A large mechanized cut-and-fill stope in hard rock is cable bolted on a square pattern as shown in the sketch. A single 5/8 in. (1.59 cm) diameter cable bolt is installed per hole; cables have an ultimate strength of 56,000 lbf (251kN). Bond strength between grout and cable steel is 700 psi (4.83 MPa). Shear failure is expected to occur at the steel???grout interface rather than at the grout???rock interface.

(a) How many inches of hole length are required to provide anchorage capacity in shear equal to the bolt tensile strength?

(b) Develop a formula that relates the thickness h of a slab that could be supported in the stope back to the spacing S of the bolts assuming that there are n bolts per hole.

(c) Plot the formula with h on the x axis and S on the y axis for slab thickness ranging from 0 to 10 ft (3 m); use the number of bolts per hole as a parameter and plot curves for n = 1, 2, 3, and 4.

(d) If the spacing is 10 ft (3 m), what slab thickness could be supported with two bolts perhole?

S= spacing Fill