The reduction in the shaft friction and end-bearing resistance of piles in fine-grained soils
is the result of a reduction in the shearing strength of these soils under cyclic loading. The
amount of reduction for an infinite number of load repetitions depends on the ratio of the
applied stress to the ultimate stress of the soil. It is the usual practice to double the safety
factor on the combined shaft friction and end bearing to allow for the dynamic application
of load (see Section 6.2.2).
The torque of rotating machinery can cause lateral loading on the supporting piles. The
deflection under lateral loading can be calculated by the methods described in Chapter 6. To
allow for dynamic loading the deflections calculated for the equivalent static load should
be doubled.
The type of pile, whether driven, driven and cast-in-place, or bored and cast-in-place, is
unlikely to have any significant effect on the behaviour of piles installed wholly in fine-
grained soils. It is possible that the lateral movements of piles with driven pre-formed shafts
(e.g. precast concrete or steel H-piles) will be greater than those of cast-in-place piles, because
of the formation of an enlarged hole around the upper part of the shaft (see Figure 4.5).
The frictional resistance of a pile to static compressive loading in a coarse soil is relatively
low. This resistance is reduced still further when the pile is subjected to vibratory loading, and
it is advisable to ignore all frictional resistance on piles carrying high-frequency vibrating
loads. If such piles are terminated in loose to medium-dense soils there will be continuing
settlement to a degree which is unacceptable for most machinery installations. It is therefore
necessary to drive piles to a dense or very dense coarse soil stratum and even then the
settlements may be significant, particularly when high end-bearing pressures are adopted.
This is due to the progressive attrition of the soil grains at their points of contact. The
continuing degradation of the soil particles results in the slow but continuous settlement of
the piles. If possible, piles carrying vibrating machinery should be driven completely
through a coarse soil stratum for termination on bedrock or within a stiff clay. The ACI
Report(9.6)considers the complex interaction of piles in a group under dynamic loading
when piles are closer than 20 diameters and recommends suitable computer programs to
consider group dynamic stiffness and damping effects in such cases.
A problem of piled foundations for machinery sensitive to small differential settlements
was experienced at John Brown Company’s shipyard at Clydebank. At this site 18.6 m of
loose to medium-dense silty sand were overlying stiff glacial till. Gear-cutting machinery
comprising large hobbing, shaving and grinding machines had to operate to an accuracy of
0.009 mm and each machine was installed in a separate enclosure under conditions of con-
stant temperature and humidity. It was essential to avoid any appreciable settlement of the
machines due to vibrations caused by their own motion, or transmitted from elsewhere on
the shipyard and the adjacent main road. It was expected that settlements of raft or piled
foundations terminated in the medium-dense sand (with a standard penetration test N-value
of 20 to 30 blows per 300 mm) would be excessive and a type of pile had to be selected
which could be driven through the deep sand layers to reach the glacial till. The possibility
of the compaction of the sand due to driving a number of piles in a closely spaced group was
considered and this led to the choice of a small displacement pile in the form of a Larssen
BP2 box-section driven with an open end. It would have been possible to use water jetting
to assist the penetration of these piles, but all the piles were driven by a double-acting
hammer into the glacial till without recourse to jetting. The building surrounding the plant
was carried by driven and cast-in-place piles terminated at a penetration of about 4.6 m into
the sand stratum.
436 Miscellaneous piling problems