CONTENT NO | TITLE | PAGES | 1 | INTRODUCTION | 2 | 2 | OBJECTIVE | 3 | 3 | APPARATUS AND MATERIAL | 4-5 | 4 | PROCEDURE | 6-13 | 5 | DATA | 14 | 6 | ANALYSIS | 15-16 | 7 | DISCUSSION | 17 | 8 | CONCLUSION | 18 | 9 | REFERENCES | 19 | 10 | APPENDIX | 20 |

INTRODUCTION

Theory
This test is performed to determine the consolidation – drained shear strength of a sandy to silty soil. This shear strength is one of the most important engineering properties of a soil, because it is required whenever a structure is depended on a soil shearing resistance. The shear strength is needed for engineering situation such as determining the stability of slopes or cut, finding the bearing capacity for foundation and calculated the pressure exerted by a soil retaining wall.
Significance
The direct shear stress is a strain – controlled test: the rate at which the soil will be strained is controlled. A specimen of soil will be placed into a shear box, and consolidated under an applied normal load. The shear box is made of two separate halves, an upper and lower. After the application of the normal load, these two halves of box will be moved relative to one another, shearing the soil specimen on the plane that is separation of the two halves. The direct shear test imposes stress condition on the soil that force the failure plane occur at a predetermined location ( on the plane that separates the two halve of the box). One this plane there two force (or stresses) acting – a normal stress, due to an applied vertical load Pᵥ and a shearing stress, due to the applied horizontal load Pᶮ. These stresses are simply computed as :
Óᵑ=Pᵥ /A
T=Pᵑ/A

Where A is the normal area of the specimen (or of the shear box). It is usually not corrected for the change in sample area cause by the lateral displacement of the sample under the shear load pᶮ. These stresses should satisfy Coulomb’s equation T=c+ Óᵑ tan Ø

OBJECTIVE

In this lecture we will learn about the shear box test, which is a direct method of measuring the shear strength of a soil in the laboratory. The typical measurements taken during a shear box test tell us a lot about the behaviour of different soils in shear.

APPARATUS AND MATERIAL

1. Direct Shear Device * To place the sand sample and the filter sample in the shear box

2. Load and Deformation dial gauges * To rate of shearing is at selected constant rate, and take the horizontal displacement gauge, vertical.

3. Balance * To take the weight of the sand sample

SAFETY IN THE WORKSHOP Before we enter the workshops or laboratories so we need to know about safety in the workshop or laboratory. At early stage, the lecturer will explain safety rules to students. Safety regulations in the workshop to be learned from time-to time. This is because we will start working with electrical appliances and electronic devices. As we work then we need to be vigilant to prevent an event that is not undesirable occur. The causes of safety related electronic devices, the need is known and good work habits and safe should be practiced regularly. Similarly, to enable all the work or equipment in good condition.

WHEN SAFETY PROJECT 1. Safety is the most important things in doing a project related to electricity and electronics. 2. In the course, students must comply with security measures to prevent accidents or injuries during the project. 3. In the course of our work are not to be negligent or did not cooperate because of poor pay the price later.

OWN SAFETY 1. Eye Protection must be maintained while in dangerous places, then we need to wear eye cover. 2. Updated clothes before starting work, the long sleeves must be rolled to the elbow. 3. Use closed shoes and rubber based work in the workshop. 4. Use protective clothing such as shirts and workshops, apron and gloves when working with chemical material. 5. Avoid long hair, if long hair must be tied so not to disrupt at work. 6. Avoid wearing jewelery made of metal such as watches, gold chains, pendants, rings and others.

PROCEDURE

1. Measured the dimensions of the shear box in inches in ordered to get the cross-sectional area of soil specimen. 2. Practiced the turning rate that will assured a horizontal displacement of 0.5 mm per minute. 3. Placed the bottom plate and the porous plate in the shear box .Then pour the sand to be tested into shear box to a depth of approximately 0.5 inch .Compact the soil. 4. Placed the porous plate (parallel to the bottom plate) and top loading platen in the position a top the sand. 5. Placed the normal force bar across the top platen and adjust the loading screw so that the bar just fits in the slot atop the platen and balanced on the platen surface without imposing significant load. 6. Placed the 5-kilogram normal load weight onto the load hanger. 7. Set the deformation dials and the load dials to zero. 8. REMOVED THE SHEAR PINS FROM THE BOX. 9. Turn the machined crank so that the horizontal loading rod just contacted the load box. 10. Took initial readings of both dial gages (proving ring deformation and specimen horizontal deformation)

Apply horizontal shear load at 0.5 mm per minute rate .Took a sufficient number of dial readings to give a suitable curved of shear displacement versus shear stress.(read the proving ring gage for every 10 division of the horizontal deformation gage) 1. Continued the test until failure (load decreases or data repeat 3 times) or until there has been a shear displacement, whichever comes first. 2. Removed the shear box and empty the sand. 3. Removed Steps 3-14,above, using the same amount of new sand,by using another 10kg and 15kg weight to the hanger. 4. Weight all the plate which located on top of the sand to get total weight of load gage and plate.

BEFORE TEST AFTER TEST

EXPLANATION | PICTURE | 1. Weigh the initial mass of soil in the pan. | | 2. Measure the diameter of the shear box. Compute 15% of the diameter in millimeters. | | 3. Carefully assemble the shear box and place it in the direct shear device. Then place a porous stone and a filter paper in the shear box | | 4. Place the sand into the shear box and level off the top. Place a filter paper, a porous stone, and a top plate (with ball) on top of the sand. | | 5. Remove the large alignment screws from the shear box! Open the gap between the shear box halves to approximately 0.025 in. using the gap screws, and then back out the gap screws. | | 6. Weigh the pan of soil again and compute the mass of soil used. | | 7. Complete the assembly of the direct shear device and initialize the three gauges (Horizontal displacement gage, vertical displacement gage and shear load gage) to zero. | | 8. Set the vertical load (or pressure) to a predetermined value, and then close bleeder valve and apply the load to the soil specimen by raising the toggle switch. | | 9. Start the motor with selected speed so that the rate of shearing is at a selected constant rate, and take the horizontal displacement gauge, vertical displacement gage and shear load gage readings. Record the readings on the data sheet. (Note: Record the vertical displacement gage readings, if needed). | | 10. Continue taking readings until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 15% of the diameter | |

DATA

ANALYSIS

QUESTION

1. Beside sand, what other types of soil can be tested using the direct shear stress ?

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 discontinuties in soil or rock masses. The U.S. and U.K. standards defining how the test should be performed are ASTM D 3080 and BS 1377-7:1990, respectively. 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 discontinuties in rock. 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. 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 friction angle) . The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress on the x-axis and the confining stress on the y-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. The sample is normally saturated before the test is run, but can be run at the in-situ moisture content.

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. The test equipment and procedures are slightly different for test on discontinuities.
DISCUSSION

The ultimate values of stress and void ratio for dense and loose specimens under the same values of normal stress in the direct shear test are essentially equal as indicated in the test results. The reasons are mainly because in dense sand there is a considerable degree of interlocking between particles. Before shear failure can take place, this interlocking must be overcome in addition to the frictional resistance at the points of contact. In general, the degree of interlocking is greatest in the case of very dense, well-graded sands consisting of angular particles. The characteristic stress–strain curve for an initial dense sand shows a peak stress at a relatively low strain and thereafter, as interlocking is progressively overcome, the stress decreases within creasing strain. The reduction in the degree of interlocking produces an increase in the volume of the specimen during shearing as characterized by the relationship between volumetric strain and shear strain in the direct shear test. So, for the dense sand, after test, it will become loose sand and share the same properties with loose sand.

The horizontal displacement against vertical displacement The graphs of the loose sand and dense sand is very different from each other. The vertical displacement of the loose sample is negative during the whole shearing process while the dense sample’s vertical displacement becomes positive after some value. This is probably because when the loose sample is under shearing, it also undergoes a disturbing process so that the particles will combined together and the porosity decreases. As we know, the density of the loose sample will increase even under some slight compaction, not mentioned the abrupt shear process. This process can be shown clearly in the following pictures: On contrary, the dense sample need to overcome the internal frictional force when under shearing, it must be lifted up a little bit to keep the horizontal displacement.

For the compacted sand sample, it swells easily when the external force decrease. This process can be shown clearly in the following pictures.
Maximum Shear Stress against Normal Stress From the graph of vertical stress against the maximum shear stress, we can see that the larger the vertical stress applied, the larger the shear stress required to make the sand sample fail.

CONCLUSION

As a conclusion, we can know that the objective of the experiment is to determine the parameter of shear strength of soil, cohesion and angle of friction was achieved. As the soil used for the experiment is coarse- grained soil which is sand and the value of friction of angle is 28°.

The direct shear test can be used to measure the effective stress parameters of any type of soil as long as the pore pressure induced by the normal force and the shear force can dissipate with time. For the experiment we use the clean sands as a sample, so there is no problem as the pore pressure dissipates readily. However, in the case of highly plastic clays ,it is merely necessary to have a suitable strain rate so that the pore pressure can dissipate with time. Direct shear tests can be performed under several conditions The results of the tests on each specimen are plotted on a graph with the peak (or residual) stress on the x-axis and the confining stress on the y-axis. They-intercept of the curve which fits the test results is the cohesion, and the slope of the line or curve is the friction angle.

REFERENCES

1) Bowles, Joseph E. Engineering Properties of Soils. 4th. Boston: Irwin McGraw–Hill, 1992.

2) Das, Braja M. Principles of Geotechnical Engineering. 6th. Toronto, ON: Thomson Canada Ltd., 2006.

3) U.S.ArmyCorpsoFEngineers,“GeotechnicalInvestigations,EngineeringManual,”1110-1-1804, Department of Army, 2001.

4) U.S.Army Corps of Engineers,“SoilSampling,Engineering Manual,”1110-1-1906,
Department of Army, 1996.

APPENDIX