The principal objective of ground control is to maintain safe excavations in rock and soil (the terms strata control and slope management are also used in underground mines and surface mines, respectively). Ground control also finds many applications in civil engineering projects such as tunnels, hydro-electric power plants and nuclear waste repositories. It has been defined as the practical application of rock mechanics to everyday mining. The US National Committee on Rock Mechanics has proposed the following definition: “Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses; it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical environment”.
Rock masses exhibit extremely complex behaviour, and rock mechanics and ground control have been the subject of considerable fundamental and applied research throughout the world since the 1950s. In many ways ground control is a craft more than a science. Ground control requires an understanding of structural geology, rock properties, groundwater and ground stress regimes and of how these factors interact. Tools include the methods of site investigation and rock testing, measures to minimize damage to the rock mass caused by blasting, the application of design techniques, monitoring and ground support. Several important developments have taken place in rock mechanics and ground control in recent years, including the development of empirical design and computer analysis techniques for mine design, the introduction and wide use of a variety of ground monitoring instruments and the development of specialized ground support tools and techniques. Many mining operations have ground control departments staffed by specialist engineers and technicians.
Underground openings are more difficult to create and maintain than rock or soil slopes, therefore underground mines generally must devote more resources and design efforts to ground control than surface mines and quarries. In traditional underground mining methods, such as shrinkage and cut-and-fill, workers are directly exposed to potentially unstable ground in the ore zone. In bulk mining methods, such as blasthole stoping, workers do not enter the ore zone. There has been a trend away from selective methods to bulk methods in the past decades.
Ground Failure Types
Rock structure and rock stress are important causes of instability in mines.
A particular rock mass consists of intact rock and any number of rock structures or structural discontinuities. Major types of rock structures include bedding planes (division planes which separate the individual strata), folds (bends in rock strata), faults (fractures on which movement has occurred), dykes (tabular intrusions of igneous rock) and joints (breaks of geological origin along which there has been no visible displacement). The following properties of structural discontinuities affect the engineering behaviour of rock masses: orientation, spacing, persistence, roughness, aperture and presence of infilling material. The collection of pertinent structural information by engineers and geologists is an important component of the ground control programme at a mining operation. Sophisticated computer programmes to analyse structural data and the geometry and stability of wedges in surface or underground mines are now available.
Stresses in rock also can cause instability in mines; knowledge of the stress-strain behaviour of rock masses is essential to sound engineering design. Laboratory tests on cylindrical specimens of rock from drill core can provide useful strength and deformability information concerning the intact rock; different rock types behave differently, from the plastic behaviour of salt to the elastic, brittle behaviour of many hard rocks. Jointing will greatly influence the strength and deformability of the entire rock mass.
There are some common types of rock slope failures in surface mines and quarries. The sliding block failure mode occurs where movement takes places along one or more rock structures (plane shear, step path, wedge, step wedge or slab failures); a rotational shear failure can occur in a soil or weak rock mass slope; additional failure modes include toppling of blocks formed by steeply dipping structures and ravelling (e.g., dislodging of blocks by freeze-thaw or rain).
Major slope failures can be catastrophic, although slope instability does not necessarily mean slope failure from an operational standpoint. The stability of individual benches is usually of more immediate concern to the operation, as failure can occur with little warning, with potential loss of life and equipment damage.
In underground mines, instability can result from movement and collapse of rock blocks as a result of structural instability, failure of rock around the opening as a result of high rock stress conditions, a combination of stress-induced rock failure and structural instability and instability caused by rockbursts. Rock structure can influence the choice of an underground mining method and the design of mining layouts because it can control stable excavation spans, support requirements capability and subsidence. Rock at depth is subjected to stresses resulting from the weight of the overlying strata and from stresses of tectonic origin, and horizontal stresses are often greater than the vertical stress. Instruments are available to determine the level of stress in the ground before mining has begun. When a mine opening is excavated, the stress field around this opening changes and possibly exceeds the strength of the rock mass, resulting in instability.
There are also various types of failure which are commonly observed in underground hard rock mines. Under low stress levels, failures are largely structurally controlled, with wedges or blocks falling from the roof or sliding out of the walls of the openings. These wedges or blocks are formed by intersecting structural discontinuities. Unless loose wedges or blocks are supported, failure can continue until natural arching of the opening takes place. In stratified deposits, bed separation and failure can occur along bedding planes. Under high stress levels, failure consists of brittle spalling and slabbing in the case of a massive rock mass with few joints, to a more ductile type of failure for heavily jointed rock masses.
A rockburst may be defined as damage to an excavation that occurs in a sudden or violent manner and is associated with a seismic event. Various rockburst damage mechanisms have been identified, namely expansion or buckling of the rock due to fracturing around the opening, rockfalls induced by seismic shaking and ejection of rock due to energy transfer from a remote seismic source. Outbursts of rock and gas occur catastrophically in some coal, salt and other mines as a result of high rock stresses and large volumes of compressed methane or carbon dioxide. In quarries and surface mines, sudden buckling and heaving of rock floors has also been experienced. Considerable research has taken place in several countries into the causes and possible alleviation of rockbursts. Techniques for minimizing rockbursts include altering the shape, orientation and sequence of extraction, the use of a technique known as destress blasting, stiff mine backfills and the use of specialized support systems. Sophisticated local or mine-wide seismic monitoring systems can assist in the identification and analysis of source mechanisms, although the prediction of rockbursts remains unreliable at the present time.
In the Canadian province of Ontario, nearly one-third of all underground fatal injuries in the highly mechanized mining industry result rom rockfalls and rockbursts; the fatality frequency from rockfalls and rockbursts for the period 1986-1995 was 0.014 per 200,000 hours worked underground. In less mechanized underground mining industries, or where ground support is not widely used, considerably higher injury and fatality frequencies due to falls of ground and rockbursts can be expected. The ground control related safety record for surface mines and quarries is generally better than for underground mines.
Design Methods
The design of underground excavations is the process of making engineering decisions on such matters as the locations, sizes and shapes of excavations and rock pillars, the mining sequence and the application of support systems. In surface mines, an optimum slope angle must be chosen for each section of the pit, along with other design aspects and slope support. Designing a mine is a dynamic process which is updated and refined as more information becomes available through observation and monitoring during the mining. The empirical, observational and analytical design methods are commonly used.
Empirical methods often use a rock mass classification system (several such schemes have been developed, such as the Rock Mass System and the Rock Tunnelling Quality Index), complemented by design recommendations based on a knowledge of accepted practice. Several empirical design techniques have been successfully applied, such as the Stability Graph Method for open stope design.
Observational methods rely on the actual monitoring of ground movement during excavation to detect measurable instability and on the analysis of ground-support interaction. Examples of this approach include the New Austrian Tunnelling Method and the Convergence-Confinement method.
Analytical methods utilize the analysis of stresses and deformations around openings. Some of the earliest stress analysis techniques utilized closed form mathematical solutions or photo elastic models, but their application was limited due to the complex three-dimensional shape of most underground excavations. A number of computer-based numerical methods have been developed recently. These methods provide the means for obtaining approximate solutions to the problems of stresses, displacements and failure in rock surrounding mine openings.
Recent refinements have included the introduction of three-dimensional models, the ability to model structural discontinuities and rock-support interaction and the availability of user-friendly graphical interfaces. In spite of their limitations, numerical models can provide real insights into complex rock behaviour.
The three methodologies described above should be considered as essential parts of a unified approach to the design of underground excavations rather than independent techniques. The design engineer should be prepared to use a range of tools and to re-evaluate the design strategy when required by the quantity and quality of information available.
Drilling and Blasting Controls
A particular concern with rock blasting is its effect on the rock in the immediate vicinity of an excavation. Intense local fracturing and disruption of the integrity of the interlocked, jointed assembly can be produced in the near-field rock by poor blast design or drilling procedures. More extensive damage can be induced by the transmission of blasting energy to the far field, which may trigger instability in mine structures.
Blast results are affected by the rock type, stress regime, structural geology and presence of water. Measures for minimizing blast damage include the proper choice of explosive, the use of perimeter blasting techniques such as pre-split blasting (parallel, closely spaced holes, which will define the excavation perimeter), decoupling charges (the diameter of the explosive is smaller than that of the blasthole), delay timing and buffer holes. The geometry of the drilled holes affects the success of a wall control blast; hole pattern and alignment must be carefully controlled.
Monitoring of blast vibrations is often performed to optimize blasting patterns and to avoid damage to the rock mass. Empirical damage blast damage criteria have been developed. Blast monitoring equipment consists of surface-mounted or down-the-hole transducers, cables leading to an amplifying system and a digital recorder. Blast design has been improved by the development of computer models for the prediction of blast performance, including the fragmentation, muck profile and crack penetration behind blastholes. Input data for these models include the geometry of the excavation and of the drilled and loaded pattern, detonation characteristics of the explosives and dynamic properties of the rock.
Scaling of Roof and Walls of Excavations
Scaling is the removal of loose slabs of rock from roofs and walls of excavations. It can be performed manually with a steel or aluminium scaling bar or by using a mechanical scaling machine. When scaling manually, the miner checks the soundness of the rock by striking the roof; a drum-like sound usually indicates that the ground is loose and should be barred down. The miner must follow strict rules in order to avoid injury while scaling (e.g., scaling from good ground to unchecked ground, maintaining good footing and a clear area to retreat and ensuring that scaled rock has a proper place on which to fall). Manual scaling requires considerable physical effort, and it can be a high-risk activity. For example, in Ontario, Canada, one third of all injuries caused by falls of rock occur while scaling.
The use of baskets on extendable booms so that miners can manually scale high backs introduces additional safety hazards, such as possible overturning of the scaling platform by falling rocks. Mechanical scaling rigs are now commonplace in many large mining operations. The scaling unit consists of a heavy hydraulic breaker, scraper or impact hammer, mounted on a pivoting arm, which is in turn attached to a mobile chassis.
Ground Support
The main objective of ground support is to help the rock mass support itself. In rock reinforcement, rockbolts are installed within the rock mass. In rock support, such as that provided by steel or timber sets, external support is provided to the rock mass. Ground support techniques have not found wide application in surface mining and quarrying, partly because of the uncertainty of the ultimate pit geometry and partly because of concerns with corrosion. A wide variety of rockbolting systems is available worldwide. Factors to consider when selecting a particular system include ground conditions, planned service life of the excavation, ease of installation, availability and cost.
The mechanically anchored rockbolt consists of an expansion shell (various designs are available to suit different rock types), steel bolt (threaded or with a forged head) and face plate. The expansion shell generally consists of toothed blades of malleable cast iron with a conical wedge threaded at one end of the bolt. When the bolt is rotated inside the hole, the cone is forced into the blades and presses them against the walls of the drillhole. The expansion shell increases its grip on the rock as tension on the bolt increases. Bolts of various lengths are available, along with a range of accessories. Mechanically anchored rockbolts are relatively inexpensive and, therefore, most widely used for short-term support in underground mines.
The grouted dowel consists of a ribbed reinforcing bar that is inserted in a drillhole and bonded to the rock over its full length, providing long-term reinforcement to the rock mass. Several types of cement and polyester resin-grouts are used. The grout can be placed in the drillhole by pumping or by using cartridges, which is quick and convenient. Steel and fibreglass dowels of various diameters are available, and bolts can be untensioned or tensioned.
The friction stabilizer commonly consists of a steel tube slotted along its entire length, which, when driven into a slightly undersized drillhole, compresses and develops friction between the steel tube and the rock. The drillhole diameter must be controlled within close tolerances for this bolt to be effective.
The Swellex rockbolt consists of an involute steel tube which is inserted in a drillhole and expanded by hydraulic pressure using a portable pump. Various types and lengths of Swellex tubes are available.
The grouted cable bolt is frequently installed to control caving and stabilize underground stope roofs and walls. A Portland cement-based grout is generally used, while cable geometries and installation procedures vary. High-capacity reinforcing bars and rock anchors are also found in mines, along with other bolt types, such as tubular groutable mechanically anchored bolts.
Steel straps or mesh, made from either woven or welded wire, is often installed in the roof or walls of the opening to support the rock between bolts.
Mining operations should develop a quality control programme, which can include a variety of field tests, to ensure that ground support is effective. Poor ground support installations can be the result of inadequate design (failure to choose the correct ground support type, length or pattern for the ground conditions), sub-standard ground support materials (as supplied by manufacturer or damaged during handling or because of storage conditions at the mine site), installation deficiencies (defective equipment, poor timing of installation, inadequate preparation of the rock surface, poor training of crews or not following specified procedures), mining-induced effects that were unforeseen at the design stage (stress changes, stress or blast-induced fracturing/spalling, joint relaxation or rockbursting) or mine design changes (changes in excavation geometry or service life longer than originally anticipated).
The behaviour of reinforced or supported rock masses remains incompletely understood. Rules of thumb, empirical design guidelines based on rock mass classification systems and computer programs have been developed. However, the success of a particular design relies heavily on the knowledge and experience of the ground control engineer. A good quality rock mass, with few structural discontinuities and small openings of limited service life, may require little or no support. However, in this case rockbolts may be required at selected locations to stabilize blocks that have been identified as potentially unstable. At many mines, pattern bolting, the systematic installation of rockbolts on a regular grid to stabilize the roof or walls, is often specified for all excavations. In all cases, miners and supervisors must have sufficient experience to recognize areas where additional support may be required.
The oldest and simplest form of support is the timber post; timber props and cribs are sometimes installed when mining through unstable ground. Steel arches and steel sets are high load-carrying capacity elements used to support tunnels or roadways. In underground mines, additional and important ground support is provided by mine backfill, which can consist of waste rock, sand or mill tailings and a cementing agent. Backfill is used to fill voids created by underground mining. Among its many functions, backfill helps prevent large-scale failures, confines and thus provides residual strength to rock pillars, allows transfer of rock stresses, helps reduce surface subsidence, allows for maximum ore recovery and provides a work platform in some mining methods.
A relatively recent innovation in many mines has been the use of shotcrete, which is concrete sprayed on a rock face. It can be applied directly to rock with no other form of support, or it can be sprayed over mesh and rockbolts, forming part of an integrated support system. Steel fibres can be added, along with other admixtures and mix designs to impart specific properties. Two different shotcreting processes exist, termed dry mix and wet mix. Shotcrete has found a number of applications in mines, including stabilizing rock faces that would otherwise ravel because of their close jointing. In surface mines, shotcrete has also been used successfully to stabilize progressive ravelling failures. Other recent innovations include the use of polyurethane spray-on liners in underground mines.
In order to function effectively during a rockburst, support systems must possess certain important characteristics, including deformation and energy absorption. Support selection under rockburst conditions is the subject of ongoing research in several countries, and new design recommendations have been developed.
In small underground openings, manual ground support installation is commonly done using a stoper drill. In larger excavations, semi-mechanized equipment (mechanized drilling and manual equipment for rockbolt installation) and fully mechanized equipment (mechanized drilling and rockbolt installation controlled from an operator’s panel located under bolted roof) are available. Manual ground support installation is a high-risk activity. For example, in Ontario, Canada, one third of all injuries caused by falls of rock during the period 1986-1995 occurred while installing rockbolts, and 8% of all underground injuries occurred while installing rockbolts.
Other hazards include possible splashes of cement grout or resin in the eyes, allergic reactions from chemical spillage and fatigue. The installation of large numbers of rockbolts is made safer and more efficient by the use of mechanized bolting machines.
Monitoring of Ground Conditions
Monitoring of ground conditions in mines may be carried out for a variety of reasons, including obtaining data needed for mine design, such as rock mass deformability or rock stresses; verifying design data and assumptions, thereby allowing calibration of computer models and adjustment of mining methods to improve stability; assessing the effectiveness of existing ground support and possibly directing the installation of additional support; and warning of potential ground failures.
Monitoring of ground conditions can be done either visually or with the help of specialized instruments. Surface and underground inspections must be done carefully and with the assistance of high-intensity inspection lights if necessary; miners, supervisors, engineers and geologists all have an important role to play in carrying out regular inspections.
Visual or audible signs of changing ground conditions in mines include but are not limited to the condition of diamond drill core, contacts between rock types, drum-like ground, the presence of structural features, obvious loading of ground support, floor heaving, new cracks on walls or roof, groundwater and pillar failures. Miners often rely on simple instruments (e.g., wooden wedge in crack) to provide a visual warning that roof movement has occurred.
Planning and implementing a monitoring system involves defining the purpose of the programme and the variables to be monitored, determining the required measurement accuracy, selecting and installing equipment and establishing the frequency of observations and means of data presentation. Monitoring equipment should be installed by experienced personnel. Instrument simplicity, redundancy and reliability are important considerations. The designer should determine what constitutes a threat to safety or stability. This should include the preparation of contingency plans in the event that these warning levels are exceeded.
The components of a monitoring system include a sensor, which responds to changes in the variable being monitored; a transmitting system, which transmits the sensor output to the read-out location, using rods, electrical cables, hydraulic lines or radiotelemetry lines; a read-out unit (e.g., dial gauge, pressure gauge, multimeter or digital display); and a recording/processing unit (e.g., tape recorder, datalogger or microcomputer).
Various modes of instrument operation exist, namely:
- mechanical: often provide the simplest, cheapest and most reliable methods of detection, transmission and readout. Mechanical movement detectors use a steel rod or tape, fixed to the rock at one end, and in contact with a dial gauge or electrical system at the other. The main disadvantage of mechanical systems is that they do not lend themselves to remote reading or to continuous recording.
- optical: used in conventional, precise and photogrammetric surveying methods of establishing excavation profiles, measuring movements of excavation boundaries and monitoring surface subsidence.
- hydraulic and pneumatic: diaphragm transducers that are used for measuring water pressures, support loads and so forth. The quantity measured is a fluid pressure which acts on one side of a flexible diaphragm made of a metal, rubber or plastic.
- electrical: the most common instrument mode used in mines, although mechanical systems still find widespread use in displacement monitoring. Electrical systems operate on one of three principles, electric resistance strain gauge, vibrating wire and self-inductance.
Most commonly monitored variables include movement (using surveying methods, surface devices such as crack gauges and tape extensometers, borehole devices such as rod extensometers or inclinometers); rock stresses (absolute stress or stress change from borehole devices); pressure, load and strain on ground support devices (e.g., load cells); seismic events and blast vibrations.