July/August 2021 - 96

Residual Risks and Monitoring
At 65 WS, the strengthening was aimed
at reducing the risk to a reasonable
minimal level, but some reliance was
still placed on the existing foundations.
At No. 67-69, the residual risks were
related to the unlikely potential for the
Terrace Gravels to be significantly
weaker than assumed, and if any
weaker areas within the concrete were
more extensive than what was
identified during investigations. These
remaining risks were mitigated
through careful monitoring of the
movements of every column during
construction to ensure that movements
remained within acceptable levels. The
residual risks were also covered by
insurance mechanisms.
At both buildings, monitored
movements remained small and within
the specified limits until project
completion in mid-2020.
Conclusion
Cl ient buy-in and expectat ions
management, together with integration
of geotechnical and structural teams,
was key to arriving at the right
solutions when reusing foundations to
develop two buildings for residential
purposes within London's Charlotte
St r ee t Conse rva t i on a r ea . The
Transfer beam connected to columns to transfer load
load-settlement response was not
expected. The structural capacity was
also not affected.
Where load increases from existing
realistic load to new design load were
greater than 10%, added assessment
was carried out; final FOS on ultimate
bearing capacity and potential settlement
of the columns was calculated
with a load spread of 450 from the top to
the base of the foundation (or the
foundation width, if smaller). Due to
potential weaker areas in the concrete,
the foundations were also checked for
FOS on ultimate bearing capacity with an
ultimate load state load spread of 1 in 2.
Structurally, the pads were checked
for concrete strength and bearing was
checked at the column/pad interface,
based on the concrete strength determined
with laboratory testing of concrete
cores and a lightly or unreinforced
pad design check to Eurocode 2. The
analysis concluded that the footing
concrete was adequate for the additional
loads, and settlements were predicted to
be small.
96 * DEEP FOUNDATIONS * JULY/AUG 2021
The combined thinking of structural and geotechnical
design teams forged a considered and effective
solution for this project of significant complexity to
retain much-loved structures.
Gridline
location
Relevant
pit
Status
of GI
Increase
existing
realistic
load to
new design
load %
A1
A3
A4
A'5
C1
C3
C5
D3
D3'
D5
E5
A1a
A3d
A5b
A5b
Dynamic
Probes
Only
C3a/C3d Satisfactory
Dynamic
Probes
Only
D3a2
D3a3
D5a
N/A
Satisfactory
Satisfactory
Satisfactory
Satisfactory
Satisfactory
Foundation reuse strategy at No. 67-69
27.4
15.0
13.7
9.2
30.0
N/A
Satisfactory
Satisfactory
Satisfactory
Satisfactory
Satisfactory
7.3
5.0
7.8
9.7
13.4
Load
Balance
Load
Balance
Load
Balance
Load
Balance
Strength
Check OK
Strength
Check OK
Strength
Check OK
Strength
Check OK
Strength
Check OK
Strength
Check OK
Load
Balance
5.5
2.8
3.5
3.8
4.5
5.0
5.6
7.2
4.9
5.1
N/A
Pad
strength
strategy
Interpreted
Geo FOS
under NDL
Typical
settlement
under
NDL
(mm)
1.3
2.1
1.2
1.5
2.9
5.9
5.0
2.7
1.8
4.9
N/A
July/August 2021

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