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

3.10. Summary of the findings in this chapter

In this chapter a literature review was made on the historic development of modern surveying technology. The introduction of electronic distance measuring in the 1960’s, combined with the rapid development of electronic software and data storage made it possible for surveyors to take electronic distance measurements and store the data on-board of an electronic instrument, by 1968. The instruments available to the average Mine Surveyor now makes the observation and accurate measurement of distances to known sidewall survey stations possible. This capability coupled with the computational power of modern surveying instruments has made the immediate calculation of the position of unknown points possible in even the most extreme environmental conditions. A combination of angle and distance measurement is likely to improve the accuracy of observations to a point where the position determination of an unknown point by resection can be compared to the limits of accuracy currently obtained through the use of conventional hangingwall traversing techniques.

In the second part of this chapter a detailed investigation of current mine surveying practice to establish the spatial position of a survey station was made. The traditional surface surveying methods such as triangulation, trilateration and resection, used long before the introduction of electronic instruments, were investigated and the effect of modern technology on these methods discussed. Traditional hangingwall mine surveying traversing as well as lesser known methods such as plugging, grade pegs and random setup methods were investigated and discussed. From the historic and current knowledge of surveying instruments and techniques, it has become apparent that surveyors in the tunnelling industry and in isolated cases, on mines outside South Africa, adapted to these technological changes far more rapidly than the South African Mine Surveyor. The traditional legislative responsibilities of the South African Mine Surveyor coupled with the pressures of day-to-day production requirements, have made the Mine Surveyor in South Africa reluctant to test and adapt versions of surveying for the establishment of primary survey networks in the underground environment.
In the final section of this chapter, current and historic tunnelling projects were evaluated with reference to the survey control used and the method of establishing such control. From the current knowledge available it evident that many tunnelling operations use some combination of “zig-zag” traverse methods with multiple orientation rays, combined to a varying extent with normal traversing techniques.
The analysis of current surveying techniques in the tunnelling environment indicates that most tunnel surveyors make use of a modified traverse method where points are traversed on alternating sidewalls, normally in the form of a sidewall bracket. This method of surveying seems to minimize the effect of refraction. In most cases a “double zig-zag” method can be strengthened by adding redundant measurements to additional reference points, thereby creating a strong network of braced quadrilaterals that can be rigorously adjusted. It can be argued that the establishment of sidewall brackets for the use of the zig-zag method of traverse preferred by the tunnelling industry will be impractical in the narrow tunnel environment found in the typical South African mining environment. The possibility of damage to the brackets will be of too great a risk to be considered.
The hangingwall method of surveying traditionally used by Mine Surveyors has proved to be effective, but in cases where the hangingwall of an excavation is too high to reach safely with a ladder or the footwall of the excavation is too uneven or steeply inclined for the safe erection of a ladder, this method becomes time consuming and hazardous. The use of gyroscopic azimuth determination as used in certain tunnelling projects should prevent error propagation but will prove too expensive for the average mine surveying department of a mine. The check survey run as the second, higher-order survey would most definitely have to incorporate azimuth correction as determined by gyroscope observations.
The use of reference points in the sidewall of an excavation discussed in the literature review points to the fact that the method is restricted to some tunnelling- and a few mining operations in Canada and Australia. The long term effect of adapting such a system for the establishment of a primary survey network apparently remains untested in the South African mining environment.

9. A comparison of survey methods using AHP

A decision matrix evaluation technique based on the Analytical Hierarchical Process (AHP) developed by Saaty [132]is used to perform an evaluation on four survey methods. The AHP is a systematic method of comparing objectives based on a structured framework of evaluation. The process incorporates knowledge as well as subjective judgments based on experience. For this research the goal is to determine the most feasible method of underground surveying based on the 3 most common methods of tunnel surveying and the proposed sidewall station method. These three methods are: the Hangingwall method, the Wall station method and the Braced traverse (zigzag) method used only in the tunneling industry. The criteria for the evaluation were selected as: Safety, Accuracy, Reliability, Refraction and Redundancy. A decision scale used to compare the preference of one method over another [133]is illustrated below:

Table . Decision scale for matrix

In order to develop the decision matrix the preference of the factors are compared to each other. Safety is given the highest preference, followed by Accuracy. The decision matrix is weighted and normalized:

Table Pair wise comparison matrix of Factors


Factor 1. Safety. The preference of one survey method over another based on the exposure to risk including working-at-height, falls of ground from roof and sidewalls, obstruction of travelling ways and exposure to moving equipment is evaluated. The hangingwall method of surveying is perceived to have the highest risk exposure based on the aspects of working at heights and the positioning of the survey reference objects in the centre of the excavation where the exposure to moving equipment is the highest. The safety decision matrix is weighted and normalized.

Table Pair wise comparison matrix for Safety

Factor 2. Accuracy. The preference of survey methods based on perceived accuracy in surveying are evaluated; these include the setup method, instrument and target centering, forced centering, the effect of ventilation on target- and instrument centering and standards of accuracy achievable. The hangingwall method will be most influenced by the effect of ventilation and height of excavation when the accuracy of centering of instrument and target is considered. The braced traverse has the most robust methods of dealing with centering and observation protocol. The advantage of the freestation methods of surveying (wall and sidewall) methods is the fact that neither instrument- or target heights are required nor instrument centering is not required. Height of excavation, illumination and ventilation flow is evaluated as factors influencing the accurate centering of an instrument under a survey station. The accuracy decision matrix is weighted and normalized:

Table Pair wise comparison matrix for Accuracy

Factor 3. Reliability. The preference of survey methods based on perceived reliability based on factors such as the ability to check calculations underground, azimuth verification, immediate closure or loop surveys, removal of points from the observations, error detection in observations during surveying and least squares adjustment of observations are evaluated. The possibility of damage to survey control points is also evaluated. The reliability decision matrix is weighted and normalized:
Table . Pair wise comparison matrix foor Reliability


Factor 4. Redundancy. The preference of survey methods based on the perceived redundancy of reference objects are evaluated on the basis of the minimum points required to obtain a position and the amount of reference objects available to orientate a survey set up. The redundancy decision matrix is weighted and normalized:
Table Pair wise comparison matrix for Redundancy



Factor 5. Refraction. The preference of survey methods based on influence of refraction on observations based on the specific observation protocols designed to minimize the effect of refraction. In the case of the hangingwall survey it is generally considered that this method of surveying is the most effective in countering the effects of refraction that may occur closer to the walls of the tunnel. The refraction decision matrix is weighted and normalized:

Table Pair wise comparison matrix for Refraction influence

Using the normalized factor matrices developed, a decision matrix was calculated

Table Normalized Matrix


The results indicate that the braced traverse is the most effective method of surveying when considering the decision factors. In the underground mining environment this method is not considered as a viable option as bracket mounted instruments and targets will be damaged during blasting and cleaning activities. The sidewall survey station method at 23.2% is considered to be the most suitable method of underground mine surveying based on the factors of Safety, Accuracy, Reliability, Refraction and Redundancy.

9.