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

2.2. Recent Mine safety incidents related to mine surveying

The importance of accuracy in Mine surveying has been highlighted in the recent past by mining incidents where the accuracy of mine plans has had a direct impact on the health and safety of mine workers and the communities living around the mining area. It is important to understand how these incidents relate to the accuracy of mine survey networks as in these cases accuracy and safety is inter-related.

1. The Gretley Coal Mine Disaster

The disaster at Gretley coal mine in Australia highlighted the need for accurate mining plans. According to a report based on the findings of the investigation into the matter when “Disaster struck the Gretley Colliery near Newcastle (New South Wales. Australia) on 14 November 1996. Miners inadvertently broke through into the flooded workings of an old abandoned mine, and four miners died in the inrush of water.” [23] It was found during the investigation that “The Mine Surveyor at the time assumed that the plans provided by the department were accurate and the manager, relying on his surveyor, made the same assumption.” [23] The incorrect plans were described as a classic example of latent error, “as the judge in an earlier inquiry noted, the plans “sat like a loaded gun in the archives”, waiting to be fired.” The investigation found that the “inrush from old workings was a well-known hazard with the potential to cause multiple fatalities, …” [23]. In South Africa the accuracy of surveying and the plans generated from this surveying information is aimed at preventing such occurrences. In the case of the Gretley coal mine disaster the company was fined $730 000 [23]. It is interesting to note that subsequent to these findings the MHSA was amended to include a provision stipulating that the employer must take reasonable measures to ensure that a competent person determines the accuracy of any plan not prepared by him or her where such a plan may create a risk of endangering the health and safety of any persons. [11]

2. Beaconsfield Mine 25 April to 9 May 2006

During the Beaconsfield Mine tragedy on ANZAC Day 2006 the role of the Mine Surveyor was one of monitoring the progress of the rescue effort and guiding the mining operations towards the position of the trapped miners. The report describes how “Probe holes were drilled with one breaking through that the two miners could touch. A few hours later in the early hours of the 9th of May, 2006 the two miners were freed and the rescue mission successfully completed.” [24]. This sterile description of the events leading up to a drill hole 15.5metres being aligned, drilled and holed exactly between the heads of the two trapped miners lying ion their sides in a small cavity does not do justice to the skill of the surveyor who had to “employ every skill of his profession…” [25]. The 54mm diameter drill hole would establish a link between the trapped miners and the rescue team through which food, water and medication could be provided to the miners. The margin of error normally afforded to align a 4.5 metre by 4.5 metre excavation would not be sufficient to ensure a successful holing. The surveyor, Simon Arthur, had to align the drill rig with a small spot 15.5 metres into solid rock between the two men. “Piscioneri, following Simon Arthur’s calculations, had drilled…precisely between the heads of Todd Russel and Brant Webb. The whirling drilling head, still spewing water, had rained all over them.” [25] This dramatic account of the rescue attempt underlines the often forgotten role of the accuracy required of work routinely performed by mine surveyors under extremely difficult circumstances. The accuracy of the survey network and survey techniques used by Mr. Arthur could be said to be the main contributing factor to the successful of two miners trapped for 14 days in a cavity of rock barely large enough to accommodate the two miners.

3. The Chilean Mine Rescue

The recent event (October 2010) in which 33 Chilean miners were trapped underground, further emphasises the importance of up-to-date and accurate mine plans produced by the Mine Surveyor, as well as the accuracy of the survey techniques used to construct these plans. According to news reports at the time of the discovery of the miners “Rescuers had tried seven times before to reach the shelter, most recently drilling 2,300 feet (700 meters) and missing the target on Thursday. They blamed the error on the company’s maps of the mine.” [26] When eventually located “The miners sent up notes attached to probes drilling into the area of a refuge located 2,297 feet -- almost one-half mile -- underground. [27] According to Livingstone-Blevins more than 14 000 measurements were made on the 10 000metres of boreholes drilled during the rescue operation [4]. The drill holes served as the only contact to the outside world for the trapped miners, serving as the only access to food, water and communications. The rescue was eventually successful but it could be speculated that updated and accurate mine plans would probably have enabled the rescuers to locate and contact the trapped miners faster than was the case. The accurate location of the refuge chambers would possibly have saved some of the tremendous drilling costs that were incurred during the rescue.

2. An Overview of relevant standards of accuracy.

The extreme environment in which the Mine Surveyor must perform his duties is strictly regulated by the accuracy requirements imposed by the state and the mining companies. As has been illustrated by the previous section, the lives of mine workers depend directly on the accuracy of the survey network that controls the workings of the mine in which they work. Bannister stated that “understanding the minimum standards of accuracy that limit the accuracy of the measurement techniques is but one step to ensuring specifications are achieved” [28]. In order to understand the numerous standards of accuracy employed in the mining and tunnelling environment a summary of the most relevant Acts, Regulations and standard Procedures will be discussed in the next session.

1. The Mine Health and Safety Act; 1996 (Act 29 of 1996)

In the South African context all mine surveying work is regulated by the Mine Health and Safety Act. Primarily the Mine Health and Safety Act, Chapter 2 (22) requires that:

“every employee at the mine, while at the mine must: (f) comply with the prescribed health and safety measures. It is an offence to fail to do anything required by this act 91(1) Any person, including and employer, who contravenes, or fails to comply with, any; (a)provision of this act…(c) ….commits and offence and is liable to a fine or imprisonment as may be prescribed.” [11]
The first time that the minimum standards of accuracy for Mine surveying were described in South Africa was in a “Volksraad Besluit” dated the 25^th of July 1894, Article 997, titled “ Instructies voor Mijnopmeters in de Zuid Afrikaanse Republiek” regulating mine surveying activities in the Transvaal province of South Africa. It stated that the Mine Surveyor was to be held responsible for the accuracy of all the work performed by him and would be held responsible for any damage that resulted from any inaccuracies. According to these regulations a surveyor needed to perform all work to the following “allowable error” in the horizontal plane of 1/500 and a lateral deviation of 1/750 from the measured length. In the case of special surveys that required the determination of the position of shafts and connecting drives in the case of closures, the allowable error would be half of the defined allowable error [29].
The most recent update of the Mine Health and Safety Act, 1996 (Act 29 of 1996) Chapter 14 (5) requires that the employer must take all reasonable measures to ensure the safety of all persons that may be endangered by mining operations [17]. An unplanned breakthrough caused by inaccurate surveying may result in loss of life caused by the fall of ground⁹, injury by explosives or the inrush of water or gas into a working end. The consequences of an unplanned breakthrough as the result of inaccurate survey plans, was demonstrated in the Gretley Coal Mine disaster in Australia. The investigation found that the“Inrush from old workings was a well-known hazard with the potential to cause multiple fatalities, …” [23]
The MHSA regulations make clear provision for the limits of accuracy to be expected from any survey. The minimum standards of accuracy prescribed by the Mine Health and Safety Act are as follows:

17(14)(b)”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:”

( )

“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].

The MHSA makes a clear distinction between three classes of survey accuracy required under defined circumstances, namely:

17(14)(b)(i)”the allowable error for a Primary Survey (Class A) is not greater than A metres. Primary Survey means any survey carried out for the purpose of fixing shaft positions, shaft stations, underground connections, upgrading of secondary surveys to primary surveys and establishing primary surface survey control;” [17]
The abovementioned minimum standards of accuracy will be used in the proposed research work as the primary focus will be the establishment of a primary survey network. As can be seen from the error for secondary and tertiary surveys the limit of error remains a function of the limit of error determined for the primary survey network. Chrzanowski defines three “orders” of survey networks as “control networks consist of first order loops which serve as basic control and are run in permanent mine workings, second order traverses run into headings and development areas, and third order stations used for detailed mapping of excavated areas and daily checks of mining progress in stopes and headings” [30].
According to the MHSA the second and third order of accuracy for survey networks are defined as follows:
17(14)(b)(ii) “the allowable error for a Secondary Survey (Class B) is not greater than 1,5A metres. Secondary Survey means any survey carried out for the purpose of fixing main or access development, mine boundaries and establishing secondary surface survey control;” [17].

( )

It can be argued that the accuracy of a survey network on an underground level of a mine can be defined as a secondary survey and therefore be classified as a Class “B” network with lesser accuracy. The final category of survey network is a tertiary survey defined as survey networks that are extended into the production areas of a mine for measuring purposes and is defined as follows:

17(14)(b)(iii) “the allowable error for a Tertiary Survey (Class C) is not greater than 3A metres. Tertiary Survey includes survey stations established from secondary survey stations for localized survey purposes;” [17].

( )
It is generally accepted in the South African mine survey industry that a “rule of thumb” of 20mm should apply to all surveys. In an e-mail communication with Bals, he argues that this rule of thumb “… is an adaptation of the Class A survey standard using a 60 metre steel tape: 0.015 + (60 / 30000) gives 0.017m or 20mm for easy implementation.” [31]. The standards of accuracy are tabulated here to indicate the distance at which the “rule of thumb” of 20mm applies. It is indicated that the minimum accuracy for a Class “A” survey is limited to 15mm and a Class “B” survey as 22mm, which would confirm the rule of thumb of 20mm used in common practice. A tabulation of distances to compare the various classes of accuracy is illustrated in Table . A Comparison of the various limits of error
The Mine Health and Safety Act, Chapter 17 prescribes the distances from which any mining may take place from any feature that needs to be protected from mining activities in the following manner: “The employer must ensure that –

17.6.1 no mining operations are carried out under or within a horizontal distance of 100 (one hundred) metres from buildings, roads, railways, reserves, mine boundaries, any structure whatsoever or any surface, which it may be necessary to protect,..” [17]
The regulation requires the employer to ensure that all reasonable measures be taken that no boundary pillars¹⁰ are mined without the permission of adjacent employers and the inspector of mines [17].
In the case of a survey 100metres in length the following standards of accuracy are defined. Using Equation 1, a Class “A” survey will require a closure within 0.0183m. From Equation 2 a class “B” survey for the same distance would be 0.0275m and for a Class “C” survey a closure of 0.055metres. [32]
The MHSA makes reference to the original Land Survey Act when defining the limits of allowable error. The minimum standards of accuracy described in the Mine Health and Safety Act makes provision for the conventional traverse surveying method only. In order to better understand the minimum standards of accuracy used for the fixing of co-ordinates of a point by alternative survey methods such as intersection, the Land Survey Act needs to be investigated.

2.3.2 The Land Survey Act, 1997 (Act No 8. of 1997)

The minimum standards of accuracy described in the Mine Health and Safety Act, 1996 (Act 29 of 1996) were derived from the Land Survey Act, described below. This Act describes the accuracy of survey points established by traversing, intersection and resections.

(1) “A surveyor shall determine the positions of all stations and beacons within the limits of accuracy prescribed in regulation 5 and shall check every part of his or her survey.” [33]

The Land Survey Act describes the geometry of observations that would be acceptable for the location of survey points.

1. 
   “when its position is determined by intersection or trilateration, the angle at the vertex of any triangle used in such determination shall not be less than 30 degrees nor greater than 150 degrees;”
   

The following section of the Act defines the accepted geometry for this type of surveying and will be considered during the research.

(b) “when its position is determined by resection, at least four favourably situated known points shall be used, and sufficient observations shall be made to ensure the required accuracy of determination of its position: Provided that at least one arc shall be observed;”

(c) “when its position is determined by a single triangle only, observations shall be made at all three points and on at least two different parts of the circle;”
The limits of allowable error “When the position of a point is determined by polars, triangulation, trilateration, …derived from the final co- ordinates of the point fixed shall be of the order “for triangulation, resections and other forms of fixing the position of a point is defined by the following formula:

( )

Using this equation for a triangulation over a distance of 100 metres 0.0185metres is calculated. For a Class “B” survey the following equation is used:

( )

Using this equation for a triangulation over a distance of 100 metres 0.0277metres is calculated. For a Class “C” survey the following equation is used:

( )

Using this equation for a distance of 100 metres 0.0554metres is calculated. [32]. It should be investigated if the formula for calculating a standard of errors for intersections should be introduced to the MHSA. According to the Survey Manual of the Durban Corporation the limits of error for a traverse can be determined by the formula,

( )

Using this equation for a traverse over a distance of 100 metres 0.0142metres is calculated. For a Class “B” survey the following equation is used:

( )

Using this equation for a traverse over a distance of 100 metres 0.0142metres is calculated. For a Class “C” survey the following equation is used:

( )

Using this equation for a traverse over a distance of 100 metres 0.0425metres is calculated [32]. If the formula is used to create a table with distances, it can be seen that for distances from 1 to 10 metres the accepted limit of error will be 0.010mm, increasing to 0.012m for distances up to 50m [34], which is a greater accuracy requirement than that of the MHSA for the same distances.

2.3.3 Federal Geodetic Control subcommittee (FGCS), Part 4, USA

The American Federal Geodetic Sub Committee was formed “(1) to provide a uniform set of standards specifying minimum acceptable accuracies of control surveys for various purposes, and (2) to establish specifications for instrumentation, field procedures and misclosure checks to ensure that the intended level of accuracy is achieved.” [35] . These standards from 1998, were largely based on the existing U.S. Army Corps of Engineers engineering surveying standards. The standards used for aligning tunnels are normally done to 1 part in 10 000 to 20 000 but for extensive projects may increase to 1 part in 50 000 up to 1 part in 100 000 [36]. The committee states that “these standards are independent of the method of survey and based on a 95 percent confidence limit” According to Wolf and Ghilani “triangulation, trilateration and traverse surveys are included in the 1984 horizontal control standards and specifications, and differential leveling is covered in the vertical control section [36]

Table . FGCS Minimum closure standards for Engineering and Construction Control Surveys [35]

Traditional surveys Order and Class	Relative accuracy between points	Relative accuracy required between benchmarks (Vertical)
First order	1 part in 100 000	0.5mm*√k
Second Order Class I	1 part in 50 000	0.7mm*√k
Second Order Class II	1 part in 20 000	1.0mm*√k
Third Order Class I	1 part in 10 000	1.3mm*√k
Third Order Class II	1 part in 5 000	2.0mm*√k
Construction	1 part in 2 500
*k is the distance between benchmarks in kilometers

If these values are converted to be comparable to the MHSA and Land Survey Act allowable errors, for a comparable distance of 100 metres, a third order Class 1 relative accuracy of 1:10 000 correlates the closest to the MHSA Class “A” standard of accuracy, with a distance of 0.010metres and 0.020metres for a Third Order Class II relative accuracy of 1:5 000 comparing with the Class B survey defined by the MHSA. [32] .

2.3.4. Positional accuracies for primary control systems (ISO4463)

The ISO4463 standard lists that the “permissible deviations of distances and angles obtained when measuring positions of primary points, and those calculated from the adjusted coordinates of these points shall not exceed” [37] According to Kavanagh, the formula used to calculate the standard of accuracy is as follows:

( )

( )

where:

L is the distance in metres between the primary stations in the case of angles L is the shorter side of the angle
Using these equations to compare the allowable error per distance to that of the MHSA requirements it is found that the allowable error is calculated as 0.0075metres over 100metres, or a ratio of 1:13 333. [32]. These limits of accuracies appear to be for primary high accuracy surveys, normally not found in the underground environment and will not be considered in the research.

5. Intergovernmental Committee on Surveying and Mapping (ICSM SP1.7)

The Australian committee that implemented the standards and practices defined in version 1.7 of this document “defines technical standards and specifications for surveys undertaken at a state or Commonwealth level.” and further defines the term of “Positional Uncertainty is the uncertainty of the co-ordinates or height of a point, in metres at the 95% confidence limit, with respect to the defined reference frame.”. The class of survey network is defined as follows: “Class is a function of the precision of a survey network, reflecting the precision of observations as well as suitability of network design, survey methods, instruments and reduction techniques used in that survey.” [38]. This code seems to be currently used in most parts of Australia. The standard describes “The allocation of CLASS to a survey on the basis of the results of a successful minimally constrained least squares adjustment may generally be achieved by assessing whether the semi-major axis of each relative standard error ellipse or ellipsoid (i.e. one sigma), is less than or" equal to the length of the maximum allowable semi-major axis (r) using the following formula: “

r = c ( d + 0.2 ) ( )

Where:

r = length of maximum allowable semi-major axis in mm.

c = an empirically derived factor represented by historically accepted precision for a particular standard of survey.

d = distance to any station in km.
The values of “c” assigned to various classes of survey are shown in Table .

Table . ICSM SP1.7. Classification of Horizontal Control Survey [39]

Class (one sigma)	C value	Typical Application
3A	1	Special high Precision
2A	3	High Precision National Geodetic Surveys
A	7.5	National and State geodetic surveys
B	15	Densification of geodetic surveys
C	30	Survey co-ordination surveys
D	50	Lower class surveys
E	100	Lower class projects

Using Equation r = c ( d + 0.2 ) ( ) for a comparative distance of 100metes the length of allowable semi-major axis is calculated as 0.015metres if the Class “D” category or lower class surveys are used. This method of defining the semi-major axis of the error ellipse is easy to calculate and should provide a basis for a practical alternative method of defining the standards of accuracy for the current MHSA.

5. Canadian Survey Standards, General Instructions for Surveys, e-Edition, Appendix E4 – Accuracy standard for legal surveys

The Canadian survey standards take a very practical approach to the limits of allowable error and recognize that a single formula cannot serve the purposes of very short surveys, such as those found in an underground survey as well as very long surveys: “For a very long survey traverse, the allowable error may be misleading and, in some cases, may conceal a blunder. For a 100 m measurement it can be difficult to meet a 1: 5000 error of closure specification.” [40] The error ellipse for the standards was determined using this argument “Using normal legal survey instrumentation and methods, 2.0 cm was chosen as an upper limit for an allowable error for a distance of 10 m, and 10.0 cm was chosen as a limit for a distance of 1000 m.” [40]. The limit of allowable error can be calculated using the following formula:

r = 8(d+ 0.25) ( )

Using this equation for a comparative distance of 100metes the length of allowable semi-major axis is calculated as 0.028metres.

5. The Institute of Mine Surveyors of South Africa: Guidelines for standard Mine Surveying practice

The technical procedures and guidelines for mine surveying published by the Institute of Mine Surveyors of South Africa states the original limits of error for a Primary survey. “The position of every primary control station shall be established within the limits of error of a category class "A" survey.” [41]. A minimum of two arcs must be taken with a base change between arcs and distances should be measured at least twice from each end of the line. [41] These guidelines were based on the Minerals Act, Act 50 of 1991, repealed 14 November 2004, and have not been updated after the new MHSA replaced the old Act.

These guidelines also describe the allowable limit of error in terms of the old Act with the following classes:

“The position of every primary survey control station shall be established within the limits of error of a category "A" survey. Special surveys carried out for fixing boundary beacons, shaft centre positions, and holing positions on surface relative to established connections underground are considered extensions to the primary network. All survey observations shall be made with instruments capable of accuracies within the allowable limits of error for such survey. A minimum of two arcs must be taken with a base change between arcs (see section eight on instrumentation) An arc is defined as a set of direct and transit observations.” [41].

Furthermore the guidelines define in a similar manner to the Land Survey Act the basic observation geometry to be adhered to in the following manner:

(2) Unless otherwise adequately checked, the minimum requirements for the determination of the position of a point are:

(a) when its position is determined by intersection or trilateration, the angle at the vertex of any triangle used in such determination shall not be less than 30 degrees nor greater than 150 degrees; [41]

(b) when its position is determined by resection, at least four favourably situated known points shall be used, and sufficient observations shall be made to ensure the required accuracy of determination of its position: Provided that at least one arc shall be observed; [41]

(c) when its position is determined by a single triangle only, observations shall be made at all three points and on at least two different parts of the circle; [41]

5. Department of Industry and Resources, 2005, Mines survey - Code of practice: Safety and Health Division, Department of Industry and Resources, Western Australia

The Western Australian Department of Industry and Resources prescribes in the Mine survey Code of Practice the principle of establishing baselines¹¹ “Each underground mine should establish a baseline in the underground workings of each level off a shaft or at least every 750 m of traverse in a decline access. [42]. This code describes in detail all the protocol to be observed when establishing and marking such a baseline. The Code of Practice prescribes the limits of allowable accuracy as a ratio: “The position of mine workings should be established with an accuracy of 1:5000 or to within 0.5 m absolute error to mine datum”, and angular measurements (horizontal and vertical) may have a “maximum standard error of ± 5", and distance measurements may have a “maximum standard error of ± (3 mm + 5 ppm)”. [42]
According to the Australian Mines Safety and Inspection Regulations 1995. 3.49. (1) “A person who carries out a survey at a mine must ensure that —

(a) the survey is carried out using instruments and equipment of precision equal to best current industry standards and technology; and

(b) the survey is carried out to a standard that accords with good engineering practice and is to an accuracy of not less than 1:5 000.” [42]

The use of these standards have been confirmed by Arthur who states that “The code of practice for Mine Surveyors in Western Australia issued by the DOIR stipulates that resurveys are to be conducted at least every 750m travelled to an accuracy not less than 1:5 000 or 0.5m in absolute position.” [43] Using the ratio of 1:5 000, absolute position accuracy for the comparative distance of 100metres is calculated as 0.020metres. [32] From an analysis of this ratio calculated for the allowable error is close to the South African minimum standards of accuracy as defined by the regulations in the MHSA as well as other standards of accuracy.

5. Survey and Drafting Directions for Mine Surveyors 2007 (New South Wales Coal)

The regulations, issued by the Surveyor-General of New South Wales, Australia, on the advice of the Board of Surveying and Spatial Information pursuant to clause 4 of the Surveying Regulation 2006, Mine Health and Safety Act 2004, state in paragraph 3.3 for Control Surveys and Subsidiary Surveys that the limits of accuracy for surveys should be:

3.3.1 Accuracy: “Each control survey and subsidiary survey must be planned and surveyed to ensure these surveys satisfy the conditions to achieve a standard of accuracy as prescribed in ICSM, SP1 to achieve Class D or better. All control surveys and subsidiary surveys observed survey data must be analyzed to ensure all control surveys and subsidiary surveys achieve a standard of accuracy as prescribed in ICSM SP1 to a minimum standard of Class D. When calculating compliance to ICSM SP1 Class D via the formula

r = c (d + 0.2) ( )

where: d = distance to any station in km, with a minimum value of 1(km).” [44]

and describes the accuracy for secondary surveys in the following manner: “Secondary surveys shall be employed by the Mining Surveyor where necessary to accurately locate all of the Mine workings on the Mine Workings Plan to within 1mm at 1 : 2 000 Scale. Such surveys shall be completed to the highest appropriate standards of accuracy.” These standards are a copy of the Intergovernmental Committee on Surveying and Mapping (ICSM SP1.7) for a Class “D” Lower Class survey. The calculated standard for a distance of 100metres is 0.015metres. [32]

5. Limits of error defined in the Tunnelling industry

The tunnelling industry has been making use of different survey methods for a number of years. Fowler states that “the most critical factor in a tunnel breakthrough is lateral error, that is, at right angles to the tunnel center line. This component is very much dependent on the accuracy of the horizontal angle observations. Distance observations have a greater effect on the longitudinal error which is of less importance.” [45]. According to Fowler “a general rule of thumb with tunnel surveys is that breakthrough errors of 10mm per kilometer should be attainable.” [45]These standards of accuracy are generally of a higher class than that required in the mining industry, it is however investigated to highlight the differences in accuracy standards and the different methods of surveying employed to achieve these requirements.

The limits of error in tunnelling according to the Tunnel Engineering Handbook is defined for surface establishment of control points as “Triangulation, second order class 1 closing error not to exceed 1:50 000”. [46] For the establishment of the “working line” in a tunnel the following definition is used: “Primary traverse, second order Class 1. The survey methods used to transfer working line and elevation underground and set the laser beam to the tunnel construction control line and grade should provide for this precision: [46]

• 
  Angular measurements to the nearest one second of arc
  
• 
  Stationing to the nearest thousandth of a foot
  
• 
  Benchmark elevation to the nearest thousandth of a foot
  
• 
  The precision of the target readings of the laser control system and tunnel ring measurements as performed after every shove should be in the range of one to two hundredths of a foot. The short time available for the performance of these measurements explains the lesser precision required”. [46]
  

These limits of error seem to be derived from the standards of the 1998 Federal Geodetic Control Subcommittee (FGCS), USA, which in turn correlates with the existing U.S. Army Corps of Engineers-Engineering surveying standards. The standards used for aligning tunnels are normally done to 1 part in 10 000 to 20 000 but for extensive projects may increase to 1 part in 50 000 up to 1 part to 100 000 . [36]

3. A Comparison of Minimum standards of accuracy

The South African Mine Surveyor must ensure that the establishment of all primary survey network control falls within the minimum standards of accuracy dictated by the MHSA. The minimum standards of accuracy described by the various Guidelines, Standards and Acts seem to be within a narrow range of accuracy. The current minimum standards of accuracy that all primary mine survey networks on South African mines must comply with, is prescribed by the MHSA. These minimum standards of accuracy have been changed and improved upon a number of times. The limits of error prescribed in the Mines and Works Act of 1991 was to be 1% of the distance and “in the case of survey stations which have for their object the fixing of boundary beacons, the position of shafts to be sunk and the establishment of connections, the limits of error laid down for Class “C” surveys of the Land Survey Act, 1927 (Act No. 9 of 1927)” which was defined as:

( )

to the current prescribed standard of

( )

which increased the standard of accuracy from 43 mm to 18 mm per 100m. It can be argued that one of the contributing factors to this increase in the allowable accuracy is the improvement in surveying technology and availability of more accurate survey instrumentation to the general mine survey community.

An analytical comparison of the available minimum standards of accuracy that the MHSA offers the most realistic limits of allowable error. The Limit of error for Canadian and Australian mine surveying is comparable, but less stringent than the South African MHSA requirements. The commonly accepted minimum standards of accuracy for tunnelling in the USA seems to be between 1: 10 000 and 1: 50 000 depending on the nature of the project.
In the tunnelling industry the limit of error varies depending on the purpose of the tunnel, the contractual obligations of the specific project and the country where the survey has to be done. Upon comparison with these survey minimum standards of accuracy, the South African MHSA defines a standard of accuracy that will ensure a high degree of integrity to the results of an underground survey network. The USA tunnelling standard of 1:10,000 for “tunnels of short distances” [47], although “short distances” are not further defined in this statement. The standards of accuracy discussed by Chrzanowski a similarly elusive definition is given of the accuracy required for first order survey work in “small and medium mines (1:10,000)” and “large” mines, “extending over several kilometers” of 1: 20,000 [30].
Depending on the context, the limit of error is defined as a distance in mm, as a ratio such as 1:10,000 or as a ratio expressed as parts per million (ppm). The formula used to derive the Limit of error value varies between guidelines but the common purpose of all these regulations and codes of practice are to define an acceptable value in millimetres that a survey must conform to in order to ensure the accuracy and integrity of a survey network. In most cases the defined error ellipse would include the error made in distance and angle, although the MHSA makes specific reference to the Limit of error of angular observation (2 minutes of arc). The following table draws a comparison between the various minimum standards of accuracy that were investigated and compared with the minimum standards of accuracy defined by the Mine Health and Safety Act.
Table . A Comparison of the various limits of error


Source: Limits of error spreadsheet. C:\Hennies Documents\PhD\data\[limits of error.xlsx]MHSA [32]
Over a distance of 100 metres the comparisons made between the standards of accuracy it can be concluded that the Land Survey Act has the greatest allowable limit at 43mm, followed by the Canadian standards at 28mm and then the ICSM standard at 20mm. These standards are currently more forgiving than the MHSA with a limit of 18mm over that distance. The NSW Coal standards compares favourably with these standards. The method of calculation provides a scientific approach to defining the error in a survey, allowing for the calculation of probability limits. The ISO and FGSC standards are of a high order and although achievable, are not considered to be realistic ion the mining context. As a result of this comparison it is proposed that the minimum accuracy of the survey defined by the MHSA is a reasonable standard. For this research will have to meet the requirements of a Class “A” primary survey network as defined by the regulations of the MHSA. In order for the proposed method to be accepted as an accurate alternative to conventional hangingwall surveying, it is essential that the existing standard of accuracy is met.

3.