Civil Engineering Reference
In-Depth Information
In sensitive clays heave is localized and the remoulded soil around the pile is extruded
at the ground surface. Reconsolidation of the soil in this situation may result in down-
drag on the pile. Similarly, in loose sands and gravels little heave is recorded as the pile
driving operation usually causes compaction of the deposit. Insensitive clays and silts
are the soil types most prone to heave, and the volume of the soil displaced is approx-
imately equal to the volume of the piles. Based on a study of case histories, Hagerty
and Peck (1971) suggested that the surface heave for such soils would be equivalent to
one-half the displaced volume of soil, the remainder of the displaced volume causing
ground heave in the soil surrounding the pile group.
In particular circumstances lateral soil displacement may occur, leading to the dam-
age of adjacent structures or the displacement of previously driven piles. The walls
to the Clyde Dry Dock at Greenock were overloaded and cracked when cast-in-place
piles were driven adjacent to the wall (Geddes
et al
., 1966). In this instance high pore
pressures were also thought to have been induced in the silty alluvium. Significant
lateral soil displacement may also occur when piles are driven through river banks or
other sloping sites.
In addition to soil displacement, heavy ground vibrations are transferred to the soil
close to the pile. In silty alluvium, substantial pore pressures may be developed as
a result of pile driving. On one site, piezometers adjacent to the piling were seen to
overflow for a short period. Dissipation of the excess pore pressures may lead to the
development of downdrag forces. In a recent case, pore pressures set up by pile driving
re-activated an old landslip, causing 600 mm displacement of the pile heads.
7.1.4 Noise and ground vibration
7.1.4.1 Noise from piling operations
Pile driving is an inherently noisy operation and severe environmental restrictions may
be included in the conditions of contract. Noise levels of over 85 decibels within 10 m
of the piling plant are quite common. Typical data on noise levels produced by piling
operations have been published by CIRIA (Report No. 64). These data are reproduced
in Table 7.2.
Investigations into the sources of the noise have shown that a large proportion
arises from secondary effects, rebound of the hammer, rope slap, engine noise, etc.
Improved design of the components of the piling rig can considerably reduce the
high-frequency components of noise. To dampen the noise sufficiently to be accept-
able in urban situations, it may be necessary to enclose the hammer or the leaders
in an acoustic chamber. Several piling companies have developed such chambers to
suit their particular piling machines. The use of such devices usually results in some
reduction in the efficiency of the pile hammer and creates difficulties in handling and
pitching piles.
Driving steel sheet piling is exceedingly noisy, as the driving cap usually involves
steel to steel contact. In areas where severe restrictions are placed on noise levels, noise
outputs may be reduced by using an enclosure around the hammer, the use of pile
driving vibrators or employing the Giken hydraulic pile driver. However alternative
pile driving methods such as these involve the use of an auxiliary power unit which
may, in itself, emit a high level of sound.
