A tale of two towers: Big Ben and Pisa - Royal Academy of ...

A Tale of Two Towers
Prior to any substantial excavation within the station escalator box, the 4.85m diameter running
tunnels were driven as pilot tunnels. The lowest west-bound tunnel was constructed in March
1995 and the upper east-bound tunnel in October 1995. The running tunnels were then
enlarged to 7.4m diameter to form the platform tunnels, the westbound and east-bound
enlargements being carried out in February 1996 and November 1996 respectively.
The retaining walls for the station box consist of reinforced concrete diaphragm walls. Like the
adjacent Palace of Westminster car park, excavation was carried out using the top-down
method with the struts and floors being installed progressively from the top down as excavation
progressed. In order to minimise surrounding ground movements, low-level struts were installed
in tunnels close to the base of the diaphragm walls prior to excavation below the main roof
slab. Excavation within the diaphragm walls was undertaken between September 1995 and
September 1997.
Careful computer modelling of the
tunnelling and excavation was carried
out and was greatly aided by the
measurements made during the
construction of the underground car park
in the 1970’s. Despite the provision of
very stiff diaphragm walls and low level
tunnelling struts, it was recognised that
the combination of the two platform
tunnels and the station box could lead to
unacceptable tilting of the Clock Tower.
The concern was that excessive tilting
would lead to cracking where the Tower
and the Palace of Westminster were
connected. A contingency protective measure
was called for and the relatively new
technique of compensation grouting was adopted.
Cross-section showing proximity of the new Westminster station
to Big Ben
The principle of compensation grouting is to inject grout (a mixture of cement, sand and water)
under pressure into the ground at chosen locations so as to counter any subsidence that an
overlying building might be experiencing. This is done by installing into the ground a number of
steel tubes (known as TAMs, the abbreviation for ‘tubes à manchettes’) with holes machined
into them at regular intervals, typically about 0.3m. Covering each hole is a short rubber sleeve
which acts as a one-way valve allowing grout to be pumped out under pressure without flowing
back in. Any hole can be selected for grout injection and the system allows repeated grouting
through the same hole if required.
The provision of grouting tubes below one of London’s busiest areas was not a simple matter
and the horizontal array of grouting tubes were installed by drilling radially outwards from a
vertical shaft which was located in the middle of Bridge Street. The tubes were about 50m long
and were drilled beneath the foundation of the Clock Tower and immediately to the north.
The Royal Academy of Engineering
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