During excavation for a tunnel, 250 m below ground level, a highly fractured sil

Question

During excavation for a tunnel, 250 m below ground level, a highly fractured siltstone rock mass with two major joint sets and many random fractures is encountered. The RQD from the rock cores is 40% anf the average UCS is 70 MPa. The joints with average spacing of 75 mm are rather continuous with high persistence with apertures of 3-5mm, and they are filled with silty clay. The joint surface walls are highly weathered, undulating, and slickensided. There is some water inflow into the tunnel estimated as 15 L/min per 10 m of tunnel length, with some outwash of joint filings.

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

NOTE: The answer is 40, 1.7, 20+- 5. I just need to see the work. Thanks.

Explanation / Answer

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

1) uniaxial compressive strength - 70 MPa - 7 (rating)

2) RQD - 40% - 8

3) spacing of discontinuities - 75 mm - 8

4) condition of discontinuities - ** - 10

5) ground water - flowing - 15 L/min - 7

Total = 7 + 8 + 8 + 10 + 7 = 40

** = separation(3-5 mm) and is continuous = 10 (rating)

Therefore, the RMR is 40 which means a "Poor rock"

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