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Session12

Soil lmprovement- State-of-the-ArtReport Améliorationdes Sols

J.K. MITCHELL

CA, USA Professorand Chairmanof Civil Engineering,Universityof California,rBerkeley,

marginal sites and because many soils Because of the increasinq need to utilize if properly treated, soil improvement has become a materials can be made into useful construction report the principart of many present day civil In this state-of-the-art engineering projects. methods are Presented. ples, applications, and design procedures for soil improvement using different precomdeep ccmpaction of cohesionless soils, Soil improvement methods reviewed include in-situ thermal admixture stabilization, pressíon with and without vertical drains, injection and grouting, Comprehensive recent references on each topic are listed. methods, and soil reinforcement. SYNOPSIS

INTRODUCTION yost of manrs construction is done on, in, or with soil. of suitable conAs the availability sites decreases, the need to utilize struction pcor soils for foundation support and earthwork it is In addition, construction increases. necessary to strengthen becoming increasingly structures to insure the ground under existing against adjacent excavation or tunnelstability to seismic or ing, or to improve resistance Furthermore, many hunother special loadings. have shown Creds of recent successful projects reinforcement rhat through the use of suitable :raterials and systems, the uses of nature's nost abundant constructi-on material--soil--can Tt is not'surprising, De greatly extended. t,herefore, that the general area of soil improvehas been of great intenent and reinforcement rest and rapid development in the past several The inclusion of this topic for the i'ears. Sirst time as a main session in an International :onference of our Society is clear recognition cf this fact.

aspects of soil improvereport is on practical as soil ment, to include such considerations effective types best suited for treatment, of treated soils, treatment depths, properties Because costs. and relatir/e major applications, of the extensive scope of the subject the inis not clusion of detailed case histories possible.

METHODSAND SCOPE Cornmittee has identiThe Conference organizing in topics for discussion fied the following Session 72, so they have been chosen for coverreport. age in this state-of-the-art 1.

heavy tamping and Compaction, especially deep Emphasis is on in-situ blasting. and of cohesionless soils, densification compaction in thin layers is excluded.

2.

by preloading Consolidation drains, eLectro-osmosis.

and,/or vertical

3.

Grouting, excluding flow and seepage.

control

of groundwater

4.

using admixtures and by Soil stabilization . ion exchange (chemical stabilization) Emphasis is on new developrnents and applications other than subgrades and base courses for roads and airfields.

The basic concepts of soil improvement; namely, cementation, reinCrainage, densification, :orcement, drying, and heating, were developed hundreds or thousands of years ago and remain The coming of machines in the valid today. in vast increases in both lgth century resulted of vrork that could be and quality rhe quantity developments Cone. Among the most significant of of the past 50 years are the introduction ,.'ibratory methods for densification of cohesionand grouting materIess soj.ls, new injection :.als and procedures, and new concepts of soil re inforcement .

5.

Thermal stabilization.

6.

The purpose of the present report is of sj.ze the present state-of-the-art for improvement into a form suitable by practicing engineers. application same time the author has attempted to references that interested sufficient r¡i11 be able to locate more detailed Emphasis rion, case hi.stories, etc.

Emphasis is on the Reinforcement of soil. or installaduring construction inclusion and compresof both tensile tion in-situ Geotextiles elements. sion reinforcement are included. used as reinforcement

summary of The report concludes with a tabular of the methods discussed and consideration factors governing the choice of a method for any given case.

to synthesoil direct At the identify readers informain this

509

DEEP COMPACTIONOF COHESIONLESSSOILS Introduction soils may Thick deposits of loose cohesionless the improvements in order to eliminate require and suüsequent- development of excessive total and to minimize the possettlements itiffeiential under dynamic loading' for liquefaction "iUifity improvém.ent can be achieved in many Suitablé however, the needed -ases by densificationi cannot ordinariJ-y be achieved ttensifióation or.compaction at using preload surcharge fills of loose, rn-situ densification the éuiface. soil layers is usually done by cohesionless In many methods dynamic loaddvnamic methods. iig is accompanied by displacement in the form of á probe and/or construction inseriion ot-tfr. of a sand or gravel column in-situ. deep denMethods that are used for the in-situ of cohesionless soils include blastsification and heavy tamping' ing, vibrocompaction, is used herein to refer colviÉiocompactión tle +n: to alL those methods involving lectivel! probes into the ground.with sértion óf vibrating material' of a backfill ár without the addition Compaction piles are also considered in this of any of these methods ihe ability "ullqoty. to aócornplish the needed improvement- in properincluding: ties depénds on several factors, its

Soil type, especially fines content.

2.

Degree of location

3.

Initial

relative

densitY

4.

Initial

in-situ

stresses

5.

Initial effects

including soil structure, of age, cementation, etc'

6.

Special used.

characteristics

saturation

and

gradation

1.

and v¡ater table

the

of the method

Mechanism of Densification of cohesionless soil layers with Densification accompanying improvement in mechanical propersoil structhat the initial ties ieqüirés first can be ture be-broken down so that particles In saturated mávea to new packing arrangements. cohesionless materiáls this is most readily by means accomplj.shed by inducing liquefaction In the case of loadings. ot aynamic and cyclic and heavy tamping the meth;ds such as blasting compression htave generated by the sudden large in .tt.igy release can give an immediate-bui1d-up the pore water pressure which greatly-reduces This wave is followed by a lhear sttength. of for failure shear wave wt¡ict is responsible After passagé of these waves the soil ih. *""". more into nel.¡ and, ultiriately' settle Darticles 'sta¡le positions. Vibrocompaction methods are in much the same way, except that the eftective the energy Per event is many times- smaller' continue over a much lonq¡er period, viuráiións to distances from the are felt and the effects é""tgy source of one to two meters instead of up tó- 10 m or more as is the case with blasting and heavY tamPing. For partly

510

saturated

soils,

including

some con-

densificataining fines and many waste-fit1s, tion iá mainly by collapse of the soil structure The process and escape of gas from the voids' by impact comis much lhe same as densification paction as commonly done in the laboratory' accompanying ground treatment by Densification settlement of the these methods occuri rápidIy. complete by.the giound surface is essentially as Improvement in properties, énd of treatment. tests or neasured, for example, by penetration may continue over extended Dressuremeter tests, point may be of conThis latter Lime periods. importance in the evaluation practical "idérá¡i" of ground treatment. that it is often easier Experience has indicated density to a specified hiqh relative to'densify than from an condition from a loóse initial This is because density. relative interrnediate is of the loose material structure the initial easier to break down. Soil Type Considerations Vibrocompaction rnethods are best suited for denof clean, cohesíonless soils' sificatión has shown that they are generally n"pátié"." when the percentage by weight of ittLftecti.t. than 200 mesh sieve or (particles finei iines rhis is ó:ót¡ "* diameter) exceeds 20 t'o 25' containof materials the permeability úá."""á ins greater percentaqes of fines is too low to tn" rapid drainage of pore v'ater that is .fíot liquefacfollowing requireil for densification forces' tión under the action of the vibratory may be more difficult the structure .rrd ba."o"" by the owing to cohesion contributed lá aisrupt fines. greater amounts of fines; Some soils containing sanás and loess, can be densisilty á.f.,-"á*" and/or heaw tamping, both of blasting iiéa'uy which impart laige amounts of energy alf at once More and causé large ground displacements-' concerning the inconsideiations specific of soil type are presented in the disFio.tt"." For preliminary methods. of particulár .""Ái"" hówever, it may be considered that th€ olanninq, shown by size distributíons iár,g" oi-particle by rig. f. wiit ¡e best suited for densification methods. deep in-situ of Treated

Evaluation

Ground

of deep comMeasurement of the effectiveness pá.ii"" can be nade using one or more of several Techniques thai have been used includc fr"lir"a". markers

settlement

I.

Surface

2.

craters, to Volume of soil added to fill or to carry out a form compaction piles, vibrocomPaction Process

3.

Standard Penetration

4.

cone Penetration

5.

Pressuremeter Tests

6.

Seismic

7.

Pife

8.

Plate

Tests

Tests

(cPT)

(PMT)

shear wave velocity

driving

resistances

load tests

(SPT)

determination¡

!

€ Ol

(¡, = \ a \

Most DesirobleSizeRonge

(t)

c l.¡!

c(¡) \ q.) (J

o-

)

5

2

t.0 0.5

0.2 0. 0.2 0.tt

0.05 0.02 0.05 0.0t t 0.ffi5 0.02 0.0 o n5

0.00t

P or ticle Size - m m Fig.

9.

I

Down-hole

Range of

Particre

density

meters.

size

Distributions

Settlement measurements and SpT, CpT, or pMT are the most commonly used methods. Th.e CpT is particularly useful because it provides a conp e n e t r a t i o n tj.nuous record of resistance with depth, it is fast, and it is well-suited for use in sands. Penetration tests for evaluation of ground are usually improved done at locations j.ntermediate between probe points in order to provÍde the most conservative estimate of improvement.

suitable

for

Densification

b y Vibrocompaction

Approximate correlations between equivalent penetration resistance, sand density and properpertinent ties to the assessment of foundátion stability are given in Table r. In a few cases instrumentation has been used to monitor conditions during densification. Tot,al pressure cells and piezometers have provided data useful for developing improved understanding of the densification and property improvement process. Blasting

Penetration resistance values, both before and ground improvement, after are often converted to relative densities using one or more of several correlations that have been developed for particular conditions. Desiqn criteril and specifications are many times developed in terms of relative density. A direct conversion of a penetration resistance to relative densitv is uncertain, however, because penetration résistance depends on factors other than density. The correlations are not independent of soil type. Increased lateral pressure, increased time under pressure, increased stability of structure, and prior seismic strains lead to penetration (Seed, l-9] 9) . increased resistance Fortunately, these latter factors also lead to corresponding increases in resi.stance to settlenent and liquefaction, and it is the penetration resistance values themselves that are important, not the actual relative dénsity. It has been found convenient for some applications, however, " e g u i v a l e n t to work with an relative density,' which is the true relative density a sand Ceposit ',¡ould have to possess to exhibit the measured penetration resistance if ít were freshlv deposited and normally consolidated.

Deep compaction by detonation of buried explosives can provide a rapid, lohr cost means for soil improvement in some cases. The general procedure consj-sts of: l.

fnstallation of pipe by jetting, or other means to desired depth placement

2.

Placement

3.

Backfilling

4.

Detonation established

of

charge the

in

vibration, of charqe

pipe

hole

of charges pattern.

according

to

a pre-

In some cases the pipe is withdrawn prior to detonation of the charges. In others it is reclaimed after the blast, a new sectj_on is welded to the bottom. and it can be used again. The explosives used include dynamite, TNT, and anmonite. Detailed descriptions of blasting are given by Prugh (1963), ( 1 9 6 7 ) , rvanov Mitchell (1970), Litvinov (I973, I976), (1970Damitio 72), Donchev (1980), and others.

511

TABLE I Penetration

Resistance

and Sand ProPerties

4-10

l0- 30

30-35

> 5 0

CPT cone resistance (kg/cmzl *

50-100

r00-r 50

150-200

>200

Relative Equivalent óensity (t) **

l5-35

35-65

65-85

8s-100

Dry Unit Weight (kN,/m")

1 4- 1 6

I6- I8

r8-20

30-32

32-35

35-38

0.04-0.10

0.r0-0. 35

> 0 .3 5

SPT N-value ( b 1 o w s , / 0 . 3m ) *

Friction

angle

(")

Cyclic Stress Ratio óausinq Liquefaction (r/oo') ***

overburden pressure of *At an effective vertical sand' **Fresh1y deposited, normally consolidated ***From Seed (19?9), fig. 6(a).

for denclean sands are well-suited Saturated, in anv case s u c c e s s b r a s t i n s ' b v ;i;i;;;i;; of the shock wave senelÉe auiritv ;;;;;;-;; down the initiar tá'¡rear IIM-iv-ti'á-¡tast condition an¿ creaie a liquefaction structure, to period to enable Particles for a sufficient rt in a denser packins' ;;;r;J;;-ttrárnserves that the stronger the sand therefore, follows, be the larser the charses.that wilr i;;;r;líy; Thus' the densifióaliont. required for effective rs depth to which densification "iÉIilt-tÁ; equivalent initiar the X";;;;-#-tr'á'ttist'"r the greater the explosive density, ;r;¿i"; energy required. accepted theoThere appear to be no generally by for áensification design pto.tdoít" retical are usuarlY used riera trials ;;;;;;;r'-;ná A number of fier¿l urá"li"g' ;;i;;l¿'p;áá,'.ti"" rvanov (1967) inÉv ",ttt*tized ;;":-¡.át' :;;;; "'o* tn'"'

;;i;;";-¿;.atment experiences emérge, 1. 2.

3. 4.

5.

512

tne

Lo.20 4-"oll" up guidelines general T.:..

tojtbwing

Charge size:

1/4 ilepth to bottom Depth of burial: t r e a tea¡ L/2 Lo 3/4 of t o b e l a y e r of dePth cornmon Charge sPacing in Plan:

5-15 m

l-5 with 2-3 usual' Number of coverages: number of Each coverale coásists of a Successive coveraqes inaiv:-¿uaf óharges' by hours or days' are usually-sepárated Total .explosive use: 30 gm,/mi tYPical

8 - I 5 o g m ' l m 3' 1 0 -

6.

100 kPa'

Surface settlement: thickness '

2 to 10t of layer

can be used The maximum depth to which blasting known' tor soil compaction i" lgt successfully a associated-with The author is currently have which charsés of up to 30 ^ks "r"llli'-i; aéóÉt's more than 40 m below t"áí"á!.á""1éá-"t surrace-settlements sié'niticant ;;;";-;;;;;;. i" Éhe equivar:"t l:l?ti"' ;;á-i;p;;;;^á"t than áf 1oo"" zones,a€ q"plh:-g1:iter á;;";¿; feature interesting An acntá""d' 30 m have ueen although that of this work is ttre-observation is irimediate' which means settlement ;;r;;;; im¡nediate' essentially i; ;i;" that densificatÍon inditests-do'not p " t " l t á l i o " c o n e o f results density relative t"-tq"i""ftnt Íncrease an cate severtr weeks' reflect'"ioe-iár il-ih."t.s"ir.a disfolrowins effect itá"ij'ne ;;";";"-;;i"g-"r and rormation átructure ini[iái ;;-;ñ" i"lii"ii of a new one. maximum depth for It would be expected that the limitecl by the pracbe wiII treatment effective oi piácing concentrated charges itifficurty tical to create a shock wave *tgttitüae of sufficierrt structhe initial-soil íiquerv to ;;.;;-;;;;;t¡ errec"o q?-th' deptÉ-inóreases' ;;-¿tre i;;;: .1:"9'9'-is1Y':l: ii"!'"ttá""á" .,'á strensth' ano stréss-Yi11-i":.:?ase' áisruptive iá"iii.á decrease' will influence of iaaius ñ:-"?;;";rve Pmax wave pressure' The maqnitudes of the shock

pá' Yl-i!-?""' r' in iii=*ü7Zii;"';;áLh.l;pii"é (196'71as forlows gtt"i"'[v-i""it"" kq-seé/cm-, ^t.

nr.* = kr (

)ut

(r )

in

which

I

= n,(*l-)('€)"

c

= size

of

(2) (k9

charge

of

TNT)

= distance from center of R charge (m) from U 1r U 1 = e m p i r i c a l c o e f f i c i e n t s Table II. can be used for comparative These relationships studies of the probable influences of charge size, charge spacing, and sand saturation. It may be seen that the presence of even smaLl danping of amounts of gas leads to significant the P-wave pressure. It has been possible by blasting to densify relative densities sands to equivalent of 75 to In some cases, however, the results 30 percent. :ray be erratic, initially dense zones may be ).oosened, and the method is not likely to be in the upper one or two meters below effective Typical behavior may be :he ground surface. s'¡mmarized as follows. Almost immediate settlement of the ground further surface, with little settlement with tine.

-.

-.

Initially loose zones show little immediate change in penetration resistance. Penetration resistance increases slovrly with tirne until after several weeks the material a marked improvenent in properties indicates compared to its initial condition.

.

zones which are initially very dense may be permanently loosened or weakened by the however, ttre resultant blastt condition is to be satisfactory. sti1l likely

.

'Jl.timateIy,

program an effective blasting results in a deposit in which all the init:a11y loose zones have been suitably :mproved.

. t:. á series of coverages is used, the surface -----lement accompanying each coverage is usually -:s chan the one precedinq.

Attempts have been made to compact using surface because of sirnplicity, 1ow cost, and explosions, Because of energy loss above ground, speed. lack of confinement, and the formation of surface depressions, however, this method has been of limiteil ef fectiveness. A hydro-blasting technique has been used very for cornpaction of successfully and economically L973, 1976¡ collapsible loess deposits (Litvinov, Donchev, 1980). Although collapse of the loess can often be accornplíshed by flooding alone, it has been found that more uniform results can be achieved more quickly and economically by this The procedure, which is illustrated in method. cutting a contour Fig. 2, consists of first trench 0.2 m to 0.4 m wide and several meters deep around the perimeter of the area to be Boreholes spaced a few meters apart densified. are then used to pump r{tater in a grid pattern into the loess, over a period of several days, ideally until the water content is increased to above the liquiil limit. Slurry walls or plastic membranes can be installed to prevent lateral of the r.rater and softening of adjacent migration ground. Explosive charges of about 5 kg each are then at spacings of inserted down tubes installed three to six meters in grid patterns and detonof up to 10 percent Surface settlements ated. of the layer thickness and reduction in porosity of several Dercentase points are not unconmon. A r e a s o f 1 0 - 0 0m 2 t o ' 1 0 ; o o o m 2 i n v o l v i n g 1 0 , 0 0 0 I O O , O O Om 3 o f l o e s s c a n b e t r e a t e d a t o n e t i m e . Successful cornpaction of saturated sand and loess has been accomplished in the collapsible USSR using high energy, high voltage electrical discharges from probes inserted in the ground (Lomize et al., L963, 1973). Each discharge, which may release 50 to 100 kJ of energyr has an to that of an explosion of comeffect similar parable magnitude. A number of discharges are of several seconds released spaced at intervals at each level as the probe is moved upwards from It appears that use of this method the bottom. has not yet been widely adopted.

TABLE TI Parameters for

Estimating B1ast Pressures (from Ivanov, 1967)

Gas Content

Moist

sand

water

table

Values

kt

u1

kz

0

600

r.0s

0.080

1.05

0.05

450

r.5

0.075

I .10

I

250

2.0

0.045

r.25

4

4J

2.5

0.040

1.40

3.0

0.03s

1.50

0.o32

1.50

t*l

Sand below

and lmpulse

(8-10t

water)

7.5

(2-4*

water)

3.5

513

amplitude of 10-25 nm. 15 Hz and a vertical About 15 probes per hour can be done at spacings of l- to 3 m. It is of marginal in the upper 3 to 4 m of the eifectiveness zone to be densified.

Conlour Trcnchag

"

aa o t Droin Jfclb d o o

.

/ Elotling Holcs t o o t

o

aa

o

Vibro-rods developed by Saito (1977), Fi9. 3, pile drivare also driven using a vibratory and ing hammer. Several cycles of insertion withdrawal are used in the densification process. t 3 O O ¡

Uncompoclcd Soirs

tgl'--::ll:ÉÉ= ll - :ll = :ll:

- ll =--ll- ill--1

!>K{l-H,|l.l:€ [=_;]59=1, l\,-24+-'stu'¡,ping

Soil tin Flooded Zoni I t

lI

:1]

1 . . - _ -

Section A-A

2

Loess

y.ibrocompaction

Compaction

by

Hydro-Blasting

and

of cohesionThese methods for deep compaction by the insertion are characterized less soils probe into oi torpedo-shaped of a cylindrical by vibration by compaction the ground followed In a number of the methods withdrawal. during a comPacted is added so that backfill a granular a behind within column is left sand or gravel Sinking volume of sand compacted by vibration. depth is treatment of the probe to the desired methods, using vibratory accomplished usually jets at the tip. by water supplemented often at the same time has been found of air lnjection Updepths. penetration to large to facilitate has along the sides $tater jets ward directed Soil in some cases. also been found helpful by vibrofor densification gradations suitable 1. Compaction in Fig. are indicated óompaction formed by these methods piles of sand and gravel in which soils, cohesive áre also used in soft and shear as compression case they function section in a later as discussed reinforcement, of 20 n depths Ground treatment report. of this by these methods. routinely can be achieved in some Depths in excess of 30 m can be attained cases. A brief sively below. 1.

of some of the more description methods is used vibro-compaction

Vibrating

extengiven

Probes

in the developed method, The Terraprobe Vibro(Anderson, L974), uses a Foster U.S.A. pile hamrner on toP of a 0.76 m dia. driver (pipe pile) is 3 to that probe open tubular penetration than the desired 5 m longer of at a freguency operates The unit depth.

514

A

il-.

r\\.¡,

tl'.----il lt =ll- --ll- s{lF; ll. --ll.¡.$ llrF ='l

Eig.

i

compoclad Arao

lr5oo

/nI l.."

ffif L-"rs-l

Zh a7

(o) DoubleTube Rod

Fiq.

3

Vibro-Rods

(b) Rod Wilh Proleclrves

Used for

Sand Densificat:c:. (from Saito,

2.

l9--

Vibroflotation

in Germany alrrcs: This method was developed has cc:'.development ago, and its 50 years there and in the U.S.A. \^¡here it Y.s tinued The equipment in the 1940's. introduced the Vj.bra--:: ' of three main parts: consists -¡ crane. and a supporting tubes, extension and of the equipment diagram schematíc The vibrar'c: 4. :¡ is given in Fiq. process an ecceri,t::: tube contai.ning steel a hollow axis in the mounted on a verticaf weiqht

(i)

5!PPtY

( ¡i)

(¡ii)

(iv)

(v)

(vi)

(vii)

OF

GIA¡ULAR

sorL rAft

f,

i

aj.g.

4

Vibroflotation

Eguipment

and process Fig.

lower part so as to give a horizontal vi-bration. Vibrator diameters are in the range of 350 to 450 mn and the Lenqth is about 5 m including a special flexible coupling. One vibrator weighs about 20 kN. Units developing centrifugal forces up to 160 kN and varj_able vibration amplitudes of up to 25 ¡nm are avail_able. Most usuaL operating freguencies are 30 Hz and 50 Hz. The extension tubes have a slightly smaller liameter than the vibrator and a length Sependent on the depth of penetration reguired. '.':broflot sinking rates of I to 2 m,/min and ;:thdrawal,/compaction rates of aboui ,'.3 rn,/min are typical. water pressures of j.: to 0.8 MPa and flow rates up to 3,OOO :'min may be used to facilitate penetration. j¡:ld backfill i s c o n s u m e d a t a r á t e ñ r rq rY ñ +L v^ ' i -32¡"-r-^-*;; : ;m ^ ;p ; ^a : ction ::::,v+ ; h e :c: o process. The z:ne of improved soil extends from 1.5 m to { ::t from the vibrator, depending upon soil -,-.'pe and vi_brof 1ot por4rer. Additional details r:e presented by Baumann and Bauer (1974), ( 1 9 7 5 ) , :eL1 and Brown (I9'?7), among others.

: - g!!r9:ggrpgsr_-{9999 :.-.is sand compaction pile method was deve-::ed by Murayama in Japan in 1958 (Murayama, -J;8). The apparatus and procedure used in ::e compozer system are shown schematically -:. Fig. 5. A casing pipe is driven to the :i-srred depth by a vibrator at the top. A ::::3 charge is then introduced into the : ::e, the pipe is withdrawn part way while ::::ressed air is blown down inside the -rs:rg to hold the sand in place. The pipe -: '.'ibrated down to compact the sand pile .-.i enlarge its diameter. the process is :::eated until the pipe reaches the ground :^j::ace. The resulting pile is usually 600 - - 800 mm in diameter. The actual diameter '::. be estimated from the sand volume dis::.¿:sed into the ground. i.

Soj,1 Vibratory

Stabilizing

Method

:::s method, termed both the SVS method ':.! Toyomenka method, comb.ines both the

5

Construction the compozer

of Compaction system

Piles

by

vertical vibration of a vibratory driving hammer and the horizontal vibration of a ViJ.ot depth compactor. The Vilot is a probe of about the same size as special vibroflot units. Sand backfill is used, but water is not used in either the sinkinq or compaction process. The gradation of both the in-situ and the soil backfil1, which may or may not be the same material, influence the leve1 of improvement that may be obtained. Coarse sands give greater densification than fine sands, evidently because the coarser material is better able to transmit vibrations. Brown (1977) has defined a suitability number for vibroflotation backfills that is given by

-ñil;;;---,

,-= 1.7 +?6i.T, +#tT, /l5:-T, r"2O' ,"10, r.-50,

(3)

in which D5O, D2O, and D16 are the 50, 20, and 10 percent size grain diameters in mm. Corresponding suitability numbers and backfill ratings are 0 10 20 30

The lower vibroflot acceptable

- 10 - 20 - 30 - 50 > 50

the suitability can be withdrawn compaction,

ExcelLent Good Fair Poor Unsuitable number whife

the faster the stil1 achievinq

The influence of fines on the level of improvement that can be obtained by vibrocompaction is shown clearly by the data in Fig. 6 (Saito, 19'7'7\. The data provide excellent support for the rule of thumb that vibrocompaction is ineffective in soils containing more than 20 per(1977) describes cent fines. Bhandari a case in which compaction piles 1ed to soil densification only in zones where the fines content was less than 20 percent.

and

515

+ eo)

40

¡n(r s = r.or \_-=-

.After iml)rovement

!

a\t

o

2

)

(s)

u u

7(b) ' in pattern,.Fig' for piles in a triangular whici¡ d is the sand pile diameter (up to 800 mm)' curve method that can be used to An influence probe spacings to give estimate vibroflotation density is given by minimum relative a specified Brown's curves are based on the srown (L97?). (1953) ' procedure developed by D'Appolonia

a

o,30

\t"

8 2 0 3

o

\

I

t

=

o o

ao

1- @

I

I

ta\

O

oa o . \.t

tI T

l - { - a-a -r---t-,-----: \

.

@

@ @-

3ond pilo

L

(b) TriongulorPotlarn

(o) Squorc Pollorn

l

Before improvement

0

F i

^

20 Finerfraction: tp)

s€ + C^ H^ 1o9(t,/t^) I f r

' " z tI f + s -

-

s

f

s -

y

(r4)

r+s

-.e nature of secondary compression is such that ---e time after surcharge pe is removed, seconr:'/ compression will reappéar under P¡. This ::ect is smaIl, however, and can usually be .¡;lected (Johnson, 1970a). -e increase in undrained shear strength which :c::panies preloading may be the most important ::ect in many cases. The undrained strength ':er a given duration of preloading may be esti::e3 using the principles o f t h e S H A N S E Pp r o ::re (Ladd and Foott, 1974) together with an :-i'sis to estimate the increase in effective -.sclidation pressures withín the preloaded

Ít,L

''ae5 tt

tl

SOt.l ¡ COr¡P¡t3SrlLt

) l l SANO OP/lN

(13)

-:. which Co is the vertlcal per 1og cycle strain -r.:rease in time subsequent to the end of prl-::1' consolidation at tpr and Hp is the layer '-.:ckness at time tp. The analógous equation ' r (II) for the cri-tical point for thls case l-s

rttt

ccet.J!¡tf

Secondary compression may represent a very signiportion or the total compression of some iicant presoi.Is, especially organic clays and peats. ,-ompression using surcharge loadings may be for minimizing the effects of subseeffective ;uent secondary compression under permanent The concept is to estimate the total loads. settlement under p¡ as the sum of that due to :rimary consolidation and that due to secondary :.:mpression, ssec, anticipated to occur during ::e life of the structure. s"." is determined ::cording to rsec = Co Hp log

cl.rt

ftUDae.cT

Fig.

18

Typical

Ftet

SrOü Oear¡,¡c

SOtL

\

Vertical

Dc,

Drain Installation

The theory for consolidation by radial drainage and by combined radial and vertical drainage is welL developed (Barron, 1948¡ Carillo, l - 9 4 2 ). The results are summarized in Figs. 19 and 20. ft is i¡nportant to note that drain spacing is a parameter in the determinafar more significant tÍon of consolidation time than is drain diarneter. Olson et al. (L974') have developed a finite difference computer program that can be used to investigate the effects of time-dependent loading, variable consolidation coefficients, nonlinear soil stress-strain behavior, the influences of larqe strains, and the installation of drains after one-dimensional settlement has begun. Until a few years ago vertical drains of sand, typically 200 to 500 mm in diameter and spaced anywhere from 1.5 to 6.0 m on centers, were widely used. Installation was accomplished using a variety of techniques of both the displacement and non-displacement type. Displacement drains, while generally less expensive and faster to

523

r.--j------{ ¿ wELL a

a.

t--

tñc¡¡G

,l :--r-r

,O FLO,

I ¡

.cEoss oUfEP

-_ -\ .{

SUPF'CE

H

t

EX I

a

-¡'bfl

*ra-_

n =d¡5

d r = l . O SS

s. PLAN OF DRAIN tttLL

I sEcrto?{ A-A

PAfTERN

vERTICALcoatOLlOaTlo. :

.-

l r vr Y - {t r il . c" o-r - rH 7

,

*

t

c

TrHl *

I RADIALCOi9OLlDATl0rl kh c,+'tf

il - ?or clT¡

i,

T¡dl

oR | ' c,*

CO¡Eltl€ORADIALArO VERTICaLFL(t' A f A X YT l r E r

- /l\

PoeEI ExcEss

\". / "-n \ u o l "

fl)

f^rER PRESSURE )

RATros

(;)"-,'(;),

^T ^ PorxT

\uo,/¡

(;),

a v E R ^ vc ^EL u E s

)

Fig.

t9

u . l -u o'

AT A POII{T

ú-I--l

AvERAGEVALUE

uo

Theory Summary of Consolidation Drain Design for Vertical

The the surrounding soils' can disturb install, "smear" zone can impede,drainage, and iésultiirg soil may be weakened. These the disturbed as earlier may not be as detrimental effects of irowever, owing to the possibilities believed, to a higher strength than (1) reconsolidation and (2) the opening of.cracks and the original, l^'ith sand during installation fissureá tha¿ fill drainage area' and thereby increase the effective data Akagi (197i , Ig79l concluded that reliable whether non-displacement are lacking to establish than the disdrains are indeed more effective placement tYPe. sand are that conventional Present indications of confor the acceleration drains instalfed may soon be things of the past, as a solidation drains are coming into of préfabricated variety band-shapeil drains of the order of wide uie. 100 mm wide by 1 to 7 nm thick are produced by These drains can be several manufácturers. to depths up to 50.m by rapidly installed Drain spacings rn.óftirré" with special mandrels. Both dynamic ái tn" order of- 1 m are typical. a¡e used' methods of installation and static drains or wicks comPosed ci;.;i;;-;táiabricatecl fabric contáiners are of sand within cylindrical also used. A detailed discussion of these new features, and methods of árains, their specific is beyond the scope of this report' instaliation Hansbo (1979) has presented a comprehensive using prefabricated overview of consolidation Only a few general charband-shaped drains. are summarized here' acteristics K'iellmanrs cardboard wick, introduced in 1937, drains ' of the prefabricated wás the first Presently there are several types-on.the market Geodrain, castle under suóh names as Alidrain, The Ááita, Colbond, Mebradrain, and PVc Drain' clrainá usually consist of a core of plastic and or sleeve of paper, fibrous material, a filter The cross section design proporous plastic. channels for v:.aes ¡ór a system of vertical from the lhe Colbond Drain differs water floh¡. in that it is a non-rdoven fabric througháth.r" wider (30 cm) than ár-ri, .tt¿ it is signif icantly The mandreLs used for drain the others. are of cross section considerably installation Fig' 2l is an larger than the drain itself. of the drain necesThus installation exaáple. causes some disturbance of the surroundsariiy i -^ ¿¡¡Y

¡¡arr¡A Yrvs.¡Y.

f

o

6

l

ñ

E

.n'lO

0 o

t

7 3

()

n = 6"/dr1

o o o (tr o 6

- --

Vert¡colf low Rodiot f tow

o,ol

o.l Time Foctor, T" ond T,

Fig.20

524

for Radial Flot¡t Solution Consolidation at Ground strain and Equal Vertical surface (earron, 1948)

Fiq.2t

Cross Section and Mandrel

of

a Plastic

Drain

sleeve surrounding The filter must satisfy several criteria:

the plastic

core

l.

should not be signlficantly Its permeability less than that of the surrounding soil.

2.

Fine soil particles prevent clogging of core.

3.

4.

should be retained to flow channels in the

enough so as should be stiff The filter not to be pressed into core channels by soil pressures and strong high lateral enough not to be damaged during installation. should not undergo physical, The filter deterioration chemical, or biological during the intended Iife of the drain.

of required drain determination Theoretical spacing can be made in the same lttay as for sand Crains; i.e,, with the aid of Figs. 19 and 20. due lo vertical if consolidation .\Iternatively, compared to that due to :low is negligible radj-al flow, which is often the case, then the t can be determined :ime of consolidation accordinq to d-tu +-

=

_:-

8".r_h

|n -'-

r I _ ün

(rs)

-----i--

.:.ere Ú¡ is the average degree of consolidation. to a good approxima::e parámeter l, is given, bV ::,on, in terms of n(=d"rzd.)

u = ln (n) - 0.75

(16)

the values of n (>12, except for colbond, ::r (>0.8 m) used in practice. spacing 3) and drain do, of a band-shaped diameter ::e equivalent t is taken as of wiilth b and thickness ::ain

-

o*

2( = --;-

b+r)

(17)

during and disturbance resistance well :::h tirnes for conmay cause the actual -:.stallation by than predicted to be greater ;:!rdation Both of these considera:--i. 20 or Eq. (15). ::cns are discussed by Hansbo (1979). can be indrains band-shaped . :efabricated than vertical, other at orientations ;:alled applicause for special ..-.:ch enables their and on slopes. ::.ons such as under-drainage je3ause they can tolerate displacesignificant -e::ts without are prefabrícated dralns rupture, -:: due to loss of effectiveness as susceptible in the case as can occur i r shear displacements -: sand drains, , ,: : e

Practrcai

.::

drains the without vertical precompression by the is controlled of consolidation :r:e flow, for vertical of consolidation :eÍíicient may be tests and fiefd Both laboratory and there is extendetermination, -ieá for its accuracy .:'.'e literature their on the methods, r:.: limitations. e:tlement

predictions

for

two"

and

three-

dimensional deformations and drainage conditions are more complex than the one-dimensional proCornputer analyses can cedure described herein. Some solutions be developed for special cases. of approximate for determination are available cases. or limiting If the compressible layer thickness H is large to the wiilth of the loaded area B, then relative sr in there \.úi11 be an immediate settlement Its magnitude can be estimated Equation (9) . settleThe consolidation theory. using elastic rnent, saorr" in these cases may be reduced depending on the magnitude of the pore pressure coeffisettlements of consolidatíon cient Á. Estimation according to the procedure of Skempton and Lateral Bjerrum (1957) may be appropriate. the dliainage becomes important in accelerating when the value of H/B rate oi consolidation exceeds 0.25 to 1.0, depending on the shape of loaded area and whether the compressible layer The rate of settleis singly or doubly drained. be even greater if the horizontal ment will c'-¡, is greater and therefore permeability, permeability and crr-.\r. A than the vertical quantitatíve estimate of both of these effects developed by óan be made using the solutions Davis and Poulos (L972). vühen vertíca1 drains are used, the coefficient due to horizontal consolidation of vertical the rate of consolidation. flow, co,-¡, controls tests on large samples, Both special laboratory e.g., Hansbo (1960), Rowe (1964), Berry and w i Í k i n s o n ( 1 9 6 9 ) , a n d P a u t e ( 1 9 7 3 ), a n d i n - s i t u and Gardner (L975), summarized by Mitchell tests, A major difhave been used to determine c,,-". deter¡nination testing'fór in laboratory ficutty of c,,-" is that very large samples are required of the representative in máni cases if results are to be obtained. stratification true in-situ An approach that has yielded reasonable results and compute cn-¡ is to measure k¡ in the field value determined by conusing a compressibility condition A typical tests. láboratory ventional is that .rr_h/.rr_., = 2 to 10. Precompression

by Electro-osmosis

Because vtater can be made to flow through finegrained soils from anode to cathode in a direct by electroi.e., field; éurrent electrical will result if water is osmosis, consolidation removed at the cathode but not replaced at the and limited soil conditions For certain anode. may be an economisoil volumes, electro-osmosis ff, means for consolidation. ca1 and effective at the same time as water is being removed at chemicals are injected the cathode, stabilizing at the anode, soil improvement by electrocan be achi-eved. Electroinjection kinetic inJection is discussed in a later section' kinetic has been elaboThe mechanism of electro-osmosis (1967) . The theory rated by Gray and Mitchell has been by electro-osmosis for conlolidation developed by Esrig (1958), Wan and Mitche1l (1976), anil Mitchell and !üan (1977) . Recent case (1977) . are sumrnarized by Pilot histories The water flow direct current

rate, field

9n, in a one-dimensional is initialtY

q h = k e i e A

( m 3/ s e c )

(18)

525

= I x lO-e to

where k.

7 x 10-s m/sec per volt/m

ie = electrical

= cross-sectional

A

the

Alternatively,

flow

area (m2)

rate

where k, = water flow per unit a {m37sec7anp) = current

can be expressed by (19)

time per ampere

and kr are related o, conduótivity,

by the

values of o range from about 0.02 mhor/m for low salt content soils to 0.30 mho/rn for high salt Values of k¡ for vtater contents content soils. in the range of 50 to 1OO pércent are given in The power consumption P is given by Table III. P - qh AVlk. x 10-3 where Av is

the voltage

(kwh)

(21)

Water Va1ues of Electro-Osmotic Transport coefficient (water content range 50-100t)

Pore Water Salt Concen(N) tration

*,2"!*2.*p

clay, Silty kaolinite

lo- 3

I x l o - s- 5 x 1 0 - 7

Silty clayr kaolinite

10-2

5xlo-s-lxlo-7

Clay ( illitic

IO

3xio- I -6x10- I

Type

)

associated with this The amount of consolidation stress increase is obtained from a effective pressure relationship void ratio vs. effective for the soil determined in the usual manner. in the absence of electroStrength increases, can be estimated in chemical hardening effects, the same way.

Clay (illitic)

.

2xlo-8-3x10-8

= efectrode

L v

= coefficient

the water flow rate deDuring consolidation, It ceases when a hydraulic creases with tine. gradient, caused by a decrease in pore r,rater to that at the pressure at the anode relative cathode, causinq flow from cathode towards inanode, exactly balances the electrically gradient causing flow from anode duced hydraulíc the increase towards cathode. At this condition stress, Lo' , from that at the in effective start of treatment is

526

= (k./k¡)

yrv

degree the specified and boundary condi-

spacing of consolidation.

TABLE IV Time Factor for Various Degrees of Consolidation by Electro-osmosis Between Paralle1 Plate Electrodes

Degree of Consolidation U

(r) 0

¡s'

(23)

= tirne factor for of consolidation tions

where T

c

TL2 "v

degrees of consolidavalues of T for different plate electrodes tion for the case of parallel in voltage betrreen them with a linear variation Measurements by Johnston are given in Table Iv. (1977) indicate that rather than and Butterfield in voltage betvteen electrodes, a linear variation gradient infinite electrical an instantaneously which decreases develops at the anode initially way, to a uniform gradient at in a consistent values of T for the completion of consolidation. these conditions are also listed in Table IV. It occurs more raPmay be seen that consolidation case. idly for the latter

drop.

TABLE III

SoiI

Y' = the unit weiqht of water t^t (a function of position) V = voltage

(20)

= k.,/o

conductivity

is governed by the The rate of consolidation that apply to consolidation same relationships The time t for applied loading. under directly is a given degree of consolidation

(amps)

k" The coefficients specific electricaL k,

(volt,/n)

( m3 , / s e c )

c¡, = k. I a

I

gradient

potential

where kn = the hydraulic

(22)

Time Factor, Linear v Variation 0

T

Infinite Initial Gradient

n

10

0.05

0.001

20

0.10

0.007

30 40

0.16 0.22

0.017 0.020

50

0.29

0.05

60

0.38

0.07

70

0.50

0.10

80 90

0.66 0.95

0.14 0.20

ActuaL electrode arrangements are an array of rods or.pi-pes, spaced typically at 2 !o 4 meters in patterns, rather than parallel flat plates. In addition, variations-in pioperties, especially in the ratio ke,/kh, that áé"éf"p during consolidation lead to deviations from the theory (Mitchell and vüan, Lg77). rñus tfre values in Table fV can be used onlv as .r, approximation. for prediction of coisolidation E.rme. AnaJ.ysis of the electrical flows for different electrode arrangements, Fig. 22, shohrs that a hexagonal arrangement is efficient in terms of _(f) power consumption, (2) average voltage (the higher the aizerage voítage ttre gl:a,ter the.average amount of-consoliáation that can De oDtalned for_ a given applied voltage), (3) and anode to cathoáe ratió.

Fq

F¡.i + o + o + o + _ r . o + o + o + o - J . + o . o + o + o t o , ¡ , o . o * o

-./

80, xud

F'rl ,^ Ct'l'col ltaot

9 s

;

:

cro'¡¡

1-vc't'co'r'c¡

t--

--72- 5 : grouting

consistently

< 2 z grouting

not possible

K

N.

possible

(D1o)soi1

6 : grouting


11 : grouting N

For

Grout Types

/' " 1 n 5 \' =rñ_-T- soil t"85'qrout

N > 24 : grouting N < 11 : grouting

lique-

Volume change control of expansive soils through pressure injection of lime slurry. This technique is controversial and likely to be effective only under special condi( I n g l e s tions and Nei1, 1970; Wright, 1973; Blacklock, 1975¡ Thompsonand Robnett, 1976)

i s Volyme 4

Forsoils:

support

stabilization,

not be used.

grouts a soil volume of four to In soil-cement six times the loose volume of cement is common. Water volumes from one third to twice the soil volume per bag of cement are used. The 1ow water content mixes are typical of high viscosity displacement grouts. Zero slump compaction grouts with 30 to 60 second gel times can be made using cement, c1ay, and flyash mixes with an alkaline accelerator. ff bentonite is used, expanded particles may collapse if the groundwater has a high salt content. Care should be taken in the use of cement in the presence of sulfate-bearing soils or groundwater.

the lateral

loose sands aqainst

could

Neat cement and soil-cement grouts are the most commonly used particulate grouts, although soilwater grouts have been used in some cases. In vtater-cement grouts water:cement ratios of 0.5:L to 6:1 have been used. l{ith low urater:cement ratios there is less segregation and filtering, and higher strengths are obtained, but they are harder to inject than grouts with a higher water content. Chemical additives are sometimes used penetration, to facilitate to prevent cement flocculation, and to control set times. Set times can be as short as 30 seconds or very long.

Void filling

4.

9.

why other

possible

AdditionaL gruidelines relating to particulate grout types and particle size are: Types I and II Portland cement are suitable soils coarser than 0.60 Íun. Type III Portland cement is coarser than 0.42 m¡n. Bentonite is 0.25 mm.

suitable

for

suitable

soils

for

coarser

for

soils than

Chemical crouts grouts Chemical offer the advantages grouts culate that they can penetrate

over partismaller

529

pores, as may be seen in Fig. 25, they have a and there is a better control lower viscosity, On the other hand their technology of set time. Soils conis more complex and costs are high. taining less than 10 percent fines (