Civil Engineering Reference
In-Depth Information
Any contractual arrangement which brings together design and construc-
tion expertise such as partnering, joint ventures or early contractor involve-
ment can help mitigate risk in the tunnel during construction through
sharing experience and improving buildability.
6.1.6 Ground risk
This has the potential to affect the most people in the event of a tunnel
collapse. Those affected by a collapse include the client who suffers
financial loss, those building the tunnel who are at risk of death or injury
and the public who may also be at risk of death or injury. Spectacular
tunnel collapses, such as that at Heathrow in 1994 in the UK (HSE 2000),
do occur with disappointing regularity, hence engineers should always
consider them in their risk assessments and plan their emergency measures
accordingly. When they occur, the consequences will most likely be so great
that there will be political repercussions in addition to the disruption to
the works.
For all tunnel projects, adequate site investigation is essential. The
designer must know the geology and hydrogeology in order to adequately
address all the risks from the ground. The most comprehensive site inves-
tigation possible is required to identify ground parameters, discontinuities,
water, gas and contamination (see Chapter 2).
Designers should liaise closely with contractors to ensure the stability of
the tunnel under construction. This liaison must go beyond just the stability
of the permanent works to include the stability of the tunnel at all stages
of construction. In rock tunnels, the stability of the ground through which
the tunnel is being driven has to be considered along with the stability of
the ground around the tunnel intrados.
Often a primary sprayed concrete lining is classed as temporary works
and considered to be the contractor's responsibility, however it is also the
primary means by which the tunnel is supported during construction which
makes it of fundamental importance for the safety of all those in the tunnel
(and also for safeguarding the client's asset). The sequencing of the excava-
tion process, particularly in complex tunnel layouts, can be crucial in
ensuring safety. Designers should always ensure that the construction
sequence which they envisaged in their design is adhered to.
Contractors should have a proper appreciation of the engineering prin-
ciples behind the design and ensure they adhere to the design and specifica-
tion and do not sacrifice quality of materials and workmanship to achieve
cost savings and productivity. Quality assurance schemes have a place in
tunnelling, but are no substitute for good engineering practice and
supervision. It is important to learn from the mistakes of others.
