Table of Contents
A spent ore pile is located approximately 2,000 m. southwest of the mine and approximately 1,000 m. northeast of the active pad and plant facilities. The pile was constructed on a gentle northwest slope dipping 12 to 13 degrees. The pile is bordered by a small river to the northwest and by a larger river to the northeast. Both rivers eventually flow into Bolivia.
In early 1991, elevated cyanide levels in surface water samples collected from the adjacent rivers was identified. The logical source of cyanide appears to be associated with leakage from the pile or the active pads.
Soil Index Test
Laboratory index tests were conducted on samples recovered from the test pits. Select but representative samples were subjected to the following index tests:
- Moisture Content (ASTM D2216)
- Grain Size Analysis (ASTM D422)
- Atterberg Limits (ASTM D4318)
- Specific Gravity (ASTM D854)
Interface Shear Strength Tests
Most direct shear interface tests on goesynthetic liner materials are conducted with rigid backing material. For application were the subgrade for a liner is prepared with extreme care these types of tests are expected to produce appropriate design values. However, in applications where the subgrade is not carefully prepared and there is a certain amount of texture to the subgrade or undulation, rigid backed test results may be conservative or inappropriate for design.
Preliminary Remedial Design
Five remedial design options were evaluated in detail including:
- Surface Water Diversions and Groundwater Dam
- Interceptor Trench
- Groundwater Dam
- Well Points
- Well Points with Slurry Wall
Stability Assessment of Current Spent Ore Pile
Existing Conditions and Objectives
The interbench slope are constructed at their angle of repose which is generally in the range of 36 to 40 degrees. The pile was constructed on a gentle slope extending down to the adjacent river. No signs of instability in the current pile were reported by the owner or observed during the field study.
The liner system beneath the spent ore pile consists of either a HDPE or PVC liner sandwiched between two layers of geotextile. The original design was based on the assumption that a fine sand would be placed directly above and below the liner. However, a field modification replaced the sand layer with a geotextile. In the upper portion of the pile a series of benches were excavated into the subgrade to facilitate placement of the pregnant solution collection system.
Review of existing reports indicated engineering properties of the spent ore and subgrade materials. The strength parameters for the geosynthetic interfaces were developed form back-analysis of the existing pile assuming a factor of safety of 1.0 (lower bound strength parameters) and the direct shear testing conducted for this study.
All of the material properties used in our analyses are summarized on Table 2.
The HOEK-SARMA program just analyzes sliding block failure planes using the Sarma method of analysis. Two cross-sections through the spent ore pile were constructed and analyzed. Due to the presence of the underdrain system and the fact that they are not currently washing the pile, the water levels in the current pile are expected to be low, near the base of the liner.
Stability Analysis During Washing Activity
To determine the affects washing would have on the pile in its existing configuration stability analyses were conducted using water levels of 5, 10, 15, and 20 meters. The results indicate a factor of safety ranging from 0.74 to 0.93 depending on the depth of water. Based on these results it is highly probable that the pile in its current configuration would fail during the washing operation.
Stability Analysis With Toe Buttress
The simplest method to increase the stability of the pile is to place a buttress at the toe of the pile. The buttress acts to increases the resisting force without a corresponding increase in driving force.
Displacement Under Dynamic Loading
Displacements under dynamic loading from the Maximum Credible Earthquake (MCE) were determined for two cases; case 1-the pile in its current configuration (under drained condition without washing) and; case 2 the pile with toe buttress configuration 1 (assuming drained conditions).
If a buttress (buttress configuration 1) is constructed at the toe of the pile, both the static and dynamic stability will be improved. However, if washing operations result in a substantial head buildup on the liner and a major earthquake occurs then substantial displacements may take place that could rupture the liner.