Basics of Soil Mechanics- Shear Box Test

Question 1

What does the shear box test provide information about?

(2)

Answer 1

• Provides information about the resistance of soils:

1. The cohesion c’ and the friction angle φ’ for cohesive soils
2. Friction angle φ’ for granular soils

Question 2

What is step 1 in the shear box test?

(3)

Answer 2

1. Soil sample placed in a stiff metallic box composed of two separate parts, sandwiched between two porous stones
2. A top cap is then placed on top of the upper porous stone
3. The whole system is fixed to a supporting frame

Question 3

What is step 2 in the shear box test?

Answer 3

Total vertical stress is applied to the soil sample and it is maintained constant during the consolidation stage and the subsequent shearing stage.

Question 4

What is step 3 in the shear box test?

Answer 4

The upper part of the box is pushed forward relative to the lower part which is fixed to the testing frame. This produces shear failure.

Question 5

Once the test is repeated 3 times at 3 different levels of vertical σ’, sketch a graph of the results for cohesive soils.

Answer 5

Question 6

Once the test is repeated 3 times at 3 different levels of vertical σ’, sketch a graph of the results for granular (cohesionless) soils.

Answer 6

Question 7

How do we calculate:

• Shear stress undrained (short term)
• Shear stress drained (long term)

Answer 7

• Short term = Shear stress (τ) = C_u
• Long term = Shear stress (τ) = C’ + σ’tanφ’

Question 8

What are the strength parameters we obtain for granular and cohesive soils from the shear box test?

Answer 8