Dealing with slope failures

Accurate tracking of slope movement is a significant step forward in ensuring safe mining operations. Reliability of the information recorded is fundamental for strategic decision making and slope failure prevention. Mining activities involve considerable risks to people and equipment.

Slope failures can result in serious injuries, accidents or even loss of life, and can lead to significant economic loss. These negative outcomes can be abated with effective risk management.

Through comprehensive planning and consultation with geotechnical experts, you can work to ensure accurate digging to design and uninterrupted day-to-day operations. However, as geotechnical constraints, geology, extraction methods, climate and weather events all have an impact on slope stability, the only way to reliably predict and prevent failures is by tracking the trend of movement through a dedicated monitoring system.

By investigating location, type of movement, material characteristics, and velocity, you can predict rock mass behaviour and further use the data to conduct stability analyses. Best industry practices include:

• Continuously monitoring and measuring ground movement for real-time reporting and analysis on captured data.
• Appropriate identification and reporting on geometry and geological structure of the mining area.
• Applying data correlations and analysis for presentation and forecasting.
• Designing and implementing risk management programs based on accurate data.

The geotechnical information extracted from a monitoring system allows mines to establish comprehensive structural databases, ensuring control measures are put in place to predict potential rock mass failures and consistently carry out safe mining. Present day monitoring systems allow mining operations to set custom thresholds for alarms, which trigger notifications when movement exceeds set tolerances. This enables management to implement safety procedures such as evacuation of personnel and equipment.

Real world insights

Slope failure analysis conducted in a quarry in New Zealand provided interesting insights. Data collected and parameters set, in Maptek PointStudio software (formerly I-Site Studio), allowed a 3-D model to be created from surface geometry to back analyze and illustrate the levels of stress acting on a failure surface.

In this scenario, an assumption was made that no groundwater pressure was applied. Back analysis of the slope failure was conducted using different combinations of Mohr Coulomb parameters.

Implementation of the third-party software tool TSLOPE provided a solution using Spencer’s method (see figure 1).

3-D spatial measurements and visualization

Interactive 3-D models are insightful tools for analyzing and reporting on captured monitoring data. Effective decision making is empowered by providing fast and accurate scenario visualizations, accessible to everyone who needs to know the status of the operation.

Detailed 3-D models provide valuable background information to:

• Define, simulate and test exploration models and design mine plans.
• Identify structural weaknesses and possible failures that may affect operational decisions.
• Devise effective productivity improvement systems and strategy.
• Design practical, achievable expansion strategies and appropriately allocate resources including forecasting machinery deployment.

Spatial modelling can be enhanced by heat map features that allow you to compare surfaces and display changes over time. When managing tailings dams for example, material and sediment build-up can be explored and presented in a high quality and easy to understand format.

This provides additional advantages in predicting slope movements and identifying areas that are expected to change.

Effective management of tailings dams is essential to mine site safety, productivity and compliance with environmental standards. Tailings dam walls and surfaces can be safely surveyed from a remote standoff to record changes in deposition as the material dries, shifts and settles. Operations can easily analyze changes over time, comparing sediment level to a designated plane to identify areas of recent build-up. This helps determine dam management measures, such as valve open/closure, to ensure sediment is evenly dispersed.

Laser-based stability monitoring is ideally suited to surface deformation and wall stability analysis. Movement can be tracked and trends identified for future design and planning. Results are easily visualized in 2-D and 3-D, and reports are generated quickly for communication and further analysis.

Advantages of the 3-D spatial measurement approach include:

• Monitoring and mapping structures and the surrounding area produce a true digital terrain model for contextual analysis of structures.
• Heat maps coloured by displacement or velocity provide an overview of surface movement.
• Time lapse videos allow you to see changes in material deposition before and after inflow/outflow, and use this information to predict changes in other areas.
• Single source collection of survey grade data can be applied to geotechnical studies and quantify volume of material moved.

Interpretation of patterns and history of movement

Continuous laser-based monitoring allows for accurate recording of wall and soil movements or other structural failure events. The acquired 3-D point cloud data provides the source for analyzing patterns and reporting surface movements. Accurate information is then available to guide safety management programs and protect personnel and equipment during active mining, including rehabilitation and remediation activities.

For instance, a copper mine in South Australia used Maptek Sentry to monitor wall movements during three-week remediation work after a wall failure from a steep wall 24 metres above the pit floor. The laser-based technology provided accurate point cloud data that was used for geotechnical analysis of the rockfall. The volume of the rockfall was calculated at 6,321 tonnes, and consequently 11,985 tonnes was removed during the remediation.

Remediation after a slope failure:

Scans at three-minute intervals allowed remediation work to continue on the west wall without compromising safety.

Routine monitoring and inspections are necessary for:

• Recording actual slope stability performance and real-time data.
• Identifying design elements that do not comply with industry standards.
• Ensuring timely intervention before any failure occurs. Historical comparison of data collected at different points of time and at various locations can be used to forecast and report on any ground movements and changes to slope stability.

Stabilization and safety management actions

A critical aspect of managing mining day-to-day is to monitor movement to establish the safety of the operation. Policies, procedures, tools and technologies are implemented to reduce interruptions to active mining, and ensure the correct information is collected at the right place at the right time.

What can be achieved with innovative technology:

• Survey measurements and monitoring data can be used for geotechnical analysis, helping to understand the cause of the failure and avoid future events.
• Animated zones can be generated to display movement over time for assessing failures in the context of site history.
• Intuitive 3-D models and visualization assist in identifying trends and rates of movement for implementing safety mitigation programs, such as removing excess material from slopes and pit floors.
• Remote, continuous monitoring methods capture accurate data, providing a diagnostic tracking tool with clear visualization and analysis capabilities for making reliable decisions.
• Rockfall data, including points of origin and final destination, can be measured, recorded and analyzed, to maintain a site rockfall database.

Innovation and beyond

The increasing number of innovative technologies and systems available in the market can be overwhelming. Managers and geotechnical team leaders look for a system that reduces the risk associated with the complex interactions of a fast-moving mining operation, and delivers accurate real-time (or near real-time) data without sacrificing system flexibility or reliability for wider mine measurement applications.

Key considerations for mine managers include:

• Capability to monitor multiple areas at the same time.
• Visualization of the data for decision making process.
• Compatibility or interoperability with other technical data systems at the site.
• Reliability and efficiency of the technology, including power consumption.
• Ability of the system to operate in rugged, extreme weather environments.
• Ease of set up and ease of use.

By ensuring a safe and healthy workplace environment, you contribute to a more efficient and productive mining industry.

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This article was provided by Maptek. See www.maptek.com for more information.

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