Monday, August 30, 2010

Lateral Development and Ramps

1. Introduction

For underground hard rock miners, the term “lateral development” means the horizontal headings in a mine, such as the drifts and cross cuts at a mine level. Lateral development includes the inclined headings (ramps and declines) between levels. 

Because it constitutes by far the major portion of mine development, lateral development is of significant consequence. Pre-production mine development concerns are well recognized by the mining community (and discussed in other chapters of this handbook). Ongoing development during operations is not given similar attention. Part of the reason is that the major portion of the development costs in an operating mine are capitalized and not accounted for in mining costs statements. As a  result, the significance of ongoing lateral development is partially disguised.

Despite significant efforts directed at developing new equipment, techniques, and procedures, the productivity and advance rates of lateral development have shown no significant gain during the past twenty-five years. Part of the reason is that most research and development efforts have been directed at modifying mechanized equipment, such as the tunnel boring machine (TBM), continuous miner (gathering arm mucking unit), and the road header. Such machinery is capable of achieving acceptable advance rates in hard rock; however, other problems remain unresolved. These include dealing with high silica dust counts, poor visibility, low cutter wear life, squeezing ground, highly stressed ground, difficult equipment access, poor equipment mobility, difficulty in mechanizing ground support, and inflexibility with respect to gradient and curvature.

As a result, the traditional drill and blast method remains the least expensive and most practical means of advancing lateral headings. For this reason, the balance of this chapter is primarily devoted lateral headings driven by drill and blast. No universally accepted standard definitions exist for terms that refer to inclined lateral headings. In this chapter, a “ramp” is a heading containing horizontal curves used as a transport corridor for rubber-tired mobile equipment. A “decline” is a straight heading suitable for installation of a belt conveyor that may also permit travel of mobile equipment.

2. Rules of Thumb
General
• Laser controls should be used in straight development headings that exceed 800 feet (240m) in length. Source: Tom Goodell
• The overall advance rate of a lateral drive may be increased by 30% and the unit cost decreased by 15% when two headings become available. Source: Bruce Lang

Trackless Headings
• The minimum width for a trackless heading is 5 feet wider than the widest unit of mobile equipment. Source: Fred Edwards
• The back (roof) of trackless headings in hard rock should be driven with an arch of height equal to 20% of the heading width. Source: Kidd Mine Standards
• The cost to slash a trackless heading wider while it is being advanced is 80% of the cost of the heading itself, on a volumetric basis. Source: Bruce Lang
• For long ramp drives, the LHD/truck combination gives lower operating costs than LHDs alone and should be considered on any haul more than 1,500 feet in length. Source: Jack Clark
• LHD equipment is usually supplemented with underground trucks when the length of drive exceeds 1,000 feet. Source: Fred Edwards
• With ramp entry, a satellite shop is required underground for mobile drill jumbos and crawler mounted drills when the mean mining depth reaches 200m below surface. Source: Jack de la Vergne
• With ramp and shaft entry, a main shop is required underground when the mean mining depth reaches 500m below surface. Source: Jack de la Vergne
• A gradient of 2% is not enough for a horizontal trackless heading. It ought to be driven at a minimum of 2½% or 3%. Source: Bill Shaver
• The minimum radius of drift or ramp curve around which it is convenient to drive a mobile drill jumbo is 75 feet. Source: Al Walsh
• For practical purposes, a minimum curve radius of 50 feet may be employed satisfactorily for most ramp headings. Source: John Gilbert
• The gathering arm reach of a continuous face-mucking unit should be 2 feet wider than the nominal width of the drift being driven. Source: Jim Dales
• Footwall drifts for trackless blasthole mining should be offset from the ore by at least 15m (50 feet) in good ground. Deeper in the mine, the offset should be increased to 23m (75 feet) and for mining at great depth it should be not less than 30m (100 feet). Source: Jack de la Vergne
• Ore passes should be spaced at intervals not exceeding 500 feet (and waste passes not more than 750 feet) along the draw point drift, with LHD extraction. Source: Jack de la Vergne
• The maximum practical air velocity in lateral headings that are travelways is approximately 1,400 fpm. Even at this speed, a hard hat may be blown off when a vehicle or train passes by. At higher velocities, walking gets difficult and road dust becomes airborne. However, in pure lateral airways, the air velocity may exceed 3,000 fpm. Various sources
• The typical range of ventilation air velocities found in a conveyor decline or drift is between 500 and 1,000 fpm. It is higher if the flow is in the direction of conveyor travel and is lower against it. Source: Floyd Bossard
• The maximum velocity at draw points and dumps is 1,200 fpm (6m/s) to avoid dust entrainment. Source: John Shilabeer

Track Headings
• Track gage should not be less than ½ the extreme width of car or motor (locomotive). Source: MAPAO
• The tractive effort, TE (Lbs.) for a diesel locomotive is approximately equal to 300 times its horsepower rating. Source: John Partridge
• Wood ties should have a length equal to twice the track gage, be at least ¼ inch thicker than the spike length, and 1 3/8 times spike length in width. Source: MAPAO
• Typical gradients for track mines are 0.25% and 0.30%. Source: MAPAO
• A minimum clearance of three feet should be designed between the outside of the rails and the wall of the drift to permit safe operation of a mucking machine when driving the heading. Source: MAPAO

3. Tricks of the Trade
• Compared with LHDs used for haulage, underground trucks have longer life and carry more tons per horsepower with lower maintenance costs, lower tire costs, and higher availability. Source: Jack Clark
• When an underground haul truck is stuck or hung up, place blocking behind it and raise the box just enough to lift the rear wheels. Source: Bill Middleton
• Haulage ramps should be widened on curves by 600 mm (two feet) for 50 tonne capacity trucks to prevent damage to vehicle and utility lines at high-speed travel. Source: Tom Lamb
• If the back of a heading is secured with split set rock bolts, screen is easily applied by using smaller diameter “utility” split sets that are driven inside the existing bolts to a depth of 18 inches (0.45m). Source: Towner and Kelfer
• Internal ramps are better laid out in a figure eight or oval configuration. Spiral (corkscrew) ramps are difficult to survey, provide less advance warning of oncoming traffic, continually scruff the tires of mobile equipment, and make roadway grading difficult. Source: Menno Friesen
• Where it is not practical to design a curve at a location that will experience only occasional traffic, remember that LHD equipment is bi-directional, so a switchback may be employed instead. Source: Bill Middleton
• It should be considered that a high back is difficult to scale and keep safe. It may be better to dispense with arching and drive with a flat back when ground conditions permit. Source: Douglas Duke
• In burst prone ground, top sills are driven simultaneously in a chevron (‘V’) pattern. Outboard sills are advanced in the stress shadow of the leading sill with a lag distance of 24 feet. Source: Luc Beauchamp
• Front-end loaders are excellent for mucking but poor for haulage; LHD units are designed to perform both functions but do not perform either of them really well. Source: Ron Vaananen
• One practical means of obtaining road dressing underground at the size desired (-2½ inches) is to install a tuning fork grizzly (or similar device) at a suitable location to divert a portion of the undersize material in the waste rock stream for use as roadbed dressing. Source: John Gilbert and Jack de la Vergne
• When the floor of a ramp heading is to be paved with monolithic concrete to handle heavy traffic loads, it must be cleaned to the rock surface before pouring concrete. Otherwise, the pavement will fail due to the pumping action of the wheel loading. Source: Ed Cantle
• The floor of a steep ramp heading should not be paved with asphalt. The problem is that the asphalt surface will rill with the application of wheel brakes. Source: George Greer
• On the initial drive, the rail should be installed beneath its final grade. It is a lot easier to lift and ballast the track to its final surveyed grade than it is to dig out and lower it. Source: Marshall Hamilton
• The braking power of a battery-operated locomotive is limited by the weight of the locomotive itself. This restricts the train speed and the number of cars that can be safely hauled. Source: Jack Burgess

4. Track versus Trackless
A “track” mine refers to one that has rail installed in its lateral headings to provide travel for trains drawn by battery-operated, trolley, or diesel locomotives. A “trackless” or “mechanized mine” refers to the use of rubber tired mobile equipment to advance the lateral development and haul the ore. The basic component of an operation is the LHD unit. Of course, some mines employ a combination of track and trackless headings and many employ conveyors in drifts and declines for ore handling.

The trend for some time has been away from track development. Even the smallest of mines are now considered best served by trackless methods, mainly due to flexibility. At the larger mines, rail haulage has been largely displaced by conveyor transport fed from an underground crusher placed near the ore body so that it can be gravity fed by trackless equipment enjoying a short haul distance.

Trackless headings have other significant considerations besides flexibility. Both the productivity (i.e. feet per man-shift) and rate of advance (i.e. feet per month) are normally significantly higher for trackless headings than for rail headings.

Following are the principal disadvantages of trackless headings.
• The need more ground support because trackless headings are larger in cross section
• The equipment that drives trackless headings requires more ventilation
• The roadway is more difficult to maintain
Employing electric powered LHD units and trolley-line truck haulage reduces ventilation requirements; however, flexibility is impaired.

5. Design and Function of Lateral Headings
The design starts with determining the cross-section of the drift, cross cut, ramp, or decline. Lateral headings are contoured to the minimum dimensions required to safely permit passage of the largest vehicle while providing space for roadway dressing (or rail and ballast), ditches, utility lines and ventilation duct. Safe clearance must be provided for pedestrian traffic, especially if no safety bays are to be cut. The minimum clearances and the spacing of safety bays are usually specified in the applicable statutory mine regulations. If the heading is to become a main airway, its cross section may have to be enlarged for this purpose.

In the recent past, the size of typical lateral headings has continued to increase because larger and larger haulage vehicles are employed. The philosophy has been to reduce costs by economy of scale. Larger headings are not advanced as rapidly as smaller headings, which has the effect of slowing the pre-production development schedule for a new mine and ongoing development at an existing operation.

Employing remote operation and guidance systems that enable one operator to run two or three units of equipment simultaneously allow greater productivity improvement than further increasing equipment size. In this regard, a unique system employed at the Savage Zinc mine in Tennessee may have good application elsewhere (the mine employs a trackless “road train” with a 75-ton payload that travels underground at speeds of 25 miles per hour in a relatively small heading at a gradient of 6%). High-speed haulage is also employed at several underground operations in South America using a “throw-away” haul truck. 

For this purpose, the mines have dispensed with the typical slow-moving articulated underground truck and replaced it with one designed for highway travel. The truck is a stock model modified with a supercharger and oversize inter-cooler. After a useful service life of only two to three years on mine grades of 10%, a truck is sold to the after market for light duty and replaced with a new one. Some of these same mines employ surface-type front-end loaders underground instead of LHD units to muck from draw points and load the haul trucks with apparent great success. The Australians have employed relatively high-speed truck haulage for many years using off-road haulage trucks modified for underground service at typical gradients of 9:1 or 12%.

Track headings are normally fully arched with a vent duct hung at the crown when driven. For trackless headings, it is common practice to hang the vent duct and utility lines on the ditch side to save space and help protect them from wayward vehicles. The back of these headings may be gently arched or driven flat backed, depending on the ground conditions (mine company standards may dictate an arched back).

6. Laser Controls
Historically, both conventional and trackless development headings were driven using standard line and grade plugs as a means of controlling the azimuth and gradient of the heading. As the distance between the control points and the face increases, the quality of control diminishes. This is particularly important in the conventional headings as the grade control line is used for rail installation. Typically, line and grade plugs are useful for about 280 feet (85m) and then new controls must be installed.

Grade plugs are installed in the walls of the heading. Line plugs are installed in the back, and may or may not include a survey control point. Both types of controls are frequently damaged during regular mining practice. The damage may occur due to neglect or by accident. In either case, reinstallation of these controls generates additional work for the surveyors; therefore, additional delays for the mining crews. Furthermore, line and grade plugs are prone to inaccuracy, depending on the diligence and skill of the persons who use them.

For straight development headings that exceed 800 feet (240m) in length, the solution to these problems is the use of laser beams. Following are the benefits of using laser survey controls.
• A single installation will remain useful for a much greater distance than with conventional line and grade plugs reducing the overall amount of production shut down due to surveying.
• A single laser installation provides both line and grade control.
• A laser installation provides more accurate basis for each crew to correctly determine the orientation required for each drill hole.
• A laser installation is less likely to be damaged than conventional line and grade plugs.
• A laser installation can be mounted in such a manner that will allow traffic to pass by without disturbing the setup.
• A laser installation is not affected by ventilation.
• Check points can be installed to verify the laser’s alignment prior to marking the face.
• Laser lines provide a ready source of control points for starting additional excavation control.