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

Evaluation of the precision of the sidewall survey station network

At the onset of the investigation the question was posed: “Will the precision of the position of a survey point using a resection to sidewall survey stations meet the minimum standards of accuracy as specified by Chapter 17 of the regulations of the Mine Health and Safety Act no. 29 of 1996, in a narrow tunnel environment typical to South African mines?”. The question of accuracy has two components. The MHSA provides the minimum standards of accuracy for three different classes of surveying. It is therefore possible to utilize any method of surveying as long as the results obtained are within the minimum standard of accuracy for the specific type of work ranging from precise shaft surveying work to measuring and localized survey work where the standard of accuracy is allowed to be less. In order to evaluate this component of the research, it was deemed necessary to evaluate two components within the definition. To date Mine Surveyors in South Africa have been using hangingwall survey stations. During this time the method and accuracy of surveying has been evaluated, refined and trusted. The increase in excavation height as a result of mining mechanization, fast moving trains and vehicles coupled with the “zero-harm” objectives requires a different approach to mine surveying survey control, that encourages the change from hangingwall- to sidewall controls is required. A part of the change management component of introducing sidewall station surveying as a viable alternative it would be required to draw specific comparisons in the accuracy that can be obtained using this method against that of a conventional hangingwall survey network.

The results of the two networks established implied that the sidewall survey station method can, under specific conditions, meet the specific minimum standards of accuracy defined by the MHSA regulations and compare favourably with traditional surveying methods currently used on mines in South Africa. It has been noted by several authors that the definition of accuracy in surveying does not necessarily ensure the required precision and that both these two definitions must be met in order to ensure the integrity of a survey network. The accuracy of a conventional underground hangingwall survey network can be verified by check survey, but the precision thereof cannot be verified until a breakthrough has been achieved. A least squares network adjustment may improve the confidence in the accuracy of network but such adjustments require a certain amount of redundant observations in order to calculate such adjustment. Check surveys will remain essential in a narrow tunnel environment and should therefore form part of the network design on each level of the mine. It is recommended that the logistics of check survey labour and equipment requirements be included in any labour- and budget planning that may take place during the lifetime of the operation of a survey office on a mine. It has been observed that check survey and network maintenance are rarely planned for during the development phase of a new mine, or existing mines.
The accuracy of any type of survey network will be found to improve as the number of redundant observations is increased. In the case of conventional hangingwall surveying only one back reference is normally used when extending the survey forward. It was found that the sidewall survey station will provide a significant number of additional control points which in turn will improve the robustness and integrity of the network. The minimum number of stations to be observed to ensure the accuracy of a sidewall station network was determined to be four points.
The sidewall survey station method provides a significant number of additional reference points, leading to greater redundancy within the network, which in turn allows the possibility of performing a greater number of independent checks. The built-in redundancy in the network allows the calculation of an increased number of unique solutions for the new survey stations added to the network. Such redundancy allows for least squares adjustment to be made. These factors allow for the sidewall station survey network to provide improved accuracy and precision when compared with the results possible in a more traditional method of underground surveying.
During the establishment of traditional hangingwall stations, a disproportionate amount of time is spent in the use of lifting platforms and the selection of a safe and optimal reference position in the roof of the excavation. Parameters such as rock conditions and height will dictate where new survey positions can safely be placed. However it has been found that the required position for a hangingwall reference point does not always meet these requirements. In such cases the installation process can be delayed, or worse, safety compromises are made in order to complete the work requirements.
The sidewall station method of orientating an instrument does not require the measurement of instrument – or target heights in the case where targets are observed in the sidewall of an excavation. The currently accepted method of hangingwall survey makes use of trigonometrical heighting that requires both instrument and target heights to be measured in order to determine the elevation of the roof station. The sidewall freestation setup is a “pseudo” position that will only remain useful as long as the instrument occupies the position, there is no requirement to determine the elevation of a fixed point at the instrument position. The co-ordinates and elevation of the instrument optical centre is determined and used in the further calculation of new points surveyed from this “pseudo” position. The benefit of using four reference objects in this manner is that the elevation of the “pseudo” position is not only calculated from one point as in the case of a hangingwall survey, but from at least four independent reference objects. It is argued that the use of sidewall stations for reference objects, instead of setting up under or over a fixed station, eliminates the risk of any error being introduced into the survey network associated with the centring error as a result of improper alignment with the fixed position which may be as a result from instrument-, environment- or personal factors or a combination of any of these factors. The exclusion of the risk of centring error should result in the improved accuracy of the survey network as error propagation as a result of these factors is avoided.

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