Geology Reference
In-Depth Information
6.1 Introduction
In
Chapter 2,
a brief introduction was given to civil engineering
practice and types of structure. This chapter provides more detail so
that the engineering geologist can better understand the requirements
of projects, in terms of site investigation, design and construction
issues.
6.2 Loads
Most civil engineering projects involve either loading the ground,
say from the weight of a new building, or unloading because of
excavation of a slope or in a tunnel. Load changes can be permanent
or temporary, static (due to weight) or dynamic (due to blasting,
for example). A further important consideration for most geo-
technical problems is the self weight of the ground and other
in situ
stresses.
At any point in the Earth
s crust, the stresses can be resolved into three
orthogonal directions. These are termed the maximum, intermediate
and minimum principal stresses and depicted
'
σ
1
,
σ
2
and
σ
3
, respec-
tively. By de
nition, the planes to which the principal stresses are
normal are called principal planes and the shear stresses on these
planes are zero. An important point regarding rock engineering is
that all unsupported excavation surfaces are principal stress planes
because there are no shear stresses acting on them (Hudson, 1989).
One of the principal stresses will always be perpendicular to the
Earth
is surface (Anderson, 1951) and is generally vertical.
For projects close to the Earth
'
s surface, such as cut slopes or founda-
tions, natural stresses include self weight, weight of included water and
buoyancy effects below the water table, which reduces the total stress to
an effective stress (weight of soil minus water pressure), as illustrated in
'
