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

Proposed survey network to be established for the test phase

In the following chapters the accuracy of the conventional hangingwall survey network will be compared with the accuracy of a sidewall station network where the position of a point is fixed by the resection method. In order to compare the accuracy of the two survey methods, it was necessary to establish a conventional hangingwall survey network in a controlled environment. The sidewall station method was tested for suitability in a narrow tunnel environment, focussing on the establishment of sidewall pegs in a cluster of not less than four points established at convenient positions of the sidewall of an excavation. The survey network was then carried forward approximately 50 to 60 metres before a similar cluster of sidewall stations was established. Survey stations were established using the accepted standard practice of development surveying in most South African mines. The network closed within the required accuracy standards for a Class “A” survey network as prescribed in Chapter 17 of the Regulations of the Mines Health and Safety Act. After the survey network was established and verified for accuracy, the sidewall station control network was established. The resection method will require that the instrument be accurately levelled and centred under the hangingwall survey station previously established. The accuracy of the random station setup position “fix” could then be compared to the original conventional hangingwall co-ordinates surveyed. It was found that the accuracy of such a resection position fell within the prescribed limits of accuracy of the sidewall station method and could be deemed to be a safe and accurate alternative to the hangingwall survey method.

4. . Set up of the hangingwall control network

In this chapter a description will be given on how a conventional hangingwall traverse network was established in an underground tunnel. The purpose of the establishment of the network was to simulate a traverse in a primary development tunnel of an underground mine. It was decided to establish a control network in the controlled environment of a service tunnel located under the Doornfontein campus of the University of Johannesburg. The tunnel dimensions are typical to those found in South African mines, with an average width of 2.4m and an average height of 3.2m. The tunnel sidewalls are obstructed with pipes and electrical services similar to those found on in the workings of a mine. Refer to Figure and compare this to Figure . . A total of 16 traverse points were established from a surface baseline, and the survey transferred into the tunnel through two access points. Survey stations were established in the hangingwall of the excavation and observations made to the theoretical breakthrough point at hangingwall station numbered UJ015. In order to ensure the accuracy of the network, the initial breakthrough position was to be determined within a Class “A” survey as defined by the MHSA. The network was then adjusted and analysed for maximum expected closure error and compared to the minimum standards of accuracy prescribed in Chapter 17 of the regulations of the MHSA for a Class “A” survey.

1. Introduction

Figure . A Typical Mine Tunnel


Figure . The Project tunnel
The control network was established in a service tunnel under the John Orr building on the Doornfontein Campus of the University of Johannesburg. The service tunnel serves as an access to water utilities, sewage and power supply feeding the main building. In addition the building is built on the site of a spring and the tunnel serves as a catchment area for the water seepage from the surrounding soil. The tunnel can be accessed through a vertical “shaft” in the student parking, as well as a smaller access “manhole cover” situated in the floor of the metallurgy laboratory in the building .In the case of the main access, the access is through a short vertical ladder with a vertical drop of approximately 2m and then a series of three staircases before the tunnel “levels off” under the building. The tunnel dimensions are 2.4m wide by 3m high, providing a good simulation of a common development end found in South African mines. The services and pipelines are suspended from cable racks on the sidewall of the tunnel giving a realistic approximation of the type of clutter found in most development ends (mine tunnels).



Photographs by H Grobler
The total vertical drop from the surface is 8.7m, in three levels, firstly the 2m vertical drop, with a further 3.5m drop to the second level and then a further 3.5m drop to the final tunnel elevation. The length of the access tunnel is 139.8m before reaching a “T-junction” tunnel that runs the length of the John Orr building. This portion of the tunnel has a total length of 157.2m. The total length of the tunnel is 297.0m.
The layout of the control network has a length of 583m. The purpose of the establishment of the network was to simulate a traverse in a primary development tunnel of an underground mine. The tunnel network was established from a surface baseline and transferred through two entrances at opposite ends of the “T” shaped tunnel. At the junction of the two tunnels a breakthrough point was determined on a survey hangingwall station numbered UJ015, refer to Figure .
The tunnel survey was connected to the underground network through two steep vertical sections of the access points. As a result the first two levels of the tunnel necessitated the use of very short, steep bases. The minimum base length was 3.2m (the short base used to transfer the surface survey into the tunnel) and the maximum length 97m. A detailed list of the bases used including the distances between stations and the total distances are listed in Table . The inter-peg distances obtained in this manner are within the norm for survey bases established in narrow mine tunnel environments.

Figure . Vertical Entrances to the Tunnel





Photographs by H Grobler

The bulk of the survey network was made on the flat portion of the tunnel. Standard brass spads attached to numbered brass disks were installed by drilling a hole in the concrete roof and plugging the hole with a “fisher” plug²⁹, as is current standard practice on South African mines. In places where the concrete roof proved too hard to drill, a large split pin was inserted through the disk and bent flat. This assembly was then grouted onto the roof using a commercial grout product.

The instrument was levelled and centred using a suspended brass plumb bob and sights taken to the reference objects indicated by hanging mini prisms with a prism constant of -17.5mm. The instrument was checked for alignment using the laser pointer technique described earlier.

Figure . Plan of project area.

2.