A sandstone rock mass with RQD of 76% has two joint sets and some random fractur

Question

A sandstone rock mass with RQD of 76% has two joint sets and some random fractures. The joints spaced at 130mm are generally in contact, with less than 1-mm aperture. The joint surfaces are slightly rough and highly weathered with no clay found on the surface. The uniaxial compressive strength is 95 MPa. The excavation is being carried out at a depth of 110m below the ground level, and the water table is at a depth of 15 m below the ground level.

Estimate the RMR, Q, and GSI. Assume a unit weight of 28 kN/m^3 for the rock mass.

Explanation / Answer

From the Rock Mass Rating System table, ratings has been calculated

1) uniaxial compressive strength - 95 MPa - 7 (rating)

2) RQD - 76% - 17

3) spacing of discontinuities - 130 mm - 8

4) condition of discontinuities - ** - 20

5) ground water - flowing - 0

Total = 7 + 17 + 8 + 20 + 0 = 52

** = slightly rough and highly weathered = 20 (rating)

Therefore, the RMR is 52 which means a "Fair rock"

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