Patent Abstract:
A subterranean tank assembly is provided which broadly includes a vessel, a cover, and a riser which is configured to permit the riser to be adjusted in axial length at the job site. The riser is configured to permit coupling of the cover to the topmost connector portion thereof and to a portal of the vessel. The riser includes a plurality of substantially parallel, axially spaced circumferentially extending flanges which mate with the rim of the portal surrounding the opening, whereby the flanges may be fastened to the vessel in lengths intermediate to the initial length of the riser without the necessity of altering the tank or the cover.

Full Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is broadly concerned with subterranean tanks for receiving sewage or serving as a cistern for holding water which are rotationally molded of synthetic resin material. More particularly, the present invention is concerned with a tank of the foregoing type provided with parabolic wall construction and a portal arrangement for accomodating a uniquely and complementally configured cover or a synthetic resin riser which is constructed in such a manner that it may trimmed in multiple locations along its axial length while retaining wall strength and the ability to couple to the tank and receive a cover thereon. 
   2. Description of the Prior Art 
   As used in this application, “subterranean tank” refers to vessels for containing liquid which are intended to be substantially completed covered with earth so as to lie below grade. These subterranean tanks are commonly used in residential and light commercial construction in suburban and rural areas where the tanks are used as a septic tank for receiving sewage and wastewater, or as a cistern for holding water for agricultural, commercial or household use. These tanks have long been constructed of concrete or metal. These tanks are generally heavy and difficult to transport and install. Over time, the concrete tanks may crack or leak, and the metal tanks may corrode. Thus, attempts have been made to develop a practical and suitable subterranean tank out of synthetic resin which is durable, lightweight and economical to produce, while being able to withstand the soil pressures and overloads as well as impacts caused by machinery and the like inherent in their use. Among the various approaches taken by the prior art is that shown and described in U.S. Pat. No. 4,254,885. This patent discloses a subterranean tank having a plurality of wall corrugations and an arched top wall, and a depression for the ingress and egress of liquids. 
   While the construction of subterranean tanks has been advanced by this design, in many instances there has developed a need to bury the tanks significantly below grade due to ambient temperatures, the conformation of the earth surface, or other factors. In these circumstances, it is necessary to be able to gain access to the interior of the tank from the surface for such purposes as repair or pumping of the contents of the tank. Rather than specifically configure the tank to the site, a riser is often employed to compensate for the depth of the tank while permitting access to the opening through the riser. One such riser is shown, for example, in U.S. Pat. No. 6,047,724. The riser shown and described in that patent employs a corrugated wall construction and engagement structure for permitting a plurality of risers to be connected in tandem, one atop the other. In this way, the bottom portion of the riser may be connected to the tank at the opening of the latter or to similarly constructed risers therebelow, and the top portion of the riser being adapted to receive a cover or to connect to the bottom portion of a similar riser placed thereon. 
   While the riser shown in U.S. Pat. No. 6,047,724 is designed for use with other risers to provide a suitable depth or height adjustability, it does not admit for adjustments in height less than the axial length of the riser. That is to say, the construction of riser of the U.S. Pat. No. 6,047,724 necessitates that risers of a variety of different lengths be available on site to permit the cover to be placed at grade level as each riser is of a definite length and not capable of adaptation to the differences of depth as the tank is buried and covered with earth discoverable only after installation is complete. 
   Furthermore, the covers used with the tanks of this type have traditionally been of concrete or metal which are both expensive and heavy. There has thus developed a need for a cover having increased structural strength and durability, and a cover which facilitates positioning and coupling to either an opening on the tank itself or to a riser positioned thereon. 
   SUMMARY OF THE INVENTION 
   Applicant has thus recognized a need for a subterranean tank and riser assembly with greater adaptability to height variances at the site, for a tank assembly which provides improved durability and insulating qualities, for a vessel having consistency of wall thickness throughout, for improved load transfer regarding the weight of the earth over the vessel after it is buried, for a lightweight but structurally enhanced cover, for a complemental fitting between the cover and the vessel or riser which promotes positioning of the cover and securement to either the vessel or a riser, and for isolated inlet and outlet passages. These needs have been largely met by the subterranean tank and riser of the present invention. 
   The subterranean tank hereof broadly includes a vessel having spaced and isolated inlet and outlet passages and at least one and preferably a plurality of openings, a cover having upper and lower walls, and a riser especially configured for ease of attachment to the vessel and to the cover and having radially extending substantially horizontal flanges to enable trimming of the riser wall to various intermediate lengths but still enabling the remaining portion of the riser to receive the cover and couple to the tank opening. The vessel may be advantageously formed by rotational molding whereby a vessel wall of substantially capacity (e.g., 1500 gallons) may be formed in a single molding operation, and has a vessel wall which is of a corrugated, substantially parabolic shape which includes a plurality of ribs. The parabolic shape of the vessel wall is preferably provided both along the upper wall in a substantially transverse direction to the longitudinal axis and in the end walls in the longitudinal direction. The ribs alternate with arcuate, substantially flat surfaced vessel wall sections, the ribs being oriented substantially transverse to the longitudinal axis of the vessel along the arcuate top wall portion and in parallel planes to the longitudinal axis at the opposite axially spaced ends of the vessel. The rounded ribs are designed to maximize the modulus of the wall, and the large radii of the ribs enable the vessel to be rotationally molded with a uniform and consistent wall thickness. The arcuate, and most preferably parabolic top wall portion is preferably provided with portals surrounding and defining openings adjacent each end wall section at the highest part of the top wall portion to receive the covers thereon, and the vessel may be provided with a divider permanently fused to the vessel wall in the interior liquid receiving chamber oriented transverse to the longitudinal axis to divide the chamber into two compartments. The portals preferably including frustoconical rims which both help to distribute loads placed on the cover and limit stress concentrations as well as facilitating receipt of the covers thereon. 
   The cover is provided of double walled construction for improving durability, leak resistance and thermal insulating properties. The cover includes a plurality of circumferentially spaced, radially inwardly projecting lugs for bayonet-type locking engagement with receptacles on the rim of the vessel surrounding the opening. Further, the cover most preferably is provided with a plurality of circumferentially spaced depressions in the upper wall to facilitate fastening of the cover to the rim of the vessel around each of the openings by mechanical fasteners such as self-tapping screws. These depressions in the upper wall are fused during molding with the lower wall to enhance the strength of the cover around its circumferentially extending margin. The lower wall has a circumferentially extending frustoconical surface proximate the margin which aids in positioning the cover onto the rim of the portal and in the distribution of loads onto the vessel. In addition, a plurality of radially extending circumferentially spaced indentations are formed in the lower wall of the cover which are fused with the adjacent upper wall. These indentations provide additional strength for the cover to resist loads placed on the upper wall and help the cover to resist deformation of the rim of the portal due to horizontal forces such as impacts from machinery or thermal stresses. 
   The riser is configured with inwardly projecting lugs on the bottommost connector portion which are similar to those provided on the cover, and the portal also includes receivers adjacent a surrounding rim which are configured to receive the lugs of either the bottommost portion of the riser or the cover. Further, the riser is provided with at least one and preferably a plurality of circumscribing shoulders which present flanges oriented radially with respect to the longitudinal axis of the riser. The shoulders are substantially frustoconical in shape to mate with the rim of the portal and are, as a result, largely self-centering. The flanges are continuous and uninterrupted, and substantially horizontal and sized and configured complementally with respect to the rim surrounding the opening, whereby the flanges may mate with the rim when the riser is trimmed adjacent one of the flanges. A circumferentially extending seal may be provided by a bead of elastomeric sealant in a circumferentially extending slot in the rim, whereby either the cover, the bottommost portion of the riser or, when trimming of the riser has taken place, the bottomost flange may mate with the seal. The synthetic resin material of the riser permits trimming to appropriate lengths at intervals along the axial length where the flanges are provided. The riser may be held in place by mechanical fasteners such as self-tapping screws. Thus, if the grade of the earth around the tank may be estimated as falling within the initial axial length of a riser, only a single riser need be provided at the site which may then be trimmed to place the cover at or very near the grade. 
   These and other advantages of the present invention will be appreciated by those skilled in the art with reference to the drawings and description which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded, perspective view of the subterranean tank of the present assembly showing the vessel with one of its two covers detached; 
       FIG. 2  is a fragmentary, exploded view of the subterranean lank shown in  FIG. 1  having a riser in accordance with the present invention coupled to the rim of the portal surrounding each of the openings of the tank and another riser and cover shown in detached position; 
       FIG. 3  is an enlarged, fragmentary, vertical elevational view in partial section along the longitudinal axis of the vessel showing one of the passages and piping and reinforcements between adjacent ribs of the vessel wall; 
       FIG. 4  is an enlarged, fragmentary, vertical cross-sectional view of the cover, riser and rim of the vessel with the riser in an untrimmed full length condition; 
       FIG. 5  is an enlarged, fragmentary, vertical cross-sectional view similar to  FIG. 4  showing the riser trimmed to a portion of its original length adjacent a flange extending inwardly from an outer rib wall of the riser and fastened to the rim of the vessel; 
       FIG. 6  is an enlarged, fragmentary, vertical cross-sectional view similar to  FIG. 5  but showing the riser trimmed to a portion of its original length adjacent a flange extending outwardly from an inner riser wall and fastened to the rim of the vessel; 
       FIG. 7  is an enlarged, bottom plan view of the cover showing circumferentially spaced lugs on the lower wall and a plurality of radially extending circumferentially arrayed indentations in the lower wall radially inboard of the frustoconical surface; 
       FIG. 8  is an enlarged, vertical cross-sectional view of the cover in an inverted position taken along line  8 — 8  of  FIG. 7  showing the fused surfaces of the upper and lower walls at the depressions and indentations and the cross-sectional configuration of the indentations; and 
       FIG. 9  is a vertical sectional view taken through one of the connecting wall flat sections substantially longitudinally centered between the end walls along line  9 — 9  of  FIG. 3  through a flat connecting wall between corrugations in the vessel wall showing the parabolic configuration of the top wall portion. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings, a subterranean tank assembly  10  in accordance with the present invention broadly includes a vessel  12  and a cover  14 , and as shown in  FIG. 2 , a riser  16  which may be coupled to the vessel  12  intermediate the vessel and the cover  14 . A plurality of risers  16  may be connected together to provide a riser assembly  18  which effectively raises the placement of the cover  14  relative to the top of the vessel  12 . 
   In greater detail, the vessel  12  as shown in  FIGS. 1 and 2 , in a partial sectional view in  FIG. 3 , and in vertical section in  FIG. 9  includes a vessel wall  20  defining a chamber  21  and having a parabolic top wall portion  22 , a substantially horizontal bottom wall portion  24 , and upright end walls  26  and  28 . The vessel  12  extends longitudinally between the end walls. The vessel wall  20  is preferably rotationally molded of synthetic resin such as high-density linear polyethylene. The top wall portion  22  is substantially parabolic in configuration, whereby the load imposed on the vessel wall  20  by the covering earth is more evenly distributed and carried by the bottom wall portion  24 . A parabola is the locus of a point which is equidistant from a fixed straight line and a line not on this point. The arcuate top wall portion  22  includes a plurality of substantially parabolic corrugations  30  which are integrally molded with and alternate with connecting wall flat sections  32  which are also parabolic and complemental in shape to the corrugations  30 . The corrugations  30  are designed to maximize section modulus and the large radius of the arcuate corrugations  30  enable the vessel wall to be formed with a uniform and consistent wall thickness. The general shape of the top wall portion  24  including the corrugations  30  and alternating connecting wall flat sections  32  when viewed along the longitudinal axis of the vessel  12  as seen in  FIG. 9  is generally parabolic, which as used herein includes a modified ellipsoid. Bracing ribs  33  are located along each side of the top wall portion  22  proximate to the bottom wall portion  24 . Further, each of the end walls  26  and  28  have upright side corrugations  34  and a center corrugation  36  having a passage  38  which includes a gasket  40  and receives a tubular T-shaped pipe  41  therethrough. 
   The top wall portion  22  includes a portal  42  sized to admit the passage of a human into the vessel  12  through an opening  44 . A rim  46  surrounds and defines the opening  44 . The rim  46  includes a substantially horizontal circumferentially extending normally upwardly facing substantially flat closure surface  48  and a frustoconically shaped upwardly and inwardly tapering receiving surface  50  positioned radially inward and normally extending upward from the closure surface  48 . The receiving surface  50  includes a plurality of circumferentially spaced recesses  52  therein,  0  the recesses  52  each having a substantially horizontal circumferentially extending slot portion  54  and a substantially vertical slot portion  56  extending upwardly and communicating with the horizontal slot portion  54 , the radial depth of the vertical slot portion  56  progressively decreasing in an upward direction. The receiving surface  50  is configured complementally with the cover  14  as described below to facilitate positioning of either the cover or the riser  16  onto the rim  46 . 
   The cover  14  is preferably rotationally molded of synthetic resin such as high-density linear polyethylene and includes an upper wall  58  and a lower wall  60  spaced from one another along at least a part of each the upper wall and lower wall to present a cavity  61  therebetween as shown in  FIG. 8 . The double wall construction of the cover  14  provides improved impact resistance and thermal insulation as well as improving the strength and sealing capabilities of the cover  14 . The upper wall  58  is generally flat or dome-shaped and includes a plurality of circumferentially spaced depressions  62  adjacent the circumferentially extending edge  64  connecting the upper wall  58  and the lower wall  60 . The depressions  62  aid both in seating, opening and closing the cover  14 , and also aid in the use of mechanical fasteners  66  such as self-tapping screws which may be drilled through the cover  14  in the depressions  62  and into the rim  46  as shown in  FIGS. 4 and 6 . 
   The lower wall  60  of the cover  14  is complementally configured to the rim  46  and includes a plurality of circumferentially spaced lugs  68  positioned on an inwardly tapered, inclined, generally frustoconically shaped mating surface  70 . The mating surface  70  is complementally shaped to the receiving surface  50  so that the cover  14  is essentially self-centering on the rim of the portal, and the lugs  68  are sized and spaced to mate and fit into the recesses  52  whereby the cover  14  may be locked into engagement with the portal. Further, the depressions  62  are molded as kiss-offs whereby the upper wall  58  and the lower wall  60  are fused together during molding in the area of the depressions  62  to enhance the structural strength of the cover  14  around its circumferentially extending edge  64 . As shown in  FIGS. 7 and 8 , the lower wall  60  includes a plurality of radially extending, circumferentially arrayed indentations  74 . The indentations  74  include cup shaped end portions  76  and  78  and substantially planar pitched portions  80  and  82  which intersect along a peak  84 . The peak  84  is designed as a kiss-off during molding, wherein the lower wall  60  and upper wall  58  are fused therealong as shown in  FIG. 8 . The indentations  74  thus enhance the strength of the cover  14  by virtue of the provision of load transfer being the upper wall  58  and the lower wall  60  when forces are applied downwardly to the upper wall  58 , both by the cup shaped end portions and planar pitched portions and also by virtue of the fact that the upper wall  58  and the lower wall  60  are effectively welded together along the peak  84  of each of the indentations  74 . A hole  86  is centrally positioned in the lower wall  60  which faciliates drainage of any condensed moisture between the upper and lower walls of the cover  14 , but may be covered if desired, especially in cold environments. 
   The subterranean tank assembly  10  is designed to be placed into a pit or depression in the earth  88  whereby earth may cover the vessel  12 . Depending on the grade of the earth around the vessel  12 , the cover  14  may be secured directly to the portal  42  as shown in  FIG. 1 , but it is often necessary to employ one or a plurality of risers  16  as shown in  FIG. 2  to effectively raise the height whereby entry may be gained into the vessel  12  for filling or emptying the chamber, inspection of the vessel  12 , or repair. The riser  16  normally is positioned so that its longitudinal axis A is upright. The riser  16  is tubular and includes a normally bottommost connector portion  90  and a normally topmost connector portion  92 , with at least one a preferably a plurality of continuous, flat, circumferentially extending inner riser walls  94  and a plurality of ribs  96 . The ribs  96  each include a continuous, smooth and uninterrupted outer wall  98  of continuous thickness and a pair of axially spaced flanges  100  and  102 . The flanges  100  and  102  extend radially outwardly from the inner riser walls  94  and connect them with the outer wall  98  of the rib  96 , the flanges  100  and  102  each extending continuously and circumferentially around the riser  16  and being of constant thickness, the flanges further extending radially substantially perpendicular to the riser longitudinal axis A. The outer wall  98  of the ribs  96  is spaced outwardly of the inner riser walls  94  such that the flanges  100  and  102  extend outwardly a sufficient distance to mate with the closure surface  48  when the riser  16  is connected to the portal  42 , and the inner riser walls  94  are of a sufficient diameter to clear the receiving surface  50  of the portal as shown in  FIG. 6 . The bottommost connector portion  92  is configured substantially the same as the lower wall  60  of the cover  14 , and includes circumferentially spaced, inwardly oriented lugs  104 , and the topmost connector portion  90  is configured substantially the same as the rim  46  of the portal  42 . The topmost connector portion  90  thus also includes a substantially horizontal circumferentially extending normally upwardly facing closure surface  106  and a frustoconically shaped inwardly tapering receiving surface  108  radially inward and normally extending upward from the closure surface  106 . The receiving surface  108  includes a plurality of circumferentially spaced recesses  110  therein, the recesses  110  each having a substantially horizontal circumferentially extending slot portion  112  and a substantially vertical slot portion  114  extending upwardly and communicating with the horizontal slot portion  112 , the radial depth of the vertical slot portion  114  progressively decreasing in an upward direction. Thus, the topmost connector portion  92  may receive a cover  14  or another riser  16  thereon, while the bottommost connector portion  94  may mount on the portal  42  or to the topmost connector portion  92  of another riser  16 . 
   The closure surface  48  of the rim  46  preferably includes a circumferentially extending slot  116  spaced radially outward from the receiving surface  50  and into which a circumferentially extending seal  118  of elastomeric material such as synthetic rubber may be placed. The seal  118  is positioned to engage the lower wall  60  of the cover  14 , or alternatively the bottommost connector portion  94  of a riser  16  or a flange  100  or  102 . Additionally beads of sealant  120  may be applied of resilient sealant such as silicone rubber may be applied to one of the portal  42 , the riser  16  or the lower wall  60  of the cover radially inwardly of the seal  118  as shown in  FIGS. 4 ,  5  and  6 . 
   The vessel  12 , the cover  14  and the risers  16  are preferably rotationally molded in the configuration as set forth herein. After the seal  98  is positioned, the assembly  10  is ready for installation. A pit or depression is dug of sufficient depth to permit the top wall portion  24  of the vessel  12  to be covered with earth when the assembly  10  is lowered into the pit and the vessel  12  is covered. Gravel or sand is deposited into the pit to provide a base for the tank assembly  10 , and the vessel  12  is then lowered into place (such as by a crane connected to carrying eyes on the vessel wall) and leveled. Water or other liquid may be introduced into the excavated opening to aid in settling of the vessel  12 , and gravel or earth placed around the vessel wall. The pit is filled with earth and the pipe  41  connected to a sewage field or the like. The earth is then graded over the vessel  12  consistent with the surrounding grade. The covers  14  may be directly fitted onto the rims of the portals, or one or a plurality of risers  16  may be used to adjust the height relative to the vessel  12  where the cover  16  will be positioned. Because the risers  16  are of a predetermined height, the surrounding grade may necessitate only a fraction of the height of a riser  16  to effectively position the cover  14  at grade level. 
   The riser  16  may be trimmed to a desired axial length at intervals adjacent the flanges  100  and  102 . The riser  16  may be cut to length through either the outer wall  80  of the ribs as shown in  FIG. 6  or through the inner wall  76  as shown in  FIG. 5 . Where the vertical length of the outer wall  98  of the ribs  96  is a multiple of 2 inches (some outer walls may be 2 inches vertically and others may be 4 inches, etc.), and the riser inner wall  94  is a multiple of 2 inches, this permits effective adjustment of the height of the riser  16  in even increments which are a fraction of the overall length of the riser. Because the flanges  100  and  102  are sized and configured to mate with the closure surface  48  of the rim and of sufficient length to engage the seal  118 , the trimmed riser  16  as shown in  FIGS. 5 and 6  may be placed directly on the closure surface  48  and screwed thereto to hold the riser in place. The riser  16  is preferably trimmed from the top up; that is to say, the topmost connector portion  90  remains with the riser  16  while the bottommost connector portion  92  is removed with the trimmed portion. This permits the cover  14  to mate with and lock into the topmost connector portion  90 , which, as noted above, includes the frustoconical receiving surface  108  so that the cover  14  readily positions itself in proper alignment on the riser  16 . 
   By providing a riser  16  which may be trimmed to shorten its axial length, only one standard size of riser  16  need be supplied at the job site. This avoids the requirement of keeping several different risers of different lengths on hand to provide the cover at ground level. Because the riser  16  may be trimmed at various locations along its length by virtue of the provision of multiple flanges complementally sized with the receiving surface  50 , the subterranean tank assembly  10  in accordance with the present invention advantageously may very closely position the cover  14  at the desired height relative to the surrounding grade of the earth. 
   Although preferred forms of the invention have been described above, it is to be recognized that such disclosure is by way of illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
   The inventor hereby states his intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of his invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set out in the following claims.

Technology Classification (CPC): 1