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BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to liquid drainage systems used on site for footings, open trenches or nitrification fields used as discharge points for septic tanks, and more particularly to a novel drainage system which is easy to install and which maximizes the size of a storage chamber or area for the liquid until it can be absorbed by or percolated into the surrounding soil.  
           [0002]    In the past conventional drainage systems have typically comprised a horizontally extending perforated conduit placed within a drainage trench and surrounded by a quantity of loose aggregate material such as rock or crushed stone and covered with compacted soil. The space between the conduit and ground occupied by the aggregate serves to define a drainage cavity in fluid communication with the perforations of the conduit. An example of such a drainage system is found in the nitrification field of conventional ground absorption sewage disposal systems wherein effluent is discharged form a septic tank through the perforated vent pipe of a nitrification line which is surrounded by aggregate material such as rocks or crushed stone. The nitrification field creates a storage chamber or area for the sewage affluent until it can be absorbed by the soil.  
           [0003]    These conventional systems suffer a number of drawbacks as discussed in U.S. Pat. No. 5,015,123 (owned by the assignee of this invention), and the novel drainage system described and claimed in the &#39;123 patent represents a substantial improvement over the conventional system. The description of that improved system as set forth in the &#39;123 patent is incorporated herein by reference in its entirety. Briefly, that system utilizes pre-assembled drainage line units illustrated in FIG. 2 of the patent in which loose aggregate in the form of lightweight materials is provided in surrounding relationship to a perforated conduit and bound thereby by a perforated sleeve member. These units used in combination with pre-assembled units illustrated in FIG. 3 of the patent which do not include the perforated pipe replace the gravel system used in the conventional systems as illustrated in FIGS. 4 b  and  4   c  of the patent to provide the storage chamber or area for the effluent until it can be absorbed by the soil. The system of the &#39;123 patent represents a substantial improvement over the prior conventional gravel system for the reasons set forth in the &#39;123 patent and has enjoyed substantial commercial success.  
           [0004]    Recently, another drainage system has been proposed which includes a pair of drainage pipes such as those illustrated in FIG. 2 of the &#39;123 patent extending longitudinally within the trench and laterally spaced from each other to define an open storage chamber and a cover placed on top of and spanning the laterally spaced pipes to prevent top dirt fill from falling down into the storage chamber. While this system conceptionally shows some promise, the design of the cover has not been strong enough to support the weight of the top fill dirt and bends and deflects downwardly to decrease the size of the storage chamber and reduce the overall efficiency of the drainage system.  
           [0005]    Thus, there is a need in this most recent proposal for a cover which has sufficient strength and stability to support the weight of the top fill dirt and thereby avoid the problem associated with prior covers. The cover of this invention as described and claimed herein below was developed to perform that task.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, the primary object of this invention is to provide a drainage system which includes a pair of longitudinally extending drain pipes placed within a drainage trench laterally spaced from each other to define a liquid storage chamber therebetween and a novel cover placed on top of the drain pipes. The cover is sufficiently strong and stable to support the weight of the fill dirt placed on top thereof, thereby substantially maintaining the chamber at its original size for storage of the drainage liquid until it can be absorbed by the soil defining the bottom of the trench.  
           [0007]    Another object of the invention is to provide the above drainage system wherein the novel cover includes two side portions extending longitudinally over and generally conforming to the shape of the drain pipes and a center portion connecting the two side portions to maintain the drain pipes in laterally spaced relationship, the cover further including reinforcing elements extending between the side portions and the center portion to prevent downward deflection of the center portion under the weight of the top soil. As a result the size of the chamber is maintained to create maximum storage area for the liquid drainage until it can be absorbed by or percolated into the soil at the bottom of the trench.  
           [0008]    A further object of the invention is to provide various embodiments of the novel cover which can be used in the above described drainage system and wherein all of the embodiments contain reinforcement elements which prevent downward deflection of the cover under load.  
           [0009]    Still another object of the invention is to provide a novel cover as described above wherein the cover includes a plurality of vented openings which allows the system to breathe to thereby retard development of the clogging mat within the chamber, that is the mechanical loss of infiltrative capacity at the soil surface interface due to suspended solids, bacteria growth and ferrous sulfide precipitation.  
           [0010]    Other objects and advantages of the invention will become apparent from reading the following detailed description of the invention wherein reference is made to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is an end perspective of a fragmentary portion of the drainage system and cover of the invention illustrated with pre-assembled drainage units such as those illustrated in FIG. 2 of U.S. Pat. No. 5,015,123;  
         [0012]    [0012]FIG. 2 is a fragmentary end section illustrating the drainage system placed within a drainage trench;  
         [0013]    [0013]FIG. 3 is a top perspective view of a first embodiment of the novel cover which may be employed in the system of FIG. 1;  
         [0014]    [0014]FIG. 4 is a top plan view of the cover illustrated in FIG. 3;  
         [0015]    [0015]FIG. 5 is a side view of the cover taken along line  5 - 5  of FIG. 4;  
         [0016]    [0016]FIG. 6 is an end view of the cover taken along line  6 - 6  of FIG. 4;  
         [0017]    [0017]FIG. 7 is a view similar to FIG. 2 with the drainage system employing a second embodiment of the novel cover of the invention;  
         [0018]    [0018]FIG. 8 is a top perspective view of the second embodiment of the cover shown in FIG. 7;  
         [0019]    [0019]FIG. 9 is a top plan view of the cover illustrated in FIG. 8;  
         [0020]    [0020]FIG. 10 is a side view of the cover taken along line  10 - 10  of FIG. 9;  
         [0021]    [0021]FIG. 11 is an end view of the cover taken along line  11 - 11  of FIG. 9;  
         [0022]    [0022]FIG. 12 is a top perspective view of the prior art cover referred to above under the Background of the Invention;  
         [0023]    [0023]FIG. 13 is a side perspective view taken along line  13 - 13  of FIG. 12; and  
         [0024]    [0024]FIG. 14 is a bottom view of the prior art cover illustrated in FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    [0025]FIGS. 1 and 2 generally illustrate the drainage system  20  with which the invention is concerned and include a pair of horizontal perforated drain pipe units  22  and  24  placed within and extending longitudinally along trench  26  and laterally spaced from each other to provide the central open chamber  28 . While drain pipe units  22  and  24  may be of various types, they are preferably pre-assembled drainage units such as those illustrated in FIG. 2 of U.S. Pat. No. 5,015,123 and include a central perforated pipe  30  surrounded by a plurality of lightweight plastic aggregate  32  held in place by a surrounding net  34 .  
         [0026]    A cover  40  constructed according to the first embodiment of FIGS.  3 - 6  extends across the top of drainage units  22  and  24  and includes a retaining section  41  having side portions  42  and  44  which conform in shape and size to the outer surface and configuration of pipe units  22  and  24 , and a center portion  46  connected to the side portions and functioning to maintain the pipe units  22  and  24  in laterally spaced relation to form the center chamber  28 . In a typical installation the width of trench  26  may be 24 inches and the depth may be approximately 22 inches. Units  22  and  24  may be approximately 10 inches in diameter and the perforated pipe  30  within the units may be approximately 4 inches in diameter. When the storage system  20  is placed within the trench the distance between the top of the cover and the top of the trench will be approximately 10 to 11 inches and the space defined thereby will be filled with top soil  38  which was removed from the trench originally. In such an installation, assuming the density of the soil to be about 96 pounds per cubic feet, the weight or load imposed upon cover  40  and system  20  will be about 100 pounds per square foot.  
         [0027]    The prior art plastic cover  50  illustrated in FIGS. 12, 13 and  14  and described hereinabove, included a pair of elongated continuous side sections  51  and  52  of generally circular configuration to conform to the outside surface of drainage units  22  and  24  and an inverted V-shaped center section  53  joined to sections  51  and  52  along sharp junction lines  54  and  55 . Recessed depressions  56  are provided in center section  53  to prevent the flattening at the apex of that section under load. Each of the side sections  51  and  52  include a plurality of downwardly projecting lugs or dimples  57  which penetrate into the outer surface of units  22  and  24  and help to hold those units in their laterally spaced relation in the same manner as shown in FIG. 2.  
         [0028]    In storage systems such as this liquid is fed into one end of the perforated pipes  30  from a collection basin or from a septic tank and passes outwardly through the perforations in the pipe and the lightweight aggregate into chamber  28  where it is collected and stored until it can be absorbed by or percolated into the soil defining the trench. It is desirable that the liquid or effluent storage area defined by units  22  and  24  and space  28  substantially maintain its original size so as to maximize the efficiency of the drainage system.  
         [0029]    Even though the prior art cover illustrated in FIGS.  12 - 14  was about 0.125 inches thick, it encountered substantial downward deflection under load of the top soil such as bending along the junction lines  54  and  55  and the tendency of the side sections  51  and  52  to flatten out under load. As a result the size of chamber  28  and the overall size of the storage area defined by the units  22  and  24  and chamber  28  decreased to reduce the drainage capacity and overall efficiency of the system.  
         [0030]    Accordingly, applicants have developed the novel covers of FIGS.  3 - 6  and FIGS.  7 - 11  incorporating the invention which overcome the problems associated with the prior art design of FIGS.  12 - 14 .  
         [0031]    The first embodiment of the invention includes the one-piece cover  40  illustrated in FIGS.  1 - 6 . Cover  40  is molded of plastic, preferably of high density polyethylene plastic and is constructed so as to be light in weight but yet strong enough to resist downward deflection under the weight of the top fill soil  38  placed therein to fill the upper portion of trench  26 . For example in an installation such as in trench  26  shown in FIG. 2, the thickness of the cover may be about 0.080-090 inch, the width W may be about 21¼ inches and the length L may be any desired length for example, 63 inches long. The one piece cover  40  includes pipe retaining sections  41  and reinforcing sections  62  which alternate in corrugated undulating fashion along the length of the cover forming peaks and valleys defined by sections  62  and  41 , respectively. Each section  41  includes side portions  42  and  44  which are shaped and configured to correspond to the outer shape of pipes  22  and  24  and are connected together by the central radiused portion  46 . For example, in the configuration shown in FIG. 2 side portions  42  and  44  are formed on the radius of about 5 inches and the center portion is formed on a radius of about 4½ inches. Each reinforcing section  62  is formed on a single large radius to provide a crowned arch extending along the full width of the cover with each arch  70  being integrally joined to adjacent sections  41  by downwardly and outwardly tapering side walls  72  and  74 , the upper edges  76  of which are radiused to avoid any sharp stress points. Similarly, the junction lines  78  at which walls  72  and  74  join sections  41  are radiused to avoid any stress points. The width of sections  41  and  62  along the longitudinal direction is essentially the same. For example about 3½ to 4 inches.  
         [0032]    The radius on which arches  70  are formed is about 38½ inches, large enough so that the top surface of the arch at the longitudinal center of the cover is spaced about 1 inch above the center of the portion  46  and a hollow space  76  is created beneath the bottom surface  78  of the arch so that that bottom surface does not engage the pipes  22  and  24  in the drainage system as shown in FIG. 2.  
         [0033]    Each portion  42  and  44  includes retaining lugs  80  which project downwardly from the bottom surface thereof and as illustrated in FIG. 2 are pressed into the outer surface of pipe assemblies  22  and  24  to help retain those pipes in spaced relationship within trench  26 .  
         [0034]    When placed in use as illustrated in FIG. 2, portions  42  and  44  of sections  60  engage the top of laterally spaced pipes  22  and  24  with the center section  68  overlying chamber  28 . The sections  41  and  62  cooperate to completely cover pipes  22  and  24  and central chamber  28  to prevent any of the fill dirt  38  from falling down into the chamber. In addition the crowned arches  70  forming sections  62  and the tapered side walls  72  and  74  provide substantial strength and rigidity to the cover so that it is able to support the weight of the top soil and resist any significant downwardly deflection which would decrease the size of chamber  28  and the total liquid storage area defined by chamber  28  and pipes  22  and  24 .  
         [0035]    Cover  40  may also be provided with a plurality of vent holes  100  in sections  41  which allow the drainage system to breathe. This helps prevent the development of clogging mat in chamber  28 . The sides of the holes  100  is smaller than the particles of soil  38  to prevent soil from falling into chamber  28 .  
         [0036]    A second embodiment of the invention is illustrated in FIGS.  7 - 11 . The novel cover  100  mounted in place on pipe units  22  and  24  as illustrated in FIG. 7, similarly includes a plurality of retaining sections  102  and reinforcing sections  104  arranged in alternating fashion along the length of the cover to form valleys and peaks, respectively, in the same way as the construction of the first embodiment of the invention. Sections  102  include radiused sections  106 ,  108  and  110  joined along lines  112 , with each of the sections  106 ,  108  and  110  being formed on the same radius, for example, 4.985 inches, to substantially conform to the outer radius of pipe units  22  and  24 .  
         [0037]    Each of the reinforcing sections  104  includes a plurality of radiused crowned arches  114 ,  116  and  118  longitudinally aligned with portions  106 ,  108  and  110  respectively. Crowned portions  114 ,  116 , and  118  are formed on essentially the same radius as portions  106 ,  108  and  110  but on a raised center line so that the top surface of those portions extend above the top surfaces of portions  106 ,  108  and  110 , for example about 1 inch thereabove, so as to provide the corrugated or undulating configuration of cover  100 . Arches  114 ,  116  and  118  are connected to portions  106 ,  108  an  110  by way of side walls  120  and  122  which taper downwardly and outwardly at an angle of about 5 degrees from sections  104  to sections  102 .  
         [0038]    The raised crown arches  114 ,  116  and  118  of section  104  provide a hollow space  130  therebelow so that the bottom surfaces of those crowned arches do not contact the pipe units when installed in place.  
         [0039]    The width of sections  102  in the longitudinal direction is slightly larger than the width of sections  104 . For example, the width of sections  102  may be about 3½ inches whereas the width of sections  104  may be about 2½ inches.  
         [0040]    As shown in FIG. 7, cover  100  mounts on pipe units  22  and  24  and functions in much the same way as cover  40  shown in FIG. 2. The raised crown arches  114 ,  116  and  118  provide strength and ridgity to the thin walled cover  100  and resist downward deflection of the cover under the weight of the top fill soil which is on top of the cover. Consequently the size of the liquid storage area defined by pipe units  22  and  24  and chamber  28  is not reduced during use and the efficiency of the drainage system is maximized.  
         [0041]    It is apparent that the novel reinforced covers  40  and  100  are substantial improvements over the prior art cover illustrated in FIGS.  12 - 14  which experiences substantial bending and deflection under the weight of a top fill soil thereby causing the reduction in the size of the liquid storage area including chamber  28 . In contrast, the covers  40  and  100  are substantially thinner but yet are significantly stronger and experience virtually no downward deflection thereby avoiding any reduction in size of chamber  28 .  
         [0042]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Summary:
A drainage system for delivering fluid from a source of fluid for absorption into the ground, comprises a trench, a pair of perforated drainage conduits extending longitudinally in said trench and laterally spaced from each other to define an open chamber therebetween. The conduits are connected at one end to the source for receiving fluid from the source and delivering fluid to the chamber. A longitudinally extending cover overlies said conduits and prevents soil from falling into the chamber. The cover includes side portions which engage the conduits and a center portion connected to the side portions and maintains the conduits in laterally spaced relationship. The cover also includes reinforcing arches extending between the side portions and the center portion to resist downward deflection of the cover under the weight of the top soil. Vent holes in the cover permit the system to breathe.