GL3 Engineering Geology
Engineering Geology - Principles
Geology and civil engineering
Civil engineering projects can be small, such as constructing buildings and houses or very
large, for example large dams. In all cases, geology has a major impact on the way in
which these projects are undertaken and the term “geotechnics” is sometimes used to
describe the geological techniques used in informing developers of the state of the ground
they want to build on.
The main purpose of the geotechnical surveys is to save the developer money in the long
term by identifying potential problems and suggesting ways to overcome them. The way
in which this is done is through a “site investigation”.
Stages in the site investigation
Initial stage:
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Desk study
Site visit (“walkover survey”)
Preliminary report and fieldwork plan.
Main stage:
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Fieldwork
o Geological mapping
o Geophysical surveys
o Trial pits, boreholes etc
Laboratory testing
Final report
Review stage:
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Monitoring during excavation and construction
You can see that the site geotechnical team has a lot of responsibility for insuring the
success of the project.
The Desk Study
One of the most surprising things is that you don’t start drilling holes straight away. This
is a very costly thing to do and probably the information you will obtain is available
elsewhere. The engineering geologist will begin by studying records of the ground
conditions from as many sources as possible. It is surprising just how many sources of
information are available. These include the BGS, the National Geosciences Data Centre
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GL3 Engineering Geology
(borehole data), published material, local sources (local residents, experts etc), mining
records, topographical maps, aerial photographs.
The idea behind the desk study is that it is a cheap way of producing a lot of initial data
about the site that can be used to “show the way” for the later stages.
The walkover survey
The engineering geologist next visits the site to identify obvious problems and causes for
concern. From this an initial report is produced with a plan for the more detailed surveys
later.
Geological mapping
The next stage involves fieldwork in which the engineering geologists visits the site in
order to map the principal geological features such as rock type, unconsolidated deposits,
water courses etc.
Geophysical surveys
These provide some of the most useful data for the geotechnical team. There are several
types of survey.
1. Ground penetrating radar
Microwaves are transmitted into the ground by a device mounted on a trolley. These are
reflected off rock structures and recorded at the surface. An onboard computer measures
the distance travelled and a profile of the subsurface is created.
2. Electrical surveys.
The resistance to an electrical current can be measured between electrodes fixed into the
ground (or lowered down a borehole). This is particularly useful for identifying the
presence of metallic ore deposits and the presence of ground water.
3. Seismic surveys
This uses shockwaves produced by hammer blows or explosions which become reflected
or refracted from geological boundaries.
(a) Reflection seismology – used for deep surveys such as those used to identify oil
and gas deposits.
(b) Refraction seismology – used for shallow surveys such as site investigations. As
the shockwaves pass down from the upper layer to the layer beneath, they speed
up and eventually return to the surface to be recorded at geophones.
4. Magnetic surveys.
Using a magnetometer an operator can measure minor changes in the magnetic field –
these may be caused by buried debris or shafts from old mine workings for example.
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GL3 Engineering Geology
5. Gravity surveys
Minute changes in the gravitational field are recorded. A negative anomaly may indicate
the presence of a cavity under the surface.
Boreholes
The use of boreholes is the only way to really tell what type of rock is present under the
surface and what possible problems may occur, but boreholes are expensive. There are a
number of ways in which they can be used. Some borehole methods extract a tube of
rock called a core which can be analysed in the lab. Alternatively, rock chips may be
used, these are carried back from the drill head to the surface in a fluid. Additionally,
some borehole drilling equipment are fitted with devices to measure permeability,
electrical resistivity and rock strength.
Laboratory testing
Samples of rock or soil removed from the site can be taken to the laboratory for testing.
Tests include several standard tests for rock strength.
What are we looking for?
All of these procedures are involved with finding what is called “difficult ground” in
other words parts of the site where the rocks or geological structures may cause problems
during excavation or construction.
Examples:
Ground subsidence – e.g. limestone with solution cavities, clay, peat, chalk, salt, gypsum,
mined areas.
Soft ground – e.g. alluvium, recent sediments, artificial “made” ground.
Cavities and shafts
Rockhead relief – the shape of the bedrock beneath the superficial deposits, crucial for
tunnels and foundations.
Slope failure
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