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

Summary of the findings in this chapter

The hangingwall survey control network was established to provide an accurate control to the proposed sidewall survey station method. The network was established through two independent vertical accesses into an underground tunnel with similar dimensions to those found in conventional development ends in South African deep level mines. The accuracy control through these two ends was checked by a closed traverse and the establishment of a gyro baseline in the tunnel. The closure at the theoretical “breakthrough point” was found to be within the minimum standards of accuracy prescribed by the MHSA [17]. The error of closure was balanced using the Bowditch method of traverse correction. A representative mixture of distances ranging from 3.2m to 64.9m were used between survey stations, representing most of the types of distances normally found between survey stations on mines. The final co-ordinates obtained after this adjustment will be used to calculate the co-ordinates of the newly established sidewall survey stations.

The following issues were observed during the establishment of the network under controlled conditions:

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  Accuracy of the network was affected by the short steep sightlines in the vertical components of the network, which revealed that unless very stringent control measures are in place a large error can be made over a very short distance.
  
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  The use of optical and laser plummets combined with the use of a three-tripod precise surveying technique assisted in maintaining the integrity of the transfer of bearing and elevation into the tunnels through the steep vertical sections.
  
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  the establishment of a gyro baseline is essential to ensuring the accuracy of an underground network
  
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  advanced survey equipment and software cannot replace solid basic survey practice and observation procedures
  
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  the external alignment marks painted on the sides of prisms do not always align with the true optical centre of the prism, specifically in steep sights
  
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  the laser plummet of the survey instrument can be used to accurately align the instrument to be plumb under the survey station. This method saves a great deal of time and reduces the risk of exposure to working-at-heights for the surveyor in hanging a conventional plumb bob from the survey station
  
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  The laser pointer proved an excellent guide to identifying and aligning targets in low light conditions. Using this method there was no need to use a survey assistant to illuminate the targets.
  

5. Introduction to Chapter 5

In the next chapter the survey control network will be used to accurately establish a network of sidewall survey stations. Sidewall stations will be established using the freestation method that will involve setting up a “pseudo” freestation point under each of the accurately determined hangingwall survey station positions. The results obtained from these resections will be compared to the hangingwall survey station positions determined in this chapter. Should the results of these resection positions correlate with the allowable minimum standards of accuracy, it will be argued that the resection or freestation method of surveying meets the current requirements for a Class “A” survey as prescribed in the MHSA.

5. Establishment of the Sidewall station Network

5.1. Introduction

`In Chapter 4 the hangingwall survey network accuracy was established, verified and balanced using Least squares and Bowditch correction. A network of clusters of four sidewall stations was installed at each hangingwall survey station. As a result of obstructions in some areas it was not possible to install four sidewall stations in close proximity to each hangingwall station. The sidewall stations were spaced at random intervals, although care was taken to keep the stations between 1.0 and 2.0 metres from the floor of the tunnel and spaced at least two metres away from the hangingwall station. The positioning from the floor of the tunnel was decided on to simulate the current practice of grade peg installation commonly employed in development ends on South African mines. This method makes use of two sets of two grade pegs each, placed approximately 3metres apart close to the control hangingwall station.
The focal length of the Total station used was defined by the manufacturer to be 1.7m, the focal length therefore constrained the optimal spacing of the sidewall survey stations from the hangingwall survey station. Where possible, the minimum distance to the sidewall survey station from the hangingwall survey station was kept to approximately 2metres. The random spacing of the sidewall stations approximated the situations encountered in the underground environment where the placing of grade pegs are dependent on sidewall rock conditions as well as accommodating possible obstructions from pipes, manifolds and other services routinely found in a working development tunnel of a mine.

The co-ordinates determined for the sidewall station control network were then uploaded into the total station in order to be used for the next phase of the experiment. During this phase of the experiment the co-ordinates for the sidewall control stations from the balanced survey network were used to determine the “random setup” position of the instrument. In order to verify the accuracy of the “freestation” method the instrument was orientated under the hangingwall station control network points. The position of the pseudo freestation point was then determined from a “freestation” setup using a combination of sidewall survey stations. In most cases a number of these “freestations” were made at the same control point to evaluate the effect of distance, angular geometry between observed points and the number of control points used, on the accurate position “fix” of the freestation point. The results obtained in this manner were compared to the original hangingwall survey station control point under which the instrument was carefully aligned.

The accuracy of closure using the sidewall “freestation” surveys was then compared with the closure obtained with the hangingwall survey method. The results obtained were then compared with the current minimum standards of accuracy defined by the MHSA. Should the comparison of results be within the predetermined limits of accuracy the tests would then be repeated in a working mine.