Abstract:
A grid composite for protecting men and longwall mining equipment during longwall shield recovery includes a regular polymer geogrid structure formed by biaxially drawing a continuous sheet of select polypropylene material which is heat bonded to a polyester fabric. The grid composite is secured over caving shields of longwall mining equipment during a longwall mining operation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. patent application Ser. No. 07/675,616, filed Mar. 27, 1991, for a POLYMER GRID FOR SUPPLEMENTAL ROOF AND RIB SUPPORT OF COMBUSTIBLE UNDERGROUND OPENINGS. 
     FIELD OF THE INVENTION 
     This invention relates to a high strength, lightweight polymer grid laminated with a material consisting of a non-woven polyester. It is utilized in underground coal and trona mines in the longwall recovery phase during movement of longwall mining system equipment. It can also be applied as a supplemental roof and rib control product in underground &#34;non-gassy&#34; mines. 
     BACKGROUND OF THE INVENTION 
     The recent development of polymer grids for the underground coal mining industry has created new alternatives for supplemental ground control practices. The grids utilize strong, lightweight polymers, usually special grades of polypropylene. High tensile strengths and resulting load support characteristics are achieved by molecular orientation of these polymers in the manufacturing process. 
     One of the most important applications of polymer grids as supplemental ground control is in longwall shield recovery. When shields are moved from one face to another, the determining factor in the success of the recovery is the ground control provided by roof support structures along the old face. Whereas primary support is usually provided by roof bolts and cables which run the full width of the panel, supplemental support is often provided by metallic meshes of welded wire or chain-link fence. Lightweight, high-strength polymer grids may replace these heavy, cumbersome metallic meshes, giving the operation increased productivity by decreasing installation time and reducing injury downtime. 
     However, use of polymer grids immediately over the shields during longwall shield recovery has produced potential dangers due to penetration through the polymer grid by large pieces of shale and sandstone of the gob, cutting through the polymer grid. Shield recovery is thereby hampered and mine workers are placed in danger. 
     SUMMARY OF THE INVENTION 
     By the present invention, a polymer grid is connected to a grid composite consisting of a polymer grid and a geotextile to provide a longwall screening package for use during longwall shield recovery. The grid composite is formed by use of a polymer grid which is typically heat bonded to an 8.0 oz./yd. 2 , 100% continuous filament polyester, non-woven needlepunched engineering fabric. The engineering fabric or geotextile is bonded to the polymer grid using an open flame heat source or using a heated roll as a heat source. 
     During longwall mining, a first roll of polymer grid is attached, by chain, to the shearer and pulled onto the face. When the shearer has advanced 200 feet, a second roll is attached to the tail of the first roll and the shearer is advanced another 200 feet. This is done until the rolls are laying end to end the entire length of the face. 
     A spool of 9/16 inch or 3/4 inch wire rope is placed on a spool stand in each successive crosscut. Then the wire rope is attached onto the shearer and pulled to the tailgate allowing it to run on the toes of the shields. Then the wire rope is unhooked from the shearer and a loop is made in both ends using three Crosby clamps. These loops are then hooked onto a roof bolt in the headgate and tailgate and tensioned with a come-a-long. 
     The leading edge of the polymer grid is then fastened to the rope (dinged). The seams between the 200 foot rolls are also fastened. Once the rope and seams are dinged, the rope is placed under the canopy tips. The shields can then be lowered and advanced and the remainder of the roll is hung under the canopy tip. 
     During approximately the last thirty feet of a longwall mining operation, bolts are installed, at an angle, where the roof and rib meet. This usually requires ten to twelve roof bolts with plates and turnbuckles. These are spaced 30 inches apart or the width of cut of the shearer of the longwall mining system equipment. Approximately four inches of bolt are left exposed and installed at various spaced locations. 
     A full face pass is made and the procedure of installation of the polymer grid and grid composite is performed until the stopping point of the shearer is reached. The shields of the longwall mining system are now encompassed by the grid composite as held by the wire ropes on 30 inch centers which run the length of the face. The previous problem of cutting through only polymer grid protection is prevented by falling debris initially contacting the geotextile of the grid composite as reinforced below by polymer grid of the grid composite which is supported by the wire ropes. 
     The remaining gap between the canopy tips and the coal face is then bolted and planked. Longwall equipment recovery can then begin. 
     Typically, the polymer grid and the grid composite are available in 13 foot and 200 foot roll dimensions. The final width of polymer grid is joined together with an appropriate width of grid composite on the surface to eliminate most of the time consuming fastening (dinging) underground on the longwall face. 
     Rolls of grid composite are laid out side by side with a two foot overlap at the lateral seams. The seams are then joined by means of wire or plastic tie. It is recommended to use a four inch spacing o the fasteners down the length of the seams. The number of mats required depends on the width of the longwall face. The mats are rolled up and are then ready for transport underground. Typically they are folded and placed on supply cars and stored in the headgate or tailgate. 
     The grid composite includes a regular polymer geogrid structure formed by biaxially drawing a continuous sheet of select polypropylene material which is heat bonded to a polyester fabric. 
     The polymer geogrid of the grid composite shall typically conform to the following property requirements: 
     
         ______________________________________PROPERTY    TEST METHOD   VALUE______________________________________Materialcopolymer   ASTM D 4101   97% (min)polypropylene       Group 2/Class       1/Grade 1colorant and UV       ASTM 4218     2.0% (min)inhibitorInterlockaperture size.sup.1       I.D. Calipered.sup.2@ MD                      1.8 in. (nom)@ CMD                     2.5 in. (nom)open area   COE Method.sup.3                     75% (min)thickness   ASTM D 1777-64@ ribs                    0.07 in. (nom)@ junctions               0.20 in. (nom)Reinforcementflexural rigidity       ASTM D1388-64.sup.4MD                        600,000 mg-cm (min)CMD                       800,000 mg-cm (min)tensile modulus       GRI GG1-87.sup.5MD                        20,000 lb/ft (min)CMD                       21,000 lb/ft (min)junction strength       GRI GG2-87.sup.6MD                        1350 lb/ft (min)CMD                       1350 lb/ft (min)junction    GRI GG2-87.sup.6                     90% (min)efficiencyThe geotextile of the grid composite typically conforms to thefollowing property requirements:Grab tensile       ASTM D1682    285/250 lbsstrengthEOS         ASTM D422     70 US Std Sv SzWeight      ASTM D1910    8.0 oz/syThe grid composite shall typically conform to the followingproperty requiremnts:roll length           200 ftroll width            10 &amp; 12 ftroll weight           210 &amp; 260 lb______________________________________ .sup.1 MD (machine direction) dimension is along roll length. CMD (cross machine direction is across roll width. .sup.2 Maximum inside dimension in each principal direction measured by calipers. .sup.3 Percent open area measured without magnification by Corps of Engineers method as specific in CW 02215 Civil Works Construction Guide, November 1977. .sup.4 ASTM D 138864 modified to account for wide specimen testing as described in Tensar test method TTM5.0 &#34;Stiffness of Geosynthetics&#34;. .sup.5 Secant modulus at 2% elongation measured by Geosynthetic Research Institute test method GG187 &#34;Geogrid Tensile Strength&#34;. No offset allowances are made in calculating secant modules. .sup.6 Geogrid junction strength and junction efficiency measured by Geosynthetic Research Institute test method GG287 &#34;Geogrid Junction Strength&#34;. 
    
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C are a schematic flowchart for formation of a polymer geogrid. 
     FIG. 2 illustrates a grid composite including a polymer geogrid and a geotextile secured to each other. 
     FIG. 3 is a plan view of the terminal portion of a longwall screening package including a section of grid composite secured on or between two lengths of geogrid. 
     FIG. 4 illustrates a length of geogrid secured to a length of grid composite overhanging the shield tips of longwall mining equipment. 
     FIG. 5 illustrates a grid composite located over the caving shields of longwall mining equipment to facilitate longwall shield recovery. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake in clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Production of the grid composite for underground mining applications is accomplished in a four stage manufacturing process as schematically shown in FIG. 1: 
     I. SHEET EXTRUSION 
     A multi-component blending system allows for precise control of the raw material additives mix. This on-line blender feeds directly to an extruder, which compresses and melts plastic pellets, and then pumps the molten extrudate. A gear pump and a melt mixer are included in the extrusion system, to provide for a very accurate, consistent flow of a homogeneous melt. At the end of the extruder is a sheet die, which evenly distributes the melt flow across the desired sheet width. 
     The sheetline portion of the process accepts the molten sheet, cools it slowly and uniformly, controls the sheet thickness, and provides for a smooth surface finish. The sheet thickness tolerances are very tight in the sheet process, with a +/-1.0% specification in both the machine and transverse direction. The sheet thickness is monitored at all times with an on-line thickness profiler. The finished sheet 20 is then wound onto large reel carts for transfer to the next process. 
     II. SHEET PUNCHING 
     The second stage of the polymer grid production process involves punching a solid sheet 22 with a pattern of holes, prior to its orientation. Specially designed punch tools and heavy duty presses 24 are required. Several hole geometries and punch arrangements are possible, depending upon the finished product properties of the grid, in order to meet the requirements of the ground control application. 
     III. ORIENTATION 
     The polymer raw materials used in the manufacture of the grids are selected for their physical properties. However, the very high strength properties of the finished grid are not fully realized until the base polymer&#39;s long chain molecules are stretched (oriented) for the mining grid. This is accomplished in a two stage process. 
     Initially, the punched sheet is heated to a critical point in the softening range of the polypropylene polymer. Once heated, the sheet is stretched in the machine direction, through a series of heated rollers located within a housing 26. During this uniaxial stretching, polymer is drawn from the junctions into the ribs as the orientation effect passes through the junction zones. This guarantees continuity in molecular orientation in the resultant structure. 
     In the second stage, the uniaxially oriented grid 28 enters a heated tenter frame (stenter) 30 where the material is stretched in the transverse direction, at right angles to the initial stretch. This biaxial stretch process imparts a high degree of orientation and stretch throughout all regions of the grid. 
     Exiting the stretching process the biaxial grid material 32 is quenched (stabilized), and then slip and wound into a roll 34 to meet customer roll dimension requirements. 
     IV. LAMINATION 
     A polyester geotextile is bonded to the biaxial grid material by two methods. 
     Of the two methods for forming the grid composite of polymer grid and geotextile, the flame method exposes both mating surfaces of the polyester geotextile and the polymer grid to an open flame. Immediately thereafter, the two materials are joined together in a nip roll and allowed to cool. 
     The other method, the heated roll method, is accomplished by running both the polyester geotextile and the polymer grid around a heated roll with the polyester geotextile against the heated roll surface. Upon leaving the heated roll, the composite is run through a nip roll and allowed to cool. 
     As shown in FIG. 2, the polymer geogrid 40, having nodes 42 and ribs 44, is secured across the nodes and ribs 42 to a polyester geotextile 46 by the open flame method. In the heated roll method, only the nodes are bonded to the polyester geotextile. 
     In FIG. 3, three sets of 13 foot wide grid sections are shown each having a length of 200 feet. The first grid section, as indicated by arrow 50, is a polymer geogrid. The second grid section, occupying the space indicated by arrow 52, is a grid composite of the present invention. The third grid section, as indicated by arrow 54 is another polymer geogrid, which is the same as the geogrid indicated by arrow 50. Alternately, the grid composite may be overlaid onto and secured to continuous interconnected sections of polymer geogrid so as to position the grid composite to be arranged over the caving shields of the longwall mining equipment during installation. 
     At a location above ground, the three sections of grid are overlaid upon one another so that there is a two foot overlap, as indicated by arrows 56, where adjacent sections of grid are secured to one another to avoid the difficult task of joining adjacent sections together at an underground mine site. It is understood that the location of the grid composite section between adjacent sections of polymer grid is provided so that when the longwall shield recovery begins, the grid composite overlays the caving shields to prevent penetration of the gob onto the caving shields. It is also understood that, according to the length of the longwall face, several lateral sections of polymer grid are secured to each other to form the desired length of the longwall face, which is typically between 600 and 1,000 feet. 
     It is also understood with respect to FIG. 3, that the width of the polymer grid forming one terminal edge 58 of the longwall screening package is of a width so as to locate the grid composite over the caving shields of the longwall mining equipment. It is also understood that the opposite terminal edge 60 of the polymer grid includes several widths of polymer grid sufficient to support the roof of the gob extending rearwardly from the longwall mining equipment. 
     Once the desired configuration of the longwall screening package is secured to each other by overlapping sections of approximately two feet in width, the screening package is rolled up and folded over for conveyance underground by mining cars. Once underground, the screening package is unfolded and tied along its lateral edges to form a roll of screening 62 which may be hung from shield tips 64 in longwall mining equipment 68. As the longwall mining equipment is advanced, ties along the lateral edges of a screening package are cut to allow the screening package to hang down from the shield tips. During advancement of the shields 66, the unrolled screening package is allowed to extend above the shields 66. 
     In FIG. 4, advancing longwall mining equipment 68 illustrates, as indicated from junction point 70 and extending in the direction of arrow 72, joined sections of polymer grid located above the longwall mining equipment 68 to temporarily support the gob 74 above the equipment 68. Arrow 76 indicates the initiation of playing out of grid composite which terminates in another section of polymer grid so the grid composite is secured between adjacent sections of polymer grid or on top of continuous interconnected sections of polymer grid. The grid composite is finally located above the shields 66 of the equipment 68 at the terminal portion of the longwall mining process. 
     In FIG. 5, the longwall mining equipment 68 has advanced to the terminal coal face 78 such that grid composite, as indicated by arrow 80, initiates from a point 82 to extend above the caving shields 66 so as to prevent the gob 74 from penetrating through the grid composite and damaging the mining equipment or injuring workmen during longwall shield recovery. The grid composite indicated by arrow 80 is secured to polymer grid, as indicated by arrow 84, extending from the junction point 82. As previously explained, the polymer grid and grid composite is supported by wire ropes 86, located on 30 inch centers and secured to the mine roof by vertical roof bolts (not shown). 
     Having described the invention, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.