Review and assessment of in-situ rock stress in Hong Kong for territory : wide geological domains and depth profiling

Abstract

In-situ rock stress is the essential data for the prediction of rock mass excavation behavior on underground excavation stability assessment. In-situ stress acts as part of the crucial input parameter for assessment of the excavation works design, design of support system, as well as pillar design. Understanding the magnitude and direction of in-situ stress state can facilitate the preliminary design of underground excavation opening, such as tunneling, mining and cavern.

With the pursuit of development of underground space aiming at promoting the enhanced use of rock caverns by the Government authorities, better understanding of the Hong Kong In-situ rock stress state would be beneficial the design of rock cavern and tunnel in the preliminary stage.

The main purpose of this thesis project is to update the map of orientation of the regional major principal stress directions in Hong Kong, and to review whether there is any major impact or influence on those stress magnitudes and orientations based upon the major geological domains in Hong Kong, as well as to explore depth relationships on principle in-situ stress orientation for consideration of future underground cavern development.

The in-situ stress data is acquired using hydraulic fracturing, conducted as part of site investigation for various infrastructure projects in Hong Kong in the past 30 years.

From the study, the following conclusion can be made:

··The stress ratio KH- and Kh-depth profile from the in-situ stress measurement yield similar results to the previous studies (Choy et al., 1997, Klee et al., 1999, Free et al., 2000, and Kwong and Wong, 2013). The stress ratio at shallow depth is high and scattered, and it decreases with depth. It is noted that most scattering stress ratios (KH > 7) at shallow depth are resulted from H&W-method in granitic rock. ··The hydraulic fracturing data yields an overall WNW-ESE orientation of maximum horizontal stress, which agrees well with the regional stress. The local orientation variation is likely under the influence of fault, dyke, volcanic layer or lithological boundary, instead of random scattering. The far field stress (regional stress field) might have been overprinted by near field stress, such as fault and dyke intrusion, although no significant bias could be observed. ··The lithological factor imposes no prominent effect on the stress magnitude. This can be attributed to the similar properties of the volcanic and granitic rock which have minor variation in rock density, strength and stiffness, as well as similar tectonic setting during their formation. ··Geological structures such as faults appear to have slightly more effect on the stress orientation variation than dyke and joint. There does not appear to be any effect on the magnitude of stress. ··No obvious depth relationship on principal in-situ stress orientation can be found from the data reviewed.