Ground Freezing as a Principal and Remedial Method to Control Ground Water Inflows and Excavation Support for Deep Shaft Construction

Construction of a 13m shaft through approximately 100m of water bearing soils and another 100m of bedrock was attempted using the Slurry Diaphragm Wall Method with a combination of deep dewatering wells. During excavation within the slurry wall, unexpected groundwater inflows occurred in a structure of loose silty sands. Artificial ground freezing was selected as a remedial method to seal the leaking slurry wall, and as a primary excavation support and groundwater control method for the 30m of overburden material below the base of the wall. Design of the ground freezing system required analysis of both the structural requirements of the unsupported soil, the frost effects against the existing slurry wall and the groundwater control in the area of the breech in the wall. These design procedures are summarized, and construction and excavation techniques and corresponding schedules compared in this paper. Advanced drilling techniques, ground freezing quality control and general contractor procedures for efficient shaft construction are discussed.