Pile Design and Construction Practice, Fifth edition

130 Piling equipment and methods
require ramming or vibrating. The mix proportions should be such as to ensure compliance
with the requirements regarding strength and minimum cement content of BS EN 1536 or
with any special requirements for durability (see Section 10.3.1). A dry mix should be used
for the first few charges of concrete if the pile base is wet. The concrete in the shaft is fed
through a hopper or chute placed centrally over the pile to direct it clear of the sides and the
reinforcement. After completing concreting, the lining tubes are withdrawn. If a loose liner
is used inside an upper casing, the former is lifted out as soon as the concrete extends above
the base of the outer tube. A vibrator of the type described in Section 3.1.5 is a useful expedient
for extracting the upper casings used to support soft clays or loose sand. The quantity
of concrete placed in the shaft should allow for the outward slumping which takes place to
fill the space occupied by the tube and any overbreak of the soil outside it. At this final stage
there is inevitably some laitance which has risen to the top of the concrete. The laitance may
be diluted and contaminated with water and silt expelled from around the casing as the
concrete slumps outwards to fill the gap. Thus the level of the concrete should be set high
so that this weak laitance layer can be broken away before bonding the pile head onto its cap.
The terms of the contract should make it clear whether or not this removal should be
performed by the piling contractor.
The concrete in a pile shaft may be required to be terminated at some depth below ground
level, for example, when constructing from ground surface level, piles designed to support
a basement floor. It is a matter of some experience to judge the level at which the concrete
should be terminated, and it is difficult to distinguish between fluid concrete and thick
laitance when plumbing the level with a float. Fleming and Lane (3.22) recommend the following
tolerances for all conditions:
Concrete cast under water: �1.5 to �3 m
Concrete cast in dry uncased holes: �75 to �300 mm
Concrete cast in cased holes:
the greater of (a) �75 to � 300 min � cased length/15
or (b) �75 to � 300 mm � [depth to casting
level � 900 mm/10]
The Institution of Civil Engineers Specification for Piling (2.5) specifies casting tolerances
for three conditions of placing concrete in pile boreholes with and without temporary
casing. The ground surface or piling platform level is defined as the ‘commencing surface’.
The three conditions refer to a situation where the cut-off level is at a depth Hm below the
commencing surface such that H is from 0.15 to any depth for condition (a) and below or
between 0.15 and 10 m for (b) and (c). The conditions are as follows:
(a) Concrete placed in dry boreholes using permanent casing or cut-off level in stable ground
below base of casing: the casting tolerance in metres is specified to be 0.3 � H/10
(b) Concrete placed in dry boreholes using temporary casing other than as (a) above: the
casting tolerance in metres is specified to be 0.3 � H/12 � C/8, where C is the length
of temporary casing below the commencing surface
(c) Concrete placed under water or a drilling fluid: the casting tolerance in metres is
specified to be 1.0 � H/12 � C/8 where C is the length of temporary casing below the
commencing surface.