Mine ventilation structures such as overcasts and undercasts are widely used in mines to prevent mixing of forced (or induced) ventilation air flowing in one passage with forced (or induced) ventilation air flowing in another passage at an intersection of those passages. Generally, an overcast comprises a tunnel (e.g., made of two sidewalls and a deck) erected in one of the passages and extending through the intersection with the other passage. The tunnel blocks communication of air between the passages at the intersection, but permits air in one of the passages to flow through the tunnel and permits air in the other passage to flow through the intersection in a space between the top of the tunnel and the roof. Additional details relating to the construction and operation of overcasts is provided in U.S. Pat. Nos. 6,264,549 and 5,466,187, both of which are hereby incorporated by reference. An undercast is similar to an overcast, but the tunnel is formed adjacent the roof of the intersection (e.g., the sidewalls a deck are inverted and suspended above the floor). Air in one of the passages flows through the tunnel of the undercast and the air in the other passage flows through the intersection in a space between the bottom of the tunnel and the floor of the intersection.
Ventilation air can impart substantial forces on an overcast or undercast. The static pressure difference between the two passages can be significant and can generate substantial air loading on the overcast or undercast. The velocity of air flowing through the passages generates some minor additional loading. Overcasts and undercasts have to be robust enough to withstand the air loading and other forces acting on them. The construction and operation of undercasts is generally similar to that of overcasts, except that the sidewalls and deck are inverted to form the tunnel adjacent the roof of the intersection.
Overcasts and undercasts hinder flow of ventilation air through a mine. Shock loss (i.e., energy wasted by abrupt changes in the direction of air flow) is associated with the flow of air in the passage bypassing the tunnel (e.g., between the top of the tunnel of an overcast and the roof), adding to the energy required to ventilate the mine. For example, the shock loss associated with some overcasts is equivalent to the friction loss associated with about 70 feet of an ordinary unobstructed airway. Further, a mine can have multiple overcasts and/or undercasts, each of which adds to the energy demands for the mine's ventilation system.
In order to reduce the shock loss from an overcast, tailings, dirt or the like may be piled against the sides of the overcast to form a debris ramp extending up from the floor of the passage thereby altering the flow of air around the tunnel. Although this can reduce the shock loss associated with use of the overcast, the overcast has to be made more robust to withstand not only the air loading but also the additional forces imparted by the weight of the debris against it. This increases the cost of the overcast. Further, the material piled against the overcast has to be removed before the overcast can be removed from the intersection, making it inconvenient to uninstall the overcast and use it at another location.