Prediction of Dewatering Related Settlement i n Waihi Township, New Zealand

Between 1995 and 1998, Woodward-Clyde (NZ) Limited (WCNZ) conducted a settlement and rebound study of the Martha Gold Mine and Waihi township. The objective o f this study was to support a resource consent application to extend the Martha pit.

Dewatering of the Martha pit has caused the groundwater level i n most of the soil and rock layers immediately adjacent to the Martha pit to fall. Lowering of the general groundwater level has resulted in an increase in the level of effective stress within these soil and rock deposits. This increase in effective stress, and four other unrelated factors, has caused some of the soil and rock layers around the pit to consolidate resulting in settlement of the ground surface.

In order to estimate the magnitude of settlement and rebound that is likely to occur in Waihi due to operating and decommissioning the Martha pit, WCNZ completed an extensive program of research, investigation, laboratory testing and modelling. This paper summarises the methodology and results of this work.

While there has been a change in effective stress leading to some consolidation of the ground surface, dewatering of the Martha Pit has not caused any structural distress in the township, and engineering predictions indicate that no distress due to this cause is likely in the future. The magnitude of dewatering related settlement measured in Waihi Township is low compared to that measured in New Zealand and the United States at sites which have been affected by fluid extraction.