• Author / Uploaded
• Angel Cuellar

• Categories
• Fundación profunda
• Resistencia de materiales
• Mecánica de suelos
• Suelo
• Estrés (Mecánica)

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ULTIMATE LOAD CAPACITY OF PILES

3.1 INTRODUCTION

There arc two usual approaches to the Galculation of the ultimate load capacity of piles: the "static" approach, which uses the normal soil-mechanics method to calculate the load capacity from measured soil properties; and the "dynamic" approach, which estimates the load capacity of driven piles from analysis of pile-driving data. The first approach will be described in detail in this chapter, and the second in Chapter 4. In this chapter, a general expression for the ultimate load capacity of a single pile is given and its application to piles in clay and sand is described. Approaches for groups of piles in clay and sand will then be outlined. Other topics include the design of piles to rock, the use o.f in-situ tests such as the standard penetration test and the static cone to estimate pile-load capacity, the calculation of uplift resistance of piles and grou'ps, and the load capacity of bent piles. 3.2 ULTIMATE LOAD CAPACITY OF SINGLE P1LES

;,.2.1 General Expression The net ultimate load capacity, Pu , of a. single pile is generally accepted to be equal to the sum of the ultimate 18

shaft and base resistances, less the weight of the pile; that is,

(3.1)* where Psu Pbu W

ultimate shaft resistance ultimate base resistance = weight of pile

Psu can be evaluated by integration of the pile-soil shear strength Ta over the surface area of the shaft. Ta is given by the Coulomb expression Ta = Ca

+

On tan rPa

(3.2)

where Ta pile-soil shear strength Ca = adhesion On = normal stress between pile and soil angle of friction between pile and soil rPa • It is an implicit assumption of Eq. 3.1 that shaft and base resistance are not interdependent. This assumption cannot be strictly correct, but there is !ittk doubt that it is correct enough for practical purposes for all normal-proportion piles and piers.

ULTlMATE LOAD CAPACITY OF PILES

an is in turn frequently related to the vertical stress av , as (3.3) where Ks

Pu (3.4)

Ta

and

L

"" f O C(c

a

+ avKs tan 'i 3, to allow for the possible loosening effect of installatipn (see Section 2.4). (b) For Ks tan ,P�, Fig. 3.I0c should be used, taking the value of¢\ directly. The above suggestions may also require modification in the light of future investigations. Furthem10re, if jetting is used in conjunction with driving, the shaft resistance may decrease dramatically and be even less than the value for a corresponding bored pile. McClelland (1974) has reported tests in which the use of jetting with external return flow followed by driving reduced the ultimate shaft capacity by about 50%, while installation by jetting alone reduced the ultimate shaft capacity to only about 10% of the value for a pile installed by driving orJy. Another case in which caution should be exercised is when piles are to be installed in calcareous sands. Such ° sands may show friction angles of 35 or more, but have beer:i found to provide vastly inferior support for driven piles than normal silica sands. In such cases, McClelland (1974) suggests limiting the skin resistance to 0.2 tons/ft2 (19 kN/m 2) and base resistance to 50 tons/ft2 (4800 kN/m 2 ). In such circumstances, drilled and grouted piles may provide a· more satisfactory s_olution than wholly driven piles. In many practical cases, only standard penetration­ test data may be available. The value of I/>� may be esti-

28 ULTIMATE LOAD CAPACITY OF PILES 3 For drMrn pil