Suppose that entries and crosscuts are 9 m wide and that pillars are square and 9 m wide, as in Problem 6.21. Laboratory properties are the same as in Problem 6.19; depth and specific weight of overburden are also the same as in Problem 6.19. In addition, a joint set is present in the mine. Joint set dip is 60◦; dip direction is 45◦ clockwise from North.

(a) Find the joint shear strength necessary to just prevent joint slip within a typical pillar.

(b) If the joint is cohesion less and obeys a Mohr–Coulomb slip criterion, what joint friction

angle is needed to just prevent slip?

(c) If the joint is frictionless, what cohesion is necessary?