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

Phase 2. Installation

This phase of the Standard Operating Procedures will address the initial establishment of a sidewall survey station network from an existing hangingwall network, the propagation of a sidewall survey station network and the establishment of a hangingwall survey station network from an existing sidewall survey station network. This Standard Operating Procedure must be read in conjunction with the guidelines discussed in Chapter 8, specifically with reference to the identified risks and risk mitigation for each step in the process. It is recommended that in all cases permanent hangingwall survey stations are to be used as a fixed reference baseline.

Survey Station installation
It is recommended that the survey crew should request a qualified person to inspect and “make safe the area” in which it is intended to install survey control. It is essential that the survey crew re-inspect the sidewall and hangingwall before and after a survey station has been installed. It must be assumed that drilling into a sidewall will have the potential of dislodging rock. At the identified position where a sidewall survey station is to be installed, the depth of hole to be drilled will be determined by the size of the plug or expansion bolt to be used. The hole must be drilled deeper than required to accommodate any loose material in the hole. If the plug used has a tapered profile it is recommended that the hole be reamed. If the plug is to be installed by a hammer, care must be taken against rebound as well as damaging the thread of the plug. In some cases some of the plugs investigated will require glue or putty to set the sleeve in the correct position. This method is not as effective as more time will be required for the glue to set in the correct position. The expansion bolt type of plug is considered the most effective solution.
Sidewall stations should be installed on the required gradient of the excavation in a configuration where pairs of points are installed in a cluster perpendicular to the direction of mining. In order to prevent mis-identification of targets it is recommended that only one prism at a time be used when observations are made. The survey assistant can then be requested to move the prism to the next observed point after each point has been referenced. Where a specific situation dictates alternative positioning of sidewall stations, points can be moved to any stable position in the excavation such as a concrete berm in the footwall of the excavation, timber packs or in an safe and accessible part of the excavation.
Changing to a sidewall survey station network from a hangingwall network
In an ideal situation, the sidewall station network must be established at least one survey station back from the existing baseline in order to check the accuracy of network transfer at the terminal point of the hangingwall baseline. The method to be employed to transfer the network is as follows:

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  Establish the instrument directly under the first point of the hangingwall baseline. Where possible, make use of an optical plummet or laser to ensure that the alignment error is minimized.
  
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  Ensure that the instrument is levelled and centred to the greatest possible degree of accuracy.
  
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  Install a target on a tripod, directly under the terminal hangingwall station of the baseline. Follow the same procedure of levelling and centring as discussed in the first point. If possible use the survey instrument for this procedure.
  
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  At the first point, observe the opposite hangingwall station target and resurvey the point. Verify the co-ordinates of this resurvey with the co-ordinates listed in the database.
  
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  From the setup install four sidewall stations around the first setup point and take two observations and measurements to each point. Ensure that the numbering sequence is unique to the sidewall station network.
  
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  Place four additional survey stations in a cluster around the second point of the baseline and take two observations and measurements to each point.
  
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  In the case where there is a risk of the survey stations being covered by shotcrete or a similar type of roof support survey stations should be covered by a plastic sleeve.
  
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  Remove the instrument from the tribrach without upsetting the position of the tribrach.
  
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  Remove the target at the terminal point of the baseline and replace with the instrument. Install the target on the other point of the baseline.
  
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  Ensure that the orientation of the instrument under the terminal point is level and centre.
  
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  Using the freestation program and the four adjacent sidewall stations installed, obtain the position of the setup. The co-ordinates of this resection point must correspond exactly with the original co-ordinates of the terminal point of the baseline. If the co-ordinates coincide the sidewall station network can safely be extended from this point.
  
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  As an additional check the opposite point of the baseline must be resurveyed and compared with the original co-ordinates of the survey station.
  

Changing to hangingwall network from a sidewall survey station network
In order to check the accuracy of the sidewall survey station network it is recommended that a permanent hangingwall check survey baseline be installed every 750 metres, a resection position is only valid for the time that the instrument occupies that specific position. The position of such a baseline should be determined by the ground conditions and should be in the most stable area possible. Where possible, ventilation should be minimal. The following process of establishing a roof point should be followed:

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  A baseline will require at least two hangingwall stations to be installed in a straight line at least 100metres apart, depending on local conditions and mining layout.
  
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  At the position of the second last cluster of sidewall stations, a point on the hangingwall of the excavation must be identified. Ideally the point on the hangingwall must be positioned in such a manner as to provide the strongest possible observation geometry.
  
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  A hangingwall station is installed at this point and painted in a highly visible manner. Where possible the survey station number must also be painted on both sidewalls of the excavation.
  
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  A similar process of identifying a hangingwall position for the second point of the baseline must be followed at the position of the last cluster of sidewall stations.
  
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  A tripod with tribrach must then be levelled and centred by optical or laser plummet to be directly under these newly established hangingwall stations.
  
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  The existing sidewall stations are then used to determine a resection position of the hangingwall survey station at the first point. As an additional check the second point of the baseline can be surveyed from this setup.
  
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  The back check of the resection point should be confirmed by using combinations of different cluster points. It is advisable to use at least four points for observation and that the most robust method of resection available such as least squares weighting be used and confirmed from another setup.
  
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  Once the surveyor is satisfied with the position of the hangingwall station, the instrument and target are reversed and the position of the second hangingwall station determined in the same manner.
  
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  These hangingwall survey stations must be numbered and identified in the database as unique check survey baseline points. Conventional check survey procedures can then be utilized to check the co-ordinates of this baseline from the last check survey baseline.
  
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  In the case where there is a risk of the survey stations being covered by shotcrete or a similar type of roof support it is recommended that the pegs should be covered by a plastic sleeve.
  
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  It is suggested that the error in bearing is propagated to a greater extent than distance it is required to check the azimuth of the baseline against the direction determined by a gyroscope.
  
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  Survey the forward point and recheck using freestation directly underneath the point.
  

The reverse process is to be used when continuing the sidewall station network from the checked and verified baseline.
Positioning of sidewall survey stations
The placement and clustering of sidewall stations will depend on local conditions such as rock structure and condition, wire meshing, visibility and the presence of mine services such as ventilation pipes, water and power cables. The person who is installing the point must take sidewall conditions into consideration and attempt to place the point in such a position where it cannot be damaged by blasting or machinery. In order to provide sufficient redundancy in observation points a minimum of four stations must be installed at each new cluster position. Should this not be possible the new cluster must be positioned in such a manner that at least two other points from the previous cluster of points can be observed. Factors to be taken into consideration when deciding on the positioning and configuration of a cluster of points must include:

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  the stability of the sidewall at the position of the survey station
  
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  The effective spacing of survey stations to provide the best possible observation geometry
  
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  the safety of the survey station with reference to the protection of the point from blast or other mechanical damage
  
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  the visibility of survey stations with reference to irregular sidewall conditions and obstructions caused by pipes and cables
  
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  the placement geometry between clusters of points
  
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  the minimum focal distance of the instrument used must be taken into consideration when survey stations are installed.
  

Marking of survey stations
All survey points are to be marked in a permanent, legible manner according to the mine standard. It must always be kept in mind that any survey station could be the only surviving reference point in the future, it is therefore important to choose the most durable markers possible. Fluorescent paint should be used to mark survey points. The number of the survey point is normally painted in large legible letters next to the point. Blasting fumes, oxidization, water or cement will inevitably obscure the painted number. It is considered good practice to paint a circle or triangle around the survey station. This will assist in the relocation of the point in future. It is recommended that all survey stations be regularly maintained by cleaning and repainting the numbers after verifying the correct survey station number. It has been found that mines have experimented with “smart-pegs” , plastic plugs and stainless-steel bolts as an alternative to the conventional copper tallies normally used in South African mines. The plastic fisher-type plug is currently the most cost-effective survey station available.

Protecting survey stations

The most important method of protecting the survey point integrity is to ensure that the point is installed in a stable and secure position. Survey stations should be protected by a rubber mat or similar material in order to protect them from blast damage or damage from moving equipment. It is important to ensure that any survey stations that may be covered by secondary support such as wiremeshing or shotcrete be protected by a plastic sleeve or similar mechanism in order to prevent the survey station from being covered or made inaccessible.