CORRUGATED STORMWATER CHAMBER HAVING SUB-CORRUGATIONS

Abstract

A plastic arch-shape cross section corrugated stormwater chamber has a multiplicity of crest corrugations and valley corrugations which run transverse to its length. Sub-corrugations run along part or all of the arch-curve lengths of either crest corrugations or valley corrugations, or along both of them. A sub-corrugations are smaller in dimension than an associated crest corrugation or valley corrugation. Sub-corrugations may taper in width and depth and may taper to nothingness. A compound convex shape end cap, useful for closing off the ends of stormwater chambers, has substantially vertical corrugations with analogous sub-corruptions.

Claims

1-24 (canceled)

25. A chamber comprising: a base; a plurality of crest corrugations and a plurality of valley corrugations positioned along a length of the chamber; at least one crest sub-corrugation running along at least a portion of a crest corrugation; at least one valley sub-corrugation running along at least a portion of a valley corrugation; an end crest corrugation positioned at a terminal end of the chamber; and an end crest sub-corrugation running along at least a portion of the end crest corrugation.

26. The chamber of claim 25, wherein the end crest corrugation includes a substantially constant width.

27. The chamber of claim 25, wherein the end crest corrugation includes a tapering width.

28. The chamber of claim 25, wherein the end crest corrugation includes a height less than a height of the crest corrugations.

29. The chamber of claim 25, wherein the end crest sub-corrugation is positioned along a top portion of the end crest corrugation.

30. The chamber of claim 25, wherein the end crest sub-corrugation includes terminal ends positioned above the base.

31. The chamber of claim 25, wherein the end crest sub-corrugation includes terminal ends positioned at the base.

32. The chamber of claim 25, wherein the end crest sub-corrugation includes a tapering height.

33. The chamber of claim 25, wherein the end crest sub-corrugation includes a substantially constant height.

34. The chamber of claim 25, wherein the at least one valley sub corrugation includes a tapering height towards the base.

35. The chamber of claim 25, wherein the at least one valley sub corrugation includes a substantially constant width.

36. The chamber of claim 25, wherein the end crest corrugation includes terminal ends positioned at the base, and the end crest sub-corrugation includes terminal ends positioned above the terminal ends of the end crest corrugation.

37. A chamber system, comprising: a chamber including: a base; a plurality of crest corrugations and a plurality of valley corrugations positioned along a length of the chamber; at least one crest sub-corrugation running along at least a portion of a crest corrugation; at least one valley sub-corrugation running along at least a portion of a valley corrugation; at least one crest sub-corrugation running along at least a portion of a crest corrugation; at least one valley sub-corrugation running along at least a portion of a valley corrugation; an end crest corrugation positioned at a terminal end of the chamber; and an end crest sub-corrugation running along at least a portion of the end crest corrugation; and an end cap coupled to the chamber, the end cap including a flange configured to overlap the end crest corrugation of the chamber.

38. The chamber system of claim 37, wherein the flange of the end cap includes a recess having a protrusion configured to engage the end crest sub-corrugation.

39. The chamber system of claim 38, wherein the protrusion is shaped to follow contours of the end crest corrugation and the end crest sub-corrugation.

40. The chamber system of claim 38, wherein the protrusion is configured to abut against the end crest sub-corrugation to couple together the chamber and the end cap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is perspective view of a stormwater chamber having crest and valley corrugations with associated sub-corrugations.

[0023] FIG. 2 a side elevation view of a portion of the chamber shown in FIG. 1.

[0024] FIG. 3 is a vertical plane transverse cross section of the chamber shown in FIG. 1

[0025] FIG. 4 is a cross section through a portion of the sidewall of the chamber shown in FIG. 1.

[0026] FIG. 5 comprises FIG. 5(a) through FIG. 5(f) and shows portions of sidewall cross sections at different chamber elevations, as points illustrated in FIG. 3.

[0027] FIG. 6 is a partial side view of a chamber having three different styles of sub-corrugations.

[0028] FIG. 7 is a partial side view of a chamber having sub-corrugations with terminal ends which are blunt.

[0029] FIG. 8 is a partial side view of a chamber having sub-corrugations only on crest corrugations.

[0030] FIG. 9 is a fragmentary side view of a chamber having crest and valley sub-corrugations which run over the top of the chamber, from one base flange to the other.

[0031] FIG. 10 is an end view of an end cap suited for closing the open end of a chamber.

[0032] FIG. 11 is a side view of the end cap of FIG. 10.

[0033] FIG. 12 is a portion of a horizontal plane cross section view of the end cap of FIG. 11.

DESCRIPTION

[0034] An embodiment of stormwater chamber 20, shown in FIGS. 1,2 and 3, has a curved arch shape cross section. The opposing side walls 44 rise upwardly from opposing side bases 26 and curve inwardly to top 24. The opposing side bases 26 comprise horizontal flanges 46 which provide bearing area upon the soil upon which the chamber rests. The base of a chamber is sometimes referred to as the foot. In the chamber embodiments which are detailed below, the arch curve of the chamber cross section is smooth and continuously curving. Chambers within the invention may have other arch shape cross sections. For example, the arch curve may comprise interconnected flat portions, or the cross section may be nominally trapezoidal, as shown for instance in U.S. Pat. Nos. 5,017,041 and 5,511,903. Thus, the term “arch curve” and analogous verbiage of the description and claims which follows shall encompass the contour of the chamber arch as seen in a chamber end view, regardless the shape is not truly a curve. Chambers of the present invention may have cross sections which preferably are truncated semi-ellipses as described in U.S. Pat. No. 7,052,209 of Kruger et al. Alternatively, the cross section may have the shape of a parabola, a truncated semi-circle, or approximations those and other regular geometric shapes, as well as irregular and asymmetrical shapes. For strength chamber 20 has alternating crest corrugations 28 and valley corrugations 30 which run over the arc shape cross section. More information about the design and shape and use of corrugated chambers of the present invention is disclosed in U.S. Pat. Nos. 7,052,209 and 7,118,306 both of Kruger et al., the disclosures of which are hereby incorporated by reference. The disclosure of provisional patent application No. 61/217,905 filed Jun. 5, 2009, from which this application claims benefit, is also hereby incorporated by reference.

[0035] Stormwater chambers are typically buried within crushed stone aggregate or other water permeable granular medium that typically has 20-40 percent or more void space. The medium which overlies, underlies or surrounds a chamber may vary in character according to its location, and according to the material which extends to the surface of the earth. The medium within which a chamber is buried during use is generally referred to here as soil. That term should be understood to comprehend the commonly used crushed stone aggregate, as well as other manufactured media.

[0036] A simple description of some of the complex load-related phenomena associated with a chamber buried in soil is as follows: With reference to the transverse cross section of chamber 20 shown in FIG. 3, there is a vertical unit area load Fv, as a result of the weight of overlying soil and any transient load (e.g., a motor vehicles). The load is applied to the upper surface of the chamber by the soil which is in contact with the chamber. There is a resultant upward reaction force P at the opposing side bases 26 of the chamber, according to the total downward force on the chamber. The applied vertical load creates in the curved chamber sidewalls 44 compressive stresses Fp and shear stresses Fn. The compressive stress direction in a stable chamber is nominally in a direction which is tangent to the local mean curve of the chamber wall. The shear stresses are nominally perpendicular to the local mean curve of the wall. The soil load also creates bending stresses in the chamber sidewall. The stresses in the chamber wall vary with elevation from the chamber base. For example, compressive stress increases with proximity to the base.

[0037] When the load bearing capacity of a chamber is exceeded, the chamber sidewall can fail on a short term or long term basis. Typically, failure occurs when the chamber wall is crushed under soil load. Prior to failure by wall crushing, elements of the corrugation wall may buckle in a local manner thus reducing the load capacity of the buckled elements and causing the stable elements of the corrugation to be more highly stressed. As mentioned in the Background, stresses can be reduced, and the strength and stability of a chamber can be increased, by increasing wall thickness. But that is undesirable; and the invention provides an effective alternative way of strengthening the chamber. With reference again to FIG. 1 through FIG. 3, an exemplary embodiment chamber 20 has a curved top 24 (also referred to as the crown) and sidewalls 44 which run upwardly to the top from opposing side bases 26. The bases comprise horizontal flange portions 46 bearing on soil during use. Extending upwardly from the base flanges are a multiplicity of spaced apart fins 38, commonly called stacking lugs, the use of which is well known. The lugs 38 support the base flange of an overlying nested chamber, to stop nested chambers from jamming during shipment or storage. Generally, the height of the stacking lugs is chosen so that the corrugations of nested chambers may come very close, or into light contact with each other, without wedging together. See Brochu et al. U.S. Pat. No. 7,500,805 for information about how chambers nest, the disclosure of which is hereby incorporated by reference. An outer fin 29 runs lengthwise along the outer end of each base flange, to add lengthwise bending strength to the flange.

[0038] Chamber 20 has a multiplicity of corrugations which run transverse to the chamber length axis CL. The corrugations are comprised of crest corrugations 28 and valley corrugations 30; they are spaced apart along the length axis CL of the chamber with a period (also called pitch) P.

[0039] Each corrugation and sub-corrugation (i.e., the corrugations generally) has a width which is measured in a first plane which is parallel to the length axis of the chamber. Each corrugation has a depth which is measured a plane perpendicular to the length axis, typically normal to a tangent to the surface of the chamber/corrugation at the point of measurement. The depth of a corrugation is sometimes also referred to as the height of the corrugation. The length of a corrugation is a reference to the dimension of the corrugation as it runs along the arch-curve of the chamber. For brevity, crest corrugations are sometimes referred to as crests, and valley corrugations are sometimes referred to valleys. In prior patents, crest corrugations have been referred to as peak corrugations.

[0040] As seen from FIG. 2, each crest corrugation becomes narrow in width with elevation; and each valley corrugation increases in width with elevation. That shaping facilitates compact nesting. See Brochu et al. U.S. Pat. No. 7,306,399 the disclosure of which is hereby incorporated by reference. The corrugation dimensions and associated sub-corrugation dimensions are selected to provide a desired chamber strength, in context of the properties of the plastic material and the basic wall thickness of the chamber. Basic wall thickness is the nominal thickness of the chamber wall and top, as distinguished for example from possible locally thicker regions involving flow channels, bosses, openings, etc.

[0041] In chamber 20 and other embodiments the corrugations may comprise smaller corrugations 36, 32 which run lengthwise of along the corrugations. The smaller corrugations are called here sub-corrugations. In embodiments of the invention, a sub-corrugation has a height which is substantially less than the local height/depth of the corrugation with which the sub-corrugation is associated. Sub-corrugations alternatively may be referred to as secondary corrugations or mini-corrugations. Preferably, a sub-corrugation is centered within or on its associated corrugation. Sub-corrugations of the present invention are contours of the wall of the chamber; that is, both the inner and outer surfaces of the chamber are contoured and the wall thickness across the width of the sub corrugation typically does not change greatly.

[0042] Sub-corrugations are distinguished from flow channels that aid injection molding. Flow channels are relatively small thickened bands on the chamber wall that aid the flow of plastic during injection molding. They may project inwardly, outwardly, or both inwardly and outwardly from the wall on which they are positioned. See U.S. Pat. No. 7,500,805 of Brochu et al. Sub-corrugations are also distinguished from ribs, which in the lexicon used here are upstanding solid or hollow fin-like members which project inwardly or outwardly from the chamber wall.

[0043] In a chamber of the present invention, a sub-corrugation is present on one or more of the crest corrugations or valley corrugations. Typically, a plurality, and most often all, crest corrugations will have sub-corrugations. Likewise, when valley sub-corrugations are present they will be present in a plurality, most often all, of valley corrugations along the length of the chamber. In the generality of the invention, sub-corrugations may be present in only some of the valley corrugations or crest corrugations.

[0044] In embodiments of the invention, a sub-corrugation runs along at least a portion of the length of an associated corrugation; and it may run along the entire length. With reference to FIG. 1 and FIG. 2, a first set of sub-corrugations 32 runs upwardly in the center of the crest corrugations 28 from the elevation of the base. The sub-corrugations 32 taper in depth and width, and approach nothingness, as they reach an elevation hp, which in some embodiments of the invention, is between one-third and half of the height h of the chamber. In some other embodiments, the crest sub-corrugations may reach a height hp which is from one-quarter to two-thirds of the chamber height h. The direction of taper of a sub-corrugations 32 corresponds in sense with the taper of the crest corrugation, i.e., they both get narrower in width as they run upwardly. Dimensions of exemplary corrugations and sub-corrugations are given in FIG. 5 and are discussed below.

[0045] As may be seen in FIG. 1 and FIG. 2, a second set of sub-corrugations 36 runs along the respective centers of valley corrugations 30. The sub-corrugations 36 taper to nothingness in depth and width as they run downwardly and approach the base flange. Each exemplary valley sub-corrugation 36 runs along virtually the whole of the arch curve length of the associated valley. The direction of taper of a valley sub-corrugations corresponds in sense with the taper of the valley corrugation, i.e., each gets narrower as it approaches the elevation of the base. The width of a crest or valley sub-corrugation may alternatively be constant along part or all of the associated corrugation.

[0046] FIG. 4 is a cross section of a portion of the sidewall 44 chamber 20. As illustrated, crests and valleys share webs 76. Each crest corrugation 28 has a portion, running between the webs, with a width we, and each crest sub-corrugation 32 has a width wp. The maximum dimension of wp is about one-quarter of the locally associated dimension we. Each valley has a portion between webs with a width ww, and each valley sub-corrugation 36 has a width wv. The maximum dimension of wv is about is about one-fifth of locally associated valley dimension ww. The portion of a crest corrugation or valley corrugation which lies between the opposing side webs is sometimes referred to as the “flat”(portion) of the corrugation. Of course, in other embodiments of the invention, the corrugation cross section shape may vary. For example, the outermost part of the crest corrugation may bulge outwardly. In such instance, the portion referred to as the flat will be curved.

[0047] Again with reference to FIG. 4: Each crest corrugation has a height d which is measured relative to an adjacent valley corrugation. Each crest sub-corrugation 32 has a height dp and each valley sub-corrugation 36 has a height dv, as such are measured relative to the adjacent outer surface of the crest or valley, as applies. The maximum height dp of a crest sub-corrugation 32 is less than the locally associated height de of the crest corrugation 28 on which it is positioned. The maximum height dv of a valley sub-corrugation 36 is less than locally associated depth dd of the valley corrugation 30 on which it is positioned. (As mentioned above, the terms height and depth are used interchangeably for the same dimension on a corrugation or sub-corrugation.)

[0048] The cross sections of FIG. S(a) through (f) show how the shape of the sidewall varies, in particular the shapes of corrugations and sub-corrugations, with elevation from the base. As reference to FIG. 3 will show, the FIG. 5 cross sections are as follows: FIG. 5(a) is at the peak of the chamber; FIG. 5(b) is at about two thirds elevation, from the base; FIG. 5(c) is at a point just above the crest sub corrugation terminal end; FIG. 5(d) is at about one third elevation (and is the same section which is pictured in FIG. 4); FIG. 5(e) is near the base and the point where the valley sub-corrugation is diminishing to nothingness; and FIG. 5(f) is just above the upper surface of the base flange and thus the cross sections of stacking lugs 38 are present.

[0049] The cross section shapes of sub-corrugations may be vary from those which are pictured here. For instance, they may be characterized by greater or lesser included angle in cross-section, or they may have flattened tops or bottoms, etc. In embodiments of the invention, the shape of the sub-corrugations are preferably chosen so that the stacking height, or vertical separation between nested chambers, is not adversely affected, compared to a chamber having the same configuration but lacking sub-corrugations.

[0050] In an injection molded chamber, the precision of the process means that wall thickness of the chamber at the location of a sub-corrugation may be made substantially the same as the thickness of the adjacent corrugation portions, as visually evident in FIG. 5. When the invention is applied to products made by thermoforming or another comparatively less precise dimension-producing process the thickness of a sub-corrugation may be somewhat thinner (or thicker) than the adjacent corrugation wall.

[0051] Despite the small increase in cross sectional area, a surprisingly large benefit in strength is realized through use of sub-corrugations, despite the sidewall weight being increased by a very modest amount. This is shown by the test data in Table 1. Short, straight polyethylene segments representative of portions of the chamber wall were subjected to compressive loading. The specimen behavior was measured to determine load bearing capacity up to the point of failure. Each segment comprised a valley with two adjacent crests.

TABLE-US-00001 TABLE 1 Corrugated specimen test data Wall area Load per unit capacity per width of unit width of specimen specimen Relative Relative Specimen Description (inch.sup.2/inch) (lb/inch) weight strength A 0.25 inch 0.255 240 1 1 thick wall D 0.375 inch 0.413 579 1.62 2.41 thick wall B 0.25 inch 0.258 349 1.01 1.45 thick wall with sub- corrugations
With reference to the table, Specimen A represented a baseline chamber wall which was nominally 0.250 inch thick and had no sub-corrugations. Specimen D was similarly shaped but had a nominal 0.375 inch thickness. Specimen B was nominally 0.250 inch thick; it had the same shape as Specimen A, with the addition of a sub-corrugation at each of the valley corrugation and the two crest corrugations.

[0052] The first data column shows the cross sectional area per unit width of the specimen, in a plane perpendicular to the direction of the applied load. (The width of the specimen corresponds with the lengthwise direction of a chamber wall.) The weight of plastic material in the specimen is of course proportional to the cross sectional area of the specimen. The third data column gives the normalized relative weight of the specimen. The second data column shows the load capacity of the specimen; those data are normalized as relative strength, in the last data column.

[0053] As might be expected, the thicker 0.375 inch thick Specimen D has a substantially greater load bearing capacity than does the baseline specimen A. However, the weight is increased by somewhat more than 50 percent; and, the disadvantages mentioned in the Background arise—namely increased material cost, reduced injection molding manufacturability, and reduced ability for installers to manually handle.

[0054] The performance of Specimen B is surprising. The addition of sub-corrugations provides about 45 percent increase in load capacity with only about one percent increase in weight. The behavior of the specimens is qualitatively reflective of the behavior of walls in actual chambers, where the mechanics are more complex.

[0055] Specimens having the same configurations as the specimens A and B were subjected to beam flexure testing based on ASTM D 6272 Procedure B. The result was that the specimens B, with sub-corrugations, were somewhat stiffer, but were not substantially stronger at flexure failure, than were the comparable thickness specimens A, which lacked sub-corrugations.

[0056] Referring again to the chamber 20 shown in FIG. 1 through FIG. 5, it is both feasible and desirable to reduce the size of a crest sub-corrugation, as by the tapering down to nothingness, with increasing elevation. Generally, a sub-corrugation can be diminished or reduced to nothingness in chamber regions where structural analysis and or testing show that a sub-corrugation would not be of much value. Simply put, if the “flat” portion of the crest becomes become sufficiently small, so that the local buckling resistance is good, then the sub-corrugation need not be present. The same approach and rationale apply to the tapering in size and or presence of sub-corrugations in valleys. When a sub-corrugation is reduced in size, or not present, less plastic is used in making the chamber. Nonetheless, in the generality of the invention, a crest sub-corrugation, or a valley sub-corrugation, may run along the whole arch curve of a chamber.

[0057] Typically a chamber of the present invention will be made of commercial grade polyethylene or polypropylene, virgin or recycled, or some other polyolefin or combination thereof. Alternatively, the chamber may be made of any of a variety of other plastics, including fiberglass reinforced plastic, or other materials. The invention chambers are preferably made by injection molding but may be also made by rotational molding, thermoforming, by layering or lay-up (as with certain fiberglass reinforced plastics), and by other plastic molding methods.

[0058] An exemplary polypropylene chamber like chamber 20 may be about 90 inches long, about 77 inches wide at the base, about 45 inches high at the top, and will weigh about 120-130 pounds. It will have a typical wall thickness of about one-quarter inch. The depth of corrugation (i.e., the difference in elevation between a crest and adjacent valley) is about three inches. The period P of the crest corrugations is about 12 inches.

[0059] Another exemplary chamber may be about 52 inches long, about 100 inches wide at the base, about 60 inches high at the top, and will weigh about 120 to 130 pounds. It will have a typical wall thickness of about 0.25 to 0.30 inches. The depth of corrugation (difference in elevation between a crest and adjacent valley) is about 5 inches. The period P of the crest corrugations is about 15 inches.

[0060] Another exemplary chamber may be about 90 inches long, about 51 inches wide at the base, about 30 inches high at the top, and will weigh about 75 to 80 pounds. It will have a typical wall thickness of about 0.175 to 0.20 inches. The depth of corrugation (difference in elevation between a crest and adjacent valley) is about 2.5 inches. The period P of the crest corrugations is about 7 inches. The sub-corrugations are along the lines of those shown in FIG. 8, discussed below. In this chamber embodiment, the calculated load bearing capacity of the chamber is increased by-about 30 percent through the use of sub-corrugations, while the weight is only increased by about one percent.

[0061] Sometimes, for providing increased strength to a chamber design, the wall thickness of a corrugated chamber will be increased somewhat in combination with adding sub-corrugations, notwithstanding the disadvantages which have been mentioned in connection with using more weight of plastic. The dimensions of the chamber corrugations, and the period of the corrugations, may vary substantially in other embodiments of the invention. The invention may be used with chamber designs known in the prior art. Exemplary chambers meet performance requirements related to the AASHTO specifications and NCHRP Report mentioned in the Background.

[0062] FIG. 6 shows in chamber 20A in side elevation. The numbered features of chamber 20A, and chamber 20B, etc., correspond with those of chamber 20, with addition of the suffix. The overall shape and corrugations of exemplary chambers 20A and 208 are like those of chamber 20. In chamber 20A of FIG. 6, the sub-corrugations 32A on the crest corrugations are nominally the same as previously described. But the valley corrugations are different. Valley corrugations 36A run downwardly in the valleys to somewhat blunt-end termination points 42, which points are at an elevation hv that is lower than the elevation hp at which the upper ends of the crest corrugations 32A terminate. Thus the crest and valley sub-corrugations complement each other in strengthening the chamber. In addition, there is an optional second set of valley corrugations 40 which run upwardly from the base.

[0063] FIG. 7 shows chamber 208 in side elevation. The sub-corrugations 32B and 36B have approximately constant width and approximately constant depth. Instead of tapering down to nothingness, they have blunt ends.

[0064] FIG. 8 is a fragmentary side view of exemplary chamber 20C which has crest corrugations 28C that have sub-corrugations 32C which taper to nothingness part way up the chamber, and valley corrugations 30C which are free of sub-corrugations.

[0065] FIG. 9 is a fragmentary side view of exemplary chamber 20D which has crest corrugations 28D that have sub-corrugations 32D, and valley corrugations 30C which have sub-corrugations 36D. Both of the sub-corrugations run up and over the top of the chamber and down to about the elevation of the flange on the opposing side of the chamber.

[0066] Thus, in the embodiments shown and in the invention in general, the sub-corrugations may alternatively have tapered ends or blunt ends; or they may run all the way along the arch curve. Sub-corrugations which taper or diminish to nothingness, may do that by way of the height only diminishing or the width only diminishing, or both dimensions diminishing simultaneously. Sub-corrugations may alternatively have taper along their lengths, or they may have constant widths. When the sub- corrugations do not go the whole length of associated valleys or crests, the elevations at which sub-corrugations terminate may be the same for all sub-corrugations; or the elevations may differ. A chamber may have a combination constant dimension sections and tapering dimension sections.

[0067] Other chamber embodiments of the invention may have sub-corrugations only in crest corrugations or only in valley corrugations. As mentioned, a chamber may have sub-corrugations in only some of the crests and or in only some of the valleys or in only some both crests and valleys.

[0068] Use of sub-corrugations compares favorably with other alternatives for obtaining better strength in a chamber, including increasing wall thickness or applying ribs to the interior or exterior. An associated benefit of sub-corrugations is that there is a small but desirable increase in interior volume of the chamber, thus increasing its capacity to store stormwater.

[0069] In use, chambers of the present invention are placed on a graded surface, and connected end to end to form a string of chambers. After suitable end caps or closures are placed at the ends of the strings, and desired piping is installed, the chambers are back-filled with soil. Sometimes chambers are set on a geotextile covered surface and sometimes they are covered in geotextile. Chambers of the present invention may have features like those associated with prior art chambers, including that they may have a multiplicity of relatively small sidewall ports, spaced apart along the sidewalls, to allow lateral water flow out of the chambers, providing strength is not unacceptably compromised by the ports.

[0070] While the invention has been presented primarily in terms of chambers for receiving stormwater, the invention will also be useful in arch shape cross section corrugated chambers which. are useful for other purposes, such as receiving wastewater, or for providing arch shape cross section enclosures for creating spaces in soils and storing or protecting things.

[0071] End Caps

[0072] Typically, end caps are placed on the outermost ends of strings of interconnected chambers, to keep the surrounding medium, e.g., stone aggregate, from intruding into the interiors of the chambers. End caps which have outwardly bulging dome shape contours. Those shapes may also be referred to as presenting as compoundly concave shapes. Prior art end caps of such type are described in U.S. Pat. No. 7,237,981 of Vitarelli et al., U.S. Pat. No. 7,118,306 of Kruger et al., and U.S. Pat. No. 7,491,015 of Coppes et al., the disclosures of all of which are hereby incorporated by reference. As reference to the foregoing patents will show, typical prior art end caps have had a multiplicity of ribs on the concave interior side.

[0073] In embodiments of the present invention, an end cap has a plurality of upward running crest corrugations and valley corrugations. In one embodiment there are sub-corrugations in the valleys and crests, and there is an absence of interior ribbing. FIG. 10 is an end view and FIG. 11 is a side view of and an exemplary end cap 50 of the present invention. In FIG. 10, the end cap is illustrated a portion of a chamber 20, shown in phantom, to indicate how it is used to close off the end of the chamber. FIG. 13 is a partial horizontal cross section view of the cap, at an elevation somewhat above the elevation of the base. The line CP in the Figures indicates the vertical axis of the cap. The cap body has a nominal maximum height H and a nominal maximum depth D, as indicated in FIG. 10 and FIG. 11.

[0074] End cap 50 an attachment end 54 which defines an arch shape opening for mating with the arch shape cross section of a chamber. Preferably, the end 54 comprises a flange as pictured, for overlapping or underlapping the end of a chamber. End cap 50 has an arch shape base 52. The base preferably comprises a flange as shown, to provide bearing area for better supporting the cap on soil. End 54 has downwardly extending terminal ends; and base 52 has horizontally extending terminal ends. The terminal ends are connected to each other at points 72.

[0075] End cap 50 comprises a compound convex shape wall 62, which connects the arc of the attachment end 54 with the arch of the base 52. In prior patents the wall may have been referred to as an outward bulging dome or a dome-shape body. End cap wall 62 is comprised of a plurality of alternating crest corrugations 56 and valley corrugations 58 which run upwardly from the base flange. The corrugations curve inwardly along the contour of wall 62. As seen in FIG. 10, the corrugations may be characterized as running substantially vertically, as may be seen when they are projected into a vertical plane which runs through the connection points 72 of the terminal ends and parallel to vertical axis CP. Within the meaning of substantially vertical, the corrugations may have a tilt or curve, for instance as appears in FIG. 10.

[0076] Sub-corrugations 60 run upwardly within each valley corrugation 58. Crest corrugations 56 have corresponding sub-corrugations 68. In the center portion of the body, the sub-corrugations run up to a maximum height of about 60 percent of the total or maximum height H of the peak of the end cap, as such heights are projected into an aforesaid vertical plane. Near the left-right outer edges, as seen in FIG. 10, the sub-corrugations run up to about 25 percent of the peak height.

[0077] The principles of the chamber inventions which involve sub-corrugations, described above, can be applied in end caps; and the foregoing disclosure with respect to chamber corrugations and sub-corrugations is hereby incorporated by reference. In brief, the corrugations provide stiffness and structural strength to the body of the end cap, and the sub-corrugations increase the strength and buckling resistance of the end cap body structure. The benefit is that a strong end cap can be made in an efficient way with less weight of material than would otherwise be required.

[0078] An embodiment of end cap comprises corrugations having a plurality of sub-corrugations, where each sub-corrugation runs upwardly from the elevation of the base on a plurality of either or both crest corrugations or valley corrugations. Each sub-corrugation has a depth less than the depth of the corrugation with which it is associated. Preferably, in an exemplary cap, each sub-corrugation diminishes in width and depth with elevation. In another exemplary end cap, each sub-corrugation terminates at an elevation which is less than the elevation of attachment end at the location of the particular corrugation with which the sub-corrugation is associated. In another embodiment exemplary cap, the sub-corrugations terminate at an elevation which is no more than about 60 percent of the overall height of the end cap.

[0079] In alternative embodiments of the cap invention, some valley corrugations and or some crest corrugations may not have sub-corrugations; or some or all of the sub corrugations may run all the way up the respective crests or valleys, from the base to the attachment end.

[0080] End caps may be fabricated of materials and in ways which are described above for the chambers. An exemplary end cap for a large chamber may have a height of about 57 inches, a base flange width of about 98 inches, and a depth D of about 33 inches, as measured at about the elevation of the base flange. Such a chamber may be made of polyethylene or polypropylene by rotational molding, and it may have a basic wall thickness of about 0.35 inches. Rotational molding materials as a class have lower strength than comparable composition injection molding or thermoforming materials. They are also less reliable in producing uniform thickness or repeatable dimension. Thus, the use of sub-corrugations can be advantageous beyond the reasons already given. In another alternative, it may be practical to form from sheet metal an end cap of the present invention.

[0081] Although the inventions have been described and illustrated with respect to several embodiments, those embodiments should be considered illustrative and not restrictive. Any use of words, such as “preferred” and variations thereof, is intended to suggest a combination of features which is desirable but which is not necessarily mandatory; and, embodiments lacking any such preferred features or combination may be within the scope of the claims which follow. Persons skilled In the art may make various changes in form and detail without departing from the spirit and scope of the claimed invention.