TUNRA Bulk Solids engineer Ben McPherson and engineering manager Dr David Bradney discuss the challenges surrounding gravity-reclaim stockpiles and the risk of stockpile dozer engulfment.
Gravity reclaiming is a widely used method in bulk material handling systems, particularly in industries dealing with coal, ores, and similar granular materials. The process involves a series of ground-level hoppers buried beneath a stockpile of material. These hoppers remain closed while the stockpile is being built. Once the hoppers are opened, the material begins to flow down through the base of the stockpile, feeding onto a conveyor or cart system below. This method leverages the natural flow properties of bulk materials, providing an efficient means of reclaiming large volumes of material.
However, the operation is not without challenges. The flow within the stockpile is often uneven, with distinct zones of movement and stagnation. The flowing regions form around the hoppers, while large portions of the stockpile remain static, creating risks of instability.
Arching occurs when a stable obstruction occurs across a hopper outlet, effectively blocking material flow. Arching may result from the mechanical interlocking of larger particles or cohesive bonds forming between material fines. Mechanical intervention is often required to break arched obstructions.
Ratholing is often exhibited by materials with a high proportion of fines or high cohesive strength and involves the flow of material through narrow vertical paths above the hoppers, leading to stagnant areas surrounding the hoppers.
When flow problems like ratholing occur, heavy machinery, such as dozers, are often deployed to dislodge stagnant material. While effective, this practice carries significant safety risks. The use of dozers in areas prone to collapse, such as unexposed ratholes, has led to numerous incidents of equipment engulfment and operator injury. This underscores the importance of establishing and maintaining safety radii for stockpile dozer operations.
The flow properties of bulk materials are fundamental to understanding how they behave during handling and storage. These properties are influenced by factors such as particle size and shape, moisture content, bulk density, consolidation pressure, and internal friction. These properties govern the material’s ability to flow, form arches, or resist movement, making them critical for the design and operation of stockpiles, hoppers, and conveyors. The flow characteristics can vary significantly depending on environmental conditions, such as humidity or compaction, further complicating handling processes.
The design of gravity reclaim stockpile storage facilities requires knowledge and precise quantification of the flow properties of particular materials under a range of operating conditions, including varying consolidation pressures, storage times and moisture contents. The comprehensive laboratory facilities at TUNRA Bulk Solids permits the characterisation of bulk material properties using well-established test procedures. Flow properties testing is typically conducted to identify the “worst-case” scenario for material handling. This is often associated with the material exhibiting its highest strength and cohesion. Under these conditions, the material is more prone to forming arches or cohesive blockages, resulting in issues such as hang-ups in chutes, buildup in hoppers, and poor flow through reclaim systems.
In contrast to the aforementioned “worst-case” scenario for bulk material properties, safety concerns for stockpile dozers arise when they operate in proximity to areas of low bulk material strength in a stockpile. These areas of low bulk material strength are much more likely to collapse and fail around any unexposed ratholes that may have formed, leading to increased probability of equipment engulfment accidents (as seen in Figure 1).
TUNRA Bulk Solids have been engaged on several occasion to use flow properties testing to determine the minimum dozer safety radii. Using supplied material samples, the TUNRA Bulk Solids laboratories can conducted flow properties testing to determine the moisture state in which the coal samples exhibited the maximum and minimum bulk material strength. The bulk material sample can then be tested in its weakest moisture state to determine key aspects of its behaviour via, direct shear testing, angle of repose (AoR), and drawdown testing.
The flow properties results obtained by the TUNRA Bulk Solids laboratory can then be used to determine the critical rathole geometry for a stockpile at the weakest-state moisture content. The predicted rathole shape and geometry are subject to variability depending on the applied theory, as certain approaches are known to predict varying degrees of conservativeness of the estimates. TUNRA Bulk Solids use two main theories to determine the minimum safety radii of the dozer. One theory stems from the measured AoR of the bulk material, and the other is based on the materials measured internal strength. The final rathole safety radius is chosen to represent key aspects from the main predictive theories, acting as a middle ground in terms of conservativeness.
It should be noted that the safety radii represent the outer bounds of the estimated rathole, and so a stockpile dozer safety zone exceeding this is advised.
Overall, gravity reclaiming is a widely used method for handling bulk materials in high-throughput applications, but it comes with inherent challenges, such as ratholing and arching, particularly when material flow properties are not fully understood. These challenges pose significant safety risks, especially for stockpile dozer operations. To address this, TUNRA Bulk Solids uses flow properties of the stockpile material to identify critical conditions that contribute to stockpile instability. Through laboratory testing, including the angle of repose, drawdown angle, and flow property assessments, TUNRA can determine the weakest moisture state of the material and use predictive theories to calculate minimum safety radii for dozers, allowing clients to establish safer operating zones, minimising risks associated with unexposed ratholes, and ensure enhanced safety for their stockpile management practices.