This paper discusses the measurements of in situ stresses and their influence on the behaviour of large-span tunnels and caverns in hard rock in a major rock cavern project in granite rock. In situ stresses were measured using hydraulic fracturing during site investigation and then by 3-D overcoring during construction, with both measurements showing horizontal stresses two to three times the vertical stress. Instrumentation and monitoring included measurements of rock deformation using multiple borehole extensometers and convergence tapes, and load on rock bolts using strain gauges. Results of the deformation measurements show that cavern and tunnel roofs moved upward while cavern walls moved inward. Measured rock bolt loads suggest most of the rock bolts are loaded to about 25–40 % of their load capacity.
For rock caverns located in relatively shallow depths, the in situ stresses are often ignored in rock support design, especially when using rock mass classification methods. However, it has been demonstrated in recent studies that relatively high horizontal stress of a certain magnitude can be favourable to cavern stability. This favourable horizontal stress condition allowed the construction of the world's largest rock cavern of 61m span with only an average of 40–50m of rock cover [1]. In fact, Broch et al. [1] concluded that fully grouted rock bolts of 3.5m long spaced at 2.5m with 75–100mm thick steelfibre reinforced shotcrete would have been sufficient for the 61m span cavern. Myvwang [2] also presented several mining case studies where high horizontal stresses allowed 100 % pillar recovery, leaving stable caverns up to 65 m wide without any roof support. In a major underground cavern facility in granite rock in Singapore, in-situ stresses were measured using hydraulic fracturing during site investigation and 3-D overcoring during construction. The project involved construction of rock tunnels and cavern ranging from 10m to 30 m in span.
Figure 1 shows a composite profile combining results from the various geophysical surveys and drilling investigations. The residual soil overlying the bedrock varies in thickness from 3m to 62m, with the average being about 20–30m. The transition from residual soil to fresh rock is generally very rapid, with only 2–3 meters of heavily weathered granite. Other geological features include deep weathering trenches and intermittent sub-vertical strips of fractured zones. Fresh intact rock has a density of 2,650 kg/m3 and an average uniaxial compressive strength of 165 Mpa. Rock reinforcement included fully grouted rock bolts and steel-fibre reinforced shotcrete. The rock mass was classified according to the Q-system, which has been adopted as the method for support design in the project. Table 1 summarises the rock mass quality values based on logging of rock cores. Details of the site geology, design and construction, and rock support can be found in Zhou [3] and Zhou et al [4].
