The Stability of Very High Spoil Piles

ACARP Project C20019 was commissioned to investigate the strength of spoil materials and groundwater conditions within spoil dumps, and provide guidance for reliable geotechnical design. The design reliability issue arose because current designs are based on precedent testing and experience when dumps were much smaller and the consequences of dump instability were simpler to manage. Embankment dam engineering knowledge applied to spoil as a rockfill material indicated that spoil strengths could be significantly lower under the high stress states within modern built or planned dumps.

The report consists of 8 chapters covering different aspects of the project investigations and findings:

(1) Introduction and Background;

(2) Degradation of Mine Spoils;

(3) Moisture Conditions in Mine Spoil Dumps;

(4) The Shear Strength of Partially Saturated Spoils;

(5) The Tilt Test;

(6) Design and Construction of the Large Direct Shear Machine;

(7) Shear Strength Testing;

(8) Implications for Reliability of Stability Analysis for Very High Spoil Dumps.

Each chapter has its own introductory synopsis, to which the reader is directed for an overview of the report details for each aspect. The central achievement of the project was the generation of reliable strength data for spoils at stresses equivalent to modern spoil dump design demands, which are much higher than had previously been attained in geotechnical testing of rockfill. In addition to strength testing, a range of studies were carried out to provide explanations for the roles of unsaturated water content and particle characteristics in the mobilised strengths.

A key finding from the project was that certain types of routinely encountered spoil materials do not have the previously expected rockfill curvilinear strength envelope characteristics. For these spoil materials, which display a limited sensitivity to particle wetting effects, the existing design knowledge based on the BMA Coal categorisation process is adequate and reliable, and the project data suggests additional benefits for efficient designs.

Identification of spoil materials that are compliant with the BMA Coal category strengths is readily achievable using simple criteria described in the report. For materials that do not comply with these criteria, determination of adequate and reliable strengths will depend on the nature of the materials, and the report provides examples and specific guidance. Spoil strength under the very low stress conditions that exist at the toes of dump profiles is also reviewed and specific guidance for design purposes provided.

Reliable information regarding the movement of water and role of groundwater pressure in dump profiles is difficult and expensive to obtain. The report provides a detailed review of spoil groundwater conditions based on both theory and observations. Specific guidance for reliable representation of spoil groundwater for design purposes is also provided.

The project outcomes are expected to have a significant influence on future spoil dump design, because they highlight significant limitations in the current design approach and provide clear guidance for designers to identify and rectify these limitations for specific design applications.