To avoid crumbling into apologies: reinforcing quarry slopes

TO AVOID APOLOGISING AFTER THE FACT: STABILISING QUARRY SLOPES

Let us start with the fundamentals: requirements for working with quarry slopes are specified at the technical documentation preparation stage. Without substantiation of the stability parameters for quarry slopes, benches and waste-rock dumps, a project will not pass state expert review, and a mining company will not obtain a licence. In practice, however, things are not always so straightforward — as industry professionals point out.

SMOOTH ON PAPER

Vladimir Rakhmanin, Head of the Geotechnical Department at PJSC Polyus Magadan, listed the main factors increasing the likelihood of quarry slope failures. These include the seismic impact of blasting on slopes, water saturation of the rock mass and unacceptable slope parameters. The specialist also named insufficient knowledge of the deposit’s geological structure. However, there may be other causes besides these “classic” ones.

Climate change is one example. People have been sceptical about Russian weather, particularly in certain regions, for centuries, but today nature is increasingly delivering surprises. Winters may be unusually warm, followed by exceptionally severe frosts; spring may be dry, while summer may instead bring long periods of heavy rain. As a result, even mining companies that previously did not need to undertake complex slope-stabilisation measures now have to consider doing so.

“Today, it is increasingly necessary to ensure the overall stability of slopes at various deposits, including open-pit mining facilities. In areas of high seismicity, with steep slopes, low soil physical and mechanical properties such as cohesion and angle of internal friction, as well as groundwater or substantial surface runoff, there is a high probability of displacement and failure of the soil mass,” listed Konstantin Gorkov, Head of the Engineering Centre at CJSC TECHPOLYMER, as the key factors.

“During the rainy season, increased attention is required. The mass itself is hygroscopic (even when the soil is not clay), and if it also contains cracks, it is clear that the material will lose its original properties under the impact of a large and continuous amount of moisture. As a result, the measures initially adopted may no longer provide the required effect,” explained Yury Yunakov, Professor, Candidate of Technical Sciences and Head of the Mine Surveying Department at the Mining Institute of Siberian Federal University.

Moreover, the country’s economy, particularly the extractive industry, is going through difficult times despite all support measures. Slope-stabilisation work requires investment. Consequently, the situation may arise where the plan required to obtain a licence and permit has been developed, but the solutions specified in it are not implemented.

“Under all applicable standards, a mining work schedule must form part of the technical project for a deposit. This package of documentation undergoes approval, and enterprises obtain a subsoil-use licence on this basis. It must be confirmed annually by Rostechnadzor throughout the licence period. In addition, mine surveying must be carried out quarterly: based on this data, the subsoil user monitors production volumes and pays taxes, as well as controls the slope angle and determines whether it complies with the project and whether additional measures are required. This plan also contains a schedule for planned audits. However, unscheduled inspections are not provided for by law; they may be conducted only following a complaint or when discrepancies are identified between the annual reports and the project. For example, the subsoil user extracted more than stated in the licence, meaning that the technology was violated.

In other words, if mining companies treat slope formation and stabilisation procedures negligently, there is a possibility that no one will learn about the problems,” cited Natalia Sipkina, Head of the Environmental Control Service at JSC Sibagropromstroy, as an example of a possible situation.

Delayed inspections can cause harm to people and the environment. Minor rockfalls can eventually develop into failures, which in turn may result in fatalities, equipment damage, soil erosion and increased dust levels. Groundwater levels may also rise.

“Negligent quarry operation precedes environmental disasters. The 2010 accident in Hungary or the dam failure in Krasnoyarsk Krai in 2019 immediately come to mind. Yes, neither tragedy was caused by quarry slope problems, but the condition of the dams should also have been monitored and timely work carried out. In that case, the accidents could have been avoided, or at least their consequences reduced,” added Natalia Sipkina.

It should not be assumed that slopes and benches are not dams and that nothing serious can happen to them. Many experts point out that the fines resulting from such violations can be as substantial as the damage to a company’s reputation. This is particularly true if a slope failure causes injuries or fatalities. It is therefore much less expensive to invest in maintaining slopes and inclines in proper condition.

There are also sufficient slope-stabilisation techniques, with an accessible option available for each situation. Mining companies are not expected to make slopes indestructible. The main objective is to ensure safety.

“The practice of deposit development shows that ensuring the slope and quarry-slope stability required for safe deposit extraction remains a relevant task. The objective is not to maximise stability, because higher stability reduces economic efficiency. The stability of quarry slopes and inclines must ensure the safety and completeness of deposit extraction. An excessive safety margin represents an economic cost,” noted Pavel Koltsov, Candidate of Technical Sciences and Head of the Laboratory for Quarry Slope Stability and Rock Movement at the Department of Mining Science of JSC Uralmekhanobr.

AREAS REQUIRING PARTICULAR ATTENTION

Experts recommend paying particular attention to specific areas and monitoring their condition more closely.

“Priority areas for implementing stability-improvement measures are those with potential signs of possible deformation (tectonic faults intersecting the slope contour, increased rock fracturing, zones of weakened rock and others), as well as especially critical areas, namely zones within the quarry infrastructure and working areas,” listed Konstantin Grebenyuk, Head of Bench Scaling at Polyus Krasnoyarsk.

According to the expert, measures to manage the risks of adverse deformation processes should be carried out periodically in these areas. This is necessary, for example, during drilling and blasting operations.

“Stabilisation measures include optimising drilling and blasting parameters near the final contour while minimising the impact of blasting on the area beyond the contour. This measure is used in masses of hard and semi-hard rock. A relatively simple way to determine the blast impact zone is to measure the displacement of benchmarks installed directly behind the blast block during blasting operations. However, under specific conditions, the zone size must be determined through field observations. When approaching the design contour, at a distance equal to the estimated zone of adverse blast impact, special blasting techniques must be used. These include buffer blasting, controlled blasting, presplitting or intermediate slot formation, blasting after slot formation and so on,” said Konstantin Grebenyuk.

It is also advisable to implement compensatory measures aimed at reducing or eliminating the severity of the consequences of possible deformations.

The Polyus Krasnoyarsk specialist noted that this type of risk management also includes designing a backup transport ramp (if the main ramp is damaged) or an enlarged safety berm. The latter is used when there is a risk of deformation developing in areas containing infrastructure, as well as where high groups of benches are located between transport ramps — more than 180 metres high.

A wide berm is usually designed at twice the width of a standard berm and is approximately 20-25 metres or more wide.

TECHNOLOGIES ADAPTED TO SITE CONDITIONS

When slope-stabilisation procedures are nevertheless required, mining companies have a wide range of technologies at their disposal.

The variety of methods is related to the wide range of soils and rocks. Therefore, experts’ recommendations on this issue will always differ. No universal solution or single method for addressing this task has yet been developed.

“Various technologies are available, including stabilising slopes and inclines with reinforced-concrete piles, cable ties, anchoring, grouting, sprayed concrete (shotcreting), installing various mesh barriers, injecting different strengthening solutions into boreholes to improve rock strength, and loading slopes (constructing buttresses and retaining walls).

The choice of technologies for stabilising quarry slopes and inclines depends on a combination of factors, including the characteristics of the deposit, climate, physical and mechanical properties of the rock, financial costs and others. When analysing a deposit, specialists select the most suitable methods,” explained Vladimir Rakhmanin.

“The method of strengthening with injections of cement grout or resins is quite popular abroad, for example, in Germany, the United States and Canada. There it is used for a wide range of rocks, including fractured hard and semi-hard rock, as well as sandy and clay soils. Many in Russia have adopted these methods, particularly grouting. However, it is important to remember that our climatic conditions and rock compositions are different. Therefore, one should not rely solely on the popularity of a technology. In many regions, for example, the pile-stabilisation method is often used to stabilise hard rock,” said Yury Yunakov.

“In northern deposits, particularly in permafrost zones, piles may reach 20-30 metres; this is the only way to achieve effectiveness in such a climate and with highly mobile soils. More modern methods, such as geosynthetics, have been experimentally used over the past decade for road construction in the Far North. Like any material, geosynthetics have their advantages and disadvantages. In conditions where temperatures can sometimes fall to -40 degrees or below, such a material may crack,” Natalia Sipkina noted.

GEOSYNTHETICS: IS THERE POTENTIAL?

As an expert from CJSC TECHPOLYMER notes, not all geosynthetics are the same. Geotextile, one type of material, does not crack at all due to the characteristics of its composition. If we are talking about a geomembrane, the market today offers options specifically designed for use in severe climatic conditions, such as the Arctic. The question is whether it is appropriate to use geosynthetics for quarry slope stabilisation.

Despite certain limitations, geosynthetics, which are also widely used in residential and road construction, are now entering the mining sector. The material has effectively become mandatory for tailings storage facility lining. However, as a tool for slope stabilisation, it still lags behind more traditional and long-tested methods. Nevertheless, specialists in the field not only place geosynthetics alongside these methods but are also confident that interest in the material will continue to grow.

“The most common methods for increasing the stability of quarry slopes and inclines are flattening them, creating additional berms to intercept surface water, and reinforcing unstable waste-dump masses with geosynthetic materials.

At present, this type of reinforcement of the dump body is quite promising for achieving slope stability. In addition, because geosynthetic reinforcement allows steeper slopes, it is economically feasible, as larger volumes of extracted rock can be accumulated over a smaller area.

The number of layers, their arrangement and the material strength are determined by calculating the stability of the dump slope, based on which the reinforcement layout is then specified. Structural stability can be considered ensured when the calculated slope-stability value exceeds the standard stability coefficient. Taking the design scheme into account, geosynthetic materials are installed during dump construction as the material is placed in layers. Geosynthetic interlayers are arranged as partial wraps or closed reinforcing wraps,” explained Konstantin Gorkov.

The use of such new methods must also be included in the project, substantiated and approved by both the geologists and mine surveyors who prepared the project and the supervisory authorities. However, specialists clarify that if a company wishes to test geosynthetics or any other slope-stabilisation method at its deposit, this can be done after completing a number of procedures.

“It is prohibited to use any new methods independently without amending the technical project; the subsoil user may face penalties for this. However, this does not mean that amendments are impossible in principle. Technologies are constantly improving, while subsoil-use licences may be issued for more than 20 years. It therefore makes sense to discuss new developments with specialists rather than use methods from the 1980s or 1990s. They can advise not only whether the selected method is suitable for a specific facility, but also how effective it will be, both economically and environmentally. If the decision is positive, it is sufficient to prepare an amended technical project and obtain approval from the relevant authorities. The use of the methods will then be considered lawful,” clarified Natalia Sipkina.

The issue is that some people simply do not want to spend time preparing and approving the necessary documents.

FORECASTING AND COMPETENT OPERATION

Almost all industry specialists agree that the frequency of such measures can be significantly reduced, or they can even be avoided, by monitoring potentially hazardous areas and operating the deposit correctly.

“Deformation areas can and should be forecast at an early stage. Various methods exist. In 2006, JSC Uralmekhanobr developed a method for monitoring the overall stability of quarry slopes at control points using various geodetic equipment: digital levels, satellite navigation systems and total stations. Since 2013, laser scanning has also been incorporated into slope monitoring,” listed Pavel Koltsov as possible tools.

“I will give our company’s experience as an example. To improve operational safety at the Natalka quarry, delivery of a second high-precision slope-stability monitoring radar is expected by the end of the current year. It will enable simultaneous scanning of the quarry’s entire contour. A similar radar was commissioned two years ago. This equipment has demonstrated high efficiency in operation.

An automated total station is also used at Natalka to assess the condition of all slopes,” Vladimir Rakhmanin shared.

“To identify potentially hazardous areas at the earliest stage, detailed geological and structural mapping of the quarry is carried out, along with the creation of a database and a geological and structural 3D model of the deposit. Geological and structural data and other geological and geotechnical information must be collected during geological surveys in order to update and maintain current maps, zoning schemes and the geological and structural model of the deposit.

During operation of an open-pit mining facility, benches must be scaled in a timely manner to remove overhangs and ledges. These works make it possible to identify weakened areas in time, develop measures and direct them towards improving stability. Such measures may include placing a buttress, installing passive rockfall protection or stabilising the slope with piles. The choice depends on the time available to carry out the measures, communications in the weakened zone, life-support facilities at the quarry and financial costs,” added Konstantin Grebenyuk.

At the same time, some experts insist that all the numerous technologies will in any case be aimed at solving local problems in specific areas. Therefore, the most reliable approach is still to plan this work at the design stage.

“There are no effective methods for stabilising quarry slopes to ensure their global stability. There are effective methods for assessing stability at the design stage and creating a stable quarry profile that ensures safe extraction. Slope stabilisation is a highly costly and, as a rule, ineffective measure aimed at correcting design errors. The deformation process of an entire quarry slope is irreversible and cannot be stopped by stabilisation. If modelling of the deformation process indicates risks to mine safety, fundamental measures are required: flattening the slope or loading it with hard rock, strictly in accordance with the results of the stability assessment,” Pavel Koltsov stated.