Effect of weathering on the gravitational stability of Questa rock piles

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Courtesy of SoilVision Systems Ltd.

The Questa Rock Pile Weathering and Stability Project (QRPWASP) is a scientific research project whose purpose was to determine how and to what extent weathering affects the gravitational stability of the Questa mine-rock piles in 100 and 1000 years. The project studied a theoretical composite rock pile taking characteristics from several on site, and was not a site-specific engineering or regulatory analysis of a specific rock pile. Gravitational stability refers to the static stability evaluated along circular (and other-shaped) failure surfaces.

The Questa molybdenum mine (Chevron Mining Inc., formerly Molycorp, Inc.) is located in the Sangre de Cristo Mountains in north-central New Mexico and is on southward facing slopes at elevations of approximately 2290 to 3280 m. During the period of open-pit mining (1969-1982), approximately 317.5 million metric tons of overburden rock was removed and deposited onto mountain slopes and into tributary valleys, forming nine rock piles surrounding the Questa open pit.

SoilVision Systems Ltd. was entrusted with the numerical modeling of the hydrological and slope stability aspects of the waste rock pile. The research aspect of the project required the furthering of current state-of-practice in order to employ new technologies for the analysis of rock piles. The project required the inclusion of unsaturated aspects of slope stability analysis and therefore a corresponding calibrated hydrological model in order to accurately describe the distribution of pore-water pressures in the rock pile. The interaction between processes involved in the waste rock is illustrated in the figure to the right.

Numerical modeling of a rock pile at this level of sophistication had not previously been attempted. Therefore the project involved new challenges, which needed to be resolved through the implementation of new technologies in software. Some of the specific challenges and findings are outlined in the following sections.

Slope Stability

Gravitational stability analysis of the rock pile involved several unique numerical modeling challenges. A traditional total stress stability analysis would not account for the effects of suction on stability. There were also the questions of: 'Can a weak layer compromise the stability of a slope?' and 'What strength properties would a weak layer need to demonstrate in order to cause failure conditions to occur?'

Specific technical challenges for the slope stability model included:

  • Solution of exceptionally complex layered waste rock geometry.
  • Identification of critical slip surface in complex layered geometry using advanced searching techniques.
  • Advanced stress-based methods were needed to provide more in-depth insight into the analysis.
  • Methods of slope stability analysis needed to be combined closely with the hydrological analysis and resulting pore-water pressures on the site.
  • A method of performing statistical analysis of the large amount of data on the site was needed. Monte Carlo techniques were not possible due to the complexity of this site.

Specific new technologies employed in the project included:

  • Dynamic programming critical slip surface searching technique.
  • SVSlope limit equilibrium slope stability software package.
  • New APEM probabilistic slope stability analysis technique was applied to provide statistical evaluation of the factor of safety (FOS).
  • New critical slip surface searching techniques were employed.

A unique calibration opportunity arose as the rock pile slope was being 'flattened' by bulldozers. A crack formed during the 'pushing down' process and provided a unique opportunity to perform a 2D back-analysis, shown in the diagram on the right.

Hydrology

The purpose of the hydrological numerical modeling was primarily to perform a calibration to the existing flow in the waste rock system. Once a calibrated model was achieved it could then be further used: i) as the basis for prediction of future pore-water pressure stress states for input into slope stability modeling and ii) the model would then form the basis for future geochemical modeling.

The primary numerical modeling challenges were flow in a waste rock system being i) unsaturated and ii) the material was primarily coarse-grained. These two aspects typically lead to a high degree of non-linearity in solving Richards flow equation. New techniques were employed in resolving these numerical obstacles.

Specific technical challenges of the hydrological model were:

  • Numerical solution to highly non-linear gravel
  • How to represent 3D deposition structure in 2D
  • Numerical challenges associated with representing thin layers
  • Methodology needed to model long-term climatic impacts on the hydrology
  • Methodology needed to represent uncertainty
  • New methodologies for representing crusts needed to be implemented

A series of conceptual models were set up in SVFlux and solved. A number of improvements were implemented in the software package in order to i) improve numerical convergence and ii) handle the upper crust boundary condition in a reasonable manner. Automatic mesh refinement was used in order to handle the challenges of infiltration resulting from precipitation events along the upper boundary of the model. A methodology for handling uncertainty in the hydrological modeling was also established.

A key aspect of the project was that it was impossible to obtain a properly calibrated numerical model unless the crust at the top of the rock pile was properly represented in the numerical model. Several methodologies for handling crusts were developed as a result of this project.

Some of the finite element models solved for this project may be seen in the screenshots shown above.

The project was completed in December, 2008 and represents a landmark study which establishes the methodology for the analysis of future rock piles. 

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