Rock Load Transfer Mechanisms and Interactions at Cavern Junctions

Abstract

Rock at depth is subjected to stresses resulting from the weight of the overlying strata. When an underground opening is excavated, the stress field in this rock mass is locally disrupted and induces a new set of stresses surrounding the new opening. At tunnel and cavern associated junctions, the re-distributed stresses will alter the stress fields of adjacent openings. For example, loadings from a taller cavern will be transferred through the rock arch and concentrated as additional vertical stress above the crown of the shorter cavern. The load transferring mechanisms in this paper refer to the construction of the cavern complex, which involves developing new sewage treatment works in caverns to be constructed at Nui Po Shan, A Kung Kok, Sha Tin, to replace the existing Sha Tin Sewage Treatment Works (STSTW). Upon functioning of the new STSTW, the existing site will be released for other uses beneficial to the development of Hong Kong.The works at the new STSTW occupies about 14 hectares in the area comprising of Main Access Tunnel (MAT), Secondary Access Tunnel (SAT), fifteen Process Caverns, the Main Driveway (MD), Secondary Driveway (SD), four Branch Driveways, Ventilation Shaft, Ventilation Adit, two Effluent Pipelines, and lining and portal structure of MAT and SAT. These structures are excavated mainly by the drill-and-blast method in hard rock, with rock covering more than half of the excavation span/height above the crown. They are designed as drained and are primarily supported by the rock arch, reinforced by systematic permanent rock bolts with permanent sprayed concrete. In addition, drained cast-in-situ reinforced concrete lining is proposed for poor ground conditions.For the proposed cavern complex, most of the Branch Driveways are taller than Process Caverns and MD/SD except for the middle cavern for sludge treatment (STC) purposes. STC's design span and height are 30 m and 35 m, respectively. Therefore, additional stresses are expected to transfer from Branch Driveways and STC to other Process Caverns and MD/SD. Numerical modeling using finite element methods has been established, where two-dimensional design models and three-dimensional verification models in accordance with the varying excavation profiles, overburden depth, and rock mass quality have been carried out. By observing the stress redistribution from the taller STC to other Process Caverns, the two-dimensional and three-dimensional models aim to study the stress concentration zones and the extent of the influence zone at tunnel and cavern associated junctions. The maximum deformation is located along with the crown of STC and intruding corners at the associated junctions, in which the Process Caverns with the largest excavation span and height are proposed.This paper provided a detailed description of the geology, cavern complex geometrical arrangements, rock mass properties for the modeling, methodology of modeling, and mechanism of load redistribution observed at the junctions.