Introduction
For an undisclosed location and commodity, an open pit slope was monitored for slope stability purposes and to identify areas of interest for assessment, to classify the rate of movement accumulation and to propose site calibrated alarm thresholds. An image of the pit slope and the live view of the displacement and average velocity is shown in Figure 1. The distance to the identified areas of interest range between 600 m and 2 260 m, the scan area is approximately 970 m by 4 130 m.
*An observational scale is one that is outside of the alarm threshold application and shows the displacement and average velocity at increments defined by the user to show as much detail as is required.
Identification of the Areas of Interest
Three principal areas of interest were identified (Figure 2), for which detailed polygons were drawn (Area 1, Area 2 and Area 3), with defined areas covering the concentrated movement signature (Defined Area 1 to Defined Area 5). A single point/pixel was also allocated at the centre of the concentration of movement to track the highest level of movement being measured by MSR Halo (Points 1 to Point 7). Two mask areas were applied (MA1 and MA2) for data that was not relevant to the study.
Observations of the identified areas of interest are for a 24 hour review period of the collected data:
- Area 1: An active instability over an approximate 60 m slope height. Area 1, Designated Area 1 and Point 1 (Figure 3), show an almost linear accumulation profile with minor acceleration and a highly fluctuating average velocity (one hour time window).
- Area 2: A post collapse failure scarp with continued movement accumulation for a slope approximately 75 m in height. Area 2, Designated Areas 2, 3 and 4 and Points 2, 3 and 4, (Figure 4) similarly show an almost linear accumulation profile with minor acceleration and a highly fluctuating average velocity (one hour time window).
- Area 3: A backfilled portion of the slope for which there is displacement on the muck pile, the slope is approximately 140 m in height. Area 3, Designated Area 5 and Points 5, 6 and 7, (Figure 5) once again show an almost linear accumulation profile with minor acceleration (almost a stick/slip trend) with a highly fluctuating average velocity (one hour time window).
Note that the movement convention is positive, that is, above the zero line on the trend plot which indicates movement towards the radar.
Alarm Threshold Application and Trend Plot Observations
Alarm threshold values were selected based on the observed accumulation rate of displacement for the Areas 1, 2 and 3 which was also applicable to the Defined Areas (1 to 5) and the Points (1 to 7). The alarm thresholds were applied for both trend plot alarm exceedance and an area threshold (10 m x 10 m = 100 m) criterion (Figure 6). Please note that the alarm thresholds are set for a 24 hour period for the displacement. The alarm exceedance instructions are an example and may be configured as per the user’s requirements and operational processes and procedures.
Table 1 details the movement type, values and alert level ratings based on the alarm threshold application in Figure 6 for 24 hour period for Areas 1, 2 and 3.
Based on the assessment of the synthetic map, trend plots, and the alarm exceedance warnings, an Alert Level Rating Synthetic Map was generated for this specific dataset and period of observational time (Figure 7). A time period of 24 hours is applied to the displacement with a one hour time window utilised for the average velocity.
Note that Level 4 is illustrated on the synthetic map for the average velocity specifically as it shows the latest synthetic map data. This higher value may be representative of an acceleration phase for Area 3 (the Figure 4 graphs corroborate this).
Conclusion
In summary:
- The displacement and average velocity values provided a baseline of anticipated displacement accumulation and the associated speed for each of the three areas, distinct in geotechnical condition and monitoring requirements.
- The alarm threshold values provided the foundation for an Alert Level Rating scale to be applied to the synthetic map, outside of the observational scale used for visual assessment of the synthetic maps.
- As the dataset represents a 24 hour period, more refined average velocity alarms will be required for Area 3 due to the highly fluctuating nature of the trend plots, to represent acceleration and deceleration profiles and mitigate false alarm activation.
- The alarm threshold values will need to be updated to represent a higher level of accumulation for both the displacement and the average velocity when assessed in conjunction with a longer dataset.
- The deployment of MSR Halo highlighted the effectiveness of a 360 scan envelope, which can be used for the detection of multiple types of ground movement taking place in one operational pit, at long distances and complex geometric conditions.
