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distrubed but essentially
in situ
Jourong Formation with washed-in
red clay infill (probably terrestrial)
Current Sea Level
Kallang Formation (mostly
marine clays)
Zone of Jurong rock avalanche
deposits (sea cliff collapse?) with
variable matrix and possible
interbedded sediments
Jurong
Formation
Figure B3-2.4
Schematic geological model that can explain the features of the Fort Canning
Boulder Bed. The hard red clay is probably in
fissures in deteriorating
rock cliffs, and probably derived from weathered mudstone units in the Jurong. The clay became
hardened and cemented in a terrestrial environment in the same way as terra rossa in weathered
limestone environments. The colluvial facies of the FCBB was probably deposited as rock and soil
avalanches, at the coast. It remains unsorted, although may be intercalated with alluvial or beach
deposits. The FCBB was later buried locally by Kallang marine clay and
ll, washed in to open joints and
fluvial horizons and by
reclamation.
Broms & Lai (1995) present two sketches of caisson excavations for the Republic Plaza, one of which
appears to have a vertical fabric. If this was a transported colluvial deposit, boulders would be expected to
be essentially random or preferentially
flat lying. The same sketch shows vertical zones containing few
boulders, with a form similar to gulls found in areas of cambering (Parks, 1991).
An alternative model for the origin of the FCBB, therefore, is rock cliff deterioration, regression
and local collapse as sea levels rose from -150m at the end of the Holocene glacial periods. The red
clay is probably partly washed-in in
ll to the open fractures within the deteriorating rock mass, in
a terrestrial environment, as illustrated schematically in
Figure B3-2.4.
If the model is correct, the
collapsed colluvial facies of the FCBB would grade laterally into essentially in situ deteriorated
Jurong Formation rocks.
Whatever the truth regarding origin, the FCBB is a most dif
cult founding stratum and one that
might be called unforgiving in the context proposed in
Chapter 4,
but situated in an area of one of
the most valuable real estates in the world. Clearly, if dealing with a site which comprised weathered
but essentially undisturbed Jurong, one might use rather higher design parameters than for a
colluvial soil that might contain weaker horizons. Similarly, tunnelling through the weathered
Jurong, the potential hazards might be different from tunnelling through a colluvial and possibly
interbedded section of the FCBB with weaker horizons that could flow or ravel. The literature has
many examples of problems that arise through getting the ground model wrong and as Shirlaw et al.
(2003) note, identification of the FCBB can be difficult if adopting a routine approach to ground
investigation.
3.3.4.2.2 OFFSHORE
Sediments are deposited offshore in tectonically controlled, continually
subsiding basins and can be thousands of metres thick (Leeder, 1999).
Skempton (1970) compiled data on rates of deposition for argillaceous
