Pile Design and Construction Practice, Fifth edition

122 Piling equipment and methods
Reinforcing
cage for pile
38 mm (1½��) grout
injection pipe
25 mm (1��) circumferential
bars
Basket fabricated
from 14 mm ( 9 16��)
bars
Bottom plate and rubber
sheet perforated
25 mm (1��)
spacer
Rubber or neoprene sheet
Uniform size gravel
Lifting rope
6.4 mm (¼��)
steel plates
Figure 3.37 Preloading cell for compressing loosened soil beneath base of bored piles by grouting
(after Bolognesi and Moretto (3.14) ).
lowered to the bottom of the borehole. A flexible metal sheet covers the whole of the underside
of this base plate. A grout injection pipe is connected to the space between the plate and the
sheet, and a peripheral ring of grout pipes is attached to the reinforcing bars for a predetermined
height above the pile base. All grout pipes are extended to a pump and metering unit
at the ground surface. The pile is then concreted. After a waiting period to allow the concrete
to harden a cement grout is injected into the peripheral injection pipes with the object of
bonding the lower part of the pile shaft to the surrounding soil. A further period of a few
days is allowed for the grout to harden, then the space between the metal sheet and steel
plate is injected with grout under high pressure. The uplift on the steel plate is resisted by
the peripheral grout–soil bond stress on the shaft and the soil beneath the flexible sheet is
thus compressed. The height of the peripheral grouting above the pile base depends on the
required base pressure and hence on the design base resistance of the pile. The important
control criteria are the grout pressure (a function of the required ultimate pile capacity), an
upward displacement limit to minimize degradation of the side friction, and a minimum
grout injection volume to ensure lines are not clogged.
Direct injection of cement grout beneath the pile base was used to re-compress sand
disturbed by drilling 1.2 m diameter bored piles supporting an office building at Blackwall
Yard, London. Yeats and O’Riordan (3.15) described the installation of a 38.2 m deep test pile.
The shaft was drilled by rotary auger under a bentonite slurry through the alluvium and stiff
to hard clays of the London Clay and Woolwich and Reading formation (Lambeth group)
into very dense Thanet Sands. The upper 31 m of the shaft were supported by casing. After
completing the drilling four separate grout tube assemblies as shown in Figure 3.38 were
lowered to the base of the borehole. The injection holes in the tubes were sleeved with rubber
(tubes-à-manchette). The pile shafts were then concreted under bentonite, and 24 hours after
this water was injected to crack the concrete surrounding the grout tubes. Base grouting
commenced 15 days after concreting. The injections were undertaken in stages with pressures
up to 60 bar and frequent checks to ensure the pile head did not lift by more than 1 mm.