Assertion (A): In the box shear test, the failure plane is predetermined and horizontal. Reason (R): The shear stress is applied in the vertical direction. Select the correct answer from the following:
A is true but R is false.
Understanding the Box Shear Test and Failure Plane
The question asks us to evaluate an assertion and a reason related to the box shear test, a common laboratory method used to determine the shear strength parameters (cohesion and angle of internal friction) of soil.
Analyzing Assertion (A): Failure Plane in Box Shear Test
Assertion (A) states: In the box shear test, the failure plane is predetermined and horizontal.
Let's consider the setup of a box shear test:
- The soil specimen is placed in a shear box, which is split horizontally into two halves.
- A normal load is applied vertically to the top of the specimen.
- A horizontal shear force is applied to the top half of the box, causing it to slide relative to the bottom half.
Because the shear force is applied horizontally and the box is split horizontally, the soil is forced to fail along the predefined horizontal plane located at the split of the box. This plane is fixed by the apparatus geometry before the test begins, making it a predetermined failure plane. Also, this plane is horizontal. Therefore, Assertion (A) is true.
Analyzing Reason (R): Direction of Shear Stress
Reason (R) states: The shear stress is applied in the vertical direction.
In the box shear test, the shear force is applied parallel to the intended failure plane. The failure plane in the box shear test is horizontal (as established above). Shear stress ($\tau$) is defined as the shear force divided by the area over which it acts. This shear force is applied horizontally to cause sliding along the horizontal plane. Normal stress ($\sigma$) is applied vertically, perpendicular to the failure plane. Therefore, the shear stress is applied in the horizontal direction, not the vertical direction.
Thus, Reason (R) is false.
Evaluating the Options
Based on our analysis:
- Assertion (A) is true.
- Reason (R) is false.
We need to select the option that reflects this combination. Option 3 states that A is true but R is false. This matches our findings.
Conclusion
Assertion (A) is true because the box shear test apparatus forces failure along a predetermined, horizontal plane. Reason (R) is false because the shear stress in a box shear test is applied horizontally, parallel to the failure plane, not vertically.
Revision Table: Box Shear Test Key Aspects
| Aspect | Description |
|---|---|
| Test Purpose | Determine soil shear strength parameters ($\phi$, c). |
| Sample Shape | Typically square or rectangular prism. |
| Failure Plane | Predetermined by the apparatus, horizontal. |
| Normal Stress | Applied vertically, perpendicular to the failure plane. |
| Shear Stress | Applied horizontally, parallel to the failure plane. |
| Parameters Obtained | Cohesion (c) and Angle of Internal Friction ($\phi$). |
Additional Information: Direct Shear Tests
The box shear test is a type of direct shear test. While simple and widely used, the predetermined failure plane is a limitation. In reality, soil failure in the field occurs along the weakest plane, which may not be horizontal.
Other types of shear tests like the triaxial shear test allow the failure plane to develop naturally within the soil sample, making them potentially more representative of field conditions for certain applications. However, the box shear test remains valuable for its simplicity and ability to provide quick estimates of shear strength parameters, particularly for sandy soils.
Important Questions from Shear Strength
Quick sand condition occurs when:
Field vane shear is the appropriate field test for obtaining the shear strength of which of the following?
Which of following tests is conducted to assess shear strength parameter of the soil.
In a direct shear test, the soil load is subjected to more stress at the _______.
Read the following statements and choose the CORRECT answer.
(i)The unconfined compression test is widely used for determining the consistency of saturated clays and other cohesive soils.
(ii) Unconfined compressive strength is thrice the value of the shear strength of clay soil under drained conditions.
