10.6.7 Slope Stability Radar system
Slope stability is a critical safety and production issue for coal mines. A common technique to determine slope stability is to monitor the small precursory movements, which occur prior to collapse. It is a state-of-the-art development for monitoring slope movement in open pit mines. It offers unprecedented sub-millimetre precision and broad area coverage of wall movements through rain, dust and smoke. The real-time display of the movement of mine walls has allowed continuous management of the risk of slope instability at a mine operations level. There are two key roles where mines are now using the slope stability radar:
1. Safety Critical Monitoring: The radar is used during mining production as a primary monitoring tool of a designated unstable slope.
2. Campaign Monitoring: The radar is moved around the mine in a repeatable manner to compare movements at each site over an extended time, and determine problematic areas. Campaign monitoring in this manner is often used in metalliferous mines until determination of developing failure is observed.
The ‘slope stability radar’ has been developed to remotely scan a rock slope to continuously monitor the spatial deformation of the face. It is a technique for monitoring mine walls based on differential interferometry using radar waves. The system scans a region of the wall and compares the phase measurement in each region with the previous scan to determine the amount of movement of the slope. An advantage of radar over other slope monitoring techniques is that it provides full area coverage of a rock slope without the need for reflectors mounted on the rock face. The system offers sub-millimetre precision of wall movements without being adversely affected by rain, fog, dust, smoke, and haze. The system is housed in a self contained trailer that can be easily and quickly moved around the site.
It can be placed in the excavation, or on top of a wall or on a bench to maximize slope coverage whilst not interfering with operations. The scan area is set using a digital camera image and can scan 320 degrees horizontally and 120 degrees vertically. The system provides immediate monitoring of slope movement without calibration and prior history. Scan times are typically every 1-10 minutes. Data is uploaded to the office via a dedicated radio link. Custom software enables the user to set movement thresholds to warn of unstable conditions. Data from the SSR is usually presented in two formats. Firstly, a colour ‘rainbow’ plot of the slope representing total movement quickly enables the user to determine the extent of the failure and the area where the greatest movement is occurring. Secondly, time/displacement graphs can be selected at any locations to evaluate displacement rates.
The SSR units have operated within highly variable geotechnical conditions including massive hard rock, intensely fractured, foliated ultramafics, weathered oxide pits, coal strata and waste dumps of variable characteristics. It permits users to enter parameters that define the conditions for alarm generation. Four alarms are often used at an operation
The selection of alarm triggers is done on a custom basis by the mine geotechnical personnel as alarms can be set up on threshold displacement, time (using time and displacement to get a velocity trigger) and size of failure (figure 14). The SSR data is continuously dispatched to the control room and screened. When an alarm is triggered, on screen instructions with the alarm ensures that the appropriate target action response plan is undertaken.
Figure 14: Visual of Alarm Trigger and Failure Surface
The typical system consists of two main parts: the scanning antenna and radar electronics box connected via an umbilical cable (Figure 15). The scanning antenna consists of a 0.92m diameter parabolic dish mounted on a sturdy tripod and controlled by separate motors and gears for azimuth and elevation movement. The beamwidth of the antenna is approximately 2o.
Figure 15: pictorial view of slope stability radar system
The electronics box can position the parabolic dish to anywhere between –15° and 165° in elevation from the horizontal, and between –170° and 170° in azimuth. The 2D scan region is set manually for the application. The scan speed is approximately 25 minutes for 4000 pixels on the wall. The pixel size on the 2D image is determined by the range extent of a 1° angle increment. For a rock slope at 100 metres range, the pixel size is 2m x 2m approximately. Two-by-two pixels constitute one spatial resolution cell provided by the 2o beam divergence of the antenna.
Figure 16: A SSR deployed in a surface mining site
One of the primary roles of the SSR is identifying unstable slopes. The broad area coverage and almost real time scanning means that large expanses of slope (e.g. 500,000 m2) can be scanned and results obtained in less that 10 minutes. After a relatively short time, areas of stable slope can be identified, as well as those areas that are showing greater deformation than expected (providing they show deformation greater than a millimetre). This increased deformation may represent areas of slope instability, possible leading to collapse.