Rock-Properties Estimation by TBM Cutting Force

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3. Niken-Goya tunnel

The Niken-Goya tunnel is an aqueduct tunnel for a hydraulic power station located in central Japan. The length of the tunnel excavated by TBM is 4.7 km. This tunnel was excavated through primarily sandstone (compressive strength 100 - 150 MPa) and slate (50 - 100 MPa). A semi-shielded type TBM having a diameter of 2.75 m was used to excavate the tunnel.

For sandstone, the maximum compressive strength is 150 MPa, and maximum FN/p is about 900 MN/m. Substituting these values into Equation (5) yields a c1 of 6 m. For slate, the maximum compressive strength is 100 MPa and the maximum FN/p is 600 MN/m. Again, substituting these values into Equation (5) yields a of 6 m.

The rock strength estimated using Equation (5) for a section of the tunnel (from 800 m to 1,000 m) is indicated by a rigid line in Fig. 1. Almost of bedrock, from 825 m to 875 m, is hard sandstone. Estimated rock strength over this tunnel distance is relatively high, and reaches maximum value of 120 MPa, as shown in Fig. 1. Weathered slate is present at various locations throughout the observed section, causing rock mass strength decrease at 840 m and 860 m. Slate from 900 m to 930 m is fractured and very soft, and the estimated rock mass strength reaches a minimum value of 15 MPa at 920 m. Rock from 930 m to 1,000 m is composed of alternating strata of sandstone and weathered slate. Rock mass strength is relatively low, but increases greatly at a few locations, perhaps due to the presence of hard sandstone. As mentioned previously, rock strength estimated using the Equation (5) agrees well with a previous bedrock survey.

Rock strength estimated from torque is indicated by a solid line in Fig. 2. Constant C2 is calculated in a similar way to c1. Comparing Figs. 1 and 2, rock strength estimated from torque agrees with that estimated using thrust. From Equations (5) and (6), two values of rock strength can be estimated independently. The two rock estimates were found to be identical, indicating the reliability of the proposed method.

A Schmidt-hammer rebound hardness test was performed on the side wall of the tunnel at every 5 m. The relationship between Schmidt-hammer rebound hardness and rock strength Q (MPa) depends on rock type. Thus, in the present study, following equation is assumed:

log(Q) = 0.0165 S + 1.13 (7)

Rock strength estimated using the Schmidt-hammer rebound hardness is indicated by solid circles in Figs. 1 and 2. Because rock from 900 m to 925 m is very soft, the Schmidt-hammer rebound hardness test cannot be performed over this section. The estimated strength reaches a maximum (110 MPa) from 825 m to 875 m and a minimum (25 MPa) at 925 m and 950 m. Figs. 1 and 2 show that rock strength estimated using thrust or torque agree well with that estimated using the Schmidt-hammer rebound hardness test.