Optimization of Underground Caverns for the Relocation of the Sha Tin Sewage Treatment Works

Abstract

This master thesis "Optimization of underground caverns for the relocation of the Sha Tin Sewage Treatment Works? is written to look at the opportunity to build the new underground facility of Sha Tin Sewage Treatment Works more compact than what is planned at this stage. The preliminary design is seven parallel caverns of 25m span, built with a 25m pillar between them, and a minimum overburden of 25m. During the feasibility study a question was raised; Is it possible to build the facility with 3 caverns of 46m span instead of the seven 24m span caverns planned so far. This thesis has been written to look at the opportunity for building those three caverns with 46m span, and to see how small the pillar between them could be, to make the facility as compact as possible. The preliminary feasibility study has been used as a background to get an overview of the geological conditions in the proposed area, and these geological parameters have later been used in different analysis methods to look at the feasibility of the project with three caverns instead of seven. The minimum pillar width is found by using the calculation method tributary area learned from the mining industry. Possible unstable wedges have been found using the analytical software UnWedge, and support and stability issues have been looked at with help of the numerical modeling program Phase2.Through the background studies and the different analysis methods, it is at this stage found that it is feasible to build the facility with 46m wide caverns, and from analysis it has been concluded that it is possible to build the caverns with only 15m pillar between them. This is because of the good rock mass in the area, and the assumed favorable horizontal stresses. To be sure of this conclusion it is important to conduct stress measurements in the area before further deciding on the final design.