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

Required Standard of Accuracy

The defined standards of accuracy for surface and underground surveys are defined in Chapter 17 of the Mine Health and Safety Act, 1996 (Act 29 of 1996). Regulation 17(14)(b) states that the minimum standard of accuracy and class of survey for the fixing of survey stations on both horizontal and vertical planes are in accordance with the following formula:

A = 0,015 + s / 30000

“where “s” is the distance in metres between the known and the unknown survey station; provided that in the case of a traverse, after a check survey has been completed, the error in direction of a line between any two consecutive survey stations must not exceed 2 (two) minutes of arc, provided that the horizontal and vertical displacement between the measured position and final position of a survey station does not exceed 0,1 (zero comma one) metres” [17];

1. Interpretation of the MHSA Regulation 17(14)

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: _. This interpretation was verified through e-mail discussion with McGill and Pfafferot [143]. The regulation should therefore be interpreted to mean that the start point of the survey would be considered the “known” point” and the terminal point of the traverse would be considered to be the “unknown” point. The distance “s” would signify the sum of the distance of the traverses between the known and unknown points.

The Land Survey Act of 1928 [144] defines a traverse closure as “when the position of a point is fixed by a traverse , the traverse shall proceed from a known point to a known point,...” This regulation would imply that a closed traverse starts at a point of known co-ordinates and finishes at a point of known co-ordinates. In the mining environment it is considered that all traverses are “open”, with the only known point being the survey station at the station of a level. The position of which has been transferred from the shaft wires, or alternatively a survey station on the “check survey” gyro baseline, for which both the co-ordinates and bearing have been verified.

4. The traditional hangingwall survey method

In the traditional hangingwall survey method, the backsight is set up by suspending a small prism attached to a brass hook and aluminium rod assembly from the backsight. The backsight number is written down and the target height from the roof to the optical centre of the target is measured. Normally an assistant is left with the backsight to assist in stabilizing any swing of the target caused by ventilation or interference by personnel moving around the target. The terminology used in the description of the conventional surveying method is defined in Figure .

A plumb bob is suspended from the station and the instrument placed directly below the plumb bob. Care needs to be taken to ensure that the plumb bob swings as little as possible during the centring operation. The instrument is levelled out, then loosened slightly from the tripod and moved to have the optical centre of the instrument coincide with the plumb bob position. The instrument is then levelled out again. If necessary this procedure is repeated until the instrument is levelled and exactly centred. The instrument is checked for stability and to ensure that the instrument is properly clamped to the tripod. The plumb bob string is then untied and the zero of a small steel tape is tied to the string. The zero of the tape is then pulled up to the survey spad using the string and the instrument height is measured and recorded.

Figure . Conventional hangingwall surveying terminology

Graphics by H Grobler
Once the instrument has been orientated, the foresight can be installed. Normally two assistants are required for this operation. By using the laser pointer or manual signalling, a mark is made on the hangingwall on the required bearing. A hole is drilled, a plug is inserted into the hole and a spad and numbered disk is hammered into the plug. A small prism is suspended from the survey station, and the target height is measured. The foresight and backsight targets are then illuminated by the survey assistants and observations to the reference point and new survey station are made.

Figure . Underground setup Figure . Tunnel on campus

Photographs by H Grobler

In conventional mine development surveying prior to the use of modern surveying equipment, sightings were made to buttons placed on the plumb bob string and illuminated by a cap lamp. The introduction of EDM instruments into the underground environment led to the introduction of optical prisms suspended from thin aluminium rods or cables, replacing the conventional bob string and button setup. In a conventional setup the surveyor had to ensure that the sighting point (button³⁰) was not moved and that the wrong button was not illuminated and sighted (when using the “Two button setup method”) The “double-button”³¹ method was introduced to offer a second independent check on the front sight elevation also called a “double elevation”, in cases where a second check setup could not be made from the foresight for whatever reason. In the case where only one button was used, it had to be ensured that the same button was used for sighting as well as when the slope and target distance were measured. The introduction of prisms on rods and EDM measuring introduced a number of variables into the measuring of distances. Constant target heights were made possible by standard aluminium rod sections that can be screwed into each other to increase the length of the target where required. These target-rods bring with them a number of inherent problems that could influence the accuracy of particularly the elevation component of the survey network which include:

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  Non-standard target-rod lengths;
  
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  Damage to the thread of the target-rods; and
  
• 
  Target-rods not screwed in properly or becoming unscrewed in the process
  

Constant target distances can lead the surveyor to make the assumption that for example three target-rods are used the target distance is equal to x mm. Should the target-rods be bent, damaged or not screwed in properly this assumption can introduce a gross error into the observations and lead to poor accuracy on elevation. Good survey practice should remain in place forcing the surveyor to measure each target prior to moving to the next setup.

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