A rock mass instability was identified at the base of an inter-ramp slope, above a ramp at a large open pit mine. The distance between the radar and the pit slope was 1 400 m. There was no physical evidence such as tension cracks on the bench crest prior to the collapse of the rock mass. The relative range provides an indication of a progressive movement class, with a distinctive transitional movement patterns leading up to the cracking and dislocation of this portion of the pit slope, and the eventual collapse.
The synthetic map of the point data collected by the radar system shows the relative range measurements at a scale of 30 mm. There are three identified areas of the instability, represented by movement trends on the schematic image of the instability data for the relative range, average velocity and velocity delta. The first trend encompasses the entire portion of the instability to illustrate the overall movement trend (100 m in width by 50 m in height in green), the second trend is an area threshold size to replicate the alarm triggers (15 m in width by 15 m in height, dark blue) and a single trend point defined by the user (light brown) to identify the maximum amount of movement taking place on that portion of pit slope.
The movement of this portion of the pit slope starts prior to the commencement of the illustrated database. In the 36 hours leading up to the collapse, – 15 mm of relative range was measured with varying cycles of acceleration and deceleration which is mirrored in the average velocity and velocity delta trend plots. Numerous geotechnical and critical alarm settings were triggered for the average velocity and velocity delta leading up to the onset of the collapse movement stage, which was indicative of the impending collapse.
At approximately 21:00 on Day 2, the geotechnical relative range alarm was triggered, followed by the average velocity and velocity delta alarms which indicated the principal onset of the collapse movement stage. The critical alarms were then triggered. The sensitivity of the alarms was corroborated by applying a two hour time window.
In this case, due to the transitional movement pattern of the rock mass, the relative range plot enabled the user to interpret the tolerance alarm for the trigger action response plan, as the average velocity and velocity delta alarms where cyclical in nature due to the transitional movement trend. A secondary collapse of smaller portions of the rock mass can be observed at 06:00 of Day 5, indicated by the selected point data. The post collapse movement of the failure scarp, and the possibility for further instability of the areas was highlighted and monitored in the three days following the event. Smaller movement patterns and pockets of instability were observed.
It is important to note that for this case study, the reference time was selected to show the key movement stages for the 36 hours leading up to the collapse of the rock mass, thus, the full movement history of the pit slope is not illustrated. Once relative range alarms are met and exceeded due to collapse, they should be adapted in order to identify post collapse movement trends.
The temperature and reflectivity trend plots are illustrated for reference purposes. This operation is located in a very cold region. The pit slope is typically covered in snow with ice areas and the temperature typically stays below zero. The refractivity plot is considered acceptable with little scan to scan differenced, and there is little to no effect on the trend plots illustrated.
