Geology Reference
In-Depth Information
Other sediments are deposited as a result of chemical processes.
Many limestones, including chalk, are essentially biochemical, invol-
ving microplankton and algae in their formation. Other types include
bioclastic and reef limestone. Limestone can be
fine-grained rock,
uniform and extremely strong, as per the Carboniferous Great Scar
Limestone of northern England. Other limestones such as oolite can
be weak and porous. Limestone is readily dissolved by acidic water,
leading to karstic conditions (irregular rockhead and cavernous con-
ditions) with obvious implications for site formation and design.
Rocks susceptible to dissolution, including limestone, dolomite and
salts (discussed below), can constitute natural hazards due to sudden
collapse of sinkholes and general subsidence. Relatively minor geo-
logical differences can give rise to very different engineering proper-
ties. Within the chalk of southern Britain there is a gradational
change from the characteristic White Chalk, which is relatively
strong and brittle, down into grey, clay-rich rocks. The latter,
Chalk Marl, intermediate between the overlying fractured, brittle
White Chalk and underlying Gault Clay, was recognised as the
ideal tunnelling medium for constructing the Channel Tunnel
(Varley & Warren, 1996). The presence of hard
flint and chert
bands in chalk can give rise to considerable dif
culties during
construction, because of abrasion and wear on cutting tools, as
illustrated by a case example in
Chapter 7.
Limestone can become dolomitised, which involves partial replace-
ment of calcium carbonate by magnesium carbonate and leads to
greater porosity. Dolomitisation often occurs in the presence of eva-
porates (salt deposits) and dissolution of underlying salts such as
gypsum can lead to the development of sinkholes and brecciation of
the overlying rock, followed by re-cementation. This process is
described for the Magnesian Limestone in the UK by Dearman &
Coffey (1981). Similarly, the Miocene limestone and dolomite
sequences in Qatar and Saudi Arabia are complex and extremely
variable due to their post-formation dolomitisation, collapse and
re-cementation (Sadiq & Nasir, 2002). The rock is sometimes strong,
elsewhere very weak, and can be cavernous. This is important for
founding engineering structures and for other activities, including
dredging (Vervoort & De Wit, 1997). Limestone is an important
rock economically, particularly as a source of cement aggregate and,
where massive and strong, is commonly used as armourstone for
breakwaters.
Deposits of salt are formed by evaporation of lakes and even seas
(the Mediterranean completely dried up about
five million years
ago), and are also very important economically as source rocks for
chemical industries such as fertilisers. They are signi
cant for the oil
and gas industry because they have low density and low permeabil-
ity and gradually rise through the overlying denser country rock as
