soil & site investigation

8/13/2019 Soil & Site Investigation

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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 iscarried out to:

Assess the general suitability of the site forthe proposed works

Enable an adequate and economic design

to be prepared


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Foresee and provide against difficulties thatmay arise during construction due to groundand other local conditions

Predict any adverse effect of the proposedconstruction on neighbouring structures.


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Preliminary Site Investigation

Mining authorities - existing and future factorseffecting ground movement.

Ordinance Survey - land contours, rights ofway, rivers, existing buildings. (Look at oldmaps - pond in wrong place)

Geological Central Record.


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Met Office - frost hollows, prevailing windsand 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 LocalAuthority

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 inorder to find out the nature and disposition of

the soil below ground level, and to obtainsamples of the soil for testing in thelaboratory

The exploration should be done at the sametime 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 onthe size and type of structure, the nature of

the site, and the availability of local geologicalinformation. The exploration should be taken deep enough

to include all strata likely to be significantlyaffected by the loading of the building. Thisdepth will depend on the weight, size andshape 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 safetyArea factor of 2

Plate is loaded and themovement measured


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Borings

Hand Boring - depths up to 6 to 9m auger isusually 100 - 150mm in diameter

Mechanical Boring Borehole Log


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Soil Samples

Disturbed brings out the soil particles butdoes not keep them in their original position

Undisturbed brings out the core of soil as itis in the ground


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Sounding Test

The devise is driveninto the ground andthe pressure recorded


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Vane Test

The vane is twisted

in the ground andthe torquemeasured whichcauses the groundto 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 allnatural or created changes of conditions andtheir 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 andits total final amount can be forecast.

Shear Strength Tests. An important group of tests asthey indicate the load carrying capabilities of the soils.

They are particularly significant in the case of plasticclays as, in these, shear strength is constant at a givenmoisture content: thus, if shear strength is exceeded,failure is likely.


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Chemical Analysis. This will indicate the presence ofharmful compounds in the subsoil/ground water: thesecompounds will usually be sulphates or acids. If

present they will affect the choice of materials and/ormethods; e.g. sulphate resisting or blast furnacecement in foundation concrete; use of separatingmembrane; use of non-metallic pipes in acid soils.

Moisture Content Tests. Done as part of theclassification test, but also needed to forecast likelyvolume changes when building on shrinkable/swellingclays.


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Other Tests

Atterburg Limits Mohr's Circles

Bulbs of Pressure

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