Underground Mining Safety (Part 2)

In part one of this we discussed a number of safety issues dealing with working underground. If you haven’t read part 1, I hope you’ll take the time to do so.

This is a continuing series; I hope to add on to as time goes on. There is so much more information I know will be helpful in keeping you safe as you work underground. In part 2 as you will see were going to be discussing oxygen testing, emergency procedures, conditions for blasting and much more. If you would like anymore general or detailed information, I have a number of recourses available to get you anything you need. Please let me know if you have any questions, comments, or requests. Be safe…

Special air monitoring requirements
The employer must assign a “competent person” to perform air monitoring. If this individual determines that air contaminants may present a danger to life at any time, the employer must immediately take all necessary precautions and post a notice at all entrances to the underground site about the hazardous condition. In performing air monitoring duties, the competent person must take into consideration the location of the jobsite (its proximity to fuel tanks, sewers, gas lines, etc.); the geology of the site, including soil type and permeability; the history of the site and the construction operation (changes in levels of substances monitored over time); and work practices at the jobsite (use of diesel engines, explosives, and fuel gas; hot work, welding, and cutting; and the physical reactions of employees to working underground.

Test for oxygen first
The competent person charged with air monitoring must test for oxygen content before testing for air contaminants. All underground work areas must be tested as often as necessary to verify that the atmosphere at normal atmospheric pressure remains within the acceptable parameters of 19.5 and 22 percent oxygen. After verifying oxygen levels, the competent person must test all underground work areas for carbon monoxide, nitrogen dioxide, hydrogen sulfide, and other toxic gases, dusts, vapors, mists, and fumes as often as necessary to ensure that levels remain within permissible exposure limits. Testing for methane and other flammable gases

The competent person must also test all underground work areas for methane and other flammable gases to determine whether the operation must be classified as potentially gassy or gassy. If the atmosphere meets the criteria for these designations, the precautions listed in the section discussing gassy or potentially gassy operations later in this booklet must be followed. Other precautions to take when testing for methane or other flammable gases include:

? If 20 percent or more of the lower explosive limit for methane or other flammable gases is detected in any underground work area or in the air return, all employees must be evacuated to a safe location above ground (except those employees required to eliminate the hazard). Electrical power (except for acceptable pumping and ventilation equipment) must be cut off to the area until concentrations reach less than 20 percent of the lower explosive limit.
? If 10 percent or more of the lower explosive limit for methane gas or other flammable gases is detected near any welding, cutting, or other hot work, the work must be suspended until the concentration is reduced to below 10 percent of the lower explosive limit.
? When 5 percent or more of the lower explosive limit for methane or other flammable gases is detected in an underground work area or in the air return, steps should be taken to increase ventilation air volume or otherwise control the gas concentration (unless all requirements of operating under potentially gassy or gassy operations are met).

Potential hazards that require special precautions; Gassy or potentially gassy operations

Gassy or potentially gassy operations present specific hazards to underground construction workers. It is essential that employers understand the terms “gassy” and “potentially gassy” and to know what precautions to take when dealing with such environments. Operations that meet the criteria for this hazardous classification must be equipped with ventilation systems constructed with fire resistant materials; have acceptable electrical systems, including fan motors; and have above ground controls to reverse the air flow. When using a mine-type ventilation system with an offset main fan on the surface, the system must be equipped with explosion doors or a weak-wall with an area at least equivalent to the cross sectional area of the airway.
Gassy operations occur under the following conditions:

? When air monitoring discloses 10 percent or more of the lower explosive limit for methane or other flammable gases measured at 12 inches (304.8 mm) ± 0.25 inch (6.35 mm) from the roof, face, floor, or walls in any underground work area for three consecutive days; or
? There has been an ignition of methane or other flammable gases emanating from the strata that indicates the presence of such gases; or
? The underground construction operation is connected to an underground work area classified as gassy and subject to a continuous course of air that contains the flammable gas concentration. The underground construction standard requires that gassy operations meet several special requirements, including both personnel and equipment safety concerns. These requirements include:
? Entrances to a gassy operation must be marked with prominently posted signage that identifies the area as gassy.
? Maintain a fire watch when performing hot work (welding, cutting, heating) in a gassy area and for a sufficient period after completing the work to ensure no possibility of fire remains.
? Use only acceptable equipment in well-maintained condition. Any mobile diesel-powered equipment must either be approved by MSHA and meet the requirements of 30 CFR parts 36 or the employer must demonstrate that the equipment is fully equivalent to MSHA-approved equipment and operated according to these regulations.
? Smoking is prohibited in all gassy operations; the employer must collect all possible sources of ignition (matches, lighters, etc.) from any person entering a gassy operation area.
? All operations in the affected area must stop when an operation is classified as gassy until full compliance with gassy operation requirements is confirmed or the operation is downgraded to a potentially gassy operation. The only exceptions are operations to control the gas concentration, installation of above ground equipment to reverse the airflow, or actions to comply with gassy operation requirements. Gassy operations can be downgraded to potentially gassy when air monitoring results remain below 10 percent of the lower explosive limit for methane or other flammable gases for three consecutive days. Potentially gassy operations, such as an unexpected pocket of gas, occur when the following conditions exist:
? Air monitoring shows 10 percent or more of the lower explosive limit for methane or other flammable gases measured at 12 inches (304.8 mm) ± 0.25 inch (6.35 mm) from the roof, face, floor or walls in any underground work area for more than a 24-hour period.
? The history of the geographical area, geological formation, or past experience indicates that 10 percent or more of the lower explosive limit for methane or other flammable gases is likely to be encountered in such underground operations. Both gassy and potentially gassy operations require special air monitoring actions under the guidance of a “competent person,” including testing for oxygen and flammable gas content in the affected underground work areas and adjacent work areas at the beginning and midpoint of each work shift. A manual flammable gas monitor should be used for gas testing and a manual electrical shut down control must be provided near the heading for the gas monitor. The use of rapid excavation machines requires continuous automatic flammable gas monitoring to monitor the air at the heading, on the rib, and in the return air duct. If 20 percent or more of the lower explosive limit for methane or other flammable gases is encountered, the continuous monitor alert should signal the heading and shut down electrical power in the affected underground work area (except for required pumping and ventilation equipment). Local gas tests must be conducted before and throughout welding, cutting or other hot work. In underground operations driven by drill-and-blast methods, the air in the affected area must be continuously tested for flammable gas when employees are working in the area as well as before reentering after blasting operations.

Hydrogen sulfide levels
When air monitoring reveals the presence of 5 ppm or more of hydrogen sulfide, the affected underground areas must be tested at the beginning and midpoint of each shift until the concentration is measured at less than 5 ppm for three consecutive days. Employees must be notified if hydrogen sulfide is detected in amounts exceeding 10 ppm and a continuous sampling and indicating monitor must be used to keep track of levels. If the concentration of hydrogen sulfide reaches 20 ppm, the monitor must be designed to provide both visual and audible alarms to warn that additional measures (respirator use, increased ventilation, evacuation) may be appropriate.

Emergency procedures
Whenever an employee is working underground at least one designated person must be on duty above ground, responsible for maintaining an accurate count of the number of employee’s underground and summoning emergency aid if needed. Every employee working underground must have a portable hand lamp or cap lamp for emergency use unless natural light or an emergency lighting system provides adequate illumination for escape. Employers must provide self-rescuers approved by the National Institute for Occupational Safety and Health (NIOSH) in all underground work areas where employees might be trapped by smoke or gas. If 25 or more employees work underground at one time, the employer must provide at least two 5-person rescue teams, one at the jobsite or within 30 minutes travel time from the entry point to the site and the other team within two hours travel time. If less than 25 employees work underground, the employer must have one 5-person rescue team at the jobsite or within 30 minutes travel time. In both situations, advance arrangements can be made for local rescue services to meet this requirement. Rescue team members must be trained in rescue procedures, the use and limitations of breathing apparatus, and the use of firefighting equipment with qualifications reviewed annually. When flammable or noxious gases are anticipated at a jobsite, rescue teams must practice using self-contained breathing apparatus once a month. The rescue teams must be available through the duration of a construction project. If a shaft is used as the means of egress, the employer must arrange for a readily available power-assisted hoisting capability in case of emergency, unless the regular hoisting means will function in the event of a power failure.

Soft-ground tunnels most commonly are used for urban services (subways, sewers, and other utilities) for which the need for quick access by passengers or maintenance staff favors a shallow depth. In many cities this means that the tunnels are above bedrock, making tunneling easier but requiring continuous support. The tunnel structure in such cases is generally designed to support the entire load of the ground above it, in part because the ground arch in soil deteriorates with time and in part as an allowance for load changes resulting from future construction of buildings or tunnels. Soft-ground tunnels are typically circular in shape because of this shape's inherently greater strength and ability to readjust to future load changes. In locations within street rights-of-way, the dominant concern in urban tunneling is the need to avoid intolerable settlement damage to adjoining buildings. While this is rarely a problem in the case of modern skyscrapers, which usually have foundations extending to rock and deep basements often extending below the tunnel, it can be a decisive consideration in the presence of moderate-height buildings, whose foundations are usually shallow. In this case the tunnel engineer must choose between underpinning and employing a tunneling method that is sufficiently foolproof that it will prevent settlement damage.

Surface settlement results from lost ground—i.e., ground that moves into the tunnel in excess of the tunnel's actual volume. All soft-ground tunneling methods result in a certain amount of lost ground. Some is inevitable, such as the slow lateral squeeze of plastic clay that occurs ahead of the tunnel face as new stresses from doming at the heading cause the clay to move toward the face before the tunnel even reaches its location. Most lost ground, however, results from improper construction methods and careless workmanship. Hence the following emphasizes reasonably conservative tunneling methods, which offer the best chance for holding lost ground to an acceptable level of approximately 1 percent.

Vertical openings: shafts and raises

The principal means of access to an underground ore body is a vertical opening called a shaft. The shaft is excavated, or sunk, from the surface downward to a depth somewhat below the deepest planned mining horizon. At regular intervals along the shaft, horizontal openings, called drifts, are driven toward the ore body. Each of these major working horizons is called a level. The shaft is equipped with elevators (called cages) by which workers, machines, and material enters the mine. Ore is transported to the surface in special conveyances called skips.

Shafts generally have compartments in which the media lines (e.g., compressed air, electric power, or water) are contained. They also serve as one component in the overall system of ventilating the mine. Fresh air may enter the mine through the production shaft and leave through another shaft, or vice versa.

Another way of gaining access to the underground is through a ramp—that is, a tunnel driven downward from the surface. Internal ramps going from one level to another are also quite common. If the topography is mountainous, it may be possible to reach the ore body by driving horizontal or near-horizontal openings from the side of the mountain; in metal mining these openings are called adits.

Ore that is mined on the different levels is dumped into vertical or near-vertical openings called ore passes, through which it falls by gravity to the lowest level in the mine. There it is crushed, stored in an ore bin, and charged into skips at a skip-filling station. In the head frame on the surface, the skips dump their loads and then return to repeat the cycle. Some common alternative techniques for ore transport are conveyor belts and truck haulage. Vertical or near-vertical openings are also sometimes driven for the transport of waste rock, although most mines try to leave waste rock underground.

Vertical or sub vertical connections between levels generally are driven from a lower level upward through a process called raising. Raises with diameters of two to five meters and lengths up to several hundred meters are often drilled by powerful raise-boring machines. The openings so created may be used as ore passes, waste passes, or ventilation openings. An underground vertical opening driven from an upper level downward is called a winze; this is an internal shaft.

Special conditions for drilling and blasting underground

Before initiating any drilling operation underground, a “competent person” must inspect all drilling and associated equipment as well as the drilling area and correct any hazards. Employees are not allowed on a drill mast when a drill bit is in operation or a drill machine is being moved. Also, when moving a drill machine, all associated equipment and tools must be secured and the mast placed in a safe position.
Working on or around jumbo decks involves special safety precautions, including the following:

? Locate all receptacles or racks to store drill steel on jumbos.
? Warn employees working below jumbo decks when drilling is about to begin.
? The top deck of a jumbo must have a mechanical way to lift unwieldy or heavy items.
? Only employees assisting the operator may ride on the jumbo unless it is equipped with seating for each passenger and protection from crushing or catching hazards.
? Jumbo decks more than 10 feet high must be equipped with guardrails on all open sides unless an adjacent surface provides fall protection. Jumbo decks and stair treads must be slip resistant, secured, and maintained to prevent slip, trip, and fall hazards.
? Jumbos must be chocked so they will not move when employees are working on them.
Whenever an underground blasting operation in a shaft is complete, a “competent person” must check the air quality and make sure that no walls, ladders, timbers, blocking, and wedges have been loosened as a result of the activity. If repairs are required, only employees involved in repair activity may be in or below affected areas until repairs are complete. All blasting wires must be kept clear of electrical lines, pipes, rails and other conductive material (except earth), to prevent explosions or exposure of employees to electric current.

Horizontal openings: drifts
All horizontal or sub horizontal development openings made in a mine have the generic name of drift. These are simply tunnels made in the rock, with a size and shape depending on their use—for example, haulage, ventilation, or exploration. A drift running parallel to the ore body and lying in the footwall is called a footwall drift; drifts driven from the footwall across the ore body are called crosscuts. A ramp is also a type of drift.
Because the drift is such a fundamental construction unit in underground mining, the process by which it is made should be described. There are five separate operations involved in extending the length of the drift by one round, or unit volume of rock. Listed in the order in which they are done, these are drilling, blasting, loading and hauling, scaling, and reinforcing. Drilling is done in various ways, depending on the size of the opening being driven, the type of rock, and the level of mechanization. Most mines use diesel-powered, rubber-tired carriers on which several drills are mounted; these machines are called drill jumbos. The drills themselves may be powered by compressed air or hydraulic fluid. In percussive drilling, a piston is propelled back and forth in the cylinder of the drilling machine. On the forward stroke, it strikes the back end of a steel bar or drill rod, to the front of which is attached a special cutter, or bit. The cutter's edges are pushed into the bottom of the hole with great force, and, as the piston moves to the back of the cylinder, the bit is rotated to a new position for the next stroke. Through the action of high energy, frequency (2,000 to 3,000 blows per minute), and rotation speed, holes may be drilled in even the hardest rock at a high rate.

A pattern of parallel blast holes is drilled into the rock face at the end of the drift. The diameter of these holes ranges from 38 to 64 millimeters, but in general one or more larger-diameter uncharged holes are also drilled as part of the initial opening. These latter serve as free surface for the other holes to break as well as expansion room for rock broken by the blast.

Explosives may be placed in the blast holes in the form of sticks or cartridges wrapped in paper or plastic, or they may be blown or pumped in. They are composed of chemical ingredients that, when properly initiated, generate extremely high gas pressures; these in turn induce new fractures in the surrounding rock and encourage old fractures to grow. In the process rock is broken and displaced.

For many years dynamite was the primer, an explosive used underground, but this has largely been replaced by blasting agents based on ammonium nitrate (AN; chemical formula NH4NO3) and fuel oil (FO; chemical formula CH2). Neither of these components is explosive by itself, but, when mixed in the proper weight ratio (94.5 percent AN, 5.5 percent FO) and ignited, they cause the following chemical reaction:

The products of the above reaction (carbon dioxide, water, and nitrogen) are commonly present in air. If there is too much fuel oil in the mixture, however, the poisonous gas carbon monoxide will be formed; with too little fuel oil, nitrous oxides, also poisonous, are formed. For this reason, gases are carried out of the mine through the ventilation system, and blasting is normally done between shifts or at the end of the last shift, when the miners are out of the mine.
Blast holes must be fired in a certain order so that there is sufficient space to accommodate the broken rock. Those closest to the large empty holes are fired first, followed by those next to the resulting larger hole. This continues until the holes at the contour are reached. To create such an expanding pattern, the timing of explosions is very important. There are both electric and nonelectric systems for doing this. In the electric system, an electric current is passed through a resistive element contained in the blasting cap. When this heats up, it initiates a fuse head, which in turn ignites a chemical compound that burns at a known rate. This combination serves as the timing or delay element within the cap. At the other end of the delay is the primer, an explosive (generally lead azide, mercury fulminate, or pentaerythritol tetranitrate [PETN]) that, upon detonation, releases a great deal of energy in a very short time. This is sufficient to ignite the larger amount of ANFO explosive packed into the hole. The most common time interval between adjacent delays is 25 milliseconds. Other caps are available in which the delays are introduced electrically through the use of microcircuitry. These have the advantage of extremely little variation among caps of the same delay period; also, the number of delay periods available is much greater than with burning-compound caps.

After blasting, the broken ore is loaded and transported by machines that may be powered by compressed air, diesel fuel, or electricity. Highly mechanized mines employ units that load themselves, haul the rock to an ore pass, and dump it. Known as LHD units, these come in various sizes denoted by the volume or weight of the load that they can carry. The smallest ones have a capacity of less than one cubic metre (one ton), whereas the largest have a 25-ton capacity. In small, narrow vein deposits, tracked or rubber-tired overshot loaders are often employed. After the bucket of this machine is filled by being forced into the pile, it is lifted and rotated backward so that it dumps into a built-in dump box or attached railcar. Overshot loaders are commonly powered by compressed air.

Another type of loading machine features special gathering arms that sweep or scrape the broken material into a feeder, whence it is fed via an armored conveyor belt into waiting trucks or railcars. Although most loading machines have an on-board operator/driver, some are controlled remotely via television monitor.
After the broken rock has been removed (and sometimes even during the loading process), the roof, walls, and face are cleaned of loose rock. This process is called scaling. In small openings scaling is normally done by hand, with a special steel or aluminum tool resembling a long crowbar being used to “bar down” loose material. In larger openings and mechanized mines, a special machine with an impact hammer or scaling claw mounted on a boom is used. Scaling is an extremely important step in making the workplace safe.
Depending on the ground conditions and the permanence of the openings, various means of rock reinforcement may be employed before beginning a new round of drifting. The ideal is for the rock to support itself; this is accomplished by keeping rock blocks in place, thereby allowing rock arches or beams to form, but often these blocks need to be reinforced by various implements, the most common being bolts inserted into holes drilled around the opening. In one technique, a steel bolt equipped with an expansion anchor at the end is inserted into the hole. Rotation of the bolt causes the anchor to expand against the wall of the hole, and further rotation compresses a large steel faceplate, or washer, against the rock, effectively locking the blocks together. A pattern of such bolts around and along an opening creates a rock arch. If the rock pieces are quite small, a steel net (much like a chain-link fence) or steel straps can be placed between the bolts. Some mines simply cement reinforcing bar or steel cables in the boreholes. Shotcrete, concrete sprayed in layers onto the rock surfaces, has also proved to be a very satisfactory means of rock reinforcement.

Ventilation and lighting

Ventilation is an important consideration in underground mining. In addition to the obvious requirement of providing fresh air for those working underground, there are other demands. For example, diesel-powered equipment is important in many mining systems, and fresh air is required both for combustion and to dilute exhaust contaminants. In addition, when explosives are used to break hard rock, ventilation air carries away and dilutes the gases produced.

Special fans, controls, and openings are used to direct fresh air to the working places and spent or contaminated air out of the mine. In very cold climates, incoming ventilation air must first be warmed by gas- or oil-fired heaters. On the other hand, in very deep mines, because of high rock temperatures, the air must be cooled by elaborate refrigeration systems. This makes the energy costs associated with ventilation systems very high, which in turn has created a trend toward sealing unused sections of the mine and changing from diesel to electric machines.

Properly lighted working places are very important for both safety and productivity. Each underground miner is equipped with a hard-hat-mounted lamp with the battery worn on the belt. In some mines this is the primary source of lighting under which the various jobs are done. In others, however, many jobs have been taken over by machinery equipped with high-powered lights that fully illuminate the working areas. Fixed lighting is installed along travel ways and at shaft stations, dumping points, and other important locations.

Water control

The amount of water encountered in underground mining operations varies greatly, depending on the type of deposit and the geologic setting. Some mines must be prepared only to reuse the water introduced in such operations as drilling; others must contend with large inflows from the surrounding rock. In extreme cases, special water doors and underground chambers must be constructed in order to control sudden large inflows. Typically, mine water flows or is pumped to a central collection point called a settling basin, or sump. From there it is pumped through pipes located in the shaft to the surface for treatment and disposal.

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