borehole or test profile over the penetration depth of the pile. Rc cal minis calculated from
equation 4.11 using the values of cuat the base and over the penetration depth of the pile
from the borehole or test pile showing the lowest results after reviewing all test data.
The characteristic total pile resistance is then obtained from the equation:
Rck (Rb calRs cal)/ (4.12)In equation 4.12 values of Rckare obtained using the correlation factor for mean values
of Rckand for minimum values (see Table 4.6) to give
Rck RbkRsk (4.13)where Rck, Rbkand Rskare characteristic values and Rckis the lower of the factored compo-
nents of the minimum or mean strength profiles.
Rbkand Rskare divided by the partial factorsbandsrespectively after using DA1 design
approach as described in Section 4.1.4 to give the design values:
Rcd Rbk/bRsk/s RbdRsd (4.14)When the alternative procedure is used Rcdis given by
Rcd (9cubkAb)/b(F (^) p usk)/s (4.15)
where cubkand uskare characteristic values of cufor the base and shaft respectively.
The foregoing procedures are illustrated by Example 4.1 at the end of this chapter.
4.2.2 Driven and cast-in-place displacement piles
The end-bearing resistance of driven and cast-in-place piles terminated in clay can be
calculated from equation 4.4. Where the piles have an enlarged base formed by hammering
out a plug of gravel or dry concrete, the area Abshould be calculated from the estimated
diameter of the base. It is difficult, if not impossible, for the engineer to make this estimate
in advance of the site operations since the contractor installing these proprietary piles makes
his own decision on whether to adopt a fairly shallow penetration and hammer out a large
base in a moderately stiff clay, or whether to drive deeper to gain shaft friction, but at the
expense of making a smaller base in the deeper and stiffer clay. In a hard clay it may be
impracticable to obtain any worthwhile enlargement over the nominal shaft diameter. In any
case, the base may have to be taken to a certain minimum depth to ensure that settlements
of the pile group are not exceeded (see Section 5.2.2). The decision as to this minimum
length must be taken or approved by the engineer.
The conditions for predicting shaft friction on the shaft are different from those with driven
pre-formed piles in some important aspects. The effect on the soil of driving the piling tube with
its end closed by a plug is exactly the same as with a steel tubular pile; the clay is remoulded,
sheared, and distorted, giving the same conditions at the pile–soil interface as with the driven
pre-formed pile. The clay has no chance to swell before the concrete is placed and the residual
radial horizontal stress in the soil closes up any incipient gap caused by shrinkage of the con-
crete. Also the gap which may form around the upper part of the driving tube (or down the full
c
c
4
3
Resistance of piles to compressive loads 159