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

Establishment of the control network

The hangingwall control survey network was established using conventional hangingwall survey stations installed at intervals commonly found in the mining environment. Under normal conditions the extension distance of survey lines are constrained by the tape length available to the surveyor and miner. Most modern surveying instruments make use of EDM technology allowing the measurement of distances of more than 100metres. This implies that survey stations could theoretically be placed at distances in excess of 100metres between points. The reality of the South African production environment is that mining personnel still make use of 60m cloth tapes to perform their daily measurements as well as using the cloth tape for line and grade control in the marking off of the next blast pattern. In addition unless use is made of a handheld EDM for month-end measuring, tapes of 60m length will normally be used. The balance between extending the longest possible bases of survey has to be weighed against the requirements for lines close to the face for the control of direction and grade of the excavation.

Holland et al recommend that, where a closure in the traverse is not possible, a traverse in a tunnel should be “closed” back on a known point at the end of each survey “…often the only method of closing them is to return to the initial reference points along the same route but using different stations…this is by no means common practice in mining surveying…” [134] . It has been established that at least one survey contractor that makes use of this practice and is finding good results [135] .

1. Check Survey

A check survey is considered standard practice to determine the possible error and make corrections prior to a breakthrough. The check survey is prescribed according to the MHSA. A conventional check survey consists of two separate components:

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  a check survey consisting of long bases, sometimes making use of the three tripod precise traverse method, skipping over intermediate points, with the purpose of taking as long as possible sightings to targets in order to compare the co-ordinates of the final base in the survey and;
  
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  a gyro check on the azimuth of the final check survey base to compare the accuracy of the azimuth determination.
  

Dennis described the typical format of mine check surveys in the following manner: “periodic check surveys are undertaken where efforts are made to take sights of as long a distance as possible between survey stations with a view of reducing the number of horizontal angles required in the traverse.… The development programme is usually carried out in such a way that closure surveys may be carried out as the development is extended…” [136]. Bannister et al suggest that the interval between check surveys should be at least every 10 legs, which in a typical mining layout would be 10 legs of approximately 60metres each which would mean that the check survey should be done at least every 600metres: “it is suggested that no traverse should contain more than 10 legs before closing..” [94]. The use of very short bases in the establishment of the survey network is open to error in the accuracy of the network. In order to prevent the introduction of error through short bases, the check survey was planned using longer bases, as recommended by Allan et al “the errors arising from short lines in traversing can frequently be minimized by bypassing the short lines and so retaining the bearing in the main traverse.” [137].

This method is considered standard practice for check surveying on South African mines. It is accepted practice on most South African mines that check-surveys of the primary network be carried out on a regular basis. Although a check survey of the primary survey network is not defined as a requirement in the current Mines Health and Safety Act, most mining companies have established check survey Standards and Procedures, similar to this procedure for a gold mine “a secondary network shall not advance 750m in advance of the primary network,..” [138]. The requirement for longer bases in such a check survey is highlighted in the following standard from the same company “A resurvey should commence at an established primary survey base and where possible short bases must be eliminated along the traverse” [138]. With the availability of EDM instruments, the length of such bases can be increased from the normal distance of bases of between 60 and 100metres constrained by the length of steel tapes available to surveyors previously. The check survey remains the only procedure that the Mine Surveyor can use to ensure the accuracy of the survey and the accurate representation on plan as required by the MHSA.
It is normally assumed that the height component, also referred to as the elevation or ‘z’ co-ordinates, is checked by the check traverse. In the case of using conventional machine bobs to centre the instrument under survey stations, the centring error becomes a major contributor to the error introduced into a survey. This is specifically important in areas where a high volume of ventilation is experienced. In some cases use is made of the instrument laser, which should provide a far more accurate fix on the centring of the instrument, but it is highly unlikely that this error will be less than 2-5mm. The levelling of survey stations in primary development ends is not normally used to determine the final elevations of the survey stations. Levelling of survey stations should serve to strengthen the traverse network by providing an “external” check on the accuracy of the elevations determined by the normal development survey and are often used for check survey purposes.

2. Bearing Check

In order to verify the accuracy of the bearing transfer a gyroscope baseline was established on the first leg of the level section of the underground network. The bearing obtained from the gyroscope survey compared well with both the original and check bearing of the baseline.

3. Elevation check

In order to ensure that the elevations within the network were of acceptable standard, should the elevations determined by trignometrical methods as used in the standard underground traverse, not have been acceptable, a conventional levelling traverse was made along the length of the traverse. This method of ensuring the accuracy of the vertical component of a survey is described in the JCI corporate guideline: “independent leveling shall be made along major development headings” [138]

4. Instrument used

The instrument used was a LeicaTCR1201+ total station. The instrument was calibrated prior to use, refer to Appendix 3. for the calibration certificate. A brief summary of the main specifications of the instrument are listed below:

Table . Specifications of Leica instrument used

Feature	Specifications
Angle Measurement (ISO17123-3) horizontal and Vertical	1” Centralized dual axis compensator
Distance Measurement (IR Mode)	1mm + 1.5ppm
Shortest possible distance measurement	1.5m
Focussing Range	1.7m to infinity
Laser dot diameter	2.5mm at 1.5m

Proprietary instrument software was used to store and calculate all observations. In addition, all observations were manually recorded in a standard fieldbook for an additional check. Calculations were checked by manual calculation as far as possible during each phase of the survey.

5. Instrument Settings

In order to ensure the accuracy of the network it was necessary to determine the settings that would be used for the duration of the survey accurately. These settings could affect the accuracy of EDM readings made. Accuracy of the observations can suffer as a result of incorrect instrument settings. In the case of a working deep level mine, with multiple levels, these settings can become crucial, as the temperature on surface and underground can differ significantly. In addition the barometric pressure on the different levels will vary and as a result may have to require different settings for each level on which the instrument is to be used. For the project the variables of temperature and barometric pressure were studied carefully before any final settings were decided on. The instrument manufacturer defines these settings as “atmospheric data” (ppm) and states in the user manual that “Distance measurement is influenced directly by the atmospheric conditions of the air in which the measurements are taken. In order to take these influences into consideration distance measurements are corrected using atmospheric correction parameters.” [139]. It is cause for concern that on some mines, surveyors will tend to leave the instrument settings standard at a ppm of 0, this is a fundamental error. One instrument manufacturer manual states that : ”When PPM=0 is selected, the Leica standard atmosphere of 1013.25 mbar, 12°C, and 60% relative humidity will be applied.” [139].The instrument user manual states that for maximum accuracy the atmospheric conditions should be determined with an accuracy of 1ppm, air temperature to 1°C and air pressure to 3mBar. [139]

6. Barometric pressure

Measurements were made with a GPB3300 Barometer set to read mPa. Random pressure readings were made on site over a two month period. The results are tabulated in

The barometric pressure at the elevation of 1614.2m AMSL was calculated to be 828.0hPa hg. This pressure was used to calculate the ppm value of the instrument EDM used in the project. It is important to note that the survey total station was pre-programmed default values of 12degrees Celsius and a pressure of 1013hPa. The formula for atmospheric correction provided by the instrument manufacturer for these parameters these settings would result in a 0ppm setting, calculates as follows:

( )

where:

and:
a ppm setting of 0 can be calculated using the following variables:

If an instrument is used on multiple levels of a mine a significant difference in air pressure can be expected. The effect of the increase in air pressure and temperature expected to be encountered on mines will not form part of this study. Although considered poor surveying practice, in reality pressure measurements are not made at each survey station surveyed underground.

Air pressure is carefully controlled in the underground environment through a system of ventilation doors and barricades. Factors causing barometric change within a mine includes: barometric changes caused by meteorological and diurnal changes on surface as well as the changes in the airflow pattern of the mine caused by shaft conveyance, underground transport and the abuse of ventilation controls such as leaving ventilation doors open. [140]. These local pressure fluctuations are not taken into account during daily surveying operations on a mine.

It is not sure whether surveyors on these kinds of projects take these fluctuations into account as observed by Le Roux:

“…atmospheric pressure at sea level is about 100 kilopascals and that it increases by 1kPa for every 90 metres below sea level and decreases by 1kPa for every 90 metres above sea level. Thus the pressure at 1700metres above sea level (the shaft collar of the highest Witwatersrand mines) could be expected to be about 81kPa while the pressure at 1700metres below sea level would be about 119kPa” [65]

A ventilation consultant was contacted to verify the calculations and assumptions made. Ramsden concluded that for a mine “…operating at a depth of 3000m the pressure would be about 30 kPa above the ambient pressure. The main surface fan operating pressures are about 5 kPa and hence for underground excavation at 3000 m the actual pressure would be atmospheric pressure + 27.5 kPa [assuming no secondary fans and that the downcast pressure drop is the same as the upcast pressure drop].” [74]

7. Temperature

Measurements obtained from Weather SA for a year, indicated that the average temperature range observed in the Johannesburg region for the month of May in 2011 was 20 degrees Celsius [141]. This temperature was selected for the standard setting used during the observations. The average temperature for the year was calculated to be 24°C. Refer to Appendix 3.As a result of the controlled environment found in a mine, the temperature on a mining level is considered to be a constant carefully regulated by refrigeration plants and forced ventilation to maintain a working temperature within the specific parameters defined by the Regulations of the MHSA, Chapter 9.2.(2)(b) [142] as discussed in Chapter 2 of this document.

8. Final Settings

The settings used throughout the survey project are tabulated in below. Using the atmospheric equation, the ppm value for the instrument setting can be calculated from equation ( ). Temperature and Barometric corrections determined was used for the duration of the survey as it is argued that the conditions are artificially maintained to be within these specific parameters in an underground environment found on a specific level of a mine.

3.