This paper discusses the behaviour of large-span tunnels and caverns in hard rock in a major underground storage project. In-situ stresses were measured using hydraulic fracturing during site investigation and 3-D overcoring during construction, with both showing a horizontal stress ratio of 2–3 times the vertical stress. Rock reinforcement design was based on the Q-system combined with numerical modelling for special design cases. Tunnel design and performance were checked and monitored by in-situ instrumentation during rock excavation, including measurements of deformation, convergence, and load on rock bolts. Deformation measurements using borehole extensometers included one borehole drilled into the virgin rock before rock excavation, which provided a direct and very insightful comparison with the various deformation measurements made during rock excavation. These measurements coupled with numerical calculations have proven that the high horizontal stresses to be very favourable to the stability of the large openings.

Introduction

Large-span tunnels and caverns were excavated in a major underground storage project in Singapore. Due to the limited land space, even the use of underground space is subject to optimization (Zhao and Lee 1996). There is less flexibility in cavern planning and design considering joint orientation. As such, it is necessary to work around traditional rules of thumb. While no stability problems were expected during construction, rock deformation and 3-D stress measurements were carried out to confirm design assumptions and to optimise the subsequent design.

Engineering geology
Weathering trenches

The project site is located in the Bukit Timah Granite Formation, about 220 m years old. Site investigation discovered three large-scale weathering trenches in what were previously thought to be faults zones based on low seismic velocities from seismic refraction surveys. The strike and location of these trenches normally coincide with those of valleys, pools, or water convergence channels, where ground water is probably an important factor in creating these trenches. Table I summarises the key features of these weathering trenches. The maximum depth of the weathering trenches can reach 80 m below the surface. The strikes of these trenches coincide with that of the dominant sub-vertical joints.

The discovery of these weathered trenches has provided some essential input in the layout design and in precautions taken during construction in anticipation of bad rock and potential ground water problems.

(Table in full paper)

Rock joints and dense joint strips

The rock joints are typical of granite rock mass. There are three dominant joint sets, with two being sub-vertical and one sub-horizontal. Some random joints also exist. Most joints are tight with an average friction angle of about 37o. The filled joints, when wet, show a noticeable reduction in its joint strength.

Strips of densely spaced joints were observed on the quarry walls and in the core logs. The strips are normally sub-vertical and sparsely distributed at more than 5-m intervals. They are normally less than 0.5 m in width, with a few near the weathering trenches being some tens of meters in width.

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