Section 3.3.1. Enlargements cannot be formed in coarse soils unless the borehole is drilled
with the support of a bentonite slurry. The size and stability of an enlargement made in this
way is problematical. Full-scale loading tests are essential to prove the reliability of the ben-
tonite method for any particular site. Reliable predictions cannot be made of the size and
shape of base enlargements formed by hammering out a bulb of concrete at the bottom of a
driven and cast in-situ pile as described in Section 2.3.2. End enlargements formed on pre-
cast concrete or steel piles, although providing a substantial increase in compressive resist-
ance when driven to a dense or hard stratum, do not offer much uplift resistance since a gap
of loosened soil is formed around the shaft as the pile is driven down.
In the case of bored piles in fine-grained soils installed using belling tools, resistance to
uplift loading provided by the straight-sided portion of the shaft is calculated over the depth
Hin Figure 6.8 minus the overall depth of the under-ream. Failure under short-term loading
takes place in undrained shear on the pile to clay interface. The mobilized resistance should
take into account the effects of installation as described in Section 4.2.3. Uplift resistance
of the projecting portion of the enlarged base is assumed to be provided by compression
resistance of the soil overburden.
Resistance to long-term uplift loading on piles in fine-grained soils is calculated by effec-
tive stress methods as described for clayey sands in the following paragraphs.
Meyerhof and Adams(6.2)investigated the uplift resistance of a circular plate embedded in
a partly clayey soil and established the equation
(6.5)
where Quis the uplift resistance of the plate, Bis the diameter of the plate, His the height
of the block of soil lifted by the pile (Figure 6.9), cis the cohesive strength of the soil, sis
a shape factor (see below), is the density of the soil (the submerged density being taken
below groundwater level), Dis the depth of the plate, Kuis a coefficient obtained from
Figure 6.9, is the angle of shearing resistance of the soil, and Wis the weight of the soil
resisting uplift by the plate.
Qu cBHs 1
2
B(2DH) HKu tan W(c)Piles to resist uplift and lateral loading 313bBHQuFigure 6.8Uplift of single pile with base enlargement in fine-grained soil ( 0).