Technical Aspects of Mine surveying
Mining regulations normally require mines to maintain plans and sections of the surface and any underground workings. Regulations may require these documents to be provided to the government. Regulations normally specify requirements for being up-to-date, in months.
It is important to maintain accurate plans for the use of current and future land users so that the positions of any potentially hazardous workings or infrastructure are permanently recorded. The information must be adequate to ensure that adjacent mine workings do not accidentally connect, with risk of flooding, contamination or other safety hazards. Regulations may require minimum boundary pillars around mining properties.
Internally, individual mines similarly require adequate information to prevent inadvertent undercutting of installations and the unplanned intersection of workings. It is important to know the positions of all points underground relative to every place on the surface, particularly for the management of emergencies and rescues.
Mine operators require detailed plans for designing and controlling the excavation of mine workings and the construction of all mine facilities.
Large mines operate survey departments staffed by competent surveyors. In some countries, for example, Zimbabwe, the government provides assistance with free survey services to the ASM sector.
Purposes Of Mine Surveying
- Comply with regulations governing mine surveying, mine plans and related matters.
- Ensure compliance with mining title arrangements (licenses, contracts): ensuring that all work is carried out within permitted boundary limits.
- Maintain an adequate geographic database (plans, sections, models) for planning the layout of all physical features: mine workings, plant, buildings and all infrastructure.
- Maintain a permanent record of all the above physical features.
- Provide control for all mining excavations: position, direction, elevation, dimensions.
- Provide similar control for the construction of buildings and infrastructure.
- Grade control: in some instances, a survey is used to control the selection of ore of different grades from zones within an orebody, both in open pit and U/G applications.
- Measuring & recording mining activities: development metres, bank cubic metres extracted in open pits, face advance or area mined in stopes (mainly in tabular orebodies), tonnage mined. This information may be used for payment of contractors or of incentive bonuses to workers; for reconciling ore mined with plant output and calculating or verifying mining and plant recoveries; for reporting to the government, confirming compliance with licensing arrangements and other purposes.
In large-scale mines, the survey department often forms part of a technical services division which may include geology, ventilation, geotechnical engineering and other functions. Survey departments are often responsible for ore sampling and grade control and may also be in charge of ventilation and other functions. The survey department is often a key component of production recording and mineral resource accounting.
Types Of Mine Plans
Regulations may specify minimum requirements for survey practice and the plans to be kept at a mine.
Survey plans are plotted using grid coordinates which may be on a national or international system or maybe a local system used by individual mines or groups of mines. The system should allow the relative positions of adjacent mines to be easily identified.
Usually, mines require at least the following:
Comprehensive surface plan
Contours, prominent natural features such as watercourses; all man-made structures: buildings, roads, fixed plant, power lines and substations, water storage facilities and pipelines, ore stockpiles, waste rock dumps, tailings dams.
Mine workings present on surface, meaning open pits, trenches and other surface mining excavations, shafts and winzes, surface fans; areas of actual or expected subsidence.
Mining license boundaries, positions of boundary beacons
Specialised plans: for example, detailed plans of tailings dams; plans for environmental management systems showing, for example, effluent discharge points & volumes, sampling points etc.
Surface plans may be at many different scales according to the size of the mine, ranging from 1:250, 1:500 or 1:1,000 in the ASM sector, to smaller scales on larger mines. Plans may be projected onto multiple sheets where necessary and these should be indexed to each other.
Underground Plans (U/G Mines)
All development: shafts, declines, adits, drives, crosscuts, winzes, raises, ore & waste passes and large service excavations.
Stopes; often indicated by shading or hatching.
All main U/G installations: shaft stations, sub-stations, workshops, explosives storage areas, refuge bays, blasting points, first-aid stations, loco charging bays, offices etc.
Again, scale varies with convenience in relation to mine size; similar scales to surface plans.
Assay Plans
Especially on gold mines, assay results from sampling (g/t) are shown along with reef width (cm). Other types of mines may have plans showing ore values in different formats.
Block tonnages and average grades are often recorded on plans.
Geological Plans
Geologists map rock types, faults, dykes & other structures on mine plans. Additional geotechnical features might also be shown.
Ventilation Plans
It is essential for U/G mines to maintain up-to-date ventilation plans showing, at minimum, the direction of airflow. Ideally, ventilation survey results should be recorded at intervals: air volumes, temperatures, pressure, dust concentrations. The plans should show positions of all fans and direction of discharge, and locations of walls, doors, regulators, measuring points etc.
Sections
Sections through workings on appropriate planes.
In modern practice, much of the above information is recorded in digital format. Mine workings, in particular, are digitally modelled and may be reviewed in many formats.
Mine Surveying Techniques
Large mine operators may employ the most sophisticated techniques for surface surveys, including aerial and satellite mapping, GPS and, more recently, mapping from drones.
In the ASM sector, if surveying is carried out at all, it may use the most basic techniques such as a compass and tape.
Surface survey control still largely requires persons on the ground using a variety of instruments ranging from GPS to the theodolite and engineer’s level. Most larger mines now use the total station which combines a theodolite, measuring horizontal and vertical angles, with electronic distance measurement technology, all generating data which can be readily transferred to a computer for calculation of coordinates and plotting of plans or production of models.
Underground, GPS and aerial methods are not applicable and surveys are carried out with total stations or theodolites and tapes. Pegs or survey stations are installed in the H/W of tunnels for this purpose.
One of the greatest challenges in U/G surveying is the transfer of coordinates from surface to U/G down vertical shafts. Shaft dimensions provide a very short baseline on each level, from which surveys must often be carried for long distances, sometimes several kilometres off a base of as little as 2m. Shaft plumbing is a tricky task, involving suspending two thin wires on plumb bobs from surface to U/G stations, or between two or more shaft stations, measuring angles of the two wires as well as the distance between them and the vertical distance down the wires. This can be a lengthy process, disruptive to shaft sinking or production activities. The gyro-theodolite measures true bearings independently and can be used to establish direction on each level, with a single wire, used to transfer coordinates and elevation.
Traditionally, pegs are installed by surveyors for control of direction and gradient of advancing development headings; however, lasers may be installed by surveyors to assist miners to mark off for drilling along the planned line.