Analysis and Monitoring of Stage 5 Deformations at QAL Red Mud Dam No. 2

Connell Wagner was commissioned by Queensland Alumina Limited (QAL) to implement a monitoring program for the Number 2 Red Mud Dam, located 14km south of Gladstone. The aim of the program was to obtain data associated with stability, deformation, pore water pressures and seepage during and after construction several downstream raises.

Dam safety inspections and subsequent geotechnical investigations in 2001 showed that deficiencies in the clay core and/or the presence of a granular layer had caused the water table to rise within the embankment. To address potential problems, modifications were made to the proposed stage 5 raising and the frequency of dam wall monitoring was increased.

This paper presents the analysis and assessment of monitoring devices before and during the Stage 5 raising. As part of the monitoring program, finite element models were developed at the monitoring locations to determine the likely deformations of the inclinometers due to the Stage 5 raising and operation of the dam. A back-analysis was performed by calibrating the deformation analyses with the monitoring results at Stage 4 to obtain reliable soil elastic parameters for the finite element models. These were then able to provide an indication of the expected future horizontal and vertical deformations at each of the monitoring sites. Using the inherent variability of the soil parameters, trigger limits were derived to indicate procedures to be followed if specified criteria is exceeded.