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

8.9. Phase 6. Check survey

The main weakness of the wallstation method of surveying has been identified as the risk of error in positional fix created by poor observation procedures. The research indicated the likelihood of error propagation in the bearing of the network. It is therefore essential to check the integrity of a survey network, on different levels of the mine. Where possible, surveys should be closed on the same point of origin. If a closure loop is not possible an attempt should be made to perform an inter-level check between survey networks. A further check on the bearing closure should be made by gyroscope. Any deviation or error propagated by the survey network can be identified by the gyroscope direction determination and the network is normally adjusted to this gyro bearing. The main method of checking the accuracy of the survey when short distances are surveyed would be to take long observation shots bypassing intermediate survey stations, thereby reducing the propagation of error by reducing the number of stations observed.

1. Check survey baseline

It is suggested that a check-survey baseline should be established at the start of the network, and thereafter at least every 750 metres along the length of the sidewall station survey network. The method of transferring the sidewall network to a hangingwall network for the purpose of a check survey is discussed in detail in Appendix 7. It is recommended that such a baseline be installed in a stable, safe area and verified by an independent check-survey. Such a check should include a gyro direction determination of the baseline bearing. The position of the two markers should be indicated in the most permanent method available. It is proposed that such check survey baselines be placed at all important junctions on every level of the mine where production takes place.

2. Accuracy

The minimum standard of accuracy of a check survey should be within the limits of a Class “A” survey as currently required by the MHSA. The requirement of a minimum of a 2 minute error between two consecutive survey stations is considered to be inadequate to define the accuracy of a check survey. The accuracy with which the direction of a bearing can be verified is dependent on the accuracy of the skill of the operator and the type of gyroscope used. A gyro direction can be accurately determined within 30 arc seconds when using current technology. It is recommended that the azimuth error between two consecutive check survey stations be a maximum of 1 minute.

At present there exists a need for the MHSA to define the method of calculating an error vector in any closure survey. The error vector should be defined to be calculated using the following formula: . In the case where a survey cannot meet the minimum reference object requirements or has less than ideal geometry it is advised that the survey stations surveyed from such a resection be downgraded in confidence and indicated as a local survey meeting the Class “C” requirements of the minimum standards of accuracy. Once the check survey of such survey stations or portion of the network is completed it is recommended that all checked survey stations be indicated with a postfix that will distinguish the check surveyed stations from the stations not checked.

3. Process of check survey adjustment

In the mining environment where development is undertaken from a shaft complex through narrow tunnels into the workings of the mine, a check survey can become a problematic task. The unique problem in a vertical shaft environment is that each level of the mine will have a completely independent survey even though all possible precautions are taken when transferring the survey underground. It is in most cases not possible to do independent closure checks during the routine surveying process. Traverses made in tunnels will almost always fall within the definition of an open traverse. In most such cases the standard for check surveying a network normally consists of a check survey made from the station making use of long baselines to eliminate most of the short bases used to extend the traverse network into the mine. The closure co-ordinates of the check survey made in this manner are then assumed to be the correct co-ordinates for the “closure” or “breakthrough” point. A second method of check surveying is to perform a closure survey between survey networks on the different levels of the mine. This very effective method requires a breakthrough between levels in order to perform such a check.

The most accurate form of check survey is not always possible in a narrow tunnel layout. This form of check survey requires the survey to be closed on the point of origin. In this manner the accuracy of the survey can be confirmed. It is argued that although the check survey of an underground network can verify the accuracy of the survey, it should not be assumed that the precision of the survey network is verified at the same time. It is proposed that precision of a survey network can only be made possible when a gyroscope azimuth determination is performed. The following risks identified are associated with the check survey phase of the survey network:
Table . Risk analysis of the check survey phase of installing sidewall stations

Risk	Hazard	Contributing factors	Mitigating Actions
Survey network not accurate Survey network exceeds the prescribed standards of accuracy Survey deemed inaccurate by DMR leading to check survey to be conducted on mine’s cost	Off-line mining Mine layout must be changed Unplanned breakthrough Mis-holing of connections Misalignment of connections Damage to structural design (pillars) Mine boundary mined through	Poor survey preparation Poor installation and maintenance of survey reference objects Incorrect calculation of survey station co-ordinates Inadequate storage of survey observations and calculation records	Install check survey baselines at important junctions in the survey network Conduct a regular check survey Keep check survey within 750 metres of any workings Use more than two reference stations for position fix Use checks from reference objects not in the same group of survey stations s
Deterioration of survey network quality Errors propagated in survey network	Error in bearing propagated through network Error propagated through small errors at each survey station	Incorrect check survey protocol in balancing and adjusting a survey network Incorrect survey observation and calculation protocol	Conduct gyroscope surveys to verify bearing accuracy Install permanent underground check survey baselines Check survey protocol in balancing and adjusting survey network Store hard copies of all special surveys

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