BowTie Analysis

BOW TIE ANALYSIS Pit Wall Collapse

Abstract Bow tie diagrams have become popular methods in risk analysis and safety management. This tool describes graphically, in the same scheme, the whole scenario of an identified risk and its respective preventive and protective barriers.

Jessica Morgana Ribeiro Santana Mining Engineering

Introduction Walls on a mine are a very sensitive case. It should be taken seriously and risk assessment to it is indispensable considering the fact that without stable walls a mine cannot work properly and a wall collapse it a very serious accident that should be avoided at all costs. A wall collapse on a mine can have many ramifications including: 1. Loss of life or injury to persons working or visiting the mine (Safety factors); 2. Loss of worker income, loss of worker confidence, loss of corporate credibility, increased legal liability (Social factors); 3. Disruption of operations, loss of ore, loss of equipment, increased stripping, cost of clean-up, loss of markets (Economic factors), and 4. Collapse of nearby infrastructure/facilities into the open pit, for example, mine waste dumps, tailings storage facilities etc, and interference with natural drainage (Environmental factors). It is recognised that mining experience and professional judgement are important aspects of risk assessment that are not easily quantified, but can contribute significantly to the formulation of various acceptable and equally viable solutions to a particular mining problem. Management at each mining operation should recognise, identify and address the issues that are unique to each particular mine, using current geotechnical knowledge, methodology, software and hardware appropriate to the situation. Using the bowtie to analyse the hazards regarding the dependence of a stable walls system allows the visualization of the relationship between undesirable event, its causes, accidental scenarios, the preventive and mitigation measures to limit their consequences, demonstrates the effectiveness of existing controls and it is affective on structured risk analysis where quantification is not possible or desired.

Threats & Preventions It is obvious that the level of exposure of the workforce to potentially hazardous conditions will govern the occurrence of injuries and fatalities at a mine site. It is also obvious that in order to reduce the level of exposure to hazards, a system must exist whereby hazards can be systematically recognised/identified and managed. This system of hazard recognition and management (in this case wall control hazards) should be incorporated within the overall mine plan. The implementation of an allencompassing mine plan presents a major challenge for mine management. A sound understanding of the mining site conditions is vital for the selection of the most appropriate mine design, mining method(s) and risk management for a new or existing mining operation. The level of risk, both in human and economic terms, will be substantially increased if the conditions of the mining site are not sufficiently well understood.

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The top event in this assessment in the collapse of the wall in mining, which can be trigged by many causes, some of them are listed below: 1. Rock fall of accumulated weathered rocks: can be prevented by removing all the unstable rocks that can cause a toppling. 2. Water flows: monitoring the weather forecasts, analysing the subterranean waterflows to avoid any surprises and install drainage bores at some strategic places should avoid that. 3. Blasting: in order to avoid accidents due to the blast activities, reinforce the walls in order to be more resistant to the seismicity that the blasting causes, calculating properly the blasting to avoid mistakes are storing the explosives in a blastproof area should be enough 4. Defective construction of walls: A inspection should be done do make sure that proper materials are be used to construct the walls taking into account the strength of materials and also a preventive slope flattening. 5. Seismic events: Monitoring the seismic events in the area of the mine to know what to expect and having a good plan to take action when a seismic event occurs are good preventive measures. 6. Improperly used or malfunctioning mining equipment: having trained people to work with the equipment and having at least monthly inspections on all the equipment should be enough to avoid the consequences.

Recovery Measures & Consequences If the controlling measures were not enough to avoid the top event (collapse of the walls), then recovery measures should be done. And it is important to have them planned before it happens, that is the importance of a risk assessment, knowing what can happen and using the bow tie it is even more important because it makes clear what should be done after the accident happened. A few of them are listed below: 1. Use of rock mesh and catchment fence: because of injuries to the stakeholders; 2. Ditching/benching: because it is necessary to remove fallen debris; 3. Sotckpilling, an emergency clean-up plan and backup equipment: in order to avoid loss of production; 4. Construct rock fall barriers and having a protocol for equipment storage: because debris can crush the equipment; 5. Having a fast emergency team response and personal protective equipment: in case of debris crushing workers. In order to detect instability at an early stage, a sufficient monitoring system should be created to allow the formulation of safety measures. Monitoring after the accident happens it is not very effective since it does not undo the damage cause by an

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unexpected failures. The purpose of a monitoring plan (adapted from Call & Savely, 1990) is to: 1. Maintain safe operational practices for the protection of personnel, equipment, and facilities; 2. Provide warning of instability so action can be taken to minimize the impact; 3. Provide crucial geotechnical information to analyse the walls collapse mechanism and design the appropriate corrective measures. The following steps should be taken when planning the instrumentation portion of the monitoring program: 1. Understand the mechanisms that cause instability; 2. Define and prioritize the geotechnical information required; 3. Establish monitoring locations; 4. Predict the magnitudes of movement and other parameters at these locations.

“Holes in the Swiss cheese” Doing a risk management is doing a systematic use of available information to determine how often adverse events may occur, the magnitude of their consequences and how acceptable the resulting harm may be. But still, sometimes there are some unexpected outcomes and the consequence of that is likely to be serious, such as death or serious injuries. That is what the James Reason’s “Swiss Cheese” model is about, it is not accepting the risks. If the holes in the cheese line up, the accident definitely is going to happen.

Figure 1: James Reason’s ‘Swiss cheese’ model. Redrawn from James Reason’s 1991 revised for the 2006 MineSafe Magazine

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A lot of shortcoming (holes in the Swiss cheese) can happen when it comes to pit wall stability, for instance: 1. Incompetent personnel at an early stage of the mine design, it needs to be made sure that the right professional with competence to do the mine design. 2. Incompetent personnel at monitoring and maintenance, making sure the a competent staff is used on the mine is indispensable; 3. Also the use of proper material on the supports, if the wrong material with a flaw strength is used, the accident is bound to happen. So it should not be used poor material in order to save money, the safety needs to be put in first; 4. Failure of machinery can be crucial, so the equipment should be monitored often and also maintenance should be taken place on at least monthly basis; 5. Poor study of natural conditions, such as seismicity and ground conditions. Knowing the ground and site where the mine is constructed is crucial; 6. Not having a response team available when the accidents happens. A response team with a proper number of member and trained in emergency first aid and a rescue team should be available at all times; 7. Emergency procedure not being followed. The procedures in case of emergency should be clear and known by all member of the mine staff and followed completely.

Conclusion It is recommended that a sensitivity assessment be carried out to determine the effect of critical geotechnical parameters involved with wall stability. This will assist with assessment of the quality of geotechnical data obtained and required and the appropriate mine design options. Common methods used to increase the effective stability of walls include reducing slope angles, controlled blasting practices, installing reinforcement, and depressurisation of groundwater. Any deficiencies that are highlighted in the analytical methods should encourage further work to remedy these matters, extend the use of the method or develop a new method. It is really import to have a good monitoring system to always know what it happening within the mine. An important aspect of any monitoring program is the development of a monitoring strategy, which is implemented rigorously within the mine's ground control management plan. The strategy should define the monitoring schedule, the time allowed and methods used for data recording, interpretation, and reporting, and provide basic courses of action to be taken in the advent of signs of impending pit wall failure. It is essential that the monitoring data collected is correctly assessed and the results and recommendations passed on to the relevant operations personnel at regular intervals for assessment of the performance of pit wall. To be able to control the stability and to have the possibility of taking the necessary stabilizing measures in time, continuous geological mapping, particularly of distinct joints, is crucial. Even when this is done, it is often difficult to get safe access into the areas that require support due to the high and steep walls. Thus, it is very important to 4

use skilled and experienced people for carrying out this type of work. In any case, good quality perimeter blasting is important for the stability, and also for ensuring safer working conditions at lower levels of the mine.

Bow Tie for Pit Wall Collapse A Bow Tie Analysis (Annex 1) with the threats, controls, mitigation measures (and it threats) and consequences was made using the BowTie Pro™, a software which graphically assists companies complete their risk assessments using the bow tie methodology. A free trial was given, what allowed the creation of the BTA.

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Annex 1: BTA for pit wall collapse