Geology Reference
In-Depth Information
(periglacial conditions).Man has inhabited Britain on and off for at least
700,000 years but has had to abandon the country because of advances
in ice several times, most recently about 13,000 years ago. Only for the
last 11,500 years has Britain been occupied continuously (Stringer,
2006). As a consequence, much of northern Britain shows signs of
glacial erosion and also materials deposited in association with the
glaciers. Much of this is poorly sorted boulder clay or till but there are
also great thicknesses of sorted and layered outwash sediments laid
down on land or in lakes as the glaciers retreated. These materials can
be very variable vertically and laterally (Fookes, 1997), so great care is
needed in interpreting ground investigations. This is illustrated by one
case history (piling for Drax Power Station) presented in
Chapter 7.
Associated with these periods of glaciations, the sea level across the
world has
fluctuated widely. About 20,000 years ago the sea level was
almost 150m lower than it is today (Pirazzoli, 1996). As a consequence,
ancient river channels that ran across the land surface have been sub-
merged to become in
lled with marine sediments, which can prove
hazardous for engineering projects such as tunnelling or foundations
for bridges across estuaries. For example, during design of the recently
constructed 3.24 km immersed tube tunnel as part of the Busan
-
Geoje
fixed link crossing in South Korea, a depression was found in the sea
floor, unexpectedly underlain by thick marine sediments, and this
required extensive engineering works to support the tunnel, involving
the use of soil-cement mixed piles. Buried channels are also found on
land, often with no obvious surface expression. Krynine & Judd (1957)
report several case examples, including a dam at Sitka, Alaska, where
river
fill was fortuitously found by drilling, extending more than 25m
below the dam foundations. If it had been missed then there would have
been considerable water leakage beneath the dam, requiring remedial
works. In Switzerland, during the construction of the Lötschberg rail
tunnel, 25 miners were killed by inrush of glacial sediments when they
blasted out of rock unexpectedly into an over-deepened glacially
scoured valley about 180m below the valley
floor (Waltham, 2008). A
case from Hong Kong is presented in Fletcher et al. (2000) and sum-
marised in
Chapter 7,
where the planned construction of a tower block
had to be abandoned because of the potentially huge cost of deep bored
piles. A large cavernous xenolith of marble had been found unexpect-
edly within granite beneath the site; the caves were partially in
lled with
soft sediments to depths of 150m. The formation of the cave and
sediment in
ll certainly occurred when sea levels were much lower.
Other poorly sorted, mixed soils include landslide colluvium. One
such deposit is called the Fort Canning Boulder Bed, which underlies
much of the Central Business District in Singapore and has caused
numerous dif
culties for construction, not least because it has some-
times been misinterpreted during ground investigation as weathered
rock, as discussed in
Box 3-2.
