There is little evidence to show that ground-borne vibrations cause structural damage to
buildings(3.9). However, if there is concern then steps must be taken to survey buildings and
measure vibrations induced by construction activity. BS7385 describes methods of assessing
vibrations in buildings and gives guidance on potential damage levels. The recommended
thresholds in BS5228 to avoid non-structural (‘cosmetic’) damage in residential property are
peak particle velocity (ppv) of 10 mm/s for intermittent vibration and 5 mm/s for continuous
vibrations at frequencies between 10 and 50 Hz. For heavy and stiff buildings the thresholds
are 30 and 15 mm/s respectively. Protected buildings, buildings with existing defects and
statutory services undertakings will be subject to specific lower limits. The human response
to vibration should also be considered. Transmission of vibrations during piling depends on
the strata, size, and depth of pile and hammer type, and predictions of the resulting ground
frequency and ppv at distance from the source are difficult.
‘Press-in’drivers such as the Dawson ‘push–pull’unit with 2 078 kN pressing force are
becoming more common particularly for sheet piling, but many of the units can be adapted
for installing box-type bearing piles and H-pile groups, particularly in clays. The advantages
of these powerful, high pressure hydraulic drivers using 2 to 4 cylinders are the low noise
levels (around 60 dBA) and the speed and vibration-less installation and extraction of piles.
The drivers can be suspended from a crane or mounted on a hydraulic crawler rig with
more than 20 tonnes of pulldown available on a rigid leader such as the Liebherr piling rig
to assist the installation; hanging leaders are not suitable. The Giken press-in rig operates
without a separate fixed leader relying on reaction from adjacent installed sheet piles; a
service crane is needed to pitch the piles. In addition, in hard ground this unit can pre-drill
a hole or apply water jets to assist in sheet piling.3.1.8 Pile helmets and driving caps
When driving precast concrete piles, a helmet is placed over the pile head for the purpose
of retaining in position a resilient ‘dolly’or cap block that cushions the blow of the hammer
and thus minimizes damage to the pile head. The dolly is placed in a recess in the top of the
helmet (Figure 3.20). For easy driving conditions it can consist of an elm block, but for
rather harder driving a block of hardwood such as oak, greenheart, pynkado or hickory is set
in the helmet end-on to the grain. Plastic dollies are the most serviceable for hard-driving
concrete or steel piles. The Micarta dolly consists of a phenolic resin reinforced with lami-
nations of cross-grain cotton canvas. Layers of these laminates can be bonded to aluminium
plates, or placed between a top steel plate and a bottom hardwood pad. The helmet should
not fit tightly onto the pile head but should allow for some rotation of the pile, which may
occur as it strikes obstructions in the ground.
Packing is placed between the helmet and the pile head to cushion further the blow on the
concrete. This packing can consist of coiled rope, hessian packing, thin timber sheets,
coconut matting, wallboards or asbestos fibre. The last-mentioned material has the advan-
tage that it does not char when subjected to heat generated by prolonged driving. The packing
must be inspected at intervals and renewed if it becomes heavily compressed and loses its
resilience. Softwood packing should be renewed for every pile driven.
Williams(3.10) has described severe conditions for driving precast concrete piles at
Uskmouth Power Station. He states that plastic dollies were used up to 40 times, compared
with elm blocks which only lasted for a very few piles. The packing consisted of up to 125 mm
of sawdust in jute bags, covered with two dry cement sacks placed at right-angles to each other100 Piling equipment and methods