Geotechnical Engineering

DHARM

PILE FOUNDATIONS 661

When a pile is driven in soft clay, the soil around gets remoulded and loses some of its
strength. However, it regains almost its full strength within a few weeks of driving, through
consolidation. Since piles will be usually loaded a few months after driving, this reduction in
strength soon after driving does not pose any problem. However, if piles are to be loaded soon
after driving, the remoulded shear strength is to be considered.
When piles are driven into stiff clays, the soil close to the pile may get remoulded and
this may also create a slight gap between the pile and the soil; consequently the adhesion is
always smaller than cohesion and α will be less than unity.
The adhesion factors for different pile materials and consistency of the clay are shown
in Table 16.4.

Table 16.4 Adhesion factors for piles in clay (Tomlinson, 1969)

S. No. Material of pile Consistency of clay Cohesion (kN/m^2 ) Adhesion factor

1. Wood and concrete Soft 0–35 0.90 to 1.00
   Medium 35–70 0.60 to 0.90
   Stiff 70–140 0.45 to 0.60


2. steel Soft 0–35 0.45 to 1.00
   Medium 35–70 0.10 to 0.50
   Stiff 70–140 0.50

Under-reamed piles

An ‘under-reamed’ pile is one with an enlarged base or a bulb; the bulb is called ‘under-ream’.

There could be one or more under-reams in a pile; in the former case, it is called a single under-

reamed pile and in the latter, it is said to be a multi-under-reamed pile (Fig. 16.4).

bu

1.5 bu

b

(a) Single under-reamed pile (b) Multi-under-reamed pile

Fig. 16.4 Under-reamed piles

Under-reamed piles are cast-in-situ piles, which may be installed both in sandy and in

clayey soils. The sides may be stabilised, if necessary, by the use of bentonite slurry, sometimes

called ‘drilling mud’. The under-reams are formed by a special under-reaming equipment. The