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

Electronic Distance Measuring technology

In surface surveying, distance measurement traditionally had to be done by means of “stretching chain” between points. If the distance was longer than the chain, such a base could take days or even weeks to measure accurately. This difficulty of measurement was said to be one of the main reasons why triangulation was found to be more effective as distance measuring was reduced to one baseline. Deeth observed that “possibly the most important development has been the application of short range electro-optical systems of distance measurements.” [12] The EDM removed the difficulty of having to measure distances using chains or tapes and the taking of elevations in order to calculate the true distance between two points. As Bannister et al acknowledge “traditionally accurate distance measurement was the most difficult part of a surveying operation, but the introduction of the electromagnetic distance measurement has completely revolutionized this” [94] . As a result of these developments, triangulation can now be supported by trilateration and resection including accurate distance measurement. Hodges and his co-authors observed that a distance meter mounted to a theodolite can provide a “…three-dimensional survey system …” [88].

3.2.1 Electronic recording and data collection

The possibility of recording data electronically and downloading such observations has increased exponentially with the development of the possibility to store larger blocks of data and the reduction in size of the recording devices. In 1984, Deeth reported that “These data loggers now offer reliable storage of sufficient capacity to cover 800 surveyed points …data is transferred error free, typically 5 minutes for 400 observations” [91], compared to 15 years later when Bannister et al reported in 1998 that “one of the features of the latest generation of total station instruments is the provision of a removable PCMIA memory card. This is about the size of a credit card and slots into the side of the instrument, typically each card has 1Mb of memory, which can store 10 000 points.” [94] At the time of writing, as much as 32 gigabyte of information can be stored on a USB flash drive that can be connected to most totalstations.

2. On-board and post-processing software

Surveyors have always attempted to reduce the time taken for calculations in order to increase their productivity. Since the introduction of manual calculators such the “FACET”, most calculations such as resections and triangulations could be done by “Machine Calculations”. Hodges and Smith remark that “Since their introduction in the late 1960’s, the capability of programmable calculators has increased 100-fold, whilst at the same their size and price have decreased considerably.” [12]. Currently total stations do not only record information but also perform a number of complex calculations “on-board” including the “freestation”¹⁸ calculation which allows a surveyor to calculate the position of an unknown point from the observations made to two or more known survey stations. It is proposed that this “freestation” application¹⁹ will be the basis of the calculations made in this research.

It is interesting to note that as early as 1984; Deeth observed that “recent developments have seen the use of the data logger in a reverse role. The information to set-out a project can be entered from the computer into the logger and replayed on site to give the required bearing and distances to the structures.” [91] Data can now routinely be downloaded from CAD²⁰ software packages to the instrument in order to stake-out points in the field and then record and transfer the field observations back to the office computer system.

3. A review of traditional mine surveying methods

The establishment of mine surveying stations has traditionally been in the hangingwall of mining excavations. The SME mining handbook describes the conventional method of mine surveying as follows: “Underground traverse stations should be located in the back²¹ or on the roof, ... prevents their being disturbed or destroyed by current mining operations” [82]. This method of establishing survey control in a mining tunnel has been used for more than a hundred years. Gillespie makes reference to this method of surveying in a survey manual dating from 1910 [16] and Metcalfe confirmed that this method was still current in his textbook dated 1951 [22].

The method of a hangingwall survey, refer to Figure as it is understood now, requires the surveyor to establish his instrument directly below a known survey station with the help of a plumb bob²² suspended from the survey station. The instrument is then orientated to a reference point (referred to as a backsight²³) before observations are made to the new survey station (referred to as a foresight²⁴). The new survey station is installed by drilling a small hole into the hangingwall of an excavation. A wooden or plastic plug is placed in the hole and a brass spad²⁵ is hammered into position. The plumb bob or survey target is then sighted from the known survey station. This method of fixing the position of the new survey station consists of measuring a slope distance, vertical- and horizontal angle to the backsight and foresight. This method of determining the position of a survey station is defined as an “open traverse” as the survey network underground normally will not allow the closure of the loop during normal activities.
Traditionally observations were made using an opto-mechanical theodolite²⁶ to measure the vertical and horizontal angles and a 60 metre steel tape to measure the slope distance between the station and the new station. By reducing these observed angles and distances a rather simple calculation could be done to calculate the Y and X co-ordinates as well as the elevation of the new survey station. With the introduction of new surveying technology the measuring of distances by steel tape has been replaced by the Electronic Distance Meter (EDM). Other than the electronic recording of angles and distances with the new total stations, arguably nothing has changed in the method of surveying the position of a new survey station in the last 100 years. The position of any new survey station must be accurate to within the minimum standards of accuracy determined by the Mine Health and Safety Act of 1996 (Act 29). It is interesting to notice the number of standards and procedures still in place today that was developed to accommodate the older technology.
In surface surveying the co-ordinates of a new survey station can be determined by a number of methods such as traversing and by using intersections. There are two types of intersections namely triangulation and resection. Triangulation is defined by the City Engineers Department, Durban Corporation, as making angular observations to an unknown point from two or more fixed points. [34] Uren and Price define a resection as angular observations made at an unknown point to two or more fixed points [95]. Triangulations and resections have not been widely used in the underground environment due to the limitations placed on the length and geometry of observations required to obtain an accurate fix of the position of a point. These limitations, coupled with the perceived reluctance to test new surveying methods and the inherent accuracies of the older generation of surveying equipment have made the introduction of alternative methods of surveying in the mining industry a slow process. The technique of determining the position of a station using triangulation and resections has been used with great success in the tunnelling industry. There have been some attempts to adapt this method of tunnelling surveying into the underground mining environment in Australia as described by Jaroz and Shepard [96] and in Europe as described by Ludvigsen [97]. It is argued that the accuracy of using resection or triangulation in a narrow tunnel environment will depend on the geometry of the observation that would be able to be within the constraints dictated by the mining layout. By implication it is proposed that it should be established whether or not it will be possible to successfully adapt traditional tunnelling surveying techniques to the South African mine surveying context and determine if these techniques will meet the minimum standards of accuracy required by current South African mining legislation.

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