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Introduction:

The main objective of the geotechnical exploration and laboratory tests on the soil is to find out the probable subsurface conditions like stratification, denseness or hardness of the soil strata, the status of groundwater table etc. and to assess the likely range of safe bearing capacity for the structure. The work carried out is comprises of collecting soil samples, define its physical and engineering properties and analyzing all field and laboratory bearing capacity of the soil. The elements of a site investigation generally should provide the following:

  • Most suitable foundation type (shallow or deep).
  • The allowable load capacity of the foundation.
  • Laboratory test on the soil to make settlement predictions
  • Position of the groundwater table
  • Identification and probable solution of construction problems
  • Effect on adjacent property/structure
  • Environmental issues and their solution

1.0

Laboratory Tests on Soil:

1.1

Field Dry Density and Natural Moisture Content:

The moisture content test on the soil is the most commonly used laboratory test on the soil. The moisture content of soils with other test data provides important information regarding the soil characteristics.

1.1.1

Test Procedure:

  • The weight of the undisturbed soil sample with a sampler (Shelby tube) is determined after removing paraffin wax and loose material. 
  • The total length of soil sample obtained after deducting empty length from the entire length of the sampler. 
  • The volume of soil mass retained in the sampler obtained from the associated inside diameter of the sampler and total length of the soil mass. 
  • After that, the soil mass is removed, and the average moisture content is defined by keeping the soil sample along with container in the oven at 100-105 degrees centigrade for twenty-four hours. 
  • The weight of soil mass is obtained after deducting the empty weight of the sampler.

Bulk density = (The weight of soil mass / The volume of the soil mass) gm/cc  

1.1.2

Objective: 

By conducting the above tests on soil, we can find out the field density, but the moisture content is possible to vary and hence the field density also. So it is necessary to report the test result in terms of dry density. The relationship that can be established between the dry density with known moisture content is as follows:

Dry density = {(Field bulk density/ ( 1+ water content )} gm/cc  

1.2

Particle Size Analysis:

The sieve analysis is carried out as per IS: 2720 (Pt-iv) for particle size analysis of soil. The results are developed in the form of Grain size distribution (i.e., gravel, sand, silty clay, etc.) curve. The representative soil sample is obtained from the bulk soil sample collected or obtained from the site and quantity of sample varies depending on the maximum size of particle present in the soil.

1.2.1

Test Procedure:

  • Wash a prepared representative soil sample through a series of sieves as per technical specification.
  • The portion retained on each IS sieve is collected dried and weighed to determine the percentage of material passing through the sieve.
     Soil fraction retained on 4.75mm IS sieve:
  • Soil sample retained on 4.75mm IS sieve is weighed. 
  • After that, the sample is separated into various fractions by sieving through 100 mm, 75 mm and 19 mm.
  • The mass of the soil material retained on each sieve is to be recorded. 
  • Then the percentage of soil retained on each sieve is calculated based on the total mass of the sample taken.
  • The percentage of passing through each sieve is calculated with the help of the above data. 
     Soil fraction retained on 4.75mm IS sieve:
  • The part of the soil passing through 4.75 mm IS sieve is oven-dried at 105-110 degrees centigrade. 
  • The portion is coned and quartered to obtain the required representative quantity of the material. 
  • The material is weighed and placed in a container filled with water for soaking and loosening the adhered cohesive soil materials. 
  • After soaking the soil specimen is then washed on 75 microns IS sieve until the water passing through the sieve is almost clear. 
  • The material retained on 75 microns IS sieve is then transferred in a tray and dried in the oven.
  • After that, sieve analysis is carried out on a nest of sieves (2.0 mm, 425 microns and 75 microns IS sieve) either by manually or by using mechanical sieve shaker. 
  • The fraction retained on each sieve is weighed individually and has to be recorded.
  • The cumulative mass of soil fraction retained on each sieve has to be calculated.
  • The weights are then transformed into cumulative percentage retained and passing based on the mass of the sample passing through 4.75 IS sieve is taken. 
  • The combined gradation with respect to the total soil sample taken for analysis is finally calculated.

1.2.2

Objective:

Particle Size Analysis is used to determine the textural classification of soils which is useful in evaluating the engineering characteristics such as permeability, strength, swelling potential, and susceptibility to frost action.

1.3

Atterberg’s Limit Tests on Soil:

The Atterberg limits give general indices of moisture content relative to the consistency and behaviour of soils. The moisture content is varied to distinguish three stages of soil behaviour in terms of consistency. The stages are as follows:

1.3.1

Liquid limit (LL):

The liquid limit (LL) is defined as the water content at which 25 blows of the liquid limit machine closes a standard groove cut in the soil past for a distance of 12.7 cm. Some time the fall cone device is used for the test to obtain better repeatability.

1.3.1.1

Liquid Limit by Casagrande Method:
  • Add distilled water into an evaporating dish containing 250 gm of air-dried soil, passed through 425 mm sieve and mixed it thoroughly to make a uniform paste.
  • Place a part of the paste in the cup of Casagrande Liquid limit device and spread it with the help of a few strokes of the spatula.
  • Trim it to a depth of 10mm at the point of maximum thickness and return the excess soil to the dish.
  • With the help of grooving tool, cut a groove along the centre line of soil pat, so that clean, sharp groove of dimension 11 mm wide at the top, 2 mm at the bottom and 8 mm deep is formed.
  • Lift and drop the cup by rotating crank at two revolutions/second until the two halves of soil cake come together for a length of about 13 mm by flow only, and record the number of blows, which is denoted by N.
  • A representative portion of soil can be taken from the cup for a moisture content test.
  • The test shall be Carry out at least four more times for different moisture contents for blows between 10 and 40. 

1.3.1.2

Liquid Limit by Cone penetrometer Method:
  • Add distilled water into an evaporating dish containing 150 gm of air-dried soil, passed through 425 mm sieve and mixed it thoroughly to make a uniform paste.
  • The Soil paste is then transferred to the circular trough, 150mm diameter and 50mm hight of the cone penetrometer apparatus and levelled up to the top of the trough. 
  • The penetrometer is set such that the cone point touches the surface of the soil paste in the trough. 
  • After fixing the penetrometer scale to zero, loosened the vertical rod.
  •  The cone penetrates the soil sample due to its self-weight. 
  •  Record the penetration after 30 seconds from the release of the cone and the reading is considered if penetration reading 20-30mm. The moisture content of the sample corresponding to this is calculated. The liquid limit of the sample corresponds to the moisture content, which would provide 25 mm penetration of the cone, is defined.

Tests on Soil Liquid Limit by Cone penetrometer Method

1.3.2

Plastic Limit (PL):

The plastic limit is as the water content at which a thread of soil sample paste, when rolled down to a diameter of three millimetres, will crumble. 

1.3.2.1

Test Procedure:
  • 20 gm of soil sample passing through 425micron IS sieve is completely mixed with water such that it can be easily moulded with fingers. 
  • 8.0-10.00 gm of sample is rolled with the help of fingers on the glass plate with adequate pressure to turn the mass into a uniform diameter thread of 3.00mm throughout its length.
  • Repeat the process until the thread crumbles. The moisture content of crumbled soil thread pieces is checked and reported as a plastic limit.

1.3.3

Shrinkage Limit (SL):

The shrinkage limit is known as that water content below which no further soil volume change occurs with further drying.

1.3.4

Objective:

The purpose of Atterberg’s Limit test is to define the consistency and plasticity of fine-grained soils with varying degrees of moisture.

1.4

Specific Gravity(G) Tests on Soil:

Specific gravity is a ratio of the weight between the weight of the soil sample, and an equal volume of distilled water weight. At the same ambient temperature, both weights are taken.

1.4.1

Test Procedure:

  • The weight (W1) of the empty density bottle of 50ml capacity is taken for the specific gravity test of the soil sample.
  • 10.00-20.00gm of an oven-dried soil sample is taken and cooled in a desiccator.
  • After that, put the soil sample into the bottle, and weight (W2) of the bottle with the sample is taken. 
  • Then the bottle is filled with distilled water, gradually and removing the entrapped air ether by applying a vacuum or by shaking the bottle. 
  • The weight (W3) of the bottle with soil and water filled up to the top is taken. 
  • Finally, the bottle is emptied and thoroughly washed.
  •  After that, cleaned water is filled up to the top, and the weight (W4) is taken. The equation from which we can determine the specific gravity of the soil sample is as follows:           

          Specific Gravity (G) = {(W2-W1) / (W2-W1)-(W3-W4)}     

1.4.2

Objective:

The specific gravity is essential for the calculation of soil properties like void ratio, degree of saturation etc.                           

1.5

Shear Tests on Soil:

1.5.1

Undrained Unconsolidated Triaxial Tests on Soil:

The shear tests are carried out as per IS: 2720 (part x, xi, xii and xiii) on saturated soil. The undrained triaxial test is a compression test, in which the soil sample is first subjected to isotropic all-around pressure in the triaxial cell before failure. It is performed with or without measurement of pore pressure, although for most of the cases measurement of pore pressure is acceptable. 

1.5.1.1

Test Procedure:
  • The undisturbed soil sample having a diameter of 38.00mm and height to diameter ratio of two is made and placed on the pedestal of the triaxial cell. 
  • After assembling the cell carefully, the sample is placed in the loading machine, and loading ram is placed on the top. 
  • The cell must be uniformly clamped down to prevent leakage of pressure during the test and make sure first that the sample is properly sealed.
  • The cell fluid is admitted to the cell, and the pressure is increased up to the desired value. 
  • During the test, necessary adjustments of pressure gauge must be made to keep the pressure constant. 
  • A first reading of the gauge measuring axial compression of the specimen is recorded. 
  • Then the test is started and a sufficient number of concurrent readings of load and compression measuring gauge being taken. 
  • The test is extended until the maximum stress, or twenty per cent of axial strain has been reached. 
  • Further tests are carried out on identical specimen at confining pressure of 1.0 kg/cm2, 2.0 kg/cm2 and 3.0 kg/cm2. 
  • The shear parameters are taken from the plot of Mohr circles.

 1.5.1.2

Objective:

Triaxial Test is necessary for strength characteristics of soils, including detailed knowledge on the effects of lateral confinement, drainage, porewater pressure and consolidation. It helps to determine the friction angle of natural clays and silts, as well as reconstituted sands. 

1.5.2

The direct shear tests on Soil:

The angle of internal friction and cohesion of the soil plays a significant role in the design of foundation, retaining walls, slab bridges, pipes, sheet piling. The direct shear test is used to determining the above parameters easily.

1.5.2.1

Test Procedure:
  •  The test is carried out using a shear box which is split in the horizontal plane of with a specimen of dimension 60.00mm x 60.00mm. Plain grid plate placed at top and bottom of the sample, is fitted into position in the shear box housing and assembly kept on the loading frame. 
  •  Place the soil in even layers (approximately 10 mm thick). If a dense sample is desired, tamp the soil.
  • The serrations of the grid plates are placed at a right angle in the direction of shear. 
  • The loading pad is kept on the top of the grid plate. 
  • Add 5 kg normal stress 0.5 kg/cm2 and continue the experiment till failure, and the rate of longitudinal displacement so adjusted that no drainage can occur from the sample during the test (1.25mm/min). 
  • The top part of the shear box is such lifted that a gap of about 1.00mm is left.
  • The test is carried out by applying a horizontal shear force to failure or to 20% longitudinal displacement whichever occurs first. 
  • Record all the readings carefully. 
  • The test is repeated on identical specimens; Set the dial gauges zero, before starting the experiment.

1.5.2.2

Objective:

The direct shear test is necessary to determine the shearing strength of the soil.

1.6

Consolidation Tests on Soil:

The consolidation test is carried out on undisturbed soil specimen to determine the settlement characteristics of soil at different depths. The tests are conducted as per guideline provided in IS: 2720 (Pt-XV).

1.6.1

Test Procedure:

  • An undisturbed soil sample is extruded to the consolidation ring of 60.0 mm diameter, and the edge is trimmed carefully in such a way that the sample flushes with the top and bottom edges of the consolidation ring. 
  • The thickness and weight of the soil sample are measured and recorded. 
  • The porous bottom stone is then centred on the base of the consolidation cell.
  • The sample is placed centrally between the porous bottom stone and the upper porous stone. 
  • A filler paper is provided with an in-between sample and porous stones. 
  • After that, the loading cap is placed on the top. 
  • The consolidometer is placed in a position in the loading arrangement and suitably adjusted. 
  • Then the dial gauge is fitted into position to record the relative movement between the base of the cell and the loading cap. 
  • An initial seating pressure of 0.05 kg/cm2 is applied to the soil sample, and the test cell is kept filled with water. 
  • After 24 hours, the test is continued using a loading sequence on the soil sample of 0.25, 0.50, 1.00, 2.00, 4.00 and 8.00 kg/cm2. For each load increment, readings of the dial gauge are taken in the following time interval of 0, 0.25, 1.00, 2.25, 2.00, 6.25, 9.00, 16.00, 25.00, 36.00, 49.00 up to 24.00 hrs. 
  • From the observations of all increment, void ratio versus log (pressure) curve is obtained. The slope of the straight-line section is designated as the compression index(Cc).

1.6.2

Objective:

The test is conducted to determine the rate of consolidation under normal load, degree of consolidation at any time, pressure-void ratio relationship, coefficient of concentration and compression index which helps to predict the time rate of settlement of structures.

1.7

Differential Free Swell Tests on Soil:

Swelling is a natural reaction of some clays to saturation, and the potential for swell depends on the mineral composition of the soil. Montmorillonite (bentonite) shows a high degree of swell potential, whereas kaolinite exhibits almost none. Differential free swell test is carried out to determine the swelling characteristics of the soil. 

1.7.1

Test Procedure:

  • Two specimens of 10g each, oven-dried and passing through 425 microns is poured into two 100ml graduated cylinder separately. 
  • One cylinder is filled with distilled water and another with kerosene up to 100ml mark. 
  • After removal of entrapped air, the sample is allowed sufficient time to attain an equilibrium state of the volume. 
  • Finally, the volume of soil in each cylinder is recorded.

Differential Free swell (%), (Sp) = {(Soil volume in water – soil volume in kerosene) / (Soil volume in kerosene)} x 100

1.7.2

Objective:

To estimate the swell potential of (expansive) soils.

Apart from the design aspect, Geotechnical exploration and laboratory tests of soil are essential to obtain sufficient data for feasibility and economic considerations for a project. Minor structures sometimes designed without site exploration and soil tests; however, the practice is not recommended. 

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Summary
Geotechnical Engineering Laboratory Tests on soil
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Geotechnical Engineering Laboratory Tests on soil
Description
Field Dry Density & Natural Moisture Content, Particle Size Analysis, Atterberg's Limit, Specific Gravity, Shear Test, Consolidation Test and Differential Free Swell Test
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ConstructionCivil
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