Patent Publication Number: US-6334738-B1

Title: Prefabricated metal overcast having a crushable lower section

Description:
This application is a continuation of U.S. patent application Ser. No. 09/237,341, filed Jan. 26, 1999, U.S. Pat. No. 6,129,483, which is incorporated herein by reference for all that it discloses. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to mine ventilation overcasts in general and more specifically to prefabricated metal overcasts capable of compensating for floor heave. 
     BACKGROUND 
     Ventilation overcasts are commonly installed at various places in the entries (i.e., tunnels) in underground mines, such as coal mines, and are used to separate the intake and return air flows used to ventilate the mines. In general, such ventilation overcasts take the form of substantially air-tight bridges which permit the crossing of intake and return airways without interference. 
     Many different types of ventilation overcasts have been developed over the years and have been used with varying degrees of success. For example, ventilation overcasts have been constructed of tile, brick, stone, concrete, concrete blocks, steel panels, or combinations of the foregoing materials, with steel beams being used where necessary to provide additional support. Ventilation overcasts made from the foregoing materials typically require 2-6 days to construct, depending on the particular design and materials used. For example, overcasts fabricated from bricks or concrete blocks generally require 4-6 days to construct, while other designs utilizing primarily steel members may be constructed in less time, typically about 2-3 days. 
     The construction of such ventilation overcasts represents a substantial portion of the time, thus cost, required to develop and maintain the underground mine ventilation system. Consequently, devices and methods are always being sought which can reduce the time and/or cost required to acquire, set-up, and maintain the ventilation overcast. Toward this end, several different types of “quick assembly” ventilation overcasts have been developed in recent years which further reduce the time required for construction. While such “quick assembly” overcasts are being used more and more frequently, many still require at least one day (i.e., one work shift) to install. 
     One problem that may complicate the design and construction of ventilation overcasts is the degree of floor heave that may be expected in the mine. While some mines experience more floor heave than others, all mines experience at least some degree of floor heave. Heaving of the mine floor can compromise the air-tight construction of the overcast, causing intermixing of the return and intake air flows. Such intermixing of the return and intake air flows may violate applicable ventilation regulations and, in any event, represents an undesirable circumstance. Moreover, if the floor heave is excessive, it can result in structural damage to the overcast, possibly requiring that the overcast be repaired or even replaced before mining can continue. 
     Primarily in an effort to address the foregoing problems, ventilation overcasts have been developed which can compensate for a certain amount of floor heave. Unfortunately, the structure required to accommodate such floor heave tends to further increase the complexity, thus cost of the ventilation overcast. Furthermore, such accommodations usually require additional time to construct, thereby removing some of the advantages associated with quick assembly type of ventilation overcasts in which they may be utilized. 
     SUMMARY OF THE INVENTION 
     A ventilation overcast may comprise a frame member that defines a structure having a substantially inverted U-shape with first and second opposed side wall portions and a roof portion. A curtain member having a plurality of longitudinal corrugations therein is positioned adjacent the frame member so that a bottom portion of the curtain member contacts the floor of the mine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Illustrative and presently preferred embodiments of the invention are shown in the accompanying drawing in which: 
     FIG. 1 is a front view in elevation of one embodiment of a ventilation overcast according to the present invention showing the arrangement of the inner and outer frame members and the panel members positioned therebetween; 
     FIG. 2 is a side view in elevation of a portion of the inside side wall portion of the ventilation overcast showing the arrangement and spacing of the vertical support columns and the horizontal cross beams; 
     FIG. 3 is an enlarged elevation view showing one arrangement for attaching a horizontal cross beam to the upper end of a vertical support column; 
     FIG. 4 is a cross-section view in elevation of a vertical support column; 
     FIG. 5 is a cross-section view in elevation of a horizontal cross beam; 
     FIG. 6 is a cross-section view in elevation of the crimp used to join a top panel to a side panel; and 
     FIG. 7 is an enlarged elevation view of the lower ends of the inner and outer vertical support columns showing the arrangement of the retractable foot members and the corrugated curtain member. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A ventilation overcast  10  according to one preferred embodiment of the present invention is best seen in FIGS. 1 and 2 and may comprise an inner frame assembly  12  and an outer frame assembly  14  between which are positioned a plurality of panel members  16 . The inner frame assembly  12  may comprise a plurality of inner vertical support columns  18 , each of which includes a lower end  20  and an upper end  22 , as well as a plurality of horizontal cross beams  30 . The inner vertical support columns  18  may be positioned in generally opposed relation on opposite sides of the mine entry (not shown) so that they define respective first and second side wall portions  24  and  26  of the ventilation overcast  10 . Each side wall portion  24 ,  26  of the overcast  10  may be made as long as necessary by providing additional inner vertical support columns  18  at spaced positions along the lengthwise or longitudinal direction  28  of the ventilation overcast  10 . See FIG.  2 . The upper ends  22  of each pair of opposed inner vertical support columns  18  (i.e., those inner columns  18  located opposite one another on respective side wall portions  24  and  26 ) may be joined together by a horizontal cross beam  30 , as best seen in FIG.  1 . The cross beams  30  define a roof portion  32  of the ventilation overcast  10 . In accordance with the foregoing structural arrangement, the inner frame assembly  12  defines a structure having a generally inverted “U” shape or configuration. 
     The outer frame assembly  14  is similar to the inner frame assembly  12  just described and may comprise a plurality of outer vertical support columns  18 ′ and outer horizontal cross beams  30 ′. Each outer vertical support column  18 ′ may comprise a lower end  20 ′ and an upper end  22 ′. The outer vertical support columns  18 ′ may be positioned in pairs in generally opposed spaced relation on opposite sides of the mine entry, so that each outer vertical support column  18 ′ is generally outboard of a corresponding inner vertical support column  18 , as best seen in FIG.  1 . Several such outer vertical support columns  18 ′ may be spaced along the length  28  of the ventilation overcast  10  as necessary to provide each side wall portion  24 ,  26  of the overcast  10  with the desired length. See, generally FIG.  2 . The upper ends  22 ′ of opposed outer vertical support columns may be connected together by respective outer horizontal cross beams  30 ′. The outer frame assembly  14  therefore defines a generally inverted “U” shaped structure that is generally exterior to the inner frame assembly  12 . Stated another way, the inner frame assembly  12  is “nested” within the outer frame assembly  14 . See FIG.  1 . 
     The spaces defined between the inner and outer frame assemblies  12  and  14  may be closed by a plurality of panel members  16 . Accordingly, the panel members  16 , together with the inner and outer frame assemblies  12  and  14 , define a substantially air-tight structure having a generally inverted “U” shape that comprises the ventilation overcast  10 . In the embodiment shown and described herein, the plurality of panel members  16  may comprise one or more top panels  34  that are sized to be received between the inner and outer horizontal cross beams  30  and  30 ′, as well as one or more side panel members  36  sized to be received between the inner and outer vertical support columns  18  and  18 ′. Curtain members  38  may be positioned between the lower ends  20  and  20 ′ of the inner and outer vertical support columns  18  and  18 ′, as best seen in FIG.  1 . Each curtain member  38  may be provided with a plurality of longitudinal corrugations  40  that extend along the length  28  (FIG. 2) of the overcast  10 . The longitudinal corrugations  40  allow the curtain members  38  to be displaced upward (i.e., crushed) in the direction indicated by arrow  42  in order to accommodate upheaval of the mine floor  44 . The lower ends  20 ,  20 ′ of the vertical support columns  18 ,  18 ′ may be provided with respective retractable inner and outer foot members  46 ,  46 ′ which yield upwardly (i.e., also in the direction of arrow  42 ) when subjected to floor heave. 
     The ventilation overcast  10  may be installed by first positioning the outer vertical support columns  18 ′ on opposite sides of the mine entry (not shown), preferably with their respective foot members  46 ′ fully retracted into the outer vertical support columns  18 ′. The inner vertical support columns  18  may then be placed adjacent the outer vertical support columns  18 ′, again, preferably with their respective foot members  46  fully retracted. Next, the inner horizontal cross beams  30  may be attached to the upper ends  22  of the various inner vertical support columns  18 . After all the inner horizontal cross beams  30  have been secured in position, the user may then slide the top panels  34  over the tops of the inner horizontal cross beams  30  so that the top panels  34  are supported thereby. At this point, the various side panels  36  also may be moved into position between the inner and outer vertical support columns  18 ,  18 ′. Similarly, the curtain members  38  may be positioned between the lower ends  20 ,  20 ′ of the inner and outer vertical support columns  18 ,  18 ′ and the inner and outer foot members  46 ,  46 ′. After the panels  34 ,  36  and curtains  38  have been properly positioned adjacent the inner frame assembly  12 , the outer horizontal cross beams  30 ′ may be secured between the outer vertical support columns  18 ′ to form the structure substantially as shown in FIG.  1 . Suitable jacking apparatus, such as one or more roof jacks (not shown) then may be used to raise the roof portion  32  of the ventilation overcast  10  to the appropriate height. Thereafter, the inner and outer retractable foot members  46  and  46 ′ may be lowered so that they contact the mine floor  44 . Any subsequent floor heave will be accommodated by the crushable corrugated curtain members  38  and retractable foot members  46 ,  46 ′, thereby allowing a substantially air-tight seal to be maintained between the ventilation overcast  10  and the mine entry. 
     A significant advantage associated with the ventilation overcast  10  according to the present invention is that the various structural components (e.g., the vertical support columns  18 ,  18 ′; the horizontal cross beams  30 ,  30 ′ and the various panels  16 ) may be prefabricated outside the mine, thereby allowing the ventilation overcast  10  to be quickly constructed once the various components have been delivered to the appropriate location within the mine itself. The structural design of the overcast  10  also limits the number of fasteners required for assembly which again speeds construction. For example, the inner and outer frame assemblies  12  and  14  securely hold in place the various panels  16 , thereby substantially reducing, or in some cases even eliminating, the need to use separate fasteners (e.g., bolt and nut assemblies) to secure the various panels to the frame members. The inner and outer frame assemblies  12  and  14  also simplify construction by allowing the various panel members  16  to be placed on the inner frame assembly  12  before the outer horizontal cross beams  30 ′ are secured into position. That is, the inner frame assembly  12  supports the various top panels  34  while construction is being completed on the outer frame assembly  14 . 
     Still other advantages are associated with the curtain members  38  and retractable foot members  46 ,  46 ′. For example, the curtain members  38  and retractable inner and outer foot members  46  and  46 ′ allow the seal between the overcast  10  and the mine floor  44  to remain substantially intact even if the floor heaves. The curtain members  38  and yielding foot members  46 ,  46 ′ also substantially reduce the likelihood that the overcast will be structurally damaged if the floor heaves. 
     Having briefly described the ventilation overcast  10 , as well as some of its more significant features and advantages, the various embodiments of the ventilation overcast according to the present invention will now be described in detail. However, before proceeding with the description, it should be noted that while the ventilation overcast  10  shown and described herein is specifically designed to be used in a coal mine with a particular type of entry system having specific configurations, it is not limited to use in any type of mine having any particular type of entry system. Consequently, the present invention should not be regarded as limited to the applications and dimensional configurations shown and described herein. 
     With the foregoing considerations in mind, one embodiment of a ventilation overcast  10  according to the present invention is best seen in FIGS. 1 and 2 and may comprise a generally inverted “U” shaped structure formed by an inner frame assembly  12  and an outer frame assembly  14  with a plurality of panel members  16  sandwiched therebetween. The inner frame assembly  12  may comprise a plurality of inner frame members  48  positioned in spaced-apart relation along the length  28  of the ventilation overcast  10 . See FIG.  2 . Similarly, the outer frame assembly  14  may comprise a plurality of outer frame members  48 ′ positioned in spaced-apart relation along the length  28  of the ventilation overcast  10 . In one preferred embodiment, the outer frame members  48 ′ are positioned so that they are substantially aligned with the inner frame members  48 , although this need not be the case. The inner and outer frame members  48 ,  48 ′ comprising the respective inner and outer frame assemblies  12  and  14  will now be described in detail. 
     Referring now primarily to FIGS. 1 and 2, each inner frame member  48  is essentially identical and may comprise a pair of inner vertical support columns  18 , each of which has a lower end  20  and an upper end  22 . The pair of inner vertical support columns  18  are positioned in opposed, spaced-apart relation, i.e., on opposite side wall portions  24  and  26  of overcast  10 , as best seen in FIG.  1 . The respective upper ends  22  of the inner vertical support columns  18  are joined together by a horizontal cross beam member  30 , as is also best seen in FIG.  1 . 
     The inner horizontal cross beam  30  may be joined to the upper end  22  of the vertical support column  18  according to any of a wide variety of structural arrangements and using any of a wide range of fasteners that are well-known in the art. By way of example, in one preferred embodiment, each end (e.g.,  50 ) of the horizontal cross beam  30  may be provided with a connecting lug  52  designed to be received by the open upper end  22  of the inner vertical support column  18 . See FIG.  3 . The connecting lug  52  may be provided with a hole  54  therein positioned so that it aligns with a mating hole  56  provided in the upper end  22  of the vertical support column  18 . A pin or bolt and nut assembly (not shown) may be inserted through the aligning holes  54  and  56  to secure the horizontal cross beam  30  to the upper end  22  of the vertical support column  18 . Alternatively, other types of arrangements now known in the art or that may be developed in the future may be utilized to secure the horizontal cross beam  30  to the vertical support column  18 . 
     With reference now primarily to FIG. 2, it is preferred, but not required, that adjacent inner vertical support columns be connected together by means of longitudinal stringers  88 . The stringers  88  provide increased structural support for the various frame members  48  which is advantageous, particularly if the top and side panels  34  and  36  are not separately fastened to the various frame members  48 . The various stringers  88  may be fastened to the inner vertical support columns  18  by any of a wide range of fastening systems or devices well-known in the art. By way of example, in one preferred embodiment, the stringers  88  are welded to the vertical support columns  18 . 
     Referring now to FIG. 4, each vertical support column  18  may comprise an elongate member having a rectangular or square cross-section with a height  58  of about 2 inches, a width  60  of about 2 inches, and a wall thickness  62  of about ⅛ inch. Alternatively, the support column  18  may have other dimensions depending on the requirements of the particular application. The vertical support column  18  may be made from any of a wide range of materials, again depending on the requirements of the particular application. By way of example, in one preferred embodiment, each elongate support column  18  is fabricated from mild steel, although other materials may also be used. 
     The horizontal cross beams  30  used to connect the upper portions  22  of the vertical support columns  18  may also comprise an elongate member, but having a generally rectangular cross section, as best seen in FIG.  5 . In one preferred embodiment, each beam  30  may have a height  64  of about 3 inches and a width  66  of about 2 inches. The beam  30  may have a wall thickness  68  of about ⅛ inch. In one preferred embodiment, each horizontal cross beam  30  is made from mild steel. As was the case for the vertical support columns  18 , the horizontal cross beam  30  may have other dimensions and/or be made from other materials, depending on the requirements particular application. 
     Each inner frame member  48  may be sized to fit within the particular mine entry for which the overcast is designed. By way of example, in one preferred embodiment, each inner frame member may have a width W (FIG. 1) of about 14′6″ and a height H in the range of about 6′9″ inches to about 8′9″, with the height adjustment being provided by the retractable foot members  46 ,  46 ′, as will be described in greater detail below. Alternatively, of course, each inner frame member  48  may be sized to other dimensions depending on the particular entry in which the ventilation overcast  10  is to be used. 
     The outer frame members  48 ′ comprising the outer frame assembly  14  may be similar to the inner frame members  48  just described for the inner frame assembly  12 . That is, in one preferred embodiment, the outer frame members  48 ′ are essentially identical, with each outer frame member  48 ′ comprising a pair of outer vertical support columns  18 ′, each of which has a lower end  20 ′ and an upper end  22 ′, positioned in substantially opposed spaced-apart relation on opposite sides  24 ,  26  of the overcast  10 . The respective upper ends  22 ′ of the outer vertical support columns  18 ′ are connected by an outer horizontal cross beam member  30 ′, as best seen in FIG.  1 . 
     As was the case for the inner frame members  48 , the outer horizontal cross beam  30 ′ of each outer frame member  48 ′ may be joined to the upper ends  22 ′ of the vertical support column  18 ′ according to any of a wide variety of structural arrangements and fasteners that are well-known in the art. For example, in the embodiment shown and described herein, each end (e.g.,  50 ′) of the horizontal cross beam  30 ′ may be provided with a connecting lug  52 ′ designed to be received by the open upper end  22 ′ of the outer vertical support column  18 ′, as best seen in FIG.  3 . The connecting lug  52 ′ may be provided with a hole  54 ′ therein positioned so that it aligns with a mating hole  56 ′ provided in the upper end  22 ′ of the outer vertical support column  18 ′. A pin or bolt and nut assembly (not shown) may be inserted through the aligning holes  54 ′ and  56 ′ to secure the outer horizontal cross beam  30 ′ to the upper end  22 ′ of the outer vertical support column  18 ′. Alternatively, other types of arrangements now known or that may be developed in the future may be utilized to secure the outer horizontal cross beam  30 ′ to the outer vertical support column  18 ′. 
     As was the case for the inner frame members  48 , adjacent outer vertical support columns  18 ′ forming the various outer frame members  48 ′ may be connected together by one or more stringers  88 ′, as best seen in FIG.  2 . The stringers  88 ′ provide increased structural support to the overcast, particularly where the top and side panels  34  and  36  are not separately fastened to the outer frame members  48 ′. In one preferred embodiment, the stringers are affixed to the various vertical support columns  18 ′ by means of welding. Alternatively, other types of fastening systems or devices may be used. 
     Each outer vertical support column  18 ′ may be essentially identical to the inner vertical support columns  18  described above. That is, each outer vertical support column  18 ′ may comprise an elongate member having a rectangular or square cross-section with a height  58 ′ of about 2 inches, a width  60 ′ of about 2 inches, and a wall thickness  62 ′ of about ⅛ inch. See FIG.  4 . Alternatively, the support column  18 ′ may have other dimensions depending on the requirements of the particular application. The outer vertical support columns  18 ′ may be made from any of a wide range of materials depending on the requirements of the particular application. By way of example, in one preferred embodiment, each outer support column  18 ′ comprises mild steel, although other materials may also be used. 
     Referring now to FIG. 5, the outer horizontal cross beams  30 ′ used to connect the upper portions  22 ′ of the outer vertical support columns  18 ′ may also comprise an elongate member having a generally rectangular cross section. In one preferred embodiment, each beam  30 ′ may have a height  64 ′ of about 3 inches and a width  66 ′ of about 2 inches. The beam  30 ′ may have a wall thickness  68 ′ of about ⅛ inch. In one preferred embodiment, each horizontal cross beam  30 ′ may be fabricated from mild steel, although other materials may also be used. As was the case for the other support members, the outer horizontal cross beam  30 ′ may have other dimensions and/or be made from other materials, depending on the requirements of the particular application. 
     With reference now to FIGS. 1,  2 , and  6 , the top panel  34  and side panels  36  may comprise ribbed panels having a plurality of upright “hat” shaped portions  70  separated from one another by inverted “hat” sections  72 . See FIG.  1 . It is preferred, but not required, that the side panels  36  be joined to the top panel  34  by means of the interlocking joint  90  shown in FIG.  6 . So joining together the side panels  36  to the top panel  34  assures a substantially air-tight seal between the panels and also improves the structural integrity of the ventilation overcast  10 . 
     The top panel  34  and side panels  36  may be made from any of a wide range of materials suitable for the intended application. By way of example, in one preferred embodiment, the top panel  34  and side panels  36  comprise panels formed from 20 gauge sheet steel. Of course, heavier or lighter gauges could also be used, again depending on the requirements of the particular application. The ribs (e.g.,  70 ,  72 ) may be formed in the panels by any of a wide variety of processes (e.g., rolling) that are well-known in the art for forming sheet metal. 
     Depending on the overall width W (FIG. 1) of the overcast  10  (i.e, the distance between opposed side wall portions  24  and  26 ) or on other factors, it may be necessary or desirable to utilize two or more separate top panels  34  to form the roof portion  32  of the overcast  10 . If so, the top panels  34  may be joined together by an interlocking joint similar to the interlocking joint  90  illustrated in FIG. 6 to provide a substantially air-tight seal between adjacent panels. Alternatively, the two or more top panels  34  may be overlapped or simply butted together, again depending on the requirements of the particular application. 
     A similar situation exists with respect to the side panels  36  that close the opposed sides  24 ,  26  of the overcast  10 . That is, it may be necessary or desirable in certain applications to utilize two or more separate side panels  36  for each opposed side  24 ,  26 . If so, the adjacent side panels may be joined together by means of the interlocking joint described above for the top panels  34 . 
     The curtain members  38  located near the lower ends  20 ,  20 ′ of the inner and outer vertical support columns  18 ,  18 ′ are best seen in FIGS. 1 and 2 and may comprise generally elongate panels having a plurality of longitudinal corrugations  40  therein extending in the lengthwise direction  28 , as best seen in FIG.  2 . In one preferred embodiment, the longitudinal corrugations  40  provided in the curtain member  38  include at least one ridge portion  74  and at least one furrow portion  76  each of which comprises a smooth curve, as best seen in FIG.  7 . 
     The curtain members  38  may be made from any of a wide range of materials suitable for the intended application. By way of example, in one preferred embodiment, each curtain member  38  is fabricated from  20  gauge steel. Alternatively, steel having either heavier or lighter gauges may also be used, again depending on the requirements of the particular application. The longitudinal corrugations  40  provided therein may be formed by any of a wide variety of processes, e.g., rolling, that are well-known in the art for forming sheet metal panels. 
     With reference now primarily to FIG. 7, the lower ends  20 ,  20 ′ of the inner and outer vertical support columns  18  and  18 ′ may be provided with retractable foot members  46 ,  46 ′ which may yield or move in the direction indicated by arrow  42  if the floor  44  heaves excessively. Each retractable foot member  46 ,  46 ′ may be essentially identical to the others and may comprise an elongate leg section  78  sized to be slidably received by the open lower end  20  of the corresponding vertical support column  18  or  18 ′ as the case may be. The lower end  80  of leg section  78  may be provided with a plate  82  which may be affixed thereto by any convenient fastening system or device (e.g., by welding). The retractable foot member  46  may be frictionally engaged with the lower end  20  of the vertical support column  18  by means of a bolt  84  threaded into a nut  86  secured to the lower end  20  of support column  18 . When tightened, the bolt  84  securely holds the foot member  46  to the support column  18 , but will allow the foot member  46  to yield (i.e., move further into the support column  18 , as indicated by arrow  42 ) when subjected to excessive pressure, such as may be caused by the heaving of the floor  44  of the mine. As each retractable foot member  46  yields, the adjacent curtain member  38  will also yield (i.e., by means of crushing or collapsing along the corrugations  40 ), thereby preventing structural damage to the other portions of the ventilation overcast  10 . 
     The leg section  78  of each retractable foot member  46  may be provided with a length sufficient to allow the desired degree of travel (e.g., extension and retraction of the foot member  46 ). By way of example, in one preferred embodiment, the leg section  78  is provided with a length sufficient to allow the foot member  46  to be extended by about 2 feet. Alternatively, other lengths may also be used. 
     The ventilation overcast  10  may be installed as follows. As a first step in the installation process, the various outer vertical support columns  18 ′ may be positioned on opposite sides of the mine entry (not shown), preferably with their respective foot members  46 ′ fully retracted into the outer vertical support columns  18 ′. Next, the inner support columns  18  may be placed adjacent the outer vertical support columns  18 ′, again, preferably with their respective foot members  46  fully retracted. Thereafter, the inner horizontal cross beams  30  may be attached to the upper ends  22  of the various inner vertical support columns  18 . Once all of the inner horizontal cross beams  30  have been secured in position, the user may slide the top panels  34  over the tops of the inner horizontal cross beams  30  so that the same are supported thereby. At this time, the various side panels  36  also may be moved into position between the inner and outer vertical support columns  18 ,  18 ′. Finally, the curtain members  38  may be positioned between the lower ends  20 ,  20 ′ of the inner and outer vertical support columns  18 ,  18 ′ and the inner and outer foot members  46 ,  46 ′. 
     After the panels  34 ,  36  and curtains  38  have been properly positioned adjacent the inner frame assembly  12 , the outer horizontal cross beams  30 ′ may be secured between the outer vertical support columns  18 ′ to complete the outer frame assembly  14 . A hoist or jack arrangement then may be used to raise the roof portion  32  of the ventilation overcast  10  to the appropriate height. Thereafter, the inner and outer foot members  46  and  46 ′ may be lowered so that they contact the mine floor  44 . The restraining bolts  84  may be tightened to a torque sufficient to provide the desired yield characteristic, i.e., to allow the foot  46  to yield when subjected to a desired force. The curtain members  38  may then be attached to the respective side panel members  36 . Alternatively, the curtain members  38  may by positioned adjacent the side panel members  36  so that the two members  36  and  38  are free to slide past one another, as best seen in FIG.  1 . In any event, any subsequent floor heave will be accommodated by the corrugated curtain members  38  and retracting foot members  46 ,  46 ′, thereby allowing a substantially air-tight seal to be maintained between the mine floor  44  and the overcast  10 . The arrangement also prevents the ventilation overcast  10  from being otherwise damaged due to floor heave. 
     It is contemplated that the inventive concepts herein described may be variously otherwise embodied and it is intended that the appended claims be construed to include alternative embodiments of the invention except insofar as limited by the prior art.