A panel block caving system operates at a depth of 4,250 ft. Dimensions of the panels are 150 ft by 300 ft in plan. The undercut is 21 ft high. Initial bulking porosity at the start of the cave, which forms rubble, is 0.37. The caved ground has a specific weight of 105 pcf; the angle of friction between muck and rock is 23◦, and the angle of internal friction of the muck is 33◦. The ratio of horizontal to vertical stress in the caved ground is estimated to be 0.32.

(a) Estimate the cave height possible before the swell must be drawn in order to maintain caving.

(b) What values of vertical, horizontal, and shear stresses are realized at the undercut level when the cave height reaches 600 ft?

(c) What bulking porosity is indicated when the cave just reaches the surface after drawing the necessary swell?