Analysis of Potential Failure Mechanisms During the Construction of a Rockfill Dyke on a Submerged Tailings Beach

CIM Montreal 2003
Michel R. Julien,
Abstract The proposed dyke is a part of a tailings project to enclose the tailings disposal area within a lake thereby preventing movements of tailings. This dyke will be approximately 1,050 m long and will be constructed with rockfill. The dyke construction will progress from west to east with rockfill being dumped and spread to advance along the length of the dyke.

The dyke crest will be at elevation 521.5 m. This compares with the nominal lake water level that is approximately at elevation 520 m.

From the beginning of the dyke until the midpoint, there is an almost linear transition from a 1.5 m high dyke on a 35.1 m thick layer of tailings to an 18.3 m high dyke on less than 0.5 m of tailings. From the midpoint to the end of the dyke, it will be built directly on native silty clay and will vary from 18.3 to 15.2 m in height.

The tailings underlying the dyke are essentially a hydraulic fill resulting from past disposal consisting of stratified layers of fine grained soils. Therefore, the tailings are considered to be normally consolidated, compressible, and may develop excess porewater pressure on loading. The tailings overlie a thin layer of relatively soft native silty clay. The till beneath the native silty clay is strong and incompressible with respect to the tailings and native soil above.

After construction, as the tailings and silty clay consolidate, porewater pressures will dissipate and there will be an increase in effective stresses, and thus improve the dyke stability. Since the dyke stability will increase with time, the primary concern is its stability during construction.

As a result of construction activities, subsurface conditions and the dyke configuration, the following potential modes of failure were identified:
• Shallow slumping of the front slopes of the dyke.
• Deep seated failures through the dyke and into the underlying tailings or native silty clay.
• Failure induced by static liquefaction of the tailings.

Stability analyses were conducted on the above potential modes of failure. It was found that there is a high risk of failure induced by the construction activities. Finally, recommendations were provided regarding the measures that should be undertaken to increase the stability of the dyke and mitigate instabilities during construction.
Keywords: Tailings Dyke, Prevention of Failure, Dyke Stability, Excess Porewater Pressure, Modes of Failure, Dyke Comstruction, Deep Water
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