Environmental Engineering Reference
In-Depth Information
0·6
Clastic
sediment
Sandstone
31·7
29
25
96·3
12·7
32·2
12·0-
25·0
Note
: MN, mega-newtons; one newton is an SI unit of force, equivalent to 1 kg m
-1
sec
-2
; the
residual (friction) angle applies once shearing has started and abraded any surface asperities
(roughness) along the failure plane.
WEATHERING
Weathering is the preliminary etching of land surfaces which eases the task of the main
sculptors, mass movement and erosion, and is everywhere around us. Signs of its
processes and rates can be seen on every building, from the corrosion of fine-carved
monuments to the cracking of artificial 'stone'. The control of 'lithology', and chemical
and mechanical weathering processes, are self-evident at home (Plate 13.3)! Outside, the
natural world reveals their full scope, from discoloured and often friable
weathering
rinds
of rock walls and pebbles or stains from the weathering of other minerals to the
complete disintegration represented by soil. Earth's surface environment is alien to most
rocks, which display varying degrees and forms of susceptibility on exposure to the
atmosphere and biosphere. Hard rock is mechanically stable but chemically unstable in
this environment; weathering converts it to a mechanically unstable (disaggregated) but
chemically more stable residue. Fractionation is then complete; the long journey of
aluminosilicate and clay minerals through the rock cycle sees their final segregation.
Weathering must overcome the tensile strength of rock mass. It is controlled therefore
by geochemistry and texture (
lithological
properties) and by discontinuity geometry and
the assemblage of different lithologies (
structural
properties). The former determine the
specific susceptibility
of mineral species and bonds, and
