January/February 2021 - 18

Site plan with pile locations set over underground
train lines (TPO1 is test pile)

Preliminary test pile reaction frame

geotechnical and delivery teams to de-risk and optimise the
construction process resulted in each pile being constructed
without incident and two months ahead of programme (by
late November 2018).
All events that may have led to the abandonment of a pile
were investigated, simulated and risk mitigation/elimination
strategies developed by the delivery team for all eventualities,
whether they were associated with the design or construction
processes ahead of commencement on site. Since there were
only 15 locations available to install the large diameter piles
(which were 71 in [1.8m] or 95 in [2.4 m] in diameter) that were
required to support the loadings of the entire scheme, a number
of piles would become the most highly loaded ones in the U.K.

The test pile was a success and actually achieved a peak
test load of 50.5 MN (5,680 tons). The final settlement was 3 in
(7.6 cm), which massively exceeded predictions of greater
than 11 in (28 cm) at this load. The test pile was fully
instrumented with state-of-the-art fibre optic cables along its
full length.

Pile Load Test
Such high working loads meant that the piles were very reliant
on generating large base capacities from the Thanet Sand
Formation, a fine grained, glauconitic sand common in London
that was some 180 ft (55 m) below ground level. A preliminary
pile test was therefore required to confirm the capacity of this
stratum, as well as providing justification to reduce the partial
factors and model factors within the pile design.
In a traditional pile test reaction system, the anchors surround a test pile. So few locations for piles were available that
that approach was not viable. The client secured temporary
access to an additional piece of land to construct a sacrificial
test pile utilising four " contract piles " to act as reaction piles in
line with the test pile. In this case, bi-directional stability to the
test truss/frame came from the bentonite silos as part of a
bespoke steel reaction frame designed to span longitudinally
across all five piles. This structure allowed a peak test load to be
achieved of 50 MN (5,680 tons) and provided sufficient
connection to the contract piles. The weight used made this the
highest loaded top down pile test to date in the U.K.
18 * DEEP FOUNDATIONS * JAN/FEB 2021

Spin off from designed spoil skip

Pile Construction
Many of the challenges faced by the construction delivery
team required innovative but simple and practical solutions.
The first of these was to effectively and efficiently deal with
the pile spoil during drilling. The physical constraints, and
the limitations on loading patterns permitted on the grillage,
meant that the spoil needed to be double (and in some cases,
triple) handled to remove it from the site. However, the
presence of clearance voids between the perimeter of the pile
casings and the grillage presented the risk of spoil and

January/February 2021

Table of Contents for the Digital Edition of January/February 2021