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INTRODUCTION
- GEOTECHNICAL SITE INVESTIGATIONS
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
geological board.
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:
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:
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
teeth.
Clay:
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:
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:
Boulders are fragments of rock larger than 200mm.
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 stable.
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 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 content changes.
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.
Collapsing
soils
Low density soils contain voids, which often cause them to collapse and
cause serious cracks to appear in the walls.
Recommendation: Wider foundations can be laid or piling can be used.
Soft soil
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.
Sandy 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.
FOUNDATION
CONSIDERATIONS
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
matter.
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:
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