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Soil & Site Investigation

Ron Gatepain

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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.

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• 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

• Trial Pits • Plate Bearing Test • Borings • Headings • Penetration Test • Sounding Test• Vane Test

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Plate Bearing Test

Stress = Load use a safety Area factor of 2

Plate is loaded and the movement measured

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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

• (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

• 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