Geology Reference
In-Depth Information
Muir Wood summarises the factors he considers as contributing to
the collapse of the Heathrow tunnels. He notes that if any one of these
had been addressed competently, in all probability the collapse would
not have occurred.
Management
: an unfamiliar system of project management based
on the New Engineering Contract (NEC) (ICE, 2005) with self-
certi
cation by the contractor of a design by others. Muir Wood
advocates a more partnering approach.
Control of works
: it was assumed that the specialist consulting
engineers advising the contractor would bring particular knowl-
edge and expertise to the project but commercial factors limited
his presence on site and in reality he had limited power to exercise
control as should have been done for a design-led system of
construction.
Compensation grouting
: due to higher than anticipated ground set-
tlement, compensation grouting was carried out beneath one of
the buildings adjacent to the works. Muir Wood describes this as,
'
in reality grout-jacking, requiring pressures in excess of pre-
existing vertical ground stress
. The grouting probably loaded
the tunnel and contributed to the collapse.
Lack of reaction to instrument data
: there is no evidence that there
was any reaction by the contractor and his advisors to the data
indicating circumferential movement of the tunnel lining together
with depression of the crown. Muir Wood argues that there was
no acceptable explanation for this phenomenon other than a
weakness of the invert. This should have been clear from distress
that had been seen in the invert. The advisors had not established
any criteria for acceptable de
'
ections or movement of the lining so
there was no quantitative trigger to warn that action needed to be
taken. Data were presented in
figures and diagrams without any
commentary or discussion. Nevertheless, according to Muir Wood,
the data caused the client (British Airport Authorities) to question
the integrity of the tunnelling, a suggestion that was dismissed
by the contractor.
In addition, an important factor was probably the presence of several
parallel openings at the time of the failure, which would have allowed
failure mechanisms to develop more readily than in the trial tunnels at
Terminal 4 (Karaku
ş
& Fowell, 2004).
Little is said in the various papers dealing with the Heathrow
collapse about the geology of the site. According to an expert for the
prosecution by the HSE,
The London Clay is a well-documented,
largely homogeneous, uniform and extensive body of over consoli-
dated sedimentary clay with very few discontinuities and none identi-
'
fied in the area of the collapse that could have caused a landslip
…'
