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

Sidewall station data analysis

A summary of the observations made is represented in Table

Table . Summary of final Freestation co-ordinates

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Table summarizes the statistics of the geometry of the setups compared to the error vectors obtained. The data is sorted according to the horizontal angles subtended at the setup station:

Table . Two point freestations


From the difference between the error vector obtained and the minimum standard of accuracy calculated for the shortest leg of the setup, it is argued that the error vector is smaller than the allowable limit of error at angles larger than 02:00:00 and distances shorter than 42.294metres.
The maximum distance surveyed was 107.156metres and the minimum distance 2.885metres. In order to determine whether a relationship exists between the angular geometry and the error vector as well as the horizontal distance and the error vector, two correlation coefficients were calculated. The correlation (r=-0.25) of the horizontal angle and the error vector displays a weak negative correlation indicating that the error vector could probably increase as the subtended angle is reduced. A weak negative relationship would imply that the angle does not have an impact on the error vector of the final fix.
The correlation (r=0.74) between the horizontal distance and the error vector shows a relatively strong positive relationship inferring that the error vector will probably increase as the horizontal distance of between the sidewall stations and freestation point is increased. The significance of the correlation between the horizontal distance and error vector was tested at a 90% confidence interval using:

( 24 )

and found to be between r = +0.9 and r = +0.41. The spread of the correlation coefficient at a 90% confidence limit is not sufficient to calculate a regression line that could be used to determine the maximum horizontal distance at which a setup could be made from a reference point. This poor correlation could most probably be explained as the result of the small sample size.
The significance of the correlation between the horizontal angle and error vector was tested at a 90% confidence interval using equation (24) and found to be between r = +0.25 and r = -0.65. The spread of the correlation coefficient at a 90% confidence limit is too large to infer any mathematical model to adequately describe the size of the subtended angle at the freestation.

5.7. Closure obtained at the breakthrough point

The minimum standard of accuracy between the Provisional and Final co-ordinate of a survey station is defined as the vector distance calculated from the difference in the Y co-ordinate and the X co-ordinate, where “s” is the distance of the total traverse from the starting survey station to the “closure” survey station in the following manner: . The results are tabulated in Table .

Table . Final closure results of Breakthrough Freestation.

The co-ordinates of the sidewall survey stations were calculated using initial field observations and the final balanced hangingwall survey stations. The total distance to the closure point was calculated to be 130.724m from the start of the freestation network to the closing point.

8. Observations on the sidewall station survey method

It was found during the establishment of the control network that a large error in direction could be caused through the transfer of the survey through the steep entrances into the tunnel. It was not possible to transfer the direction into the tunnel by traditional shaft surveying techniques due to the narrow openings. However this project brought to light that even with the use of modern instruments and optical plummets, significant errors could still be introduced over a very short distance in such situations. The purpose of the gyro baseline calibration was to ensure that no error would be transferred underground.

Problems were encountered with the close focussing to reference points occupied with large circular prisms. The alignment marks normally found on prisms are not sufficiently accurate when doing a freestation observation. It requires a high degree of concentration to ensure that the exact centre of the prism is sighted under all circumstances. This requires proper lighting of each target by an assistant where possible and the correct identification of the target when more than one target on the same side of the tunnel is sighted.
The geometry of observations as well as the distance of the freestation to the reference objects does not have an influence on the accuracy of points. However, it is apparent that the points surveyed forward using the freestation fix, does not always give accurate results. The causes for this will need to be investigated in the next phase of the project.

8. Introduction to Chapter 6

In the next chapter, the freestation method will be evaluated in the underground environment of a deep-level working mine. The evaluation includes the investigation of existing standards used on mines where the system is used to a limited extent. The purpose of the evaluation is to complete a “check-survey” over a long distance using sidewall stations and then closing on a traditional hangingwall survey station. The geometry used in such freestation surveys will be investigated in detail. Should the closure obtained be found to be within the MHSA standards of accuracy, it should be considered that the freestation method complies with the standard of accuracy required and should be deemed a viable alternative to the conventional hangingwall survey station survey method.

6.