Pile Design and Construction Practice, Fifth edition

170 Resistance of piles to compressive loads
Depth below ground surface
Pile toe
Equation 4.6 applies to the shaft resistance of piles in coarse-grained soils. The factor K is
governed by the following influences:
(1) The stress history of the soil deposit
(2) The ratio of the penetration depth to the diameter of the pile shaft
(3) The rigidity and shape of the pile and
(4) The nature of the material forming the pile shaft.
The stress history of the soil deposit is characterized by its coefficient of earth pressure at
rest, K o, in an undisturbed state. This is measured by field tests such as the standard penetration
test (SPT), or cone penetration test (CPT) and by the pressuremeter (Section 11.1).
In normally consolidated soils K o is constant with depth and depends on the relative density
of the deposit. Some typical values for a normally consolidated sand are
Relative density K o
Interface shear stress
Figure 4.15 Distribution of interface friction on shaft of pile driven into sand.
Loose 0.5
Medium-dense 0.45
Dense 0.35
If the soil deposits are over-consolidated, that is, if they have been subjected to an overburden
pressure at some time in their history, K o can be much higher than the values shown
above, say of the order of 1 to 2 or more. It is possible to determine whether or not the soil
deposit is over-consolidated by reference to its geological history or by testing in the field