Geology Reference
In-Depth Information
The Fort Canning Boulder Bed (FCBB) underlies much of the Central Business District of Singapore
and is an example of a complex mixed rock and soil deposit, the geological nature and interpretation
of which has had, and continues to have, great consequences for civil engineering construction. When
the Fullerton Building was constructed in the 1920s,
the foundations which
consisted of clay and boulders, were of a dangerous character, and this great structure had to be
placed on a concrete cellular raft, which is so designed as to give each super
it was found that
'
cial foot of soil not
more than one ton to carry
(Straits Times, 27 June 1928). Since then, in several reported cases,
engineers have struggled with construction on and through this material. Ground conditions have
sometimes been misinterpreted following inadequate ground investigation. In 1952, the original
ground investigation for the Asia Insurance Building incorrectly interpreted the site as underlain by
in situ rock rather than FCBB. The mistake was only discovered during construction and necessitated
total redesign of the foundations (Nowson, 1954). Shirlaw et al. (2003) provide several other
examples.
The FCBB is almost 100m thick in places, is not exposed at the ground surface, but is commonly
encountered in foundation construction and tunnels (Shirlaw et al., 2003). The deposit typically
comprises a poorly sorted mixture of clay, silt and sand, with boulder content between 5% and
35% (Singapore Standards, 2003). The mostly angular boulder content is probably derived from
the adjacent Jurong Formation of Triassic/Jurassic age, with strong sandstone predominating.
Figure B3-2.1
shows boulders recovered and partially drilled in large diameter bored piles from
one site near Mount Sophia. In between the boulders, the matrix sometimes comprises hard red
clay with undrained shear strength up to 2 MPa (weak rock) but sometimes sandier and silty and
of much lower strength. Broms & Lai (1995) also report encountering a highly permeable gravel
layer within the FCBB at depth.
'
large diameter piles, from a site close to Mount Sophia, Singapore.
Shirlaw et al. (2003) suggest that the FCBB is a colluvial deposit originating from Fort
Canning Hill (
Figure B3-2.2)
but the low height of the hill relative to the thickness of the
deposit makes this somewhat doubtful and the red clay is often much stronger than would
be expected, even for clay that has undergone quite deep burial. Furthermore, the clay is clearly
