Solution Exercise 4
The minimum principal stress (σ3) has to be the same value as T (tensile strength of the rock) in order for extension fractures be formed. Consequently, joints are formed in the limestone when the value of σ3 is equal to −27.5 MPa, the tensile strength T of the limestone. By the same token, the value of σ3 for the sandstone should be −6.25 MPa.
The green and red Mohr circles drawn in the Mohr diagrams below represent possible stress conditions under which extension fractures are generated in the limestone and sandstone, respectively.
