Shear Strength

Shear strength is the property that enables a material to remain in equilibrium when its surface is not level. (Smith, 2006) The direct shear test is used to determine the shear strength of soils on a predetermined failure surface. This test is used to measure the shearing resistance, undrained shear strength, and dilative and contractive tendencies in soils. (Fratta et al, 2007) The results can then be used to determine the allowable bearing capacity of foundations and stability of retaining walls, cuttings and embankments.

To determine the angle of shearing resistance of a sample of sand; the test may be carried out either dry or fully saturated but not anywhere in between.

Three samples of the same soil are placed into the shear box, which has two stacked squares to hold the sample. A confining stress is applied to the sample and the upper square is pulled laterally until the sample fails.

The normal stress applied, the vertical displacement and proving ring are recorded at regular and frequent intervals.

Normal Stress(kPa):25,050,075,0
Sheat stress at Failure (kPa):22,439,073,0
Area = 60 * 60 = 3600mm2 = 3,6 * 10-3 m2
σ1 = 25,0/Area = 6944,45 kPa
σ2 = 50,0/Area = 13888,89 kPa
σ3= 75,0/Area = 20833,34 kPa
Τ1= 22,4/Area = 6222,23 kPa

Τ2= 39,0/Area = 10833,34 kPa

Τ3= 73,0/Area = 20277,78 kPa

ɸ‘= tan-1 (6222,23/6944,45) = 41,86°

The relevance of this test is to determine whether the soil will have enough shear strength to support planned construction works upon it, such as excavation, embankments, roads, foundations and retaining walls.

The shear strength value that is calculated from this test allows the soil to be used in a way that will not be subjected to failure due to shear.

Using a 10x hand lens the following visual description was concluded:

To BS95930 :1990 the classification of the soil was determined as:
Pale brown well graded medium angular quartz sand.

The direct shear test is one of many test methods used to determine the shear strength of a soil. With the results of this test, the angle of shearing resistance (ɸ ‘) can be determined and used to calculate the active lateral pressure coefficient (Ka) which is then used in the design of retaining walls.

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