In sampling of soils,briefly describe the following: a. Sample sizes b. Soil dis

Question

In sampling of soils,briefly describe the following: a. Sample sizes b. Soil disturbance c. Soil disturbance during drilling d. Soil disturbance during sampling e. Disturbance after sampling f. Undisturbed sampling techniques g. Sand sampling h. Preparation of disturbed samples for testing i. Preparation of undisturbed samples for testing j. Preparation of cylindrical specimen direct from sampling tube k. Preparation of cylindrical specimen of diameter smaller than the sampling tube l. Preparation of cylindrical specimen from undisturbed block sample m. Preparation of disc or square specimen from sample tube.

Explanation / Answer

. a) Sample Size:

It is defined as the minimum quantity of soil required for a particular test procedure which will dictate the volume or mass that must be obtained. It is determined on the basis of both soil type and the purpose for which the sample is needed.

However, the size selected must be large enough to ensure that the sample contains a representative distribution of the particle sizes that are in the ground, and to ensure that enough material will be available for the tests that are envisaged. In addition, to ensure that any particle size distribution tests carried out are representative of the ground from which the sample has been taken, and to ensure that testing will give representative results.

It is normally considered adequate to take samples which have a minimum dimension of the order of 5—10 times the maximum particle size of the soil.  

b) Soil Disturbance:

The change in the physical or chemical characteristics of soil which can occur either during drilling, sampling, transportation or storage is termed as soil disturbance. Any sample of soil being taken from the ground, transferred to the laboratory and prepared for testing will be subjected to disturbance. The mechanisms associated with this disturbance can be classified as follows:

1. Changes in stress conditions - Reduction of the total horizontal and vertical stresses from their in situ value to zero on the laboratory bench.

2. Mechanical deformation - Shear distortions applied to the soil sample

3. Changes in water content and voids ratio - Overall swelling or consolidation of the soil sample

4. Chemical changes – As a result from contact with drilling fluid or with sampling tubes.

The importance of a particular type of disturbance depends not only upon the sampling processes being used, but also upon the type of soil being sampled.

c) Soil Disturbance during drilling:

Various types of disturbances can occur at the time of drilling:

1.Swelling - Swelling can occur at the base of the borehole before insertion of a sampler tube, during the taking of a sample and after sampling when the soil is inside the sampler tube. The amount of swelling that can occur is proportional to the change of total stress occurring at the base of a borehole. Thus if the borehole is substantially empty of water there is likely to be more swelling than if the borehole is kept full of mud or water.

2.Compaction, remolding and displacement - Soil displacement can occur as a deliberate method of advancing a borehole; many well-boring rigs operate on the percussion drilling principle, where a heavy drilling bit is alternately raised and dropped by a ‘spudding’ mechanism. This type of displacement drilling leads to significant remolding and compression of the soil.

3.Piping - Piping is used to describe the behavior of granular soil when its effective confining pressures, and hence strength, are removed as a result of high upward seepage pressures. Under these conditions the individual soil particles are free to move and finer soil particles are carried upwards with the water.

4.Caving - Caving occurs when boreholes are advanced into soft, loose or fissured soils. Material from the sides of the borehole collapses into the bottom of the hole and must be cleaned out before sampling can take place. Progress is slowed because more material must be removed from the borehole.

d) Soil disturbance during sampling:

1. Stress Relief - A reduction in the total stress applied to the soil is called stress relief. At one stage or another, the block of soil will normally experience zero total stress. This will lead to a large reduction in the pore pressures. The soil attempts to suck in water from its surroundings, during sampling, either from the soil to which it is attached, or from any fluid in the pit or borehole. This result in a reduction in the effective stress in the block.

2. Mixing and Segregation – As a result of driving, a thick walled open-drive sampler into hard soil by repeated blows of a hammer; the soil is usually heavily fractured and if any material is recovered it often has the appearance of an angular gravel.

3. Failure to recover – As a result of heavy and repetitive tampering, the soil sample collapses and loses its individual identity making it extremely difficult to reproduce original ground condition.

e) Soil disturbance after sampling:

Changes to the soil after sampling can be at least as severe as those occurring during boring and sampling. Major types of change can be recognized as-

1. Moisture loss

2. Migration of moisture within samples

3. The effects of inadvertent freezing

4. The effects of vibration and shock

5. The effects of chemical reactions.

f) Undisturbed Sampling Techniques:

Undisturbed Samples can be obtained in a number of ways:

Undisturbed samples can be obtained either by drive sampling or block sampling. In either case it is important to recognize the disturbance created by excavating the trial excavation, and ensure that disturbed material is carefully removed before or after sampling. The normal technique is to cut a column of soil about 300mm cube, so that it will fit inside a box with a clearance of 10—20 mm on all sides. A box with a detachable lid and bottom is used for storage. With the lid and bottom removed, the sides of the box are slid over the prepared soil block, which is as yet attached to the bottom of the pit.

2.In boreholes, using either drive or rotary techniques.

Drive samplers are samplers which are either pushed or driven into the soil without rotation. The volume of soil corresponding to the thickness of the sampler wall is displaced into the Surrounding soil, which is either compacted or compressed. It can be – open, piston or sliding type.

Rotary samplers (often termed ‘core barrels’) have a relatively thick and blunt cutting surface, which has hard inclusions of tungsten or diamond set into it. The sampler is rotated and pushed gently downwards, cutting and grinding the soil away beneath it. It can be –retracted, protruding or retracting inner barrel type.

g) Sand Sampling:

Undisturbed sand sampling can be very expensive, and is normally only required in special circumstances, for example to obtain values of in situ density for earthquake liquefaction problems or for compressibility studies. The techniques used for sampling sand are:

1. Thin-wall fixed piston samplers in mud-filled holes

2. Open-drive samplers under compressed air

3. Impregnation

4. Freezing

5. Core catchers

h) Preparation of disturbed samples for testing

The steps for the preparation of disturbed samples of soil from the samples received from the field and their allocation are:

i) Preparation of undisturbed samples for testing

Preservation of samples: Loss of moisture from undisturbed samples shall be prevented during transportation, preparation and storage

Five procedures by which undisturbed sample are prepared -

a) Cylindrical specimen of the same diameter as the sample tube

b) Cylindrical specimen or set of specimens, of smaller diameter than the sampling tube.

c) Cylindrical specimen from a block sample.

d) Disc or square specimen from a sample in a sampling tube

e) Disc or square section from a block sample

j) Preparation of cylindrical specimen direct from sampling tube

When a specimen is to be prepared from an undisturbed sample contained in a tube of the same internal diameter as the test specimen the procedure shall be as follows. The sample shall normally be pushed out of the tube in the same direction as it entered.

K) Preparation of cylindrical specimen of diameter smaller than the sampling tube

This procedure applies to the fine grained cohesive soil which can be extruded into tubes with Negligible disturbance.

l)Preparation of cylindrical specimen from undisturbed block sample

m)Preparation of disc or square specimen from sample tube.

This procedure is used for preparing either a square cylindrical specimen for a consolidation test or swelling test in an oedometer, or a square specimen for a direct shear test in a small shearbox apparatus. The mould with a cutting edge into which the specimen is intruded is referred to as the cutting ring, whether it is circular or square.

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