Contractors World - UK & Ireland Vol 3 No 5 - page 13

Page 13
Contractors World - UK & Ireland Vol 3 No 5
complex logistics
and planning re-
quirements.
The
contract period was
just 10 weeks to in-
stall over 466 cast-
in-situ
concrete
piles to depths up to 55 m. Piles are mostly 750
mm in diameter except for one bearing pile in
the north-east corner which is 1,500 mm in di-
ameter. Close to an underground railway line,
this pile accommodates a cantilevered transfer
slab adjacent to where the main pile retaining
wall begins to the east and south. The pile was
drilled immediately adjacent to the tunnel ex-
clusion zone.
Using the cased rotary method, this pile had
to be bored through the made ground to the
stable dry London Clay and continue through
this into the more unstable Lambeth Group
materials (clays, sands, gravel, etc) that lie
below.
As a consequence of encountering
Lambeth sands, bentonite drilling fluid was
introduced to stabilise the bore. A tremie
system was used to continuously pour the 110
m³ of S4 concrete (slump 230) at a rate of
25 m³ per hour while removing the bentonite.
Prior to concrete placement, a full length
prefabricated steel reinforcing cage was
lowered into the pile in spliced sections.
With this pile complete, the Llamada P150-tt
piling rig started work on the contiguous pile
retaining wall that surrounds the entire site
and comprises 174 piles. These piles, 750 mm
diameter, were formed using the continuous
flight auger method and are to a maximum
depth of 30 m with an average depth of 15 m.
With CFA techniques, the pile cage is plunged
into the wet concrete bore.
Simultaneously a second rig, a 75 t. Mait HR 260, using
Aerial view of the King’s
Cross development area.
P1 is middle, left hand side
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