1. AN ACTIVE LEARNING ASSIGNMENT ON Prepared By: AGARIYA DEVJI-130180106002 ARUN CHAVDA-130180106018 DAXESH CHAVDA-130180106019 DABHI ROYAL-130180106021 Prepared By: AGARIYA DEVJI-130180106002 ARUN CHAVDA-130180106018 DAXESH CHAVDA-130180106019 DABHI ROYAL-130180106021 Guided By : Prof. B.M.PUROHIT 2015 SOIL MECHANICS(2150609) DEPARTMENT OF CIVIL ENGINEERING GOVERNMENT ENGINEERING COLLEGE, DAHOD “SHEAR CHARACTERISTICS OF SAND”

2.  The cohesion less soil (sand) derives its shear strength from internal frictional resistance between the soil particles and interlocking of the particles. The angle of internal friction is a measure of the resistance of the soil to sliding along a plane. It depends upon the density of packing, particle size, shape, roughness and particle size distribution.

3.  The ‘angle of repose’ is the angle with the horizontal at which a heap of dry sand, poured freely from a small height, will stand without support. It is approximately the same as the angle of internal friction in the loose state.  In general clean sand do not possess cohesion. However, the presence of even small percentages of silt and clay in a sand give it cohesive properties which also contributes to the shear strength of sand.

4.  A sand can be tested either in the dry or in the saturated condition. If sand is dry, there will be no pore water pressure and if it is saturated, the pore water pressure will be zero due to quick drainage.  However, there may be certain situations in which significant pore pressure are developed, at least temporarily in sands.

5.  For example during earthquake, heavy blasting and operation of vibratory equipment, instantaneous pore pressure are likely to develop due to the large shocks or dynamic loads. These may cause sudden and total loss of shear strength leading to the phenomenon known as liquefaction of sand. The structures resting on such soils lack stability and may sink.

6.  The stress strain relationship of sand can be obtain from thxial compression test or direct shear test conducted e triaon dry sand or drained saturated samples. In the direct shear test the shear test is plotted again the shear displacement. In triaxial compression test the deviator test is plotted against the axial strain.  Fig. (a) shows the stress-strain relationship for sand initially in loose, medium dense and dense condition. It can be observed that for an initially dense sand it reaches a peak value and decreases at greater values of strain, while for an initially loose sand the stress builds up gradually.

8.  It is evident that the denser a sand is, the stronger it is. The hatched portion represents the additional strength due to phenomenon of interlocking in the case of dense sands.  Fig. (b) gives the volume change characteristics of sands. An initially dense sand tens to increase in volume and becomes loose with increase of strain, while an initially loose sand tends to decrease in volume and become dense. This may be due to the arrangement of the particles during shear.

9.  fig. (c) shows the changes in pore water pressure during undrained shear which is rather not very common owing to high permeability of sands. Positive pore water pressure develop in the case of initially loose sand and negative pore pressure develop in the case of an initially dense sand.  The volume change characteristics of sand depend on various factors such as the particles size, shape and distribution, density index, principal stresses and previous stress history. Volume changes are usually expressed in terms of void ratio.

10.  Fig. (d) shows the relation between void ratio and shear strain, for sands. For initially loose sand the void ratio decreases while for initially dense sand the void ratio increases. At large strains both initially loose sands and initially dense sands attain nearly the same void ratio at which further increase in shear strain will not produce any volume change. Such a void ratio is termed as critical void ratio (CVR).

11. 1.Direct shear test :  This test also called the ‘shear box test’, is the oldest shear test that is in use and is also quite simple to perform.  A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses.  For rock the test is generally restricted to rock with (very) low shear strength. The test is, however, standard practice to establish the shear strength properties of discontinuities in rock.

12.  The test is performed on three or four specimens from a relatively undisturbed soil sample.  A specimen is placed in a shear box which has two stacked rings to hold the sample; the contact between the two rings is at approximately the mid- height of the sample.  A confining stress is applied vertically to the specimen, and the upper ring is pulled laterally until the sample fails, or through a specified strain.  The load applied and the strain induced is recorded at frequent intervals to determine a stress–strain curve for each confining stress

13.  Several specimens are tested at varying confining stresses to determine the shear strength parameters, the soil cohesion (c) and the angle of internal friction, commonly known as friction angle.  The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress on the y-axis and the confining stress on the x-axis.  The y-intercept of the curve which fits the test results is the cohesion, and the slope of the line or curve is the friction angle.  Direct shear tests can be performed under several conditions.

14.  The sample is normally saturated before the test is run, but can be run at the in-situ moisture content. The rate of strain can be varied to create a test of undrained or drained conditions, depending whether the strain is applied slowly enough for water in the sample to prevent pore-water pressure buildup.

15.  Direct shear test machine is required to perform the test.  The test using the direct shear machine determinates the consolidated drained shear strength of a soil material in direct shear.  The advantages of the direct shear test over other shear tests are the simplicity of setup and equipment used, and the ability to test under differing saturation, drainage, and consolidation conditions.  These advantages have to be weighed against the difficulty of measuring pore-water pressure when testing in undrained conditions, and possible spuriously high results from forcing the failure plane to occur in a specific location.

16. 2.Triaxial compression test :  A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil (e.g., sand, clay) and rock, and other granular materials or powders. There are several variations on the test.  In a triaxial shear test, stress is applied to a sample of the material being tested in a way which results in stresses along one axis being different from the stresses in perpendicular directions. This is typically achieved by placing the sample between two parallel platens which apply stress in one (usually vertical) direction, and applying fluid pressure to the specimen to apply stress in the perpendicular directions.

17.  The application of different compressive stresses in the test apparatus causes shear stress to develop in the sample; the loads can be increased and deflections monitored until failure of the sample.

18.  During the test, the surrounding fluid is pressurized, and the stress on the platens is increased until the material in the cylinder fails and forms sliding regions within itself, known as shear bands.  The geometry of the shearing in a triaxial test typically causes the sample to become shorter while bulging out along the sides.  The stress on the platen is then reduced and the water pressure pushes the sides back in, causing the sample to grow taller again.  This cycle is usually repeated several times while collecting stress and strain data about the sample.

19.  During the test the pore pressures of fluids (e.g., water, oil) or gasses in the sample may be measured using Bishop's pore pressure apparatus.  From the triaxial test data, it is possible to extract fundamental material parameters about the sample, including its angle of shearing resistance, apparent cohesion, and dilatancy angle. 3.Vane shear test :  The Vane Shear Test (also known as the Insitu Vane Shear Test) is a method of testing soil characteristics on site

20.  Vane Shear Test is a very popular type of insitu shear test among Geo-technical engineers, geologists, and other instances that benefit from soil data.  The Vane Shear Test is mainly used to determine the in situ undrained shear strength and the sensitivity of a saturated cohesive soil.

21.  It is limited to fine grained soils which can retain water content during the testing, in other words, sample soil has to stay saturated throughout the test.  The test works by inserting a vane and rotating it until the soil fails. The undrained shear strength and sensitivity can then be calculated by analyzing the obtained torque and the diameter of the vane used.  The data acquired from vane shear test provide a simple and convenient index of shear strength and guidelines for foundation construction.  Unlike other types of shear tests, vane shear test does not require sample taken or additional lab equipment to analyze the soil. This is beneficial for initial site testing since it generally requires less time and cost to operate.

22. 4.Unconfined compression test :  Unconfined Compression Test (UCT) is a simple laboratory testing method to assess the mechanical properties of rocks and fine-grained soils.  It provides a measures of the undrained strength and the stress-strain characteristics of the rock or soil.  The unconfined compression test is often included in the laboratory testing program of geotechnical investigations, specially when dealing with rocks.

23.  The primary purpose of the Unconfined Compression Test is to quickly determine a measure of the unconfined compressive strength of rocks or fine-grained soils that possess sufficient cohesion to permit testing in the unconfined state.

24.  This measure is then used to calculate the unconsolidated undrained shear strength of the clay under unconfined conditions.  In general, The UCT can be conducted on rock samples or on undisturbed, reconstituted or compacted cohesive soil sample.  In the unconfined compression test, the sample si placed in the loading machine between the lower and upper plates.

25.  Before starting the loading, the upper plate is adjusted to be in contact with the sample and the deformation is set as zero. The test then starts by applying a constant axial strain of about 0.5 to 2% per minute.  The load and deformation values are recorded as needed for obtaining a reasonably complete load- deformation curve.  The loading is continued until the load values decrease or remain constant with increasing strain, or until reaching 20% (sometimes 15%) axial strain. At this state, the samples is considered to be at failure. The sample is then removed for measurement of the water content.

26. REFERENCE :  https://www.google.co.in/search?q=test+for+shear+s trength&oq=test+for+shear+strength&aqs=chrome.0.69i59.12589j0j8&sourceid=chrome&es_sm=93&ie =UTF-8 https://www.google.co.in/search?q=test+for+shear+s trength&oq=test+for+shear+strength&aqs=chrome.0.69i59.12589j0j8&sourceid=chrome&es_sm=93&ie =UTF-8  SOIL MECHANICS(FIRST EDITION)2015,DR.R.P.RETHALIYA