DirectShear test

ULTRA DESIGN AND RESEARCH COMPANY

PRESENTED BY: Yama Moheb

o Introductiono Procedureo Calculation

o Introductiono Definitions

Direct shear test

Shear strength in Soils

Titles:

Failure due to inadequate strength at shear interface

The strength of a material is the greatest stress it can sustain.

The safety of any geotechnical structure is dependent on the strength of the soil.

If the soil fails the structure founded on it can collapse.

Shear Strength:

ENGINEERS MUST UNDERSTAND THE NATURE OF SHEARING RESISTANCE IN ORDER TO ANALYZE SOIL STABILITY PROBLEMS SUCH AS;• BEARING CAPACITY•SLOPE STABILITY• LATERAL EARTH PRESSURE ON EARTH-RETAINING STRUCTURES

• PAVEMENT

Significance of Shear Strength:

• The shear strength of a soil is its resistance to shearing stresses.• It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles.• Shear strength in soils depends primarily on interactions between particles.• Shear failure occurs when the stresses between the particles are such that they slide or roll past each other.

Shear Strength in Soils:

Soil derives its shear strength from two sources:1. Cohesion between particles (stress independent component)

Cementation between sand grains Electrostatic attraction between clay particles

2. Frictional resistance between particles (stress dependent component)

Determination of Shear Strength ParametersThe shear strength parameters of a soil are determined in the lab primarily with two types of tests;1.Direct Shear Test2.Triaxial Shear Test

Direct Shear Test:• Direct shear test is Quick and Inexpensive

• Shortcoming is that it fails the soil on a designated plane which may not be the weakest one

• Used to determine the shear strength of both cohesive as well as non-cohesive soils.• ASTM D 3080

• The test equipment consists of a metal box in which the soil specimen is placed.• The box is split

horizontally into two halves.

• Vertical force (normal stress) is applied through a metal platen• Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen.

Digital Direct/Residual Shear Apparatus Complete with Lever Loading Assembly.220-240V 50/60Hz 1Ph.

Peak Strength

Residual StrengthShea

r str

ess

Test Data:

Volume change:

Procedure:1. Measure inner side or diameter of shear box and find the area2. Make sure top and bottom halves of shear box are in contact and fixed together.3. Weigh out 150 g of sand.4. Place the soil in three layers in the mold using the funnel. Compact the soil with 20 blows per layer.5. Place cover on top of sand6. Place shear box in machine.7. Apply normal force. The weights to use for the three runs are 2 kg, 4 kg, and 6 kg if the load is applied through a lever arm, or 10 kg, 20 kg, and 30 kg, if the load is applied directly.

Note: Lever arm loading ratio 1:10(2kg weight = 20 kg)

8. Start the motor with selected speed (0.1 in/min) so that therate of shearing is at a selected constant rate9. Take the horizontal displacement gauge, vertical displacement gage and shear load gage readings. Record the readings on the data sheet.10. Continue taking readings until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 15% of the diameter.

Calculations:1. Determine the dry unit weight, d

2. Calculate the void ratio, e

3. Calculate the normal stress & shear stress

Figures:1

Figures:2

Figures:3

References:

• ASTM D 3080• CE 3121: Geotechnical Engineering Laboratory• Soil Mechanics – Laboratory Manual, B.M. DAS (Chapter 15)• Soil Properties, Testing, Measurement, and Evaluation, C. Liu, J. Evett• Civil Engineering - Texas Tech University