Geology Reference
In-Depth Information
Inadequate site investigation that fails to identify the true nature of a
site and its hazards can result in huge losses and failure of projects.
Similarly, poorly directed or unfocused site investigation can be a total
waste of time and money whilst allowing an unfounded complacency
that a proper site investigation has been achieved (box ticked).
The engineering geologist needs to work to avoid these occurrences.
He needs to be able to communicate with the engineers and to do that
he needs to understand the engineering priorities and risks associated
with a project. Those risks include cost and time for completion. This
book should help.
In geology all naturally occurring assemblage of minerals are called
rocks, whatever their state of consolidation, origins or degree of
weathering (Whitten & Brooks, 1972). For civil engineering pur-
poses it is very different. Geological materials are split into soil and
rock, essentially on differences in strength and deformability. To
make it more dif
nitions of what is soil and what is
rock may vary according to the nature of the project. For many
purposes, soil is de
cult, the de
ned as material that falls apart (disaggregates)
in water or can be broken down by hand but, for a large earth-moving
contract, materials may be split into soil and rock for payment
purposes according to how easy or otherwise the material is to
excavate; rock might be de
ned as material that needs to be blasted
or that cannot be ripped using a heavy excavating machine. For
engineering design, the distinctions are often pragmatic and there
may be fundamental differences in approach for investigation
and analysis. This is illustrated for slope stability assessment in
Figure 1.3.
In the left-hand diagram, the soil, which might be stiff
clay or completely weathered rock, is taken as having isotropic
strength (no preferential weakness directions), albeit that geological
units are rarely so simple. To assess stability, the slope is searched
numerically to
find the critical potential slip surface, as explained in
Chapter 6.
In contrast the rock slope to the right is, by de
nition,
made up of material that is too strong to fail through the intact
material, given the geometry of the slope and stress levels. In this
case, site investigation would be targeted at establishing the geometry
and strength characteristics of any weak discontinuities (such as
faults and joints) along which sliding might occur. If an adverse
structure is identi
ed then the failure mechanism is analysed directly.
This conceptual split is fundamental to all branches of geotechnical
engineering, including foundations, tunnels and slopes, and it is
important that the engineering geologist is able to adapt quickly to
seeing and describing rocks and soils in this way.
