Spatial positioning of sidewall stations in a narrow tunnel environment: a safe alternative to traditional mine survey practice

6.3. South Deep Mine, 50 Level study

A similar project was completed at a deep level gold mine in the Witwatersrand basin approximately 60 kilometres south of Johannesburg. The mine is situated on the West Rand of the Witwatersrand complex. The shaft is at an elevation of 1626metres above mean sea level and produces gold mined from the Elsburg seams up to a depth of 3 000 metres below the surface. [142] The mining technique is a massive mining technique making use of backfill pillars. The project was completed on 50 Level, a currently inactive level approximately 1307 metres below surface. The tunnel serves as haulage for men and material as well as electricity, ventilation, water and compressed air and serves as a drainage channel for water from the workings. The tunnel has a width of 3.0metres and a height of 3.0m. The tunnel used is extensively supported by roofbolts and wiremeshing. The mine currently uses a similar type of wall station surveying in the stoping areas of the mine where the size of the excavation is prohibitively high for the use of ladders. The wall station method have not been used in development survey networks.

1. The use of the Sidewall survey station network

The wire meshing made the installation and sighting of targets more difficult as the wire sometimes obstructed the prism from view. The survey network was started from 3 existing sidewall station pegs installed for the purpose of profiling the shaft layout for a 3D CAD model of the shaft. From these 3 sidewall stations, new sidewall stations were installed and surveyed from the freestation setup made between the first three sidewall stations. The sidewall station consisted of a 10mm hole drilled into the sidewall or footwall of the tunnel depending on visibility. A plastic plug with a small diameter screw was then placed in the hole and expanded using a hexagonal spanner that fits into the screw. The number of the sidewall station was indicated by a small numbered brass disk as well as a painted number. The drilling and installation of the peg were sometimes made difficult due to the wiremeshing, refer to Figure . Installation of plug.

In one case a direct line of sight could not be obtained due to the area being obstructed by a locomotive. In this case the sidewall station was installed into the concrete berm on the floor of the excavation, used as a water sump in the tunnel. The direct sight line of pegs was obscured by a ventilation door system that consists of two doors that cannot be opened at the same time to regulate ventilation into the workings of the mine.
The project plan was to extend the sidewall survey stations to a point where a closure survey could be made to the old hangingwall survey network of the tunnel. The survey system consists of a Leica TCR1200 robotic instrument controlled by a Bluetooth enabled workabout laptop. The survey system uses proprietary software called “minemarkup”. The system uses a minimum of two survey stations to orientate and fix the freestation position of the instrument. In this project it was decided that a minimum of three sidewall stations may be used for any setup. The software constrains the geometry of the setup observation rays and will not continue the setup if any of the parameters are not met.
A total of 305 metres was surveyed using 7 freestation setups. Two hangingwall survey stations were closed on during the process. Although the software used for this method of surveying places very stringent constraints on the geometry of the observed rays, excellent closure results were observed.
The sidewall station method is mainly used for the direction and measuring of volumes of stopes. In most cases sidewall control points are installed on one side of the excavation in solid rock. The other sidewall is normally backfill and considered to be not stable enough to maintain the accuracy of survey points. Currently the system is not used in the establishment of primary survey networks.

2. Methods and Standard procedures employed

In most cases new wallstations are all installed on one side of the excavation of the stoping area as the opposite sidewall is not comprised of solid rock but rather an hydraulic backfill material. Standard observation practices requires that the angle at the instrument remains greater than 20 degrees. In addition it is recommended that the instrument setup is made closer to the existing reference points and new stations installed between 5 and 20 metres from this point. According to the mine standard procedure this will ensure the correct angular geometry at the setup point. The Standard Operating Procedures currently used defines that the geometry of such a setup should retain the internal angles at the setup of between 20 and 160 degrees. [142]

The mine uses a plastic sidewall station plug that is equipped with an internal thread and a grub screw that can be expanded using a hexagonal spanner. Once installed, the plug is very stable. A grub screw with similar thread is used to attach a Leica mini prism to the wall station for surveying. Figure to Figure illustrates the plug with the installation setup as well as the prism setup. Figure . Installation of plug.

Figure . Plastic plug and equipment.

Figure . Installed Prism.

Figure . Software and instrument setup.

The survey instrument used was a single second, robotic instrument that had a Bluetooth connection to Minesite software on a “Toughbook” laptop. The software plots the points directly onto a 3D underground plan. Unfortunately no record of observations is kept, once the instrument is orientated the point is plotted on the mine model.

3. Closures obtained

From data provided by the mine, it was noted that a total of 8 688 observations using the two point setup were made with an average of 0.0071m error for these observations. Survey error theory suggests that the same magnitude of error can be expected to be made both as a positive- and a negative error. These error distributions will follow the normal distribution curve dispersed around the mean. The data obtained does not distinguish between a positive or negative error, but only indicates the magnitude of the error in closure, based on the expected accuracy of the freestation “fix”.

A graph of two-station freestation setups indicates only the data that falls in the 50mm tolerance level of these observations, although the class of survey that these surveys were classified as is uncertain. Of these surveys, 8 152 two-point free station setups were within the 20mm range of tolerance, but due to the absence of the sign of the error the graph only displays one half of the curve. It is assumed from general survey error theory that the error will be dispersed symmetrically around the mean, indicating quite a high kurtosis in the data that would imply a narrow standard deviation in the results. The results depicted were closely distributed around a mean error of 1mm as a result of the fact that only setups where the geometry and limit of error set in the instrument had been met, would have been recorded. The results displays that the high number of freestation observations made mostly fell within a 12mm standard deviation which is within the minimum prescribed standard of accuracy.

Figure . Graph of tolerances from Two point setups

The graph depicting the results of 337 freestation setups using 3 points indicates standard deviation of 6mm, with 330 of these setups within a 20mm range.

Figure . Three point setup tolerances

From these results it is observed that the two and three point methods of surveying used on this shaft does comply with the minimum standards of error prescribed by the MHSA. The small surveying project proved that these results are feasible under the right circumstances. It was highlighted that the following observation protocol must be followed in order for the method to remain accurate.

4. Does this method meet the minimum standards of accuracy?

In the case of the two-point as well as the three-point setups the data seems to support the statement that the sidewall station method meets the minimum standards of accuracy. The proprietary software³⁸ used constrains the geometry of any setup to ensure the highest standard of accuracy. The strict geometry constraints prevent the observation of angles less than 20 degrees between any two reference objects. These constraints may force the surveyor to move the setup to a better position, which leads to additional time taken during the initial setups. The procedures recommended by this company is analysed in detail in Chapter 7.

5. Is this method a safer method of surveying?

The method removes the need to access the hangingwall by the surveyor, mitigating the risk of working at height to a great extent. It is argued that the method is a safer and more convenient method of surveying than conventional hangingwall surveying.