Ventilation planning factors
The physical layout of a stone mine significantly impacts the mine’s ventilation efficiency. NIOSH research has shown that significant improvements in ventilation are achieved by using long stone pillars to direct the ventilation air. Long stone pillars are created by leaving the break-though cut in a crosscut, creating a pillar that is much longer than normal, with some pillars being over 500 feet long compared to a 45-foot width, as shown in Figure 1. Although there is an apparent loss of stone when developing long pillars, the pillars can be mined during the last days of the mine. Long stone pillars eliminate the leakage that occurs when using fabric stoppings and substantially increase the ventilation airflow to the face. Notice that the mine in Figure 1 delivered 74 percent of the air produced by the main mine fan to the face. By observation, it was apparent an even higher percentage could have been achieved by reducing the leakage at three of the four crosscuts. In comparison tests, a different mine using conventional fabric stoppings at every crosscut, instead of long stone pillars, moved only 33 percent of the total air produced to the face.
Although conventional fabric stoppings are difficult to properly construct and maintain and are prone to leakage, they are a necessity in some locations. Where stoppings are required, a good construction technique is to use “cut-downs.” Cut-downs involve cutting the roof and rib of the entry smaller than normal, as shown in Figure 2. This allows for a solid stone backing along the rib and roof against which the material can be attached to permit a better seal. Cut-downs are helpful in any entries where stoppings are to be built. One more effective method to reduce leakage is to use piled rock in the crosscut entry and top it with a fabric stopping, as shown in Figure 2. The smaller area created by the piled stone reduces the quantity of fabric material and the opportunity for rips or tears.
Although stone mines are generally shallow operations, their natural ventilation is apparent, but it is uncontrollable in direction. In many mines, the natural ventilation changes direction several times within a day. In most cases, the main mine fan should overcome all natural ventilation. However, there are conditions that make a mine or sections of a mine much more difficult to ventilate. Mining up slope creates contaminated air pockets of warm diesel exhaust, as shown in Figure 3. NIOSH has observed large production areas being difficult to ventilate because the sections were developed up slope. In the summer months, fresh air that has been cooled by the mine ground temperature is required to move up slope to push out warm air that is collecting in the upper reaches. Mines that are developed down slope are easier to ventilate as the warm, contaminated air naturally rises away from the face.
Many mines use a slope for access from the surface or from one mining level to another. When developing the slope or the mine workings, advancement can be slow. If a single slope is used, tubing attached to an exhaust or blowing fan is necessary for ventilation. In such cases, operators may be tempted to develop a mine airshaft as soon as possible once the mining level is reached. In fact, NIOSH has observed some operations where the slope bottom is just a few breaks away from the bottom of an airshaft with no stoppings between. As the mine expands, this creates a ventilation problem, as most of the airflow will short circuit directly to the exhaust shaft rather than move past the shaft to the mine workings. This results in serious contamination problems in the mine workings due to minimum inflow of fresh air.
An alternative approach is to continue to use the blowing tubing to construct the shaft a further distance from the slope bottom and orient long stonewalls to force the air to the production faces, as shown in Figure 4. Later, the cut-downs “A” can be permanently sealed to prevent short-circuit leakage from the slope to the shaft. It is a tradeoff, as using tubing is considered a necessary nuisance while trying to locate the shaft some distance from the slope. Notice that once underground development to the shaft begins, at least two entries must be available for an escapeway, as shown by “A.” Once the mine is expanded, future mine plans may eventually call for abandoning the shaft and relocating the shaft to a location beneficial to a larger mine.
General ventilation considerations
A common question is whether to use blowing or exhaust ventilation in a stone mine. Due to the low fan exit losses, equal amounts of energy are used with either method. If a mine uses drift portals for ventilation, one disadvantage of an exhaust system is the need for check curtains on the portal where the trucks leave the mine. A mine operating an exhaust system needs two return portal exits, one for the truck haulage and one for the fan. Without check curtains, the exhaust fan would pull air from the truck portal, short circuiting airflow to the active face area. For a mine operating a blowing system, truck haulage using the returns and exhaust portals do not need check curtains. This is a distinct advantage for a blowing system in these types of instances.
Potentially hazardous circumstances can develop on a seasonal basis depending upon how and where the ventilation air enters the mine. If a mine has an exhaust shaft with an intake slope, cold air that enters the slope may create icy roadways and hazardous travel conditions during the winter months. In the summer months, warm moist intake air being cooled in the mine causes condensation on the roof and ribs, which, in turn, can promote ground control problems. This is likely less of a problem near the shaft, which is normally away from common travel areas. Therefore, local conditions may dictate the choice of an exhaust or blowing system. For a mine with only portals and no air shaft, blowing ventilation has a distinct advantage of not requiring the check curtains. Also, it has been observed that the fans can be reversed during different times of the year. In some cases, this reversal could be beneficial, although a detailed investigation into changes in traffic pattern would be required. Considerations must also be given to keeping haul trucks from traveling in the return airways until blast fumes from stone production have left the mine. This can usually be accomplished by blasting at the end of the last shift of the day (assuming the mine doesn’t work all night). This schedule allows the mine ventilation to clear out blast fumes. To determine the time required, velocity rates for intake air can be checked in the return using an anemometer. Chekan et al. (2004) found that velocities in one stone mine ranged from 60 feet/minute to 75 feet/minute by monitoring dust and exhaust clouds traveling through the mine after a production shot.
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