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

Recommendations

The sidewall control survey stations surveyed during the course of the research provided results that compared well with the survey stations in the roof of the tunnel, making a case for sidewall survey stations which are able to meet the minimum standards of accuracy prescribed in the South African Mine Health and Safety Act’s regulations. It has been shown that the sidewall station survey method has a number of advantages including additional redundancy of reference points, speed of operation, comparable accuracy, labour efficiency and minimal survey team exposure to health and safety risks.

The current minimum standards of error prescribed by the MHSA for primary mine survey networks have been changed and improved upon a number of times. These changes in the minimum standard of accuracy have mainly come about as a result of the improvement in surveying technology over the past years. The objectives of the MHSA have changed from being a prescriptive based Act to an outcomes based [180] Act that places responsibility on the Mine Surveyor to ensure compliance to the regulations through self-regulation. The current MHSA regulations stipulate only the required accuracy and do not prescribe the survey method to be used in order to obtain these minimum standards of accuracy. It is therefore reasoned that as long as the accuracies are achieved in a safe manner, any method of surveying may be employed. As a result of these findings it is recommended that the Department of Mineral Resources and the South African mine surveying industry should consider recognising the method of installing and propagating of sidewall survey station networks as an appropriately accurate and safe alternative method of underground tunnel surveying.
It has been suggested that in order to ensure the integrity of all survey networks the following changes be made to the MHSA Regulations should be considered:

1. 
   A series of check survey baselines be installed in an accessible, stable portion of the workings on each level of the mine. The distances between check survey bases may at no time exceed 750 metres and should be at maximum of 375 metres from the position of current workings on that level.
   
2. 
   The check survey baseline should consist of at least two survey stations installed in the hangingwall of an excavation.
   
3. 
   The distance and direction of the baseline should be verified by a check survey using a calibrated EDM and gyroscope surveying.
   
4. 
   The current limit of 2 minutes of arc allowed between two consecutive survey stations be reduced to 1 minute of arc because of the significant impact of an error of 2 minutes of arc over a short distance.
   
5. 
   A similar provision to the Land Survey Act, Act No 9 of 1997 be considered to specifically define survey networks established by freestation methods such as stated in the Act: “when its position is determined by resection, at least four favourably situated known points shall be used, and sufficient observations shall be made to ensure the required accuracy of determination of its position: Provided that at least one arc shall be observed;” [33]
   
6. 
   Records of the check survey should be stored both electronically and as hard copy. These should include all field observations and calculations.
   
7. 
   Check survey baselines should be indicated on the mine plan in an easily identifiable manner.
   
8. 
   Instruments should be regularly calibrated according to manufacturer’s specifications and the records of instrument calibration be filed.
   
9. 
   The scale factor, barometric pressure and EDM settings used on an instrument should be shown on all survey calculations.
   
10. 
    Instruments used for primary survey networks must be capable of single second accuracy.
    

During the course of the research a number of areas were, with reference to the method of surveying, identified that would require further investigation and development. A different methodology of research is unlikely to lead to a different conclusion. However, it is recommended that the principles of sidewall installations should be expanded to establish whether further application in vertical or shaft survey practices could be possible. The following topics that were found during the research to warrant further detailed investigation are listed and discussed briefly under the following headings:

1. National Standard Operating Procedures for Mine Surveying

A number of issues were identified during the course of the research. Although to some degree maintained and regulated by corporate guidelines and Standard Operating Procedures, certain survey procedures are not prescribed by the regulations or have been removed or repealed from the MHSA regulations. It is recommended that either the MHSA be updated or a standard guideline be produced by IMSSA to regulate mine surveying practice in South Africa. Such guidelines are in existence in Australia. It would be the ideal if produced in a generic format by the International Society of Mine Surveyors, addressing amongst other key factors, the following issues:

1. 
   Instrument capability and specifications
   
2. 
   The recommended geometry used for observations
   
3. 
   Check survey methodology and recommended distances between check surveys.
   
4. 
   The establishment and maintenance of underground baselines.
   
5. 
   Record keeping of check- and gyro surveys.
   
6. 
   The storage of electronic observations and calculations.
   

The Institute of Mine Surveyors of South Africa produced a generic guideline based on current practice during the 1990’s but it has not been updated since. It is recommended that the guidelines be revised to ensure compliance with the new MHSA and take new technology and methods into account.

2. Change management to introduce the sidewall station method of surveying

Publicizing the accuracy, safety, practicality and efficiency of the survey method in order to gain support within the South African environment will be required. The purpose of this study was to prove that the spatial accuracy as a result of survey stations installed in sidewalls of primary development was comparable to stations installed in the roof of a tunnel. The results prove that the prescribed accuracy could indeed be met. The favourable results obtained with sidewall stations are significant because survey stations installed in the roof of excavations are still the norm in South Africa. This research could therefore have the potential of creating a shift in South African mine surveying practice.

The hangingwall method, despite its obvious geometrical shortcomings, gained credibility over a long time as Mine Surveyors were able to produce exceptional results over long distances often having open-ended traverses. The protocols for establishing and maintaining a highly accurate hangingwall traverse network were developed over years of collective knowledge by what at times was seen as an isolated profession, where specific survey methods have been refined and used without major incident or critical evaluation. The highest possible standards in observation protocols developed through the years have been used to ensure high degrees of accuracy in mine surveys. It is noted that although technology such as Global Positioning Systems, Electronic Distance Measuring, laser scanning and offsetting using tacheometry have been embraced by the mine surveying industry, a certain amount of scepticism amongst Mine Surveyors remain to accept the accuracy and efficiency of the resection in the underground environment.
The added number of reference points and number of redundant observations that can be made when fixing the position of a survey point should therefore be promoted as a key point in the argument for the adoption of this survey method into the main stream surveying methods used on mines. This argument must be emphasised during any change management procedure.
The freestation method of positioning a random setup point has been accepted as a standard method of surveying by tunnelling contractors and mining companies in the USA, Canada, Europe and Australia. The correct procedure including the method of observation and minimum reference points to be observed is less clearly defined and seems to be largely dependent on the type of work, contractual obligations and the personal experience of the responsible surveyor.

3. The effect of the misalignment of the optical centre and the nodal point of a prism

This research has identified the fact that oblique observations to prisms may result in errors in the determination of the position of sidewall survey stations. As the position of a sidewall survey station is dependent on the prism attachments and the size and make of prism used while surveying the original point, it is important to fully understand the effect of using prisms not designed specifically to be a true 360 degree prism. It was found that specifically larger prisms are prone to misalignment.

4. The calculation of quality control parameters in the establishment and maintenance of underground survey networks

The need for the proper planning of survey networks both in new development and existing development is becoming increasingly important. The current skills shortage of competent, qualified surveyors highlights the need for a more efficient method of surveying. The distances between survey control, check- and gyro bases and measurement frequency should be planned and budgeted for in the working budget of a survey department on a mine. Surveying instrumentation and trained operators are essential to the efficient establishment of survey networks. In order to determine the angular and distance accuracy specifications of instruments used and the required distances between reference objects to meet the requirements of the survey regulations of the MHSA as well as corporate standard procedures, it is necessary to investigate the quality control parameters for a survey network. Specific attention should be given to the possible error propagated by inconsistent geometry in the selection of control points and the required measures to be taken to prevent such error propagation. It will be necessary to develop a network analysis method specifically designed for open traverse or resection survey methods constrained by long narrow tunnels to plan for the “breakthrough” accuracy at inter-level connections. Such a breakthrough position should be defined by an error ellipsoid, such is standard international surveying practice. A simplified method of calculating the error ellipsoid must be introduced to make the calculation method accessible to all Mine Surveyors.

5. Confidence limit of minimum standards of accuracy and error ellipses

In some countries the accuracy of a survey is described using error ellipses and confidence limits. This method of describing the accuracy of a survey is highly recommended but due to the nature of the calculation methodology it is seen as an impractical method of verifying the accuracy of an underground survey network. It can be argued that in South Africa, the average level of education of mining personnel in general is not sufficient to introduce a complex method of describing the accuracy of a survey and a simplified method of describing the minimum standards of error, such as that currently described in the MHSA regulations is sufficient for the purpose. The minimum standards of accuracy as defined are simplified and easy to understand as it stands. It is however recommended that the error vector used for the calculation of the closure of a survey or breakthrough point should be stated in a more simplified manner. This should include a definition of confidence in the results. Check survey and special survey records must be treated with additional care and where required meet a higher standard of accuracy.

6. Shaft surveying technique and monitoring of the position of shaft wires using the freestation method of surveying

Currently the freestation method of aligning shaft wires and construction is employed by only one survey contracting company using a proprietary method of aligning shaft construction using freestation software. This method was analysed and found to provide adequate control accuracies within the shaft construction phase of shaft sinking. The accepted method of establishing a survey network on an underground station from the shaft plumbing wires is the Weisbach triangle method. It is proposed that shaft plumb wires be used as reference objects and a freestation setup be established at station breakaways to facilitate the accurate transfer of the survey network into the underground workings. Such survey stations would then be the first baseline established and checked on each level and can be used to extend the sidewall survey network into the workings of the mine.

7. Measuring and profiling of underground excavations

The practice of using a freestation setup from hangingwall survey stations in order to profile or offset a mining excavation has gained popularity in recent years. The method has been tested on various sites and has proven to provide accurate and detailed information. In most cases this method of measuring is rapidly replacing the conventional tape offsetting surveyors are used to. It is proposed that the method of using hangingwall stations be extended to sidewall survey stations and that a detailed comparison between the different methods of measuring be made. The use of freestation software coupled with readily accessible sidewall stations should make it possible for non-survey personnel to monitor production and excavation using this method.

8. The effect of elastic rebound and tunnel deformation on the movement of sidewall reference points

The movement of sidewall and hangingwall in ultra-deep level mines has been studied from the premise of rock engineering. The effect of rock movement on the accuracy of the primary survey network has not been fully evaluated and studied. The proximity of a check survey base to the workings may have to be evaluated in the case where rock movement may have caused a displacement in the position and elevation of survey stations. Survey stations can be used to monitor the deformation within a tunnel, but if the same points provide accurate spatial reference to a survey network the impact of deformation has to be studied and fully understood.

9. Underground location with transponders in sidewall stations

The accuracy of sidewall stations has been determined in this thesis. It is argued that it would be possible to use a Radio Frequency Identifying Device (RFID) or transponders built into sidewall stations to be used for a number of purposes. The use of peg identifiers was investigated in the late 1990’s. Technology has improved to a point where it should be possible for a sidewall survey station to be equipped with an identifier that would not only provide information about the survey station but also its position. Technology developed for “indoors GPS” technology could be used in some manner to convert sidewall stations to a type of “pseudo satellite” which would enable a device such as a smart pad to be used to provide up to date positioning and GIS information regarding the user’s position and all relevant production information such as survey advice, blasting patterns, ventilation and rock engineering instructions in real time. Should this technology be available it should be possible for users to update information regarding geology, sampling, ventilation and safety conditions in the same manner.

A network of transponders or “smart, multi-sensor stations” would be able to monitor environmental conditions, such as heat, the presence of gas and ventilation flow and send this information to a central control centre where this information can be communicated back to the workers underground. The smart stations can be used to locate the position of workers underground through a link between the smart station and a transponder in the cap lamp of underground workers. The transponder technology is currently available but cannot report on the actual position of the transponder as it currently makes use of the underground “leaky-feeder” system. Such an underground network of smart stations can be used to develop real-time indoor-location similar to that being developed for smart-phone applications.

6.