Three different methods were used to measure stress in a deep tunnel under excavation, and the obtained values were compared. At two locations at a depth around 200m, the stress relief method (CCBO) and the hydraulic fracturing method (HF) were used. The stress states measured at these points have large horizontal stress components, and the maximum principal stresses were greater than the overburden stress. The stress states measured by CCBO and HF agreed well with each other. At a depth of location of 1,130m, CCBO and the Borehole Wall Strain method (BWS) were used. Assuming that the tunnel itself is a large borehole, stress measurements by BWS were obtained by gluing strain gauges directly to the tunnel wall and releasing stress only in that part. This BWS is a very unique and challenging method. Interestingly, the stress states measured by two different methods were in good agreement with each other.
In this study, rock stresses were measured during excavation of the Pahang-Selangor raw water transform tunnel in Malaysia. About 80% of 44.6 km was excavated by three tunnel-boring machines (TBMs) at the same time. This tunnel is very long and has an overburden of 1,246 m. To tunnel both safely and efficiently, it is extremely important to ensure the stability of the rock mass during excavation. Therefore, it was considered necessary to confirm the rock stresses that greatly affect the stability of the tunnel. The tunnel route consists mostly of granite. A schematic cross-section of the tunnel and the three measurement locations are shown in Figure 1.
The rock stresses were measured at three locations: two at the start of different TBM excavations at a depth of 200 m (shallow depth locations), and one at the middle of TBM excavation at a depth of 1,130 m (deep depth location). At the shallow depth locations, boreholes were drilled in three directions, at 90 degrees to the other. Two of these boreholes were horizontal and one was vertical. For the Compact Conical ended Borehole Overcoring method (CCBO), the horizontal borehole in the axial direction of the tunnel was used, and for the Hydraulic Fracturing method (HF), all three holes were used. CCBO used only one borehole because it is possible to measure a three-dimensional stress state in a single measurement. On the other hand, HF only measures a two-dimensional stress state in a single borehole, so the stress state in three dimensions was calculated by combining the data from the three boreholes.
