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

8.5. Phase 2. Installation

A number of aspects need to be taken into account when installing sidewall survey stations in a tunnel. The first factor to be considered must be to decide whether an existing hangingwall survey network will be changed into a sidewall survey station network or if an existing sidewall survey station network is to be extended. Some aspects to be considered when making a transition from hangingwall to sidewall reference stations are discussed under a separate heading. It is recommended that in all cases permanent hangingwall survey stations are to be used for a fixed reference baseline. The transition from sidewall stations to a hangingwall baseline will also be discussed.

Risks that will influence the accuracy of the survey are normally not considered during a risk assessment but only as to how the integrity of the survey has a direct impact on the health and safety of employees of a mine. The following risk factors impacting on the accuracy of a surveying network should be evaluated and a plan of action mitigating, avoiding or reducing the risk of each of these factors be documented. The main advantage of the sidewall station survey method is that the use of ladders and the resultant working-at-heights hazards are avoided. In the case where transition is made from hangingwall to sidewall or back to hangingwall the risk cannot be avoided and all safety protocols for working-at-heights will remain in effect. On a day-to-day basis the sidewall method will not require ladders and the resultant safety and productivity benefits will prove to be of great use. A detailed hazard and risk assessment is tabulated here, but it should be noted that the hazards identified are generic in nature and site specific hazards must be investigated and analysed:

Table . Risk analysis for the installation phase of installing sidewall stations.

Risk	Hazard	Contributing factors	Mitigating Actions
Potential injury to survey crew	Hand tools – impact or penetration injury Foreign body in eye Falls of ground Moving equipment	Drilling dislodging dust Drilling dislodging side- or hangingwall Paint entering eyes	Wear correct PPE Barring Equipment in good condition Training in use of equipment
Potential injury to survey crew	Working-at-heights	Ladder safety Footwall conditions Lifting equipment	Wear correct PPE Equipment in good condition Inspection of lifting equipment Training in use of equipment
Survey reference survey stations damaged or destroyed	Off-line mining Mine layout must be changed Unplanned breakthrough Mis-holing of connections Damage to structural design (pillars) Insufficient control for redundancy	Blast damage Theft or intentional vandalism	Use non-metal survey stations Ensure stations are placed in area where probability of damage is minimized Freestation software should be able to identify error in position fix
Incorrect identification of survey stations	Off-line mining Mine layout must be changed Incorrect survey observations	Incorrect numbering Survey stations identified incorrectly Incorrect marking of survey stations	Communication between crew members Surveyor must check all installations personally Freestation software should be able to identify error in position fix
Survey control inaccurate	Insufficient control for redundancy Off-line mining Mine layout must be changed Unplanned breakthrough Mis-holing of connections Damage to structural design (pillars)	Survey stations placed with poor geometry Survey stations too close to observe (instrument focal length) Survey stations provide weak geometry for position fix Survey stations identified incorrectly	Training in application of method Direct supervision of installation process Freestation software should be able to identify error in position fix Software should prevent final position fix if not within determined standards

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1. Survey Station installation

Before the installation of survey stations in the sidewall of an excavation can commence the area where work will be done must be thoroughly inspected by a competent person. Any aspects that could negatively influence the safety of the sidewall such as prevalent geological issues and mining layout must be taken into consideration. On coal mines making use of continuous miners, the sidewalls are normally barricaded off and a certain amount of scaling is expected. In these conditions sidewall stations would be dangerous to install and an alternative method of surveying or installation would have to be considered.

The installation of a peg cluster is recommended to be placed on the required gradient of the excavation in a configuration where pairs of points are installed perpendicular to the direction of mining. Some instruments have the ability to install side-grades using the “major-minor dip method” for gradient.

2. Changing to a sidewall survey station network from a hangingwall network

The method of establishing the first cluster of sidewall points will depend on the availability of existing survey control in the area. It is imperative that the new survey network must be established from an already established and check-surveyed baseline, preferably a baseline of which the true direction has been verified by gyroscope. The detailed method to be employed to transfer the network from the hangingwall to the sidewall is described in detail in Appendix 7.

3. 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 check survey baseline is installed every 750 metres. As a resection setup is essentially a non-permanent setup that is only valid for the time that the instrument occupies that specific position it is recommended that a permanent hangingwall baseline is established. 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. The detailed method to be employed to transfer the network from the sidewall back to the hangingwall is described in detail in Appendix 7. Such a baseline must be clearly marked underground and on all underground plans. It is recommended that these baselines should be established on every level of the line and at each junction between working areas.

4. Positioning of sidewall survey stations

The findings of this research suggest that four sidewall stations will provide the accuracy required for a primary survey network. In order to provide sufficient redundancy in observation points a minimum of four stations must be installed at each new cluster position. Factors to be taken into consideration when deciding on the positioning and configuration of a cluster of points⁴³ must include the stability, spacing, safety, visibility, and the geometry between points.

The type of prism used may influence the accuracy of observation as it has been found that under certain conditions the optical centre of the prism does not coincide with the prism alignment marks. At close range it may happen that the observer may have to find the exact optical centre of the prism by aligning the instrument cross hairs with the cross in the centre of the prism.

5. Marking survey stations

All survey points are to be marked in a permanent, legible manner. In most cases South African mines prefer to use copper disks with a number stamped on it. In recent years copper theft has become prevalent on South African mines and as a result alternatives to copper have been investigated such as plastic and stainless-steel markers as discussed in detail in Chapter 6. It must always be kept in mind that any survey station could be the only surviving reference point in the future, so it is therefore important to choose the most durable markers possible. Survey stations must be marked in a clearly visible and permanent manner in order to assist in the relocation of the points in future. It is essential to the sidewall survey station method that the survey station number is checked before painted on to the sidewall. Survey stations should be regularly maintained by cleaning and repainting the numbers.

6. Protecting survey stations

The fundamental requirement for survey station installation is to carefully select a stable and secure position for the installation. After the installation of a sidewall station it is recommended that a mechanical barrier should be installed to protect the survey station from any damage. In the case where secondary support may be installed, extra precautions must be taken to prevent damage from the wiremeshing, gunnite⁴⁴ or shotcrete installation.