschermd matenaal

3.2 SPECIFICATIONS AND TEST METHODS The basic properties of geosynthetics and the functional requirements have been discussed in Section 3.1. The suitability of geosynthetics should be checked against these functional requirements during the design of civil engineering constructions. In principle, qualificati­ on tests need only be executed once (at the start of a project), unless there is a significant change in the production process of geosynthetics or in the project. The necessary tests should be of good quality and according to the national or international standards. In order to be sure of the constant quality of geosynthetics it is necessary to subject them to quality control tests regularly, the so-called index tests. Sometimes it is also necessary co carry out the performance tests or prototype tests for design purposes (the behaviour of geosynthetics in prescribed design conditions). An objective of this section is not to catalogue standard test methods, but to stress their importance in the design process and the final performance of a structure under consid.e r­ ation. The description of test methods can be found in Fluet (1985), Koerner (1990), ICOLD (1986, 1991), Ingold (1994), Van Santvoort (1994), and in the ISO/DIN/­ RILEM/CEN/ASTM standards and publications. Most of the existing test methods refer to standard 'index tests'. The few existing performance tests still have the character of non-standard tests and need further development and standardization (Pilarczyk, 1984, 1987). The most important design parameters when designing with geosynthetics, which should be properly determined and tested are listed below (Ingold, 1994). The selection of parameters depend on the functional requirements of a project. These are: • basic material, specific gravity and mass per unit area • thickness * stable fibre network * flexibility • l.inear dimensions * loading regimes * tensile strength and modulus * tear strength * puncture and burst strength * soil-geotextile friction * pore size and percentage open area * permeability (filtration/drainage capacity) and transmissivity * appropriate retention capacity (i.e. soil tightness) * creep * ultraviolet light resistance (UV) * durability incl. chemical and biological resistance Most of these parameters are discussed in the subsequent Chapters 1n relation to specific products (i.e. geosystems) and applications. PRODUCT lDENJ'IFICATION Identification of a geotextile is important from both a contractual and a technical point of view (PIANC, 1987). Contractually, the product must be clearly designated and simple testing procedures must be available. Technically, identification data can provide basic information allowing the hydraulic and mechanical properties to be estimated, using comparisons with known products. As an example, recommendations issued by the

63 Auteursrec1telqk beschermd matenaal

RILEM Technical Commiuee on Geotextiles (1985) propose six items to be included on the manufacturer's identification sheet: l. Trade name 2. Manufacturer 3. Constituents and manufacturing characteristics - type of polymer - density of polymer - diameter of fibres or filaments - tnanufacturing process (woven, non-woven, etc.) 4. Mass per unit area 5. Nominal thickness 6. Presentation (weight and dimensions of roll)

Once the engineer becomes experienced with geosynthetics the identification data becomes a useful guide to technical properties, although standard laboratory tests must be carried out eventually. Because of the purpose of this book (the application of geosystems in hydraulic engineering), some of the relevant parameters and definitions are discussed more in detail. HYDRAUUC PROPERTIES (PIANC, 1987) The two important hydraulic properties of geotextiles are: - permeability, and - filtration/retention characteristics. Permeability. Geotextil.es are usually highly permeable materials (especially when selected for filter or drainage designs). Flow through the fabric is normally laminar when the geotextile is embedded in the soil, but may become turbulent when subjected to wave action (see Revetments, Chapter 4). Permeability is usually measured in the laboratory using values of hydraulic gradient low enough for laminar flow. With most geotextiles (particularly those used as filters), hydraulic heads lower than a few centimetres lead to laminar flow (in the Netherlands a standard 100 mm hydraulic bead is applied). Water flow through the fabric may be normal to its plane or in its plane. Permeability is normally defined in respect of water flow normal to the fabric. Permeability is the rate of flow per unit area per hydraulic gradient. Hydraulic gradient is the ratio of the head to the thickness of the geotextile. However, it can be difficult to measure the fabric thickness during the test. Alternatively the permeability may be expressed as permittivity. The equation for flow through the fabric is, (3.1)

where: 3 q = rate of flow (m /s) ag = surface area of geotextile (m2) i:.\H = head loss (m) t, = geotextile thickness (m) k1 = permeability of geotextile (m/s) 64 Aut�ursr1chtqhjk beschermd materiaal

�

You have either reached a page that is unavailable for viewing or reached your vi ewi ng Iimit for this book.

* Type of fabric - woven, thin

non-woven, thick non-woven (see Figure 3.9), * Type of soil - granular or cohesive, • Density of soil - compact or loose, * grading of soil - uniform or non-uniform * Magnitude of hydraulic gradient.

100

-

90

---

woven

� �

50 � �

pore size (mm) Figure 3.9 Comparison of fabric pore sizes

The retention criterion can be expressed as, (3.4)

where: 0• =

is the opening size of the geotextile corresponding to the diameter of the largest particles that can pass through the geotextile. 00 corresponds to the d0 of the soil passing through the geotextile; n is normally taken as 98, 95 or 90 %. is chosen as a characteristic diameter of th.e retained soil. As a representati­ on of the large diameter particles within the soil, dss or � are normally used; for medium-sized particles d50 is used. is a coefficient that depends upon the factors listed above together with the type of test used to determine 00 (dry, wet, wet turbulent or wet alternate flow method).

Many different interpretations of equation (3.4) have been proposed and this is dealt with in Chapter 4. There are various methods of determining the size of a geotextile opening. Details of the tests vary from the country to country, the chief difference being that some use dry sieving and others wet sieving, with either one-directional flow or alternate flow. Wet sieving is generally preferable. Whichever test is used, it is essential that it is compatible with the adopted design criteria. Actually, the European Standardization Committee (CEN) is preparing the European Standards on the specification and testing of geotextiles.

MECHAMCAL PROPERTIES The three important mechanical properties of geotextiles are:

* Behaviour under tensile load,

* Behaviour under concentrated stress, and * Soil - geotextile interaction.

66 Auteursrec1tel Jk beschen'ld mater1aal

Behavi.our under tensile load is an important practical aspect. Recommended tensile tests use samples which are substantially wider than the distance between the clamps. It is important to realise that using different sample shapes or different test facilities may give very different results. Behaviour under concentrated stress is of importance for situations with localised stresses as from the dropping of stones, anchoring, uneven soil, etc. In order to prevent discontinuities in the fabric, local ruptures must be avoided and where these do occur, tears must not b e allowed to propagate. - Rupture my be due to tension, compression or shear. A single test cannot differentiate between these situations although various puncture tests are sometimes used. - Tear tests measure the force necessary to propagate an initial cut in the material. Tear resistance is expressed as a force and not a stress. Soil - geotextile interaction is also an important design aspect. When laid on a slope, the geotextile must not create a preferential slip interface. Additionally, when subject to hydraulic attack the filter must remain in contact with the soil. Three types of interaction must be considered: • Friction is expressed as an angle of friction or as a coefficient of friction. A direct shear test using a shear box is used to measure the angle of friction. Tests where a strip of geotextile is pulled out from within a soil mass are not recommended because the relative displacement between soil and geotextile is not accurately known. For revetment structures the inclined-table test is recommended. • Interpenetration occurs when soil particles penetrate the geotextile or if the fabric has rigid fibres which can penetrate the soil. • Confoonation - when a flexible geotextile is placed within a compact coarse granular material it defoons locally to follow the soil. When a pulling force is applied to the fabric, friction increases considerably, resulting i n a strong anchoring of the material.

REFERENCES Berendsen, E., 1996, Dumping of rock on geotextiles, Proceedings Jsr European Geo­ synrhetics Conference (EuroGeo), Maastricht, the Netherlands. CUR/NGO, 1995, Geosynthetics in Civil Engineering, Centre for Civil Engineering Research and Codes ( CUR) and Netherlands Geosynthetics Society (NGO), Report 151, Gouda, the Netherlands. Fluet, J.E. (editor), 1985, Geotextile testing and the design engineer, ASTM special technical publication 952, Philadelphia. Giroud, J.P., 1987, Geotextiles and Related Products, in Geotechnical Modelling and Applications, Sayed M. Sayed editor. Gulf Publishing Company, Houston. Hausmann, M.R., 1990, Engineering Principles of Ground Modification, McGrmv-Hill Publishing Co. Ingold, T.S., 1994, The Geotextiles and Geomembranes Manual, Elsevier Science Publishers Lid.

67 Auteursrec1tel Jk beschen'ld mater1aal

!COLD, 1986, GEOTEXTILES as filters and transitions in fill dams, /COLD Bulletin 55, Paris. !COLD, 1991, Watertight Geomembranes for Dams, /COLD Bulletin 78, Paris. Koerner, R.M. and Welsh, J.P., 1980, Construction and Geotechnical Engineering Using Synthetic Fabrics, John Wiley & Sons. Koerner R.B., 1990, Designing with geosynthetics, Prentice-Hall Inc., Englewood Cliffs, New Jersey. PIANC, 1987, Guidelines for the design and construction of flexible revetments incorpo­ rating geotextiles for inland waterways, PIANC, Supplement 10 Bulletin no. 57, Brussels. PIANC, 1992, Guidelines for the design and construction of flexible revetments incorpo­ rating geotextiles in marine environment, P/ANC, Suppl. to Bulletin 78179, Brussels, Belgium. Pilarczyk, K.W., 1984, 1987, Filters, in The Closure of Tidal Basins, Huis in 't Veld et al (editors), Delft University Press, the Netherlands. Pilarczyk, K. W., 1990, Coastal Protection, A.A.. Balkema Pub/., Rotterdam. Pilarczyk, K.W., 1994/1995, Novel Systems in Coastal Engineering; geotextile systems and other methods, Rijkswaterstaat, Road and Hydraulic Engineering Division, Delft, the Netherlands. RILEM, 1985, Synthetic membranes; RILEM recommendations, 4 7SM - Technical Comnzittee on Geotextiles. Van Santvoort, G./Veldhuijzen van Zanten, R./, 1994, Geotextiles and Geomembranes in Civil Engineering: a handbook, Netherlands Geotextile Society, A.A. Balkema Puhl., Rotterdam.

68 Auteursrec1tel 1k beschen'ld mater1aal

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is much smaller. A high permeability of the mattress ensures that any possible pressure build-up under the mattress can flow away, as a result of which the differential pressures across the mattress remain smaller. The stability is therefore the largest with a l.arge mat­ tress permeability. In the long term, however, pollution of the Filter Points or the clogging of the geotextile can cause a decrease in the permeability. The susceptability for blocking can be reduced by increasing the gradation of the subsoil. To reduce the susceptibility for clogging it is recomrne.nded to reduce the sludge content of the subsoil.

---...

' \

\

slab

-' -

tube

Figure 5.10 Examples of coocrete mattresses The concrete mattrasses are fabricated of polyamide (nylon) or polyester, or combina­ tion of both. The tensile strength of fabric is about 50 kN/m. The opening siz.e of fabric is usually 0.1