Direct Shear Test

A H M 531The Civil Engineering Center

Objective: To obtain the parameters of shear strength (C,) for a cohesion less

sample using the direct shear test.

General discussion: Failure on any plane of any material occurs because of a critical combination of normal stress and shearing stress, where the failure plane can be represented by Formula 1; * This formula defines the failure envelope of a soil sample. When testing soils for their shear strength, we aim to fine the two parameters of shear strength defined in Formula 2; * Which are cohesion and the angle of internal friction. These two parameter are found by plotting the shear failure envelope, in which the slope represents the angle of internal friction and the Y- intercept represents the cohesion. Graphing the shear failure envelope requires knowledge in the relation between shear strength and the normal stress, so tests are done to find out how shear varies with varying normal stress on a sample. At least three tests on three samples shall be done to check for test errors and sample anomalies. Then on a coordinate axes, shear strengths and corresponding normal stresses are plotted and connected with a straight line representing the shear failure envelope. Direct shear test is useful when cohesion less soils are to be tested. In this test the failure plane is forced to occur at a predetermined location where both normal and shear stresses are acting; the sample is placed in a closed shear box, fixed at the base with the top free to translate under a horizontal force. The two portions of the box are spaced by using spacing screws to reduce the friction. The space should be at least as large as the largest sand particle. The box is then placed in the direct shear apparatus, and increasing horizontal load is applied with constant corresponding vertical load, and the horizontal deformation shall be recorded by using the dial gage. For each test shear stress-strain diagram is drawn in order to find out the ultimate stress, then the shear failure envelope is drawn by relating each

A H M 531 Direct shear test By: Ahmed Essam Mansour

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ultimate shear stress to the normal stress corresponding to it in at least three tests. In case of testing the sand, the sample can be reused for the repeated tests. Direct shear test may be categorized as follows:

1. Unconsolidated-Undrained (UU tests)

2. Consolidated-Undrained (CU tests)

3. Consolidated-Drained (CD tests) In our test we made the UU test.

Sample identification: Cohesion less sand sample.

Equipments:

1. Direct shear device

2. Two dial gages

3. Direct shear box

Procedures:

1. The shear box was assembled carefully and then placed in the direct shear apparatus.

2. A normal load of 1kg was placed by means of using a lever arm.

3. Increments of shear stress were applied, and horizontal deformation corresponding to each load increment was recorded of the answer sheet.

4. A new normal load was placed on the lever arm, which was twice as large as the prior one, and then step 3 was repeated.




Calculations:




Discussion and results: It can be obviously seen than when increasing the normal stress on the sample the shear strength is increased. The stress strain diagrams was erratic, and there existed a cohesion for the used sample which makes no sense as our sample was cohesion less sand and the cohesion should equaled zero. This can be a result of making two tests only on the sample. Sources of Error:

1. Only two tests were made, and so only two points was available to graph the shear failure envelop, which is not sufficient for checking the errors and anomalies.

2. The mass of the load block and upper half of the shear box was not included as a part of the sample vertical load.



Direct Shear Test

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