Slope Design and Slope Stability

Supporting the Mining and Geothermal Industries Since 1994

Sims Professional Services

Quarries verses Mines

Intuitively, rock slope design considerations should be the same for small quarries as they are for large mines. However, the designs often differ because of local regulations that are imposed on quarries, and the training and experience of the geologists and engineers that perform the work.

Slope design is also a critical financial consideration for large mines and small quarries. A one degree difference in design slope angle can make a big difference in whether a slope stands or fails, and failures that disrupt operations can result in lost revenue and large cleanup costs. Also, if the design is too steep and the wall must step out into the mine to gain stability, the mine may lose millions of dollars of ore under the adjusted wall. On the other side of the equation, a one degree difference in slope angle can equal millions of dollars in excess stripping costs if the slope is too shallow.

It is the job of the slope design professional to provide the mine with an assessment of the relative risks associated with a range of possible design slope angles so that informed decisions can be made.

Slope Design Services Offered to Quarries, Mines and Exploration Groups

Evaluate existing geological and structural models for application to slope design
Provide mapping and interpretation services to support slope design and to define failure mechanisms for unstable slopes
Train mine geologists
Provide slope design recommendations for feasibility studies, existing operations and reclamation plans
Work with and support in-house or consulting engineers
Manage oriented core programs and geotechnical core logging
Expert witness testimony
Present technical arguments to local agencies where regulations are contrary to best mining practices or MSHA regulations

Some Mines Where Dr. Sims Provided Engineering Geology, Slope Design, Slope Monitoring and Training Services

Slope Design and Slope Stability - Metals, Aggregates, Industrial Minerals

Dr. Sims is an internationally recognized expert in the collection and use of structure data for rock slope design in mines and quarries. Since 1994, Dr. Sims has mapped over 200 linear miles of bench faces for slope design, modeled structural geology, performed stability analysis, monitored unstable slopes, supervised oriented core drilling, and trained numerous geologists and engineers. He performs these services at world-class metal mines and at aggregate and industrial minerals mines. He is coauthor of a widely cited paper that describes state-of-the-art methods for structure data collection and use in slope design (Nicholas and Sims, 2000).

The importance of structure data collection and interpretation in rock slope design cannot be overstated. When mine slopes fail, it is typically not an engineering mistake. Instead, the failure is owing to a lack of geological understanding. The engineer correctly analyzed geological conditions that were believed to exist, but did not actually exist.

Because structural geology is so important for slope design, and it is a specialized field, Safe Work Australia (similar to OSHA and MSHA), provides that "the task of developing the structural model (for slope design) is one for an experienced structural geologist. Exploration and mine geologists are an essential part of the modelling team, but the team leader should be a structural geologist who has the specific skills and the experience in structural geology. Information on collecting and using geologic structure data for slope design can be found in Nicholas and Sims (2000)." (Safe Work Australia, 2011, Ground Control in Open Pit Mines, Draft Code of Practice).

It is Dr. Sims' experience that many small mining companies underestimate the importance of having qualified slope design geologists and instead focus on slope design engineer's qualifications, whereas larger companies that have experienced many slope failures have a greater appreciation for engineering geology and collecting geotechnical data, even at the exploration stage.

Twin Creeks Mega Pit gold mine (Photo from DD LaPointe, http://www.nbmg.unr.edu/Photos.html). After a 2.5 million ton slope failure in December, 1994, Santa Fe Pacific Gold had Dr. Sims map every accessible bench in the mine to develop a structural model for slope design purposes.

The graph on the right demonstrates an example for a quarry wall that is 200' high and 300' long. It is assumed that the limits of the quarry are property boundaries and the quarry wall is in ore. Because local ordinances for quarry slopes often look to the International Building Code for guidance, which uses slope ratios in its slope design recommendations, civil engineers commonly provide slope recommendations as slope ratios that are in increments 0.5:1. In this example, a wall that is mined at 1.5:1 will recover about 107,000 yards less material than a wall that is mined at 2:1.

Mine design professionals typically do not base recommendations upon slope ratios at 0.5:1 increments because precision is much greater than what is allowed using that criterion. Instead, slope angles are expressed in degrees, shown as blue diamonds on the graph, and labeled on the horizontal axis.

The greatest similarity is that for all large open pit mines and relatively small rock quarries, rock slope stability is a critical safety consideration. All of these operations fall under the jurisdiction of the United States Mine Safety and Health Administration (MSHA), which requires that slopes must be designed to protect miners. If a slope is improperly excavated, MSHA can fine the operator, require modifications, and even order operations to cease. At the Federal level, these regulations are generally based upon best mining practices, which have been developed by mining professionals with the utmost concern for mine safety.

State laws and local ordinances may place additional restrictions on the design and excavation of mine and quarry slopes and these typically affect small quarries more than large mines. At the local level, regulations are likely to be based upon criteria that are developed for the design of civil projects under the International Building Code. One might think that a design based upon civil criteria should be expected to provide the best safety for miners, but that is not necessarily the case. Also, these civil design slopes often recover less material than a slope design based upon best mining practices.