shear box test

Table of Contents INTRODUCTION ....................................................................................................................................... 2 THEORY ................................................................................................................................................... 3 Direct shear box .................................................................................................................................. 3 Measurement of shear strength ......................................................................................................... 3 Volume change in shear box ............................................................................................................... 4 OBJECTIVE ............................................................................................................................................... 6 MATERIAL & EQUIPMENT ....................................................................................................................... 7 TEST PROCEDURE .................................................................................................................................... 8 SAFETY AND HEALTH PROCEDURE ........................................................................................................ 11 RESULTS AND CALCULATION: ............................................................................................................... 12 Dense sand: ....................................................................................................................................... 12 i) Sample A (60kPa) ....................................................................................................................... 12 ii) Sample B (100kPa) .................................................................................................................... 13 iii) Sample C (150kPa).................................................................................................................... 14 iv) Angle of internal friction of sand samples ............................................................................... 15 Loose sand: ....................................................................................................................................... 17 Sample A: (60kPa) ......................................................................................................................... 17 Sample B (100kPa): ....................................................................................................................... 18 Sample C (150kPa): ....................................................................................................................... 19 DISCUSSION........................................................................................................................................... 21 CONCLUSION......................................................................................................................................... 23 REFERENCES .......................................................................................................................................... 24 Books ................................................................................................................................................. 24 Internet ............................................................................................................................................. 24

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INTRODUCTION 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 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 discontinuities 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 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. 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 porewater pressure when testing in undrained conditions, and possible spuriously high results from forcing the failure plane to occur in a specific location.

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THEORY Direct shear box The soil is contained in a box (Figure 1) which has a separate top half and bottom halves. A normal stress is applied onto the soil by placing weights on the lid of the box. The horizontal shear force needed to cause failure of the soil is measured.

Figure 1: Shear box test apparatus

Measurement of shear strength We carry out shear box tests on a soil with different normal stresses. We then draw a graph of shear stress at failure against normal stress

Figure 2: Shear strength of soil determined from the shear box test The shear strength of the soil (φ′) in the shear box is simply the angle under the graph as shown in Figure 2. It is usual to calculate the angle from the slope of the graph rather than to measure it, since

tanφ′ = slope of graph Page | 3

Sometimes the graph does not pass through the origin and the soil appears to have some shear strength at zero normal effective stress. This value of shear stress is called the cohesion c′ of the soil, measured in kPa. Soil cohesion should be used with caution in geotechnical design, it adds a lot of apparent strength to the soil but in many situations, particularly long-term, the cohesion may be less than apparent in the laboratory test and may even be zero.

Volume change in shear box As well as measuring the shear force and normal force during the shear box test, we also measure the vertical movement of the lid of the shear box (Figure 3). If the lid moves up during the test, the volume of the soil is increasing (dilation). If the lid moves down during the test, the volume of the soil is decreasing (compression).

Figure 3: Measurement of volume change in the shear box Usually dense soils dilate during shear and loose soils compress during shear. The reason for this behaviour is described below.

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The changing strength and volume of soils during a shear box test is best shown with typical graphs of shear box test results. Figure 4 below shows two graphs from a typical shear box test on a dense and loose sample of a soil. Both graphs have the horizontal travel of the top half of the shear box (x) on the horizontal axis. The first graph shows the shear force T needed to shear the soil during the test. In the dense soil, a peak shear strength is reached after which the shear strength of the soil reduces. The loose soil requires only a small shear force at first but the shear force increases as the test continues. Eventually, both graphs reach the same shear force and will remain at this shear force if the test is continued. Both the dense and the loose soil have reached the same critical state.

Figure 4: Typical graphs obtained from a shear box test The second graph shows the vertical movement of the lid of the shear box. You can see for the dense soil the lid may move down a little at the start if the soil is not perfectly dense but later the lid moves upwards as the soil dilates. The dilation does not continue forever but stops when the soil reaches the critical state. For the loose soil, the lid moves down as the soil compresses. Again, this does not continue forever but stops when the soil reaches the critical state. Both the dense and the loose soils will continue to shear at the critical state without changing volume (i.e. without the lid moving up or down).

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OBJECTIVE This test is performed to determine the consolidated-drained shear strength of a sandy to silts soil. The shear strength is one of the most important engineering properties of a soil, because it is required whenever a structure is dependent on the soil’s shearing resistance. The shear strength is needed for engineering situations such as determining the stability of slopes or cuts, finding the bearing capacity for foundations, and calculating the pressure exerted by a soil on a retaining wall.

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MATERIAL & EQUIPMENT

NO

NAME

1

Direct shear device

2

Load and deformation dial gauges

3

Balance

4

Shear Box And Sand

PICTURE

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TEST PROCEDURE 1. The initial mass of soil in the pan was weight.

2. The diameter and height of the shear box was measure. Compute 15% of the diameter in millimetres.

3. The shear box was carefully assembled and places it in the direct shear device. Then place a porous stone and a filter paper in the shear box.

4. The sand was placed into the shear box and level off the top. A filter paper, a porous stone, and a top plate (with ball) were placed on top of the sand.

5. The large alignment screws were removed from the shear box. The gap between the shear box halves were opened to approximately 0.025 in. using the gap screws, and then back out the gap screws.

6. The pan of soil was weight again and the mass of soil used was compute.

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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. The vertical load was set to a predetermined value, and the bleeder valve was closed and applies the load to the soil specimen by raising the toggle switch.

9. The motor were started 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. The readings were recorded on the data sheet.

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10. Continue taking readings until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 15% of the diameter.

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SAFETY AND HEALTH PROCEDURE 1. The screw in the shear box must be release before starting applying the horizontal load, otherwise, all the shear stress will be borne by the screws instead of the soil sample.

2. The thickness of the soil being placed in the shear box should be appropriate. To thin will make the sample shear off in an undesired plane.

3. The apparatus should not be started before the computer starts recording the data while the recording should not be stopped until the shear stress is constant or decreases.

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RESULTS AND CALCULATION: 1. Dimensions and Unit weight of soil sample A

B

C

Length of box (mm)

99.70

100.40

100.06

Width of box

99.80

100.00

100.06

9950.06

10040.00

10012.00

30.84

30.84

30.20

306859.85

309633.60

302362.4

512.5

517.1

504.9

16.3841

16.3831

16.3812

Applied vertical load (kg)

6.0

10.0

15.0

Normal stress (kPa)

60

100

150

Horizontal cross sectional area of box (mm2) Thickness of sand layer (mm) Volume of sand (mm3) Mass of sand (g) Unit weight of sand (kN/m3)

Average Unit Weight of Sand =

3

Dense sand: i) Sample A (60kPa)

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ii) Sample B (100kPa)

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iii) Sample C (150kPa)

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iv) Angle of internal friction of sand samples

Sample

A

B

C

Normal Stress (kPa)

60

100

150

Maximum Shear Stress (kPa)

59.2

97.7

147

Ultimate stress (kPa)

39.70

61.85

83.20

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From the above graph, equation relating shear stressed with normal stress: t = c + σ tan θ can be obtained. By setting c = 0, the equation obtained by the graph is y = 0.9798x tan θ = 0.19798

From the above graph, equation relating shear stressed with normal stress: t = c +σ tan θ can be obtained. By setting c = 0, the equation obtained by the graph is y = 0.583x tan θ = 0.583

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Loose sand: Sample A: (60kPa)

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Sample B (100kPa):

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Sample C (150kPa):

Angle of internal friction of sand samples Sample

A

B

C

Maximum Shear Stress (N/mm2)

41.7497

69.9899

71.8367

Normal Stress (kPa)

60

100

150

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From the above graph, equation relating shear stressed with normal stress: t = c +σ tan θ can be obtained. By setting c = 0, the equation obtained by the graph is y = 0.5618x

tan θ = 0.5618

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DISCUSSION 1. Specimen disturbed while trimming. The trimming of specimens must be done in the humid room with every care taken to minimize disturbance of the natural soil structure or change in the natural water content. As a rule, the effect of trimming disturbance is inversely proportional to the size of the specimen.

2. The area under the shear and vertical loads did not remain constant throughout the test, hence affecting the value of shear stress and normal stress calculated.

3. Specimen disturbed while fitting into shear box. The specimen must exactly fit the inside of the shear box to insure complete lateral confinement, yet a pre-trimmed specimen must be inserted without flexing or compressing. The specimen cutter must have the identical inside dimensions as those of the shear box.

4. Permeability of porous stones too low. Unless the porous stones are frequently cleaned, they may become clogged by soil particles and full drainage of the specimen inhibited.

5. Slippage between porous stone and specimen. When testing undisturbed firm or stiff clays, particularly under low normal loads, it may not be possible to transfer the required shear force to the specimen by means of the standard porous stone. In such a case, slippage of the porous stone will result and a portion of the shear force will be applied to the specimen by the rear edge of the upper frame. The slippage may be marked by tilting of the upper frame and the development of an inclined shear plane through the upper rear corner rather than one through the mid-height of the specimen. The use of dentated porous stones or of wire cloth or abrasive grit between the stone and the specimen may be necessary to affect the transfer of shear stress.

6. Rate of strain too fast. The time to failure in the drained direct shear test must be long enough to achieve essentially complete dissipation of excess pore pressure at failure. In general, it is safer to shear too slowly.

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7. The assumption of zero cohesion in the dry sand is invalid because there is a very small amount of cohesion actually. Also, the sand might not be 100% dry. 8. The test’s simplicity and, in the case of sands, the ease of specimen preparation.

9. The travel of the machine can be reversed to determine the residual shear strength values, which is shear strength parameters at large displacements.

10. Shear box represents a cheaper method in determining the drained shear strength parameters for coarse-grained soil. Preparing soil samples for other testing methods is relatively difficult and expensive.

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CONCLUSION In our experiment, the friction angle of the loose and dense sample has been tested. The peak frictional angle of the dense sample is 44.42° while the ultimate frictional angel is 30.24°. The loose sample’s is 29.33°. Some important findings are listed below: 

The loose sample only has an ultimate stress while the dense sample has peak and ultimate stress.



The vertical displacement of the dense sample is positive while the loose sample is negative.



The dense sample’s residual force is rather similar with loose sample’ ultimate stress.

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REFERENCES Books 1. R.F. Craig, Soil Mechanics, 7th Edition, Chapman & Hall, 2004 2. B.H.C. Sutton, Solving Problems in Soil Mechanics, 2nd edition

Internet 1. http://en.wikipedia.org/wiki/Direct_shear_test 2. http://www2.hkedcity.net/citizen_files/aa/az/gs55401/public_html/ShearStrength/shea r.html

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