July/August 2021 - 95

pi les on 65 WS, and 1930s pad
foundations for No. 67-69. The 65 WS
piled foundation investigations highlighted
a number of inconsistencies. As a
result, a foundation-strengthening
approach was included in the updated
cost plan for the building. At No. 67-69,
the pad foundation size and depth could
not be verified.
A Phase 3 geotechnical investigation
was carried out in the first half of
2016. This was required to map the pad
foundation sizes at every foundation
location at No. 67-69. The work
involved pits 9.8 ft (3 m) deep and cores
through the pad foundations to
understand their depth and strength
and durability of the 1930s concrete. At
65 WS, Phase 3 investigations were
carried out at every foundation location
to map the extents of the pile caps.
The third phase enabled final design
of the reuse schemes at both buildings,
and the strengthening works solutions
at 65 WS.
Design and Construction
at 65 WS
The investigations at 65 WS had
indicated that the building was founded
on piled foundations, but found that pile
lengths were short and variable, in
particular, related to their penetrations
into the main bearing stratum, the
London Clay. In addition, foundation
dimensions and ground conditions
varied and extrapolations could not be
made to cover other foundations not
investigated. As a result, it was recommended
that for any increase in load on
an existing foundation in the proposed
development, the existing foundations
would need to be strengthened using
additional mini piles. The structural
connections required to take load from
the existing foundations out into the
new piles as the structure settled
required careful consideration and
innovative thinking. The final solution
adopted at internal column locations
shed column load to the strengthening
piles through a reinforced concrete
collar; the collar was constructed around
Parallel seismic testing site to locate 65 WS piles, map cap dimensions
Super kitten rig for smaller mini piles
A collar for mini pile load transfer, added to pile cap
the existing pile caps, and linked
through a shear connection. At edge
column locations, the design addressed
the fact that there was a lightwell
involved. New pile caps were constructed
on either side of the existing foundation
and load taken out to these adjacent
pile caps through a beam, which
connected into the existing column.
The mini piles adopted for the
strengthening works were 9.8 in (250
mm) and 12 in (300 mm) in diameter. They
were constructed within the existing
basement space using a Super kitten rig
and a Klem 702. Mini pile construction
took place in the last quarter of 2016.
Foundation Reuse at No. 67-69
At 67-69 Whitfield Street, the strength
tes t ing carr ied out on the pad
foundation concrete was interpreted,
together with the density and thickness
of the Terrace Gravels, and strength of
the underlying London Clay. The
foundation reuse design approach was
based firstly on load balance with
existing realistic load as far as possible;
for pad foundations, provided that the
pad was currently operating at working
conditions with a reasonable factor of
safety (FOS) on ultimate bearing
capacity, for small changes in load, a
significant reduction in stiffness of the
DEEP FOUNDATIONS * JULY/AUG 2021 * 95
July/August 2021

Table of Contents for the Digital Edition of July/August 2021
