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Managing Underground Risks in Singapore Through Geotechnical Interpretative Baseline Reports


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Author J Rozek and L N Loganathan


Risk exposure in tunnel projects is extremely high due to various factors
such as variability of subsurface conditions, selection of inappropriate design
and tunnelling methods, improper risk assessment on adjacent surface and
subsurface structures and utilities, and inadequate contractual practices. The
risks in tunnelling must be borne by the contractor, designer and the owner of a
tunnelling project. There have been a number of improvements in the last ten to
15 years in the manner in which tunnel projects are developed and implemented in
terms of their design, construction, financing and operations. One significant
improvement has been the manner in which underground conditions and their risks
are being managed. Owners are less willing to transfer the full responsibility
to the contractor for managing the underground conditions, although this is a
familiar manner in which underground major urban projects have been previously
contracted and managed.

The owners of tunnel projects in Singapore have advanced the risk management
process for their projects through geotechnical interpretative baseline reports
(GIBRs), geotechnical data reports (GDRs) and project specifications (PSs). This
paper discusses industry practice and its development as well as the application
during the preparation and use in Singapore on the Deep Tunnel Sewerage System
(DTSS) and the Marina Coastal Expressway (MCE) project in Singapore. The DTSS
trunk sewer is 48 km and ranges from 3.3 m to 6 m finished diameter up to 50 m
below the ground. The underground conditions ranged from Old Alluvium, strengths
of about 1 MPa to competent Bukit Tima granite, with strengths of over 230 MPa.
The total route length of the MCE is about 5 km. About 3.6 km will be
constructed underground, consisting of dual five-lane carriageways. The
underground construction will be cut-and-cover, and 2.6 km will be constructed
about 13 m underground, and 1 km of tunnel will range from 13 m to 24 m as the
tunnel passes under a 420 m wide reach of Marina Bay, whose crossing will be
constructed as a two-phase cofferdam.