Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): In case of soils compressive normal stresses are taken positive.
Reason (R): Most of the normal stresses acting on soils are compressive in nature.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Soil mechanics reverses the usual sign convention because soils rarely sustain tension.

Answer 1

The Correct Option is A

Step 1: In soil mechanics, unlike in structural mechanics where tension is usually taken as positive, compressive normal stress is conventionally taken as positive. This makes assertion (A) true.
Step 2: Soil, being a particulate and essentially cohesionless-to-weakly-cohesive material, cannot sustain significant tensile stress, so almost all normal stresses that develop in a soil mass, overburden pressure, confining pressure, applied loads, are compressive in nature. This makes reason (R) true.
Step 3: Since compression is by far the dominant and physically meaningful type of stress in soils, it is adopted as the positive sign convention purely for convenience and consistency, which is exactly what (R) states. So (R) correctly explains why (A) is true.
Step 4: Both statements are true and (R) is the correct explanation of (A), option 1.