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

5.2. The establishment of the Sidewall survey station network

Sidewall stations were established in the controlled environment of the tunnel situated on the campus of the University of Johannesburg. In order to simulate the positioning of the sidewall stations, the points were installed in a similar layout to that found in most South African mines where hangingwall survey stations are combined with two sets of two gradeline³⁴ pegs. Figure illustrates the normal layout of the first set of sidewall stations in the tunnel, with the actual sidewall stations circled.

Figure . Idealized Gradeline Layout


Graphics by H Grobler

Figure illustrates the ideal set up of sidewall stations in the tunnel.

Figure . Sidewall stations in tunnel


Hanging Wall STATION


Photograph by H Grobler

The sidewall stations were installed using a 10mm hole drilled into the concrete sidewall. Tapered brass plugs were custom made for this project. The brass plug was tapered from 8mm to a final diameter of 12mm, which tapered profile allowed the plug to be very securely wedged into position inside the drilled hole. The tapered brass plug had a threaded centre hole that is designed to accept a threaded brass “bayonet type” prism attachment. This attachment was screwed into place and a prism (large round type) was attached. When the survey setup was complete, the prism could be removed and the prism attachment unscrewed. In order to prevent dirt contaminating the exposed screw thread in the tapered brass plug, the plug was sealed with re-useable putty. This putty could easily be removed but provided excellent temporary protection against mud and debris damaging or blocking the thread.

When surveying great care had to be taken to ensure that the correct prism constant was used. Hangingwall survey stations were established using mini-prisms with a -17.5mm constant and standard large round prisms with a 0.0 mm constant were used for the sidewall stations. A total of six bayonet attachments with six large round prisms were used in this phase of the project. Each brass sidewall station established was indicated with a luminous paint circle painted around the plug and the unique identifying number sprayed onto the sidewall. In addition due to the texture of the sidewall the number was duplicated using a black permanent marker pen.

Figure . Sidewall station Figure . Station with Large Prism

Photographs by H Grobler

5.3. The co-ordinates of the Sidewall station network

The sidewall stations were surveyed from the existing hangingwall control network by observing the horizontal and vertical angles and slope distance to each point. An additional check to the sidewall stations was made by re-surveying the same points from the different hangingwall survey stations. The sequence of control point installation was as follows:

Step 1: install the hangingwall control points; and

Step 2: from the hangingwall orientation install and survey the sidewall stations as illustrated in Figure :

Figure . Sidewall station network installation

Graphics by H Grobler
From the forward survey station the instrument was orientated on the backsight station and new sidewall stations installed and surveyed. Where possible an additional check was done to all other visible sidewall stations, as illustrated in Figure . This process was repeated through both legs of the original traverses until the breakthrough position was reached.

Figure . Sidewall station installation and check observation. Graphics by H Grobler

A list of the sidewall station coordinates as surveyed from the balanced hangingwall control network are shown in Table .

Table . Co-ordinates of the Sidewall stations

5.4. Sidewall Survey station survey method accuracy evaluation

In order to verify the accuracy of the freestation method, a “random” setup was made under the accurately established hangingwall survey stations and the “new” survey extended as far as possible, terminating where practicable under the next hangingwall survey station in order to accurately determine the accuracy of the fix of the resection point. This method is also referred to as the “freestation method; [94].

Bannister et al observed that “Although a position fix can be obtained by observations onto two stations, for accurate work the surveyor should sight a third station for a check on the data.” [94]. It was therefore decided that the observations would be made using two stations to obtain a first “fix”, then three stations would be used to obtain a second “fix”, which would be compared to the first “fix”, then a third “fix” would be made using four or more sidewall stations and so on, until all the visible sidewall stations had been observed and a number of “fixes” had been obtained.

1. Description of the observation methodology

The adopted method for the sidewall station network was to install the instrument directly under the hangingwall peg previously established. The instrument was carefully levelled and centred under an existing hangingwall survey station and the co-ordinates of the station determined by sighting to optical centre of prism instead of alignment marks. A minimum of two sidewall stations were then observed and the freestation calculated. The observations included a direct and transit horizontal angle and vertical angle as well as a distance measurement. The results were then booked and the process repeated with three sidewall stations and the result recorded. This process was continued until all the possible combinations of sidewall stations had been observed as illustrated in Figure .

Figure . Sidewall station observations

Graphics by H Grobler

Once the position of the instrument has been determined, the sidewall stations were re-surveyed. This process was repeated at each hangingwall station until the “breakthrough” point was reached. This process is illustrated graphically in Figure .

Figure . Re-surveying the control points from the freestation

Graphics by H Grobler

The final point fixed pseudo station setup as determined by a resection or “freestation” should correspond within the limits of accuracy to the original survey hangingwall station. These co-ordinates should serve as individual checks along the entire length of the sidewall station network. The closure error was then evaluated at the theoretical breakthrough point at hangingwall survey station UJ015. The accuracy of this closure was used to compare the accuracy of closure with the original hangingwall survey station network. Had the limits of accuracy proved to be less than that obtained by the original hangingwall survey it could be argued that the method could prove to be a viable alternative to the conventional surveying method. Had the limit of error not compared favourably the conclusion would then be reached that the sidewall station method does not have the same accuracy as a conventional hangingwall survey station network.

It is accepted good practice in surface surveying that in order to strengthen the accuracy of the traverse network additional reference points must be observed. In practice a Mine Surveyor will not observe more than one backsight and foresight due to time constraints as well as to minimize the disruption to the production team activities in the tunnel being surveyed. In addition it may not be possible to sight any additional points due to obstructions in the tunnel as well as the fact that old areas may be barricaded off. In addition to these considerations it will require additional assistants to illuminate any additional points. Realistically this procedure is possible as surveyors are no longer constrained by the use of 60m or 100m steel tapes for horizontal distance measurements. In a conventional mining haulage (tunnel) layout cross cuts and connecting roadways will branch off the main tunnel and survey stations should be available within these ends. In some cases as many as two points would be observable from the main haulage if two tunnels branch off opposite the setup point. These points will be in addition to any foresights or backsights that can be observed in addition to the survey base. In some mines it was standard practice before a new line extension was done, to start the survey one “base” back from the last base. The purpose of duplicating or checking the previous base before continuing the survey was to ensure that the last base was surveyed within minimum standards of accuracy and to ensure that the last survey point was not tampered with by mining personnel. Although by no means a regular occurrence, mining personnel are still found removing survey points and installing the points at a position from where their production advances when measured from the survey point a greater advance than actually achieved will be reported. Instances have also been observed where survey stations have been moved to conceal any off-line development from management during underground visits.
The sidewall resection in some respects approaches the Weisbach triangle solution as the angles observed are obtuse and the location of the sidewall stations are in a line. Most texts on the subject do not take into account the measuring of distances as well as angles to the same accuracy into account. Briggs commented on the calculation of error in the Weisbach triangle. In the narrow tunnel environment, the freestation can be broken down into a combination of a number of triangles (including triangles that exhibit the same characteristics as a Weisbach triangle) as well as a number of conventional resection problems. With the use of an EDM of the totalstation, distances are measured in addition to the observed angles increasing the number of redundant observations made. As only angles or distances are required for a unique solution, any additional measurements would then be available for a least-squares adjustment. A combination of observations were made to the sidewall survey stations to determine the minimum and maximum amount of observed points to obtain acceptable closures on point UJ015 as the virtual holing point.
It can be forwarded that one of the apparent advantages of the resection method is that as a result of the nature of the setup, no centring error would be introduced into the survey observations. Evaluating the accuracy of the survey by levelling and centering the instrument directly under the established hangingwall survey station would introduce a centring error when final comparisons are made between the two survey methods. It is expected that such centring errors would have an influence on the final results obtained. In addition, the measurement of an instrument height is not required if the instrument is set up at a random point. For the purpose of the investigation the instrument height was measured as it provided a check on the accuracy of the elevation or “z” component of the survey.
In order to minimize the effect of refraction the freestation setup was made in the centre of the excavation where the maximum number of points could be observed in the most optimum configuration. Figure shows the location of the hangingwall control points, the sidewall stations and the theoretical breakthrough point UJ015 at the junction of the two tunnels.

Figure . Plan of sidewall station network
Plan of Sidewall station network

University of Johannesburg

Doornfontein Campus

Please do not scale!

Access point from Metallurgical Laboratory

Vertical Access point from parking area

2. Description of the sidewall station Freestation calculation methodology

Figure . Idealized Freestation
The solution of final co-ordinates of the freestation position of the hangingwall survey stations were calculated using the on-board resection software of the Leica TPS1201+. According to Zimmerman and Nindel, the principle behind the “freestation” program includes calculating the provisional plan co-ordinates using a computation of the station co-ordinates using each possible unique solution including the solution of triangles, each possible combination of resections and Helmert transformation [112]. This method was discussed in detail in Chapter 3. For each solution, the result of the computed value is compared with the observed value. These results with the smallest number of observed minus calculated values are used to calculate the median of the values which in turn will provide the final result for the freestation position. [112] Figure illustrates an idealized freestation setup.

Graphics by H Grobler

For the purpose of the study the on-board storage of the instrument was used and all observations were booked manually. After each set of observations the solution was manually written down in a fieldbook for later reference. The on-board software calculated the final solution and produced a summary of the error in solution at each observed point. Where the observations to the sidewall stations did not fall within the predetermined minimum standards of accuracy, the point was identified and the user was presented with the option of rejecting the observations based on the accuracy of the fix. Figure illustrates an idealized freestation setup.
A manual check using a least squares adjustment of a combined resection and triangulation adjustment at a point was made. A freestation using four observed sidewall stations was used in order to provide the best possible comparison of results. In order to determine the provisional co-ordinates of the freestation point UJ14 a solver equation was developed to adjust all the angular and distance measurements observed at the point, similar to the method described by Zimmerman and discussed in Chapter 3. In this calculation the individual observations were broken down into 4 individual triangles, each with two measured distances to sidewall stations and an included angle observed at the freestation point, the condition set for these observations were that the observed angles at the freestation point must add up to 360°. Secondly the internal angles calculated for each individual triangle should add up to 180° and lastly the internal angles of the quadrilateral formed should also add up to 360°. The conditional equations and constraints for the calculations was set-up so that . A spreadsheet was developed in MS Excel using the solver application to determine the values to calculate the provisional co-ordinates of the freestation from 4 individual triangles. For the calculation, please refer to Appendix 13.
The provisional co-ordinates obtained in this manner were used in the least squares adjustment calculation. The least squares adjustment made at the freestation point is a combination of a trilateration and a resection least squares solution. This method takes account of all the observations made. A copy of this calculation is attached as Appendix 14.
The setup design of the original survey network allowed for a fast and accurate check of these point resolutions. The principle of the experimental procedure is to force the freestation set-up to be under a known station point that would serve as an immediate verification on the accuracy of the freestation co-ordinates with the original hangingwall survey station co-ordinates.
The following pages list the various observation setups used in the determination of the pseudo station points. Each survey station was observed using a combination of two, three and more sidewall survey stations in order to evaluate the accuracy obtained from each “fix”. A small diagram is included to illustrate the geometry of the observation at each point. A copy of the filed observations recorded with the instrument is listed in Appendix 11.