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SamplingTypes, Criteria for undisturbed samples, Transport and preservation of samples

Soil sampling Soil sampling is the process of

collection of samples of soil in order to determine the engineering properties of soil required for the proper design of foundation

Assessment of water table is also considered to be a part of soil exploration

Types of Samples

Samples

Disturbed Sample

Non -Representativ

e SampleRepresentative sample

Undisturbed sample

Disturbed SampleThe samples in which the

natural structure of the soil got disturbed either partly or fully during sampling are called disturbed samples

They are further classified as a) Non Representative Sample

b) Representative Sample

Non -representative SampleNon-representative samples consist of

mixture of materials from various soil or rock strata or are samples from which some mineral constituents have been lost or got mixed up.

These are suitable only for providing qualitative information such as major changes in subsurface strata.

Example: Soil sample from Auger and wash boring

Representative samplesRepresentative samples are

those in which all the constituent minerals are retained but only the structure of the soil got disturbed

There are changes in water content also

These samples are used in the determination of physical properties like atterberg limits and specific gravity

Undisturbed sampleUndisturbed samples are those which

are subjected to minimum disturbances. Purely undisturbed sample is an ideal

condition because all the samples will get disturbed to some extent even though precise equipments are used

These samples are used in strength and consolidation test

Example: Tube sample and Chunk sample

Obtaining chunk sampleA cylindrical container open at

both the ends is used for sampling.

The soil is trimmed to shape at the bottom of the test pit

One end of container is closed and inverted over the soil chunk and the soil sample is removed using spatula

This method is suitable for cohesive soil.

Obtaining chunk sample or hand craved sample

Criteria for undisturbed sampleArea ratio

Ar= Maximum cross-sectional area of the cutting edge X 100/Area of the soil sample

Ar = (D22- D1

2)/D12 X100

The area ratio for undisturbed sample must be 10% or less than 10%

Inside Clearance: It allows elastic expansion of the sample and reduces frictional

dragCi = (D3-D1)/D1 X100

For undisturbed sample, it should be in the range of 0.5 to 3%

Outside clearance: Co =(D2 – D4)/D4 X 100

For undisturbed sample, It should lie between 0 to 2%

Soil Sampler

4. Inside wall Friction: The inside wall should be smooth and oil is

generally applied to reduce friction

5. Design of Non return Valve:The sampler must have a large orifice to allow quick escaping of air, water or slurry. It should close immediately when the sampler is with drawn

6. Method of applying force: The rate of advancement and method of application

of force controls the sample disturbance. The sampler must be pushed not driven

Types of samplersSplit spoon samplerScraper bucket samplerShelby tubes and thin walled

samplersPiston samplerDenison sampler

Preservation and transportation of samples (IS 1892 -1979)Disturbed Samples of Soil – Immediately after being taken from the bore hole or trial

pit, the sample should be placed in a cloth bag or tin preferably in a glass jar of at least 0.5 kg capacity, and it should fill this container with a minimum of air space.

The container should have an air-tight cover. In this way the natural water content of the sample can be maintained for one or two weeks without appreciable change.

The containers should be numbered and a label should be placed immediately under the cover in a container.

The containers should be carefully packed in a wooden box with saw dust or other suitable material, to prevent damage during transit.

They should be stored if possible in a cool room.

Undisturbed Samples of Soil - : Immediately after being taken from the boring or

trial pit, the ends of the sample should be cut and removed to a depth of about 2.5 cm ( or more in the top to cover any obviously disturbed soil).

Several layers of molten wax should then be applied to each end to give a plug about 2.5 cm thick.

If the sample is very porous, a layer of waxed paper should first be placed over the ends of the sample.

Any space left between the end of the liner or tube and the top of the wax should be tightly packed with saw dust or other suitable material; and a close-fitting lid or screwed cap be placed on each end of the tube or liner.

The lids should, if necessary, be held in position by adhesive tape. If the longitudinal joint of the liner is not air-tight, this should be waxed and protected by adhesive tape in the same way as the lid.

Samples which are not retained in a tube should be wholly covered with several layers of molten paraffin wax immediately after being removed from the sampling tool, and then placed in a suitable metal container, being tightly packed in the container with saw dust or other suitable material.

If the sample is very porous, it may be necessary to cover it with waxed paper before applying the molten wax

The number of the sample should be painted on the outside of the container, and the top or bottom of the sample should be indicated.

The liner or containers should be placed in a stout wooden box, preferably with separate partitions, and packed with saw dust, paper, etc, to prevent damage during transit.

It is desirable to test the undisturbed samples within two weeks of sampling

If possible the sample is stored in cool dry place.

Planning of an excavation programmePlanning depends on type and

importance of the structureDepth, extent, thickness , composition

of each strata and the depth of water table are required to be determined

Trained persons having knowledge on geology and geotechnical engineering are necessary.

Spacing and depth are the two important aspects of a boring programme

Spacing of boringIt is impossible to determine the

spacing of borings before an investigation begins, since it depends on the uniformity of the soil deposit.

It is related to the type, size and weight of the proposed structure, variation of strata and availability of funds

Spacing is decreased if additional data are required and increased if the strata is uniform

Spacing of boring

Depth of boring Boring is performed upto the depth that could consolidate

significantly under the load of the structure In case of important structures like dams, bridges etc, boring is

done upto the depth of rock Settlement will not occur if the additional load imposed by the

structure is 10% of the initial stress in the soil due to self-weight.

Critical depths of borings for buildings are about 3.5 m and 6.5 m for single- and two-storey buildings.

For dams and embankments, the depth ranges between half the height to twice the height depending upon the foundation soil.

In any case, the depth to which seasonal variations affect the soil should be regarded as the minimum depth for the exploration of the sites.

But, where industrial processes affect the soil characteristics, this depth may be more.

The presence of fast-growing and water-seeking trees also contributes to the weathering processes

Depth of Exploration

Boring log Information on subsurface conditions obtained from the

boring operation is typically presented in the form of a boring record, commonly known as “boring log”.

It consists of1. Description or classification of various soil and rock type2. Ground water table details3.Test data in case of ‘lab log’

Comparison is generally made between the data obtained from adjacent borings in order to predict the nature of variation in the strata

A site plan showing the location of borehole is enclosed along with the boring log

A typical boring log

Report writingThe site investigation

report should contain the discussion of the results of exploration programme.

The main findings of the investigation should be brief but should clearly state the salient points

A soil exploration report generally consists of the following

1. Introduction2. Description of the proposed structure3. Location and geological condition of the site4. Methods of exploration5. Number of borings, their depth and location6. General description of sub-soil condition as

obtained from the SPT and cone test7. Details and results of the laboratory test

conducted8. Depth of ground water table and its fluctuations9. Discussions of the results10. Recommendation about allowable bearing

pressure, depth and type of foundation11. Conclusions and limitations of the investigations .