Risk Appraisal Approach of Open-pit Slope Design in Indian Mines

Mining is a high-risk business and, due to the fact that owners have an appreciation and an appetite for risky ventures, is often successful. In contrast, technical specialist is generally risk averse and have an appetite for technical excellence. In many instances, the slope angles are the dominant parameter that define the mineral reserve, and therefore become a critical decision for the owner. Suitable communication between the owner and technocrat is required to enable the best decision to be made on design slope angles.

2.1 Fundamental understanding of risk analysis

In this paper the concept of risk is addressed in terms of both chance of occurrence (likelihood) and consequence, where;

Risk = Chance of Occurrence × Consequence.

Risk implies about the future likelihood of the course of events. Therefore, the definition of risk in the context of mining would mean a certain knowledge or experience, which would be a guide to understand the likelihood of occurrence and the related consequences pertaining to a certain condition. Geotechnical Engineers, the primary designers of the open pit mine slopes analyse several factors and propose the probability of failures, factor of safety, etc. based upon the geotechnical characterization of the mine slopes and the previous knowledge obtained. The limitation with such approach is that the models proposed by them only infer that which is already known or understood.  If any new geological condition is encountered it becomes a difficult job.

Therefore the basic rudimentary question that had to be addressed during mine development is; “Has there been any previous experience with the geo-mining conditions of a particular mine slope?” In case of limited or no experience with identical geological condition, then there exists uncertainty or risk.

This index in reality can be attributed to the index of experience and expertise of the design engineer. It does not completely portray the risk associated with the slope but is just a preliminary assessment tool. Engineering judgement has to be taken at each stage of the design process, be it, ground characterization by borehole drivage or mapping for design analysis [4, 5]. The index is then used to map the probability of likelihood for occurrence of failure. It can thus be inferred that there exists an inherent uncertainty and risk even in the relatively uncomplicated engineering computations.

What can be conceived about consequence? In straightforward problems that can be practically calculated such as development of tension crack on the bench, the consequences can be effortlessly perceived and risk can be computed. However, the problems associated with large open pit mines are complicated. Some of them can be categorized under improper design of slope or bad decision of management. When these factors concur, the problem becomes apparent. It takes years for these kinds of problems to surface. In order to cater to such problems, one needs much experience to rightly anticipate, and comprehend the consequences. Hence, it can be said that the problems pertaining to open pit mine slopes are complicated and bear inherent significant uncertainty and risk.

The title of this paper Risk Appraisal Approach of Open-Pit Slope Design, implies the geotechnical input to a safe and efficient open pit mine is contained only in a single element, the pit slope design. However, it is fundamental to the understanding presented in this paper that geotechnical engineering for open pits has two components:

• Firstly, the pit slope design, which means the design of the environment from where excavation has to be done; and
• Secondly, the pit slope management, which includes numerous management techniques of the mining environment so that the overall risk is minimized, and the objectives for safe working environment is achieved.

With the passage of time, the domain of slope design has emerged to be a prominent area of research for mining professionals. The author has covered and answered certain important questions concerning risk related to safety and economy in the context of pit slope design and management.  

The design exercise that has been undertaken and proposed in this thesis renders the owner to efficiently decipher the level of risk in terms of steepest slope angle which is allowable to him and permits the mining professionals to develop the mine in that direction, thereby satisfying the risk criteria. These risk criteria are developed according to the consequences of potential failure that a mine can experience. In other words the risk/ consequence process espouses the mining venture of slope into a design criterion. As a result of it, suitable areas for geotechnical exploration can be identified in order to minimize risk.

From the Figure 1, it can be inferred that there is a need for a fresh initiative that can possibly reduce the upward trend of the fatal accidents and catastrophic disasters, which are occurring, in a repetitive fashion every year. If a hierarchy is drawn for prevention and control of accidents in open pit mines, the first priority will always be to avert disaster followed by fatal and serious accidents. The International Labour Organization (ILO) has undertaken a commendable initiative to attain zero harm and no injury in the working environment. It can be deduced from the above discussion that an integrated approach needs to be developed in terms of risk assessment and risk management techniques. As a result of this initiative, the accidents can be eliminated or reduced and minimal risk of accidents can be achieved at the vicinity of the work place.

2.2 Analysis of pit slope failures

The Indian Coal Mining Industry has experienced the pit slope failures at Dorli OC-I of M/s SCCL, SRP OC-I of M/s SCCL, Medapalli OCP of M/s SCCL, KTK OC sector-I of M/s SCCL Juna Kunada Colliery of M/s WCL Kawadi OCP of M/s WCL and Rajmahal OCP of M/s ECL. The Indian Coal Mining Industry is moving towards deeper opencast mines upto a depth of around 500m like Manuguru OC-II Extension and RG OC-II extension [6]. In India, Lot of accidents have been occurred due to slope failure (Figure 1). Some of the slope failures in India are shown in the Figure 2.

Figure 2. Photographs showing some of the pit slope failures in India

The analysis of about 100 slope failure cases by the author has converged on one common but a major factor causing the accidents that of the lack of scientific design and monitoring of the pit and the dump slopes in mines.

2.3 Pit slope stability analysis

In simple terms, slope stability is perceived to be the ratio of forces in resistance to the forces driving over an inclined surface that may lead to either collapse or slide [7]. Slope stability analysis comprises of significant steps, some of which are to observe and identify the mechanism of failure, identify critical location posing danger to slopes, identify critical factors stimulus to failure followed by optimal design of slope considering safety and profitable economics. Consequently, the engineering judgement has to be made on the basis of the assessment of the results of analysis, taking into consideration allowable risk or factor of safety [8]. The Figure 3 visualizes the flow chart for pit slope stability analysis.

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Figure 3. Flow chart for pit slope stability analysis

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Figure 4. Numerical analysis results of Dorli OCP-1 and Medapalli OCP of M/s SCCL

The evaluation of the stability of rock slopes is a critical component of open pit design and operation [9]. So, the Indian coal mining industry has identified optimum design of slope as one of the thrust areas.

Based on field observations and numerical analysis results of some of the above-said failures shown in Figure 4, it is concluded that the groundwater, higher slope angle and soft material like soil and clay leading to improper design and inadequate slope monitoring are the major influencing parameters for causing instability of pit slope.

Because of the unpredictability of slope behavior, slope monitoring can be of value in managing and preventing slope hazards, and they provide information that is useful for the design of remedial work. Slope monitoring is a key aspect of mining operations due to volatility of slope behaviour. It has to be done consistently. The information collected from monitoring of slope renders to manage and mitigate slope hazards. The system of slope monitoring can be classified into four categories. Figure 5 illustrates them as an observation through visual monitoring, surface measurements, subsurface measurements and remote monitoring technologies. In order to obtain an effective slope monitoring system, the idea of real time monitoring has been proposed which will be able to collect large volume of geotechnical information for the mine [10]. Through the analysis of Indian accident statistics, it is suggested to maintain well-developed drainage system in and around the open pit mine to avoid entry of rain/surface water into it to maintain pit slope stable and carry out continuous intensive slope monitoring to detect any instability well in advance [10].