Pile Design and Construction Practice, Fifth edition

122 Piling equipment and methods

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 predeter-

mined 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.

Reinforcing

cage for pile

Uniform size gravel

Rubber or neoprene sheet

Bottom plate and rubber

sheet perforated

Lifting rope

6.4 mm (¼)

steel plates

25 mm (1)

38 mm (1½) grout spacer

injection pipe

Basket fabricated

from 14 mm ( ) bars

25 mm (1) circumferential

bars

(^916)
Figure 3.37Preloading cell for compressing loosened soil beneath base of bored piles by grouting
(after Bolognesi and Moretto(3.14)).