Design of single piles - Piling Engineering - page 114

σ n and an interface friction angle, a function of the normal effective stress, , in much

the same way as for piles in free-draining soils. The normal stress was related to the
effective overburden stress,

δ

σ v , by a factor, K , to give

τ s = σ n tan σ v tan

δ = K δ = βσ v (4.19)

where . The value of K will vary depending on the type of pile (driven

or bored) and the past stress history of the soil. For piles in soft, normally or lightly
overconsolidated clay, Burland (1973) and Parry and Swain (1977) suggested values
of K lying between (1

β = K tan δ

sin 2

φ ). Neither of these approaches
takes due account of the stress changes that occur during and after pile installation.
For more heavily overconsolidated soils, Meyerhof (1976) showed that the value of
K consistent with measured shaft capacities varied between 1 and 2 times the in-situ
horizontal stress, with an average ratio of 1.5 - a value that was supported by data
from instrumented model pile tests (Francescon, 1983).
During the 1990s, detailed field measurements of pore pressure and normal stress
variations around full-displacement piles were obtained by a research group at Imperial
College, and these have led to a design approach that considers separately the three
stages of: pile installation, pore pressure equilibration, and pile loading (Lehane et al. ,
1994; Jardine and Chow, 1996). Chow (1997) assembled a database of results from
pile load tests, and the data for radial total stresses (less the in-situ pore pressure, u 0 ,
and normalized by

φ ) and cos 2 φ /

− sin (1 +

σ v ) immediately following pile installation, and the radial effective

σ rc /σ v , after full equilibration of excess pore pressures are plotted against
the overconsolidation (or more correctly yield stress) ratio, R , in Figure 4.13 for closed-
ended piles.
Immediately after installation, the total radial stress ratio, (

stress ratio,

/σ v , increases
linearly with the yield stress ratio from a value of just under 2 for normally con-
solidated soil (yield stress ratio of unity), with a gradient of about 0.4. As Lehane
(1992) observed, the gradient is approximately parallel to the correlation of K 0 with

σ ri − u 0 )

10

Radial stress
coefficients
( s ri - u o )/ s v o ′
&
s rc ′ / s v o ′

After installation
( s ri - u o )/ s v o ′

1

After consolidation
s rc ′

Equation (4.21)

/ s v o ′

Data from Chow (1997)

0.1

1 10 100

Yield stress ratio, R

Figure 4.13 Radial stress measurements around full displacement piles.