The
role of the engineering geologist in civil works can be related to the various
"stages" normally connected to a civil engineering project. This
short note thus discusses the role or functions that the engineering geologist
performs.
At
the feasibility study stage of a project, various parties (e.g. consultants,
contractors, etc.) are interested in the project. Information is scarce,
especially with regard to site geology, etc. The input by the engineering
geologist at this early phase of the project would be to furnish preliminary
information on regional and site geology based mainly on compilation and
collation of existing published or unpublished literature and maps. For
example, published geological maps on the scale of 1:63,360 or 1:25,000 by the
Geological Survey Department with its accompanying memoir or internal files
would serve as the first guide as to what rock formations are traversed by the
proposed highway, foundation rocks at a building site, etc.
The
main role of the engineering geologist in civil works is in the site
investigation stage. It is the responsibility of the engineering geologist to
provide , as complete as possible, the Site Characteristics. Site
characterisation would encompass a broad spectrum of topics such as detailed
site geology (surface and subsurface), physical, chemical and mineralogical
properties of soils and rocks at the site, groundwater characteristics, etc.
While the traditional surface geological mapping has its definite place in the
site investigations stage, much more subsurface data need to be acquired from
boreholes or drilling planned and executed in association with the geotechnical
engineer. The accompanying soil/rock samples, properly logged and tested in the
laboratory, coupled with various field tests (such as the Standard Penetration
Test, borehole permeability test, etc.), would provide additional information
on site characteristics from which a 3-dimensional picture of the site can be
constructed. In addition, borehole and laboratory test data provide some degree
of quantification of information essential for engineering works.
Site
visits and site surveys conducted by the engineering geologist accompanied by
the geotechnical design engineer would be most beneficial to both parties as
the geologist can highlight potential geology-related problems to the engineer,
while the engineer can bring to the geologist's attention critical zones (e.g.
highrise versus playground) where perhaps more detailed study is required.
In
the planning of boreholes or a site investigation program, it is the engineer
who decides on locations and concentrations of boreholes in relation to his
proposed civil works, and the geologist uses these borehole data for
interpreting subsurface geology. However, additional boreholes can be requested
to aid geological interpretations, such as in the case of limestone bedrock
with its associated complex karstic features. Whether the additional boreholes
are granted is another matter!
Though
practices may differ in different countries, the design phase involving various
design calculations is entirely the domain of the geotechnical engineer
in Malaysia. The design parameters obtained from the site investigation stage may
have been through the joint effort of the geologist and the engineer, but the
choice of parameters input into the design calculations is again the decision
of the engineer. As one of the objectives of the site investigation phase is to
obtain design parameters (eg. strength parameters), it is useful for the
engineering geologist to understand what design parameters the engineer is
looking for so that he can be of more assistance in the site investigation
work. To serve the needs or requirements of the engineering project, the
engineering geologist has to understand or at least be aware of some of the
engineering-related issues connected to the proposed project. Thus, issues (and
hence requirements) connected with a tunnelling project would differ from dam
foundation or highway cut-slope, etc.
The
construction stage is perhaps the most interesting stage of the project, since
various activities are on-going (and many things can happen!), such as massive
excavation works, etc. It is at this stage that new exposures are made
available, and it is ideal for the engineering geologist to check and confirm
(or otherwise) his "predicted/interpolated/extrapolated" geology or
soil profiles. Moreover, various minor or major geological features such as
dikes, faults, buried channels, etc. may be exposed for the first time since
they may have been missed by boreholes. In any case, a revision or update of
the detailed site geology is possible at the construction stage. The
involvement or input by an engineering geologist would thus be most useful at
the construction stage. Variations from anticipated or predicted ground
conditions can be highlighted, new geological features noted, and to use the
words of Rawlings(1972), ominous geological symptoms brought to the
attention of the engineer for appropriate action. The author's experience shows
that because of the considerable time delay (several years sometimes) between
site investigation and construction, the geologist's input or involvement is
heavy during the site investigation stage, but is then "phased out"
or "omitted/forgotten?" come construction time since "ALL"
the geological information is already available in the S.I. Report/Engineering
Geological Report! Occasionally, the geologist gets called in when something
happens (eg. collapse) to provide input on geology-related factors.
Post-construction
performance monitoring forms part of civil engineering works, and is very much
the domain of the engineer. In cases of post-failure (whether during or after
construction) investigations, such as ground subsidence, landslides/slope
failures, etc., geological input is again required to provide an idea of the
material involved, possible mechanisms and causes of failure, etc. (again
confining to geology-related factors only!) Post-failure investigations will
invariably lead to remedial and other engineering works outside the scope of
works of the geologist.
An
important contribution by the engineering geologist to civil works is in the
search for construction materials such as rock aggregates (quarry site
investigation), soil fill materials (borrow pits), sands, etc. Then again,
detailed quantification/evaluation of quantities would involve boreholes, etc.
is best carried out in close association with the material engineer.
This
short note briefly discusses the role of the engineering geologist in civil
works, as pertaining to the various stages of an engineering project. It is a personal
view, based on the author's experiences and interactions with
engineers/geotechnical engineers in the past 20 years.
Reference
RAWLINGS,
G.E. (1972). The role of the Engineering Geologist during construction. Quarterly
Journal of Engineering Geology, vol.4, 1972, pp.209 - 220.