Rock mechanical analysis of underground caverns at Shatin wastewater facility

Abstract

This master thesis Rock mechanical analysis of underground caverns at Shatin Wastewater facility is written to evaluate the stability of the planned facility, based on the design and layout at the feasibility study level. The rock mechanical analysis will be based upon empirical and analytical tool that are available. The output of the tools will be compared to build a robust base for evaluation and choice of layout. The thesis includes an overall evaluation of the underground cavern stability based upon empirical methods with respect to the planned layout of caverns and pillar. This evaluation has been combined with numerical modelling to make an engineering geological assessment. Additionally the thesis discusses measures to problems identified by the rock mechanical evaluation. A numerical model has been established for a layout representative for the overall situation in the underground facility. The models were constructed based on a rock mass with the strength parameters deemed most realistic, and for a possible weaker rock mass. Two different in-situ stress ratios were analysed. Models has been established both for a initial rock support design based on the Q-system, and for a rock support design proposed by AECOM in the preliminary design report. The Preliminary Design Report is used as source for geological data and planned design layout for parameters used in the analysis. The tributary area method has been used for estimating pillar capacity and the numerical modelling program Phase2 version 8.0 has been used to analyse and evaluate the cavern construction. The available geological information indicates that the rock mass in the area is strong and suitable for construction of the facility, but the rock support may experience yielding in areas with large stress redistribution during the excavation. Precautionary measures should be implemented to avoid this and achieve a secure facility. Additional information regarding in-situ stress and material strength at the planned cavern site should be obtained before deciding upon the final design.