Why do a Geotechnical site
Good soil information can lead to substantial
savings and tighter foundation design because the design
parameters are well defined. The more comprehensive the
information, the more economically can a good structure be
built. The object is to build a good building cheaply and
not to build a cheap building.
Geo-technical site investigations may cost up
to 2% of the total project cost, but is very necessary as
geo-technical engineers often say: you will pay for your
geo-technical investigation anyway!
GEOTECHNICAL INVESTIGATION PROCEDURE
Initially one should find geological
information for a specific site about the original rock type
of the area and the soils which are the weathered rock. This
can be done at either the government printer or your local
If in doubt regarding the characteristics of
the soil, consult an engineer or take soil samples from a
test hole to a soil laboratory for analysis. Based on the
results of this analysis, a recommendation can be made
regarding the best foundation structure to use.
CLASSIFICATION OF SOIL TYPES
Sand consists of discrete particles normally
visible to the naked eye. Sand is clearly distinguishable by
the presence of gritty particles which do not break down
when rubbed on the palm of the hand.
Silt consists of fine particles. In general
silt particles are barely felt when rubbed with water on the
palm. When this wetted soil is placed on the tongue, the
particles can be felt grating the against the enamel of the
Clay consists of very fine particles. The
particles are flaky and when rubbed on the palm with water
has a soapy or greasy feeling. No sensation of grittiness
can be detected when tasted.
Gravel consists of fragments of rock. The
shape of the particles should be described as: rounded
shape, oval shape, rounded corners, corners slightly
bevelled, corners sharp or irregular.
Boulders are fragments of rock larger than
Once the soil is classified and the
composition of the grain particles are determined, one can
make a recommendation as to which type of foundation or pile
structure should be used.
There are various types of clay and not all
of them may be regarded as bad. Some are soft but are still
Collapsible soils are usually not very dense
because of the high proportion of voids present.
Heaving or active clays contain minerals
which cause the soil mass to react strongly to changes
regarding moisture content. These clays will swell when more
moisture is added and will shrink as they dry out. The
treatment of soft soils is usually to make the foundations
wider to reduce the pressure to an acceptable level so that
the soil will be able to withstand the load with little or
no settlement. Should the soil prove to be too soft or wet
then a raft foundation or piling is recommended. A pile is a
concrete column driven into the earth which either carries
the load down to a stronger soil or rock below or by hanging
in friction on the soil around its shank.
RECOMMENDATIONS FOR PROBLEM SOILS
Clay is generally smooth and has no visible
particles, it retains water and does not drain well. The
presence of clay is sometimes indicated by cracks on the
surface of the ground. This type of soil could pose a
problem although some soft clays are quite stable. Heaving
clay is very problematic and changes a lot when the moisture
Recommendation: One should lay a compacted
hardcore (of broken bricks, stones etc.) under the concrete
foundations but for heaving clay a raft foundation will
often have to be used; also, depending on the extremeness of
the heaving clay, the site can be thoroughly soaked once the
trenches have been dug and building can then begin when it
has dried sufficiently. This ‘sealing-in’ of the moisture
greatly reduces subsoil movement, minimize the wet-dry cycle
and the structure will only have to copy with minimal
movement. However it is essential that a structural engineer
should be consulted when dealing with heaving clay.
Low density soils contain voids, which often
cause them to collapse and cause serious cracks to appear in
Recommendation: Wider foundations can be laid
or piling can be used.
Most buildings are subject to some settling
but soft soils will in some, although rare, instances
provoke the kind of settlement that causes the entire
structure to sink.
Recommendation: Foundations should be widened
in order to reduce pressure to a level that will hold the
load. Brickforce should also be laid on every 3rd
brick course depending on the severity of the soft soil.
This type of soil feels gritty and has no
plasticity and has good drainage properties. Generally,
sandy soil is not a problem but on a steeply sloping site,
the weathered soil from the hill gradually slides down to
the bottom and one might have collapsing soil problems.
Recommendation: The area where is to be built
can be re-filled with a stable soil type to a depth decided
by an engineer or one could use friction piling or end load
bearing piling if the soil type below is suitable. One can
also make used of wider foundations. It is also common
practice to flood the ground once the trenches have been dug
and then to compact thoroughly. This improves cohesion and
makes the soil considerably more stable to build on.
Foundations are important to ensure that the
loads of the building above are transferred to solid ground
capable of supporting these loads. Thus, the foundations
should be able to carry the weight of the building without
uneven settlement taking place, which would result in
cracking and potential collapse.
The topsoil on the site usually contains all
kinds of vegetable matter which is easily compressed and
would not be suitable for foundations, but is valuable as a
top dressing for gardens and may be disposed of in this
You should avoid at all costs any site where
there is a chance of ground or shallow sub-surface water
remaining close to the base of the building, therefore:
Avoid sites that can be flooded or be left in
Make sure that water coming off the roof and
walls of the building is moved quickly away from the base of
the walls. This can be done by compacting a protective
sloping pavement (apron).
One should be very careful when building on
unstable soils. When inadequate foundations cause cracking
of walls or other problems to occur, you will have to spend
a lot of money to rectify the problem, and it will not only
reduce your property’s market value in future, also most
house insurance policies exclude coverage against subsidence
and landslip damage caused by unstable soils. Never take any
chances, consult a qualified geo-technical engineer when you
are uncertain of your stand’s soil conditions.
Please note: This document should be used as
a guide only and the author accepts no responsibility for
losses incurred as a result of advice followed in this
document. The foundations of your proposed dwelling should
always be discussed with your engineer.