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5.2.2: Active and Passive Ground Support

Ground support can be active or passive. If we take actions that actually increase the strength of the rock, then we are providing active support. On the other hand, if we do nothing to increase the strength, but instead we take an action to prevent the rock mass from falling, then we are providing passive support. There are valid applications of both categories of support. Let’s look at an example. Actually, this is a really important example!

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The rock strata overlying a tabular deposit are often somewhat weak. They may consist of relatively thin layers of a weak rock. These layers, which may be a few inches to a few feet in thickness, may also have partings between them. These partings may be very thin, and very weak. Now, what is going to happen when we remove the ore from underneath the overlying rock mass that I just described?

Did you ever have a long bookshelf made of wood – wood that is not very thick? What happened when you place a full load of books on the shelf? It begins to sag in the middle, right? It may not have happened immediately, but over time, it sags in the middle. A similar mechanism is at play in the mine strata of our example. All of the weight of the overlying rock layers is pushing down. You removed the ore from underneath the strata, so you now have a long unsupported span of relatively weak rock layers. The rock layers begin to sag, and before long the lowest layer will separate from the others, and fail. When it fails, it will break apart and fall to the ground. Now, the layer immediately above the one that just failed will sag, and it too will fail. This will continue until the rock mass reaches some equilibrium. This is an unacceptable outcome! We’ll never make any money at this mining business if we can’t support the openings that we create! So, you must prevent the initial failure. What are your options?

As with the bookshelf, you could place a support in the middle of the span to support the load and prevent sagging. In our mine, we could set a timber post into place, and it would prevent the failure. This is an example of a passive support. Placing timber posts, steel arches, and so on is a time-honored means of supporting the rock and preventing failures. There are some drawbacks, however. If we’re mining a 6’ thick coal seam, we can set a 6’ post. What if we are mining a 60’ seam of limestone? Finding, handling, and placing a 60’post would be a supremely daunting task, and clearly not practical! Even when it would be easy to set a post, there can be unacceptable drawbacks to the practice.

Let’s think about a basketball arena like Bryce Jordan Center. The roof of that building spans a large area, and that roof needs to be supported. Assuming that we can obtain timber posts of sufficient length, why not set them across the width of the building, perhaps every 20’ or so to support the roof? For starters, it would make for an interesting new twist to the game of basketball as the players dodged the posts on their way to the basket! Again, not very practical, and that is the drawback in the mine as well. Instead of basketball players, we have large equipment moving around and between the working areas. Thus, in many locations, this kind of passive support is inconsistent with production needs. While passive support is an appropriate control strategy in many instances, we need an alternative for when it is not.

Roof Bolts

One of the most prevalent and effective means of providing ground support is an active support known as a roof or rock bolt. Remember the failure in the layered strata of our example mine? What if we were to clamp several of those weak layers together, forming a “laminated beam”? Beams are very strong, and, indeed, by clamping these layers together, we have strengthened the rock mass, and it will be able to support the weight on it without failing. Many of you may have seen laminated beams used in building construction. Several layers of thin and relatively weak plywood are glued together, and the result is beam with superior properties to a solid piece of wood of the same thickness.

In our case, we can’t access both sides of the rock mass to clamp the layers together, as is the case when they fabricate laminated wooden beams. Instead, we use a rock bolt, as illustrated in this diagram.

It works as follows. A hole is drilled into the rock mass that is to be supported. Next, the roof bolt is inserted into the hole. Then the bolt head is rotated, which causes the expansion shell to expand into the surrounding rock, locking it in place, and as the bolt rotates, it draws in the bearing plate. As a result, the layers between the expansion shell and the bearing plate are compressed tightly together, forming a beam. For this to work, the expansion shell must be anchored in competent rock. Sometimes, this can be done over 4’ and other times, 6’ or more. The length of the bolt is chosen with this in mind. The diameter of the bolt, which is between ½ - 1”, is based on the required tensile strength of the bolt, which is determined by calculating the weight of overlying rock that each bolt will have to support. In the future, you will learn how to size rock bolts. At this time, the takeaway message is:

1. bolting is an active support because it is increasing the strength of the rock mass to prevent failure by clamping weak layers together to form a strong beam; and
2. bolting is the most prevalent means of ground control in the majority of underground mines, and also in certain surface mines.

Under difficult conditions, we can place a skinny bag of resin, i.e., a strong glue, into the hole with the bolt; and then, as the bolt spins while it is being tightened, the bag will break and the resin will be mixed and will completely fill the hole around the bolt. Moreover, this resin will plug cracks and fractures in the rock in the immediate vicinity to further enhance the bolt’s holding power.

Here is a picture of roof bolting machine, commonly known as roof bolter or simply, a bolter. If you look closely, you will see the drill in front of the operator. Notice the attachment point to the machine at the bottom, i.e., the chuck. As the hole is drilled, that chuck assembly will move up towards the collar of the hole. After the hole is drilled, the operator will place a bolt in the chuck, insert it into the hole, and tighten it. The machine has plenty of space to store a supply of bolts along with the drill tools. You will also notice hydraulic canopies pressing into the top or “roof” of the mine opening. Those are passive supports to ensure that the top does not fall down while the bolter operator is installing the roof bolts.

In so-called large opening mines, where the distance between the bottom and top of the mine opening might be 20 – 30’ or more, a slightly different configuration is required, as shown here. The operator would stand in the “cherry picker basket" and operate the drill and bolter.

Cable Bolts

Sometimes, we need a bolt that is much longer than 4 – 6’. Remember, I said that for the bolt to achieve its purpose, the anchor must be in a competent rock layer. What if that layer is 15’, or 50’ or even 100’ above the opening? A bolt of such lengths would be impractical. Instead, we use a cable bolt. A reel of specially constructed steel cable is stored on a cable-bolting machine. A hole of the required length is drilled, an anchor shell is attached to the end of the cable, and it is fed to the end of the hole. An anchor plate and tensioning bolt is affixed to the end, and it is tensioned in some fashion as a traditional rock bolt. One difference, however, is that concrete grout will be pumped into the hole as well. Here is an example of a cable bolter. If you look closely, you can see the cable routed from the spool on the rear of the machine, up over the machine, and to the front where it will be inserted into the hole. The carriage housing the drill steel and the drill are also visible at the front of the machine.

Shotcrete

Shotcrete is another active support of note. Pillars of rock or ore are often left standing to help support the overlying strata, and only the ore between the pillars is recovered. The pillars can be under a lot of stress, and in some cases will begin to fail. The outer layers tend to crack and spall off, until the pillar is much skinnier and more likely to fail. The spalling of the outer layers can also present an immediate safety problem. One means of active support is to spray a Shotcrete onto the pillar. The Shotcrete may be cement or polymer mix, and fibers of various materials may be embedded in the mix. Thicknesses of an inch or more would be typical. Once the Shotcrete has dried, or really cured, it is quite strong and provides a confinement stress on the pillar. Sometimes the top surface of the span will be shotcreted as well.

Here is a picture of a shotcrete operation in progress. You may be able to see the nozzle at the end of the boom, the mixing tank, and the dry feed hopper, and water line on the equipment. The other line is an electric power cable. The hose and cable reels provide sufficient mobility to the machine to allow it to move freely among working places, at least until the limit of the hose and cable are reached. Then, it will be necessary to advance the electrical and water supplies. Advancing utilities, such as electric power and water, is another important auxiliary operation that we will discuss shortly.

We’ve covered the majority of the ground control technologies with this brief introduction to active supports such as roof bolts, cable bolts, and shotcrete; and passive supports such as timber or steel posts and arches. There are others, but this group is illustrative. There is one more ground control technique that deserves a brief mention before we move on to other auxiliary operations, and that is backfilling.

Backfilling

Backfilling is the process of adding support to a mined out area by filling it with backfill, i.e., waste rock, tailings, or a mixture of cement and tailings or waste rock. As you might suspect, the time and cost to backfill is very expensive. Nonetheless, it is a part of the cut and fill mining method, in which we routinely mine a slice of ore, backfill it, and mine out the next slice, and so on. We’ll talk more about backfilling when we discuss this mining method. For now, you should simply be aware of backfilling as another ground control practice.

Also, I want to be clear that while the ground control challenge in underground mining is generally far more complicated than in surface mining, ground control practices including inspection and scaling are part of surface mining, and the use of rock bolts is part of some surface operations.

After this brief tutorial of ground control, you’ve probably forgotten how we got started on this discussion in the first place! Ground control is a critical auxiliary operation, and although we usually speak generally of ground control, in total, we’ve now seen that there are three components: inspection, scaling and installing ground support. Depending on the mining method and the competency of the rock, the activities associated with ground control may occur at differing time points in the production cycle.

§ 56.2 Definitions.

The following definitions apply in this part. In addition definitions contained in any subpart of part 56 apply in that subpart. If inconsistent with the general definitions in this section, the definition in the subpart will apply in that subpart:

American Table of Distances means the current edition of “The American Table of Distances for Storage of Explosives” published by the Institute of Makers of Explosives.

Approved means tested and accepted for a specific purpose by a nationally recognized agency.

Attended means presence of an individual or continuous monitoring to prevent unauthorized entry or access.

Authorized person means a person approved or assigned by mine management to perform a specific type of duty or duties or to be at a specific location or locations in the mine.

Barricaded means obstructed to prevent the passage of persons, vehicles, or flying materials.

Barrier means a material object, or objects that separates, keeps apart, or demarcates in a conspicuous manner such as cones, a warning sign, or tape.

Berm means a pile or mound of material along an elevated roadway capable of moderating or limiting the force of a vehicle in order to impede the vehicle's passage over the bank of the roadway.

Blast area means the area in which concussion (shock wave), flying material, or gases from an explosion may cause injury to persons. In determining the blast area, the following factors shall be considered:

(1) Geology or material to be blasted.

(2) Blast pattern.

(3) Burden, depth, diameter, and angle of the holes.

(4) Blasting experience of the mine.

(5) Delay system, powder factor, and pounds per delay.

(6) Type and amount of explosive material.

(7) Type and amount of stemming.

Blast site means the area where explosive material is handled during loading, including the perimeter formed by the loaded blastholes and 50 feet (15.2 meters) in all directions from loaded holes. A minimum distance of 30 feet (9.1 meters) may replace the 50-foot (15.2-meter) requirement if the perimeter of loaded holes is demarcated with a barrier. The 50-foot (15.2-meter) and alternative 30-foot (9.1-meter) requirement also apply in all directions along the full depth of the hole.

Blasting agent means any substance classified as a blasting agent by the Department of Transportation in 49 CFR 173.114(a) (44 FR , May 31, ) which is incorporated by reference. This document is available for inspection at each Metal and Nonmetal Safety and Health District Office of the Mine Safety and Health Administration, and may be obtained from the U.S. Government Printing Office, Washington, DC .

Blasting area means the area near the blasting operations in which concussion or flying material can reasonably be expected to cause injury.

Blasting cap means a detonator which is initiated by a safety fuse.

Blasting circuit means the electrical circuit used to fire one or more electric blasting caps.

Blasting switch means a switch used to connect a power source to a blasting circuit.

Booster means any unit of explosive or blasting agent used for the purpose of perpetuating or intensifying an initial detonation.

Capped fuse means a length of safety fuse to which a blasting cap has been attached.

Capped primer means a package or cartridge of explosives which is specifically designed to transmit detonation to other explosives and which contains a detonator.

Circuit breaker means a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent setting without injury to itself when properly applied within its rating.

Combustible means capable of being ignited and consumed by fire.

Combustible liquids means liquids having a flash point at or above 100 °F (37.8 °C). They are divided into the following classes:

(1) Class II liquids—those having flash points at or above 100 °F (37.8 °C) and below 140 °F (60 °C).

(2) Class IIIA liquids—those having flash points at or above 140 °F (60 °C) and below 200 °F (93.4 °C).

(3) Class IIIB liquids—those having flash points at or above 200 °F (93.4 °C).

Combustible material means a material that, in the form in which it is used and under the conditions anticipated, will ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. Wood, paper, rubber, and plastics are examples of combustible materials.

Company official means a member of the company supervisory or technical staff.

Competent person means a person having abilities and experience that fully qualify him to perform the duty to which he is assigned.

Conductor means a material, usually in the form of a wire, cable, or bus bar, capable of carrying an electric current.

Delay connector means a non-electric short interval delay device for use in delaying blasts which are initiated by detonating cord.

Detonating cord means a flexible cord containing a solid core of high explosives.

Detonator means any device containing a detonating charge that is used to initiate an explosive and includes but is not limited to blasting caps, electric blasting caps and nonelectric instantaneous or delay blasting caps.

Distribution box means a portable apparatus with an enclosure through which an electric circuit is carried to one or more cables from a single incoming feed line, each cable circuit being connected through individual overcurrent protective devices.

Electric blasting cap means a detonator designed for and capable of being initiated by means of an electric current.

Electrical grounding means to connect with the ground to make the earth part of the circuit.

Employee means a person who works for wages or salary in the service of an employer.

Employer means a person or organization which hires one or more persons to work for wages or salary.

Emulsion means an explosive material containing substantial amounts of oxidizers dissolved in water droplets, surrounded by an immiscible fuel.

Explosive means any substance classified as an explosive by the Department of Transportation in 49 CFR 173.53, 173.88, and 173.100 which are incorporated by reference. Title 49 CFR is available for inspection at each Metal and Nonmetal Safety and Health district office of the Mine Safety and Health Administration, and may be obtained from the U.S. Government Printing Office, Washington, DC .

Explosive material means explosives, blasting agents, and detonators.

Face or bank means that part of any mine where excavating is progressing or was last done.

Fire resistance rating means the time, in minutes or hours, that an assembly of materials will retain its protective characteristics or structural integrity upon exposure to fire.

Flammable means capable of being easily ignited and of burning rapidly.

Flammable gas means a gas that will burn in the normal concentrations of oxygen in the air.

Flammable liquid means a liquid that has a flash point below 100 °F (37.8 °C), a vapor pressure not exceeding 40 pounds per square inch (absolute) at 100 °F (37.8 °C), and is known as a Class I liquid.

Flash point means the minimum temperature at which sufficient vapor is released by a liquid or solid to form a flammable vapor-air mixture at atmospheric pressure.

High potential means more than 650 volts.

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Highway means any public street, public alley, or public road.

Hoist means a power driven windlass or drum used for raising ore, rock, or other material from a mine, and for lowering or raising persons and material.

Igniter cord means a fuse, cordlike in appearance, which burns progressively along its length with an external flame at the zone of burning, and is used for lighting a series of safety fuses in the desired sequence.

Insulated means separated from other conducting surfaces by a dielectric substance permanently offering a high resistance to the passage of current and to disruptive discharge through the substance. When any substance is said to be insulated, it is understood to be insulated in a manner suitable for the conditions to which it is subjected. Otherwise, it is, within the purpose of this definition, uninsulated. Insulating covering is one means for making the conductor insulated.

Insulation means a dielectric substance offering a high resistance to the passage of current and to a disruptive discharge through the substance.

Laminated partition means a partition composed of the following material and minimum nominal dimensions: 1⁄2-inch-thick plywood, 1⁄2-inch-thick gypsum wallboard, 1⁄8-inch-thick low carbon steel, and 1⁄4-inch-thick plywood, bonded together in that order (IME-22 Box). A laminated partition also includes alternative construction materials described in the Institute of Makers of Explosives (IME) Safety Library Publication No. 22, “Recommendations for the Safe Transportation of Detonators in a Vehicle with Other Explosive Materials” (May ), and the “Generic Loading Guide for the IME-22 Container” (October ). The IME is located at 19th Street NW., Suite 310, Washington, DC -; 202-429-; https://www.ime.org. This incorporation by reference has been approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies are available at MSHA's Office of Standards, Regulations, and Variances, 201 12th Street South, Arlington, VA -; 202-693-; and at all Metal and Nonmetal Mine Safety and Health District Offices, or available for inspection at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.

Lay means the distance parallel to the axis of the rope in which a strand makes one complete turn about the axis of the rope.

Loading means placing explosive material either in a blasthole or against the material to be blasted.

Low potential means 650 volts or less.

Magazine means a facility for the storage of explosives, blasting agents, or detonators.

Major electrical installation means an assemblage of stationary electrical equipment for the generation, transmission, distribution, or conversion of electrical power.

Mantrip means a trip on which persons are transported to and from a work area.

Mill includes any ore mill, sampling works, concentrator, and any crushing, grinding, or screening plant used at, and in connection with, an excavation or mine.

Misfire means the complete or partial failure of a blasting charge to explode as planned.

Mobile equipment means wheeled, skid-mounted, track-mounted, or rail-mounted equipment capable of moving or being moved.

Multipurpose dry-chemical fire extinguisher means an extinguisher having a rating of at least 2-A:10-B:C and containing a nominal 4.5 pounds or more of dry-chemical agent.

Noncombustible material means a material that, in the form in which it is used and under the conditions anticipated, will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. Concrete, masonry block, brick, and steel are examples of noncombustible materials.

Non-electric delay blasting cap means a detonator with an integral delay element and capable of being initiated by miniaturized detonating cord.

Overburden means material of any nature, consolidated or unconsolidated, that overlies a deposit of useful materials or ores that are to be mined.

Overload means that current which will cause an excessive or dangerous temperature in the conductor or conductor insulation.

Permissible means a machine, material, apparatus, or device that has been investigated, tested, and approved by the Bureau of Mines or the Mine Safety and Health Administration and is maintained in permissible condition.

Potable water means water which shall meet the applicable minimum health requirements for drinking water established by the State or community in which the mine is located or by the Environmental Protection Agency in 40 CFR part 141, pages 169-182 revised as of July 1, . Where no such requirements are applicable, the drinking water provided shall conform with the Public Health Service Drinking Water Standards, 42 CFR part 72, subpart J, pages 527-533, revised as of October 1, . Publications to which references are made in this definition are hereby made a part hereof. These incorporated publications are available for inspection at each Metal and Nonmetal Mine Safety and Health District Office of the Mine Safety and Health Administration.

Powder chest means a substantial, nonconductive portable container equipped with a lid and used at blasting sites for explosives other than blasting agents.

Primer means a unit, package, or cartridge of explosives used to initiate other explosives or blasting agents, and which contains a detonator.

Reverse-current protection means a method or device used on direct-current circuits or equipment to prevent the flow of current in the reverse direction.

Rock fixture means any tensioned or nontensioned device or material inserted into the ground to strengthen or support the ground.

Roll protection means a framework, safety canopy or similar protection for the operator when equipment over-turns.

Safety can means an approved container, of not over five gallons capacity, having a spring-closing lid and spout cover.

Safety fuse means a flexible cord containing an internal burning medium by which fire is conveyed at a continuous and uniform rate for the purpose of firing blasting caps or a black powder charge.

Safety switch means a sectionalizing switch that also provides shunt protection in blasting circuits between the blasting switch and the shot area.

Scaling means removal of insecure material from a face or highwall.

Secondary safety connection means a second connection between a conveyance and rope, intended to prevent the conveyance from running away or falling in the event the primary connection fails.

Shaft means a vertical or inclined shaft, a slope, incline or winze.

Short circuit means an abnormal connection of relatively low resistance, whether made accidentally or intentionally, between two points of different potential in a circuit.

Slurry (as applied to blasting). See “Water gel.”

Storage facility means the entire class of structures used to store explosive materials. A “storage facility” used to store blasting agents corresponds to a BATF Type 4 or 5 storage facility.

Storage tank means a container exceeding 60 gallons in capacity used for the storage of flammable or combustible liquids.

Stray current means that portion of a total electric current that flows through paths other than the intended circuit.

Substantial construction means construction of such strength, material, and workmanship that the object will withstand all reasonable shock, wear, and usage, to which it will be subjected.

Suitable means that which fits, and has the qualities or qualifications to meet a given purpose, occasion, condition, function, or circumstance.

Travelway means a passage, walk or way regularly used and designated for persons to go from one place to another.

Water gel or Slurry (as applied to blasting) means an explosive or blasting agent containing substantial portions of water.

Wet drilling means the continuous application of water through the central hole of hollow drill steel to the bottom of the drill hole.

Working place means any place in or about a mine where work is being performed.

[69 FR , June 29, , as amended at 80 FR , Sept. 2, ]

Hoisting Procedures

§ 56. Availability of hoist operator for manual hoists.

When a manually operated hoist is used, a qualified hoistman shall remain within hearing of the or signal device at all times while any person is underground.

§ 56. Availability of hoist operator for automatic hoists.

When automatic hoisting is used, a competent operator of the hoist shall be readily available at or near the hoisting device while any person is underground.

§ 56. Hoist operator's physical fitness.

No person shall operate a hoist unless within the preceding 12 months he has had a medical examination by a qualified, licensed physician who shall certify his fitness to perform this duty. Such certification shall be available at the mine.

§ 56. Experienced hoist operators.

Only experienced hoistmen shall operate the hoist except in cases of emergency and in the training of new hoistmen.

§ 56. Maximum hoisting speeds.

The safe speed for hoisting persons shall be determined for each shaft, and this speed shall not be exceeded. Persons should not be hoisted at a speed faster than 2,500 feet per minute, except in an emergency.

§ 56. Maximum acceleration and deceleration.

Maximum normal operating acceleration and deceleration shall not exceed 6 feet per second per second. During emergency braking, the deceleration shall not exceed 16 feet per second per second.

§ 56. Persons allowed in hoist room.

Only authorized persons shall be in hoist rooms.

§ 56. Lowering conveyances by the brakes.

Conveyances shall not be lowered by the brakes alone except during emergencies.

§ 56. Maximum riders in a conveyance.

In shafts inclined over 45 degrees, the operator shall determine and post in the conveyance or at each shaft station the maximum number of persons permitted to ride in a hoisting conveyance at any one time. Each person shall be provided a minimum of 1.5 square feet of floor space.

§ 56. Trips during shift changes.

During shift changes, an authorized person shall be in charge of each trip in which persons are hoisted.

§ 56. Orderly conduct in conveyances.

Persons shall enter, ride, and leave conveyances in an orderly manner.

§ 56. Entering and leaving conveyances.

Persons shall not enter or leave conveyances which are in motion or after a signal to move the conveyance has been given to the hoistman.

§ 56. Closing cage doors or gates.

Cage doors or gates shall be closed while persons are being hoisted; they shall not be opened until the cage has come to a stop.

§ 56. Riding in skips or buckets.

Persons shall not ride in skips or buckets with muck, supplies, materials, or tools other than small hand tools.

§ 56. Skips and cages in same compartment.

When combinations of cages and skips are used in the same compartment, the cages shall be enclosed to protect personnel from flying material and the hoist speed reduced to man-speed as defined in standard 56., but not to exceed 1,000 feet per minute. Muck shall not be hoisted with personnel during shift changes.

§ 56. Hoisting during shift changes.

Rock or supplies shall not be hoisted in the same shaft as persons during shift changes, unless the compartments and dumping bins are partitioned to prevent spillage into the cage compartment.

§ 56. Riding the bail, rim, bonnet, or crosshead.

Persons shall not ride the bail, rim, bonnet, or crosshead of any shaft conveyance except when necessary for inspection and maintenance, and then only when suitable protection for persons is provided.

§ 56. Use of open hooks.

Open hooks shall not be used to hoist buckets or other conveyances.

§ 56. Maximum speeds for hoisting persons in buckets.

When persons are hoisted in buckets, speeds shall not exceed 500 feet per minute and shall not exceed 200 feet per minute when within 100 feet of the intended station.

§ 56. Lowering buckets.

Buckets shall be stopped about 15 feet from the shaft bottom to await a signal from one of the crew on the bottom for further lowering.

§ 56. Hoisting buckets from the shaft bottom.

All buckets shall be stopped after being raised about 3 feet above the shaft bottom. A bucket shall be stabilized before a hoisting signal is given to continue hoisting the bucket to the crosshead. After a hoisting signal is given, hoisting to the crosshead shall be at a minimum speed. The signaling device shall be attended constantly until a bucket reaches the guides. When persons are hoisted, the signaling devices shall be attended until the crosshead has been engaged.

§ 56. Blocking mine cars.

Where mine cars are hoisted by cage or skip, means for blocking cars shall be provided at all landings and also on the cage.

§ 56. Hoisting tools, timbers, and other materials.

When tools, timbers, or other materials are being lowered or raised in a shaft by means of a bucket, skip, or cage, they shall be secured or so placed that they will not strike the sides of the shaft.

§ 56. Conveyances not in use.

When conveyances controlled by a hoist operator are not in use, they shall be released and the conveyances shall be raised or lowered a suitable distance to prevent persons from boarding or loading the conveyances.

§ 56. Overtravel backout device.

A manually operated device shall be installed on each electric hoist that will allow the conveyance or counterbalance to be removed from an over-travel position. Such device shall not release the brake, or brakes, holding the overtravelled conveyance or counterbalance until sufficient drive motor torque has been developed to assure movement of the conveyance or counterbalance in the correct direction only.

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