January/February 2021 - 19

bentonite migrating through to the control rooms and
platforms of the active rail station below.
A solution capable of containing a 95 in (2.4 m) diameter
auger and the spoil contained thereon was required to
mitigate the risk of disruption to the station operation, as well
as to eliminate the need to place spoil on the grillage.
By modifying a standard shipping container to suit the
circular profile of an auger, the piling subcontractor fashioned
a container that enabled the rig to spin off spoil in the
conventional manner at one end and enabled the spoil to be
removed by the attendant excavator from the other end. This
simple and very effective solution not only mitigated risk, but
also improved efficiency and reduced the need for triple
handling the spoil.
As part of the pile load test, understanding positional
tolerances on the embedment steel was required to connect
the test frame to the anchor piles (which also served four
permanent piles). This was critical to minimise eccentric
loading during the pile load test. To achieve this, Skanska
devised a bespoke centralising guide box and positioning tool.
These tools were also used with the remaining 11
permanent piles after they had been bored to depth and the
main reinforcement installed into the bore; both the plan
position and orientation of the embedment steel was set by
installing the guide box into the pile. Four positional lugs were
attached to the top of the casing and surveyed into position.
The box was then secured to these lugs. The embedment steel
was preconnected to the positioning tool and the combined
elements placed into the guide box to the desired design level.
Once set, concreting of the pile commenced. The fins attached
to the positioning tool provided sufficient reaction within the
guide box to prevent any twisting or distortion of the
embedment steel resulting from dynamic forces exerted
during the concreting operations.

Loading limitations on the steel deck allowed for only a
single service crane to be present during piling operations.
However, lifting the heavy reinforcement cages complete
with prefabricated base grouting Tube-A-Manchettes (TAMs)
and associated pipework with a single crane would have
presented a significant risk of damage. The subcontractor
devised a lifting cradle into which the bottom of each
reinforcement cage section could be secured, allowing the
cage to be single lifted at the top whilst being pivoted safely
and securely at the toe. This approach provided absolute
protection to the grouting TAMs and pipework. Furthermore,
the cradle skids were rubber lined to minimise metal-onmetal contact that may have caused the cradle to slide or skid
during lifting.
The contract piles were also heavily instrumented with
fibre optic cables and strain gauges. The fibre optics were
initially used for our patented CemOptics thermal integrity
testing technique, which provided a safe and high-quality
assessment of pile concrete integrity along the complete pile
shaft and also, critically, within the cover zone (where the
conventional crosshole sonic logging technique cannot detect
potential defects).
The project specification called for minimal concrete
overpour above the concrete cut-off level. With pile cut-off
level several meters below the piling grillage level, later
access to break down concrete to the cut-off level presented
many hazards, most notably a confined access space for
working and the need to use tools that would avoid hand arm
vibrations (HAVs). To minimise these hazards, the piling
subcontractor devised a heavy concrete bailing tool from
materials on hand. With its pointed, pencil-like profile, the
tool was plunged into a pile to remove concrete overpour, and
to free water and potential contamination that accumulated
during concreting.

Centralising guide frame box (left); positioning tool, after concreting and box removal (middle); and anchor pile embedment steel (right)
DEEP FOUNDATIONS * JAN/FEB 2021 * 19

January/February 2021

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