Open Cut » Rock Mechanics
Windrows or safety berms are used in mine sites to protect haul trucks from rolling over an edge and to avoid collisions. Their current design is based on rules of thumb and the behaviour of windrows in mining applications is still poorly understood. It is, however, well known that the problem is extremely complex. A first attempt in analysing this complexity was made in Stage 1 where full-scale experiments and reduced-scale laboratory testing were combined with a simplified numerical model, based on the Discrete Element Method (DEM), to determine the energy absorption capacity of berms.
The results of Stage 1 indicate that the height of the berms or windrows should not only be related to the size of the vehicle, but also to the velocity the vehicle is travelling when running into the windrow. In addition, they clearly showed the need for a more advanced numerical model, which allows considering the entire body of the haul truck. This is necessary because it was found that the truck body plays a crucial role when impacting in forward motion particularly at high velocity. Hence, a realistic model, where the haul truck and its dynamics are modelled using Multi Body Dynamics (MBD), is developed in Stage 2. The model of the haul truck is then coupled with the DEM model of the windrow to allow more realistic predictions.
An extensive numerical analysis based on five representative scenarios with various windrow geometries is carried out: The scenarios include: a reversing truck at moderate velocity (Scenario S1), head-on collision at high velocity (Scenario S2), collision at shallow approach angle (Scenario S3), collision at shallow approach angle on ramp (Scenario S4) and side-wise collision due to sliding/skidding (Scenario S5).
The numerical analysis clearly highlights that the width of the windrow is equally if not more important as its height. Hence, the width should be considered in the design. In addition, the research shows that the effectiveness of the windrow in stopping an ultra-class haul truck also depends on the approach direction and approach velocity. This suggests that the windrow geometry needs to be adapted according to the most likely or most critical scenario, e.g., windrows on ramps should be bigger than the one on even haul roads. The application of high risk bunds to specific areas is encouraged.
For each representative scenario, design charts indicating the required windrow geometry (height and width) are derived for various truck velocities. Higher and wider berms proof to be more efficient in all scenarios. Trapezoidal windrows should be preferred as they are generally more effective. The suggested design charts can be used to design windrows for a specific speed limit or to estimate the admissible velocity for specific windrow geometries.
The analysis of windrows with low batter angles (α = 20°) clearly shows that a low batter angle increases the risk of the truck climbing the berm and the truck driver not noticing that contact has been made. The analysis also suggested that the amount of material (mass and volume of the windrow) is crucial as it provides the main resistance to the impact especially at high velocities where the dynamics of the collision play a critical role. Increasing the width not only adds additional resistance, but it also provides more room for a bigger braking/stopping distance. The analysis with rigid windrows shows that only windrows with batter angles of α ≥ 40° can effectively redirect the truck.