A sandstone bed with RQD = 70% and = 26.0 kN/m 3 lies beneath 1.5 m

A sandstone bed with RQD = 70% and γ = 26.0 kN/m³ lies beneath 1.5 m of overburden soil. A 2.0 m × 2.0 m square foundation is to be placed on top of the sandstone rock (i.e., at a 1.5 depth below the ground level) to carry a column load. The unit weight of the soil is 18.0 kN/m³. Assuming the rock strength parameters from Problem 7.17, determine the maximum load that can be allowed on the foundation with the safety factor FS = 3. The compressive strength f'_c of concrete is 30.0 MN/m².

Problem 7.17

A sandstone rock specimen has q_uc = 50 MN/m² and ϕ' = 35°. Find c'.