4.3 Piles in coarse-grained soils
4.3.1 General
The classic formulae for calculating the resistance of piles in coarse soils follow the same
form as equation 4.1. Expressed in the parameters of a coarse-grained soil (cu = 0), the total
pile resistance is given by the expression:
(4.16)
where is the effective overburden pressure at pile base level, Nqis the bearing capacity
factor, Abis the area of the base of the pile, Ksis a coefficient of horizontal soil stress which
depends on the relative density and state of consolidation of the soil, the volume displacement
of the pile, the material of the pile and its shape, is the characteristic or average value of
the angle of friction between pile and soil, and Asis the area of shaft in contact with the soil.
The factors Nqand Ksare empirical and based on correlations with static loading tests, is
obtained from empirical correlations with field tests.
The factor Nqdepends on the ratio of the depth of penetration of the pile to its diameter
and on the angle of shearing resistance of the soil. The latter is normally obtained from
the results on tests made in-situ (see Section 11.1.4). The relationship between the standard
penetration resistance Nand , as established by Peck et al.(4.18), and between the limiting
static cone resistance, qcand as established by Durgonoglu and Mitchell(4.19), are shown
in Figures 4.10, and 4.11 respectively.
From tests made on instrumented full-scale piles, Vesic(4.20)showed that the increase of
base resistance with increasing depth was not linear as might be implied from equation 4.16,
but that rateof increase actually decreased with increasing depth. For practical design
purposes it has been assumed that the increase is linear for pile penetrations of between
10 and 20 diameters, and that below these depths the unit base resistance has been assumed
to be at a constant value. This simple design approach was adequate for ordinary foundation
work where the penetration depths of closed-end piles were not usually much greater than
10 to 20 diameters. At these depths practical refusal was usually met when driving piles into
medium dense to dense coarse soils.
However, the use of piled foundations for offshore petroleum production platforms has
necessitated driving hollow tubular piles with open ends to very great depths below the sea
bed to obtain resistance in shaft friction to uplift loading. The assumption of a constant unit
base resistance below a penetration depth of 10 to 20 diameters has been shown to be over-
conservative (see Section 4.3.7).
The value of Nqis obtained from the relationship between the drained angle of shearing
resistance ( ) of the soil at the pile base and the penetration depth/breadth of the pile.
The relationship developed by Berezantsev et al.(4.21)is shown in Figure 4.13. Vesic(4.20)
stated that these Nqvalues gave results which most nearly conform to the practical criteria
of pile failure. The alternative is to use the Brinch Hansen Nqfactors shown in Figure 4.13.
They should be multiplied by a shape factor of 1.3 to allow for the square or circular
cross-section of the pile base. The Brinch Hansen factors may be over-conservative for
some D/B ratios, as, for example, a D/B ratio of 20 and values greater than 35. It is
important to note that the values of obtained from SPT N-values should not be
voQp Nq voAb^1
2
Ks vo tan AsResistance of piles to compressive loads 165