Most structures are earth-laid and it’s in
your best interest to know what the earth feels about what you are about to
construct over it. If you really don’t care and the earth doesn’t feel good
about your ideas, you are heading for disaster. Investigating the condition of
the sub-soil upon which a structure will be built is a process that should not
be undermined and must be carried out diligently. Every building is supported
by a foundation, making it about the most important component of a building.
The foundation itself, no matter how strong could be unreliable if it does not
sit firm on the ground hence the need to understand where and how best to place
it.
The process of determining how the earth would
accommodate the foundation first begins with carrying out a sub-soil
investigation. It goes by the saying ‘what you don’t know can kill you’; for
instance, if you don’t know that at 1-metre depth the subsoil on your land is
too weak to support your proposed structure and you go ahead to place your
foundation there, the result is likely going to be disastrous.
Soil test for construction purposes, otherwise
referred to as sub-soil investigation is different from the process carried out
to test the biological and chemical properties of a soil in order to know if it
can accommodate the growth of certain plant species; ofcourse not! It actually
involves the physical examination of the soil from layer to layer to determine
its type, density, strength characteristics (bearing capacities), consolidation
potentials, rate of soil settlement, water table and other relevant data. This
information is useful to guide the structural Engineer in determining the most
suitable type of foundation to support the proposed structure, the most
suitable depth to terminate it and the overall design of the foundation. The
test also helps predict problems that may arise during and after construction and
how they can be properly dealt with.
Usually as the major scientific step before
construction, geotechnical information from soil investigation also helps in
determining a more reliable construction cost estimate for a proposed project. On
live projects, a simple soil investigation can influence a foundation design
that can put the cost of construction significantly in excess of what was
originally budgeted; hence the need to conduct a soil investigation before
arriving at any meaningful construction budget especially in a joint venture
(JV) or concession arrangement.
In Nigeria, a standard soil test report
prepared by geotechnical personnel is a pre-requisite for structural design
approval especially for buildings that consist of more than one floor. The most
common soil tests for building projects back here are the Dutch-Cone
Penetrometer Test (DCPT) and the Cone Penetrometer Test (CPT). While the former
is quite simple; can be concluded on-the-spot within a few hour and is mostly
useful for buildings not more than two-floors, in the latter a rig is deployed,
it is more comprehensive and would normally require laboratory analysis of
samples taken from site. It is standard practice to test at least two points per
plot of land for DCPT and at least one trial pit and borehole of CPT on the
same size of land.
A standard soil test report consists of a soil
profile showing the various layers of soil and their physical and geotechnical
characteristics. It would normally show the procedure adopted in the carrying
out the test together with graphical and numerical data both from on- the-field
results and those computed based on established empirical formulae. In the end,
the geotechnical personnel or structural engineer is able to ascertain whether
or not a shallow or deep foundation is okay to sustain the proposed structure.
Most report would enlist a range of suitable foundation types based on the
information available on the size and load characteristics of the proposed structure.
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