Transcript Soil & Site Investigation

Soil & Site Investigation
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Objectives of a Site Investigation
According to BS 5930, a site investigation is
carried out to:
• Assess the general suitability of the site for
the proposed works
• Enable an adequate and economic design
to be prepared
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• Foresee and provide against difficulties that
may arise during construction due to ground
and other local conditions
• Predict any adverse effect of the proposed
construction on neighbouring structures.
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Preliminary Site Investigation
• Mining authorities - existing and future factors
effecting ground movement.
• Ordinance Survey - land contours, rights of
way, rivers, existing buildings. (Look at old
maps - pond in wrong place)
• Geological Central Record.
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• Met Office - frost hollows, prevailing winds
and severity, rain fall.
• Water Authorities - water tables, flood planes.
• Planning and Building Control. (tree
preservation orders)
• Utilities - gas, water, electricity, British
Telecom, sewage.
• Listed and historical buildings.
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Information obtained from the Local
Authority
• The likely site conditions.
• Possible subsidence due to ground movement
or underground workings.
• Previous land use and buildings on the site.
• Problems likely to be encountered with regard
to water tables and flooding.
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Site Exploration
• The aim is to provide a survey of the subsoil in
order to find out the nature and disposition of
the soil below ground level, and to obtain
samples of the soil for testing in the
laboratory
• The exploration should be done at the same
time as the preliminary design of the building;
this prevents obtaining insufficient or
unwanted data. constructionsite
• The extent of the exploration will depend on
the size and type of structure, the nature of
the site, and the availability of local geological
information.
• The exploration should be taken deep enough
to include all strata likely to be significantly
affected by the loading of the building. This
depth will depend on the weight, size and
shape of the loaded areas.
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Methods of Sub Surface Investigations
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Trial Pits
Plate Bearing Test
Borings
Headings
Penetration Test
Sounding Test
Vane Test
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Plate Bearing Test
Plate is loaded and the
movement measured
Stress = Load
Area
use a safety
factor of 2
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Borings
• Hand Boring - depths up to 6 to 9m auger is
usually 100 - 150mm in diameter
• Mechanical Boring
• Borehole Log
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Soil Samples
• Disturbed – brings out the soil particles but
does not keep them in their original position
• Undisturbed – brings out the core of soil as it
is in the ground
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Headings
Soil is taken from the side of
the ground to be sent for
testing
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Sounding Test
The devise is driven
into the ground and
the pressure recorded
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Vane Test
The vane is twisted
in the ground and
the torque
measured which
causes the ground
to shear
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Classification by Grain Size
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(BS 1377:1967)
Clay : particles smaller than 0.002mm
Silt : particles between 0.002 - 0.06mm
Sand : particles between 0.06 - 2mm
Gravel: particles between 2 - 60mm
Boulders: particles above 60mm
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Basic Subsoil Classifications
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Rock - a very wide strength range.
Cohesive Soil - clay and silt.
Non-cohesive Soil - sand and gravel.
Made up Ground - unsuitable until
consolidated.
• Peat - unsuitable because it is organic and will
therefore decompose.
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The objectives of a ground investigation
• To assess the general suitability of the site for
the proposed works.
• To enable an adequate and economic design to
be prepared.
• To foresee and provide against difficulties that
may arise during construction due to ground
conditions.
• To investigate the occurrences or causes of all
natural or created changes of conditions and
their results.
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Laboratory Tests
• Soil Classification.
• Consolidation Tests. These enable time/settlement
graphs to be drawn so that the rate of settlement and
its total final amount can be forecast.
• Shear Strength Tests. An important group of tests as
they indicate the load carrying capabilities of the soils.
They are particularly significant in the case of plastic
clays as, in these, shear strength is constant at a given
moisture content: thus, if shear strength is exceeded,
failure is likely.
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• Chemical Analysis. This will indicate the presence of
harmful compounds in the subsoil/ground water: these
compounds will usually be sulphates or acids. If
present they will affect the choice of materials and/or
methods; e.g. sulphate resisting or blast furnace
cement in foundation concrete; use of separating
membrane; use of non-metallic pipes in acid soils.
• Moisture Content Tests. Done as part of the
classification test, but also needed to forecast likely
volume changes when building on shrinkable/swelling
clays.
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Other Tests
• Atterburg Limits
• Mohr's Circles
• Bulbs of Pressure
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