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This application is a continuation in part of both patent application Ser. No. 14/025,782, filed Sep. 12, 2013, now U.S. Pat. No. 9,016,979, and patent application Ser. No. 14/025,773, filed Sep. 12, 2013. This application claims benefit of both provisional application Ser. No. 61/700,313, filed Sep. 12, 2012, and provisional application 61/700,315, filed on Sep. 12, 2012. The disclosure of each of the foregoing patent applications is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to molded plastic chambers having arch shape cross sections, for receiving, containing and dispersing stormwater when buried beneath the surface of the earth. 
     BACKGROUND 
     Arch shape cross section storm chambers made from injection molded plastics have been used for a number of years to handle stormwater. In a typical installation, multiple rows of strings of interconnected chambers are placed on the floor of a cavity made in the earth surface and are then backfilled with crushed stone or the like. Stormwater, such as might run-off from a paved parking lot or roofs of buildings is channeled to the chambers so the waters can accumulate and then be dispersed over time by either percolation into the surrounding soil or by controllably flowing to a water course. 
     Some types of arch shape cross section chambers, exemplified by a corrugated chamber described in Detullio U.S. Pat. No. 5,087,151, have closed ends and are interconnected by pipes. Those chambers might be made by thermoforming of thermoplastic sheet. Another type of chamber, of more relevance to the invention described herein, is exemplified by the chambers shown in Kruger U.S. Pat. No. 7,118,306. Those kinds of chambers are preferably made by injection molding. The chambers have open ends. A string of chambers is assembled by overlapping a first end of one chamber on the second end of a like chamber, when the like chamber has been previously placed within a cavity in the earth. After installation, the chambers are backfilled, typically with crushed stone, and the stone is covered to create a soil surface, often a paved surface which can be used by motor vehicles. When so installed beneath the surface of the earth, stormwater chambers should have requisite strength and durability, particularly for bearing the overlying load of soil and any vehicular or other traffic. 
     Systems comprised of molded plastic arch shape cross section stormwater chambers are in functional- and cost-competition with other stormwater systems, including buried systems comprised of steel conduit and detention ponds. Generally, it is an objective to have storm chambers with larger and larger volumetric capacity per unit length, while of course still meeting the load bearing requirements. Whereas early plastic chambers used 20 years or more ago had a peak height of 12 inches, more recent chambers may be quite large. For example, a commercial Model 4500 stormwater chamber sold by Stormtech LLC, Rocky Hill, Conn. is 100 inches wide at the base, about 60 inches high, about 48 inches long, and weighs about 120 pounds. There is a generalized desire to commercialize even larger chambers. 
     There are practical problems encountered with large chambers. Among them are: First, it is not easy to mold large chambers because they require large molding machines and machinery for handling the just-molded products. Large and thus less common injection molding machines can be costly. 
     Second, large chambers present problems with respect to storing and shipping in economic fashion by truck—the most common mode. Typically chambers are nested one within the other to form a stack for shipment, typically mounted on a pallet on the bed of a truck. But because the height of each chamber is large, only a limited number of chambers can be nested upon one another in upright fashion, before the height capacity of an ordinary highway truck is exceeded. For example, if the load height capacity of a truck is about 100 inches from the bed surface, and the first or bottommost chamber is 60 inches high, then there is only about 40 inches of space for containing nested chambers. If the stack height is about 6 inches (the spacing between one chamber and next-nested chamber), then only 6 to 7 chambers can be stacked on top of the bottom chamber. 
     This application is a continuation in part of both patent application Ser. No. 14/025,782, filed Sep. 12, 2013, now U.S. Pat. No. 9,016,979, and patent application Ser. No. 14/025,773, filed Sep. 12, 2013, now U.S. Pat. No. 9,233,775. The disclosure of each of the foregoing patent applications is hereby incorporated by reference. 
    
    
     SUMMARY 
     An object of the invention is to provide large stormwater chambers which have improved characteristics with respect to manufacturability, shipment and handling. Another object is to provide a chamber and an associated handling and shipping method which minimizes storage and shipping costs. 
     In accord with the present invention, a stormwater chamber is comprised of two half chambers having coupling features, so they can be mated and joined at the top of the chamber. Half chambers of the present invention may be stacked as a nested multiplicity of half chambers on a pallet or the like for economical shipping, particularly by means of a motor vehicle transport truck. At or near the point of use, the half chambers are mated at their respective coupling features, to form a chamber which has a joint at the top of the chamber. In embodiments of the present invention, the coupling features are secured to each other by means which includes one or more of welding, fasteners, or at least one lengthwise running locking rod. Optionally, each half chamber coupling feature comprises a plurality of stirrups spaced apart by slots. When mated, the stirrups of one half chamber fit into the slots of the other half chamber to form a passageway, and a locking rod slips into the passageway. Preferably the locking rod has an eccentric shape cross section, so that it can be rotated when put in place, thereby to exert a camming action that draws the coupling features/flanges at the top of the chamber toward each other. 
     Different embodiments of coupling features and joints may be used. Typically the joint is comprised of mating flanges, intermittent or continuous, which run along the length of the top of the chamber. The flanges may interlock. Clamps and latching means may be used to hold the half chambers relative to each other with or without a locking rod. 
     The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a stormwater chamber comprised of half chambers mated at a top joint. 
         FIG. 2  is an end view of the chamber of  FIG. 1 . 
         FIG. 3  is an exploded view showing the upper portions of two half chambers which comprise a chamber like that shown in  FIG. 1 , showing how the half chambers couple to each other at a joint at the top of the chamber. 
         FIG. 4  is a vertical transverse cross section through the top portion of the chamber of  FIG. 1 . 
         FIG. 5  is a view like  FIG. 4 , showing an alternative joint configuration which comprises fasteners. 
         FIG. 6  is a view like  FIG. 4 , showing a chamber embodiment where C channels hold the mated half chambers to each other. 
         FIG. 6A  is a partial detail of a variation on the joint shown in  FIG. 6 . 
         FIG. 6B  is a partial detail of another variation on the joint shown in  FIG. 6 . 
         FIG. 7  is a partial perspective view of the top end of a half chamber, like that shown in  FIG. 6 . 
         FIG. 8  is a perspective view of a channel which may be used to hold to half chambers together at a top joint. 
         FIG. 9  is a view like  FIG. 4  showing a joint comprising an integral J channel and a C channel clamp. 
         FIG. 10  is a semi-diagrammatic illustration of half chambers mounted on a pallet for shipping to an assembly point. 
         FIG. 11  is a partial perspective view of a chamber made of separately formed half chambers which are held together by a joint feature which comprises two locking rods. 
         FIG. 12  is a partial view of the joint feature region of one of the half chambers shown in  FIG. 11 . 
         FIG. 13  is a view of the joint feature region of a half chamber very much like that shown in  FIG. 11 , where there is a continuous flange running between the peaks. 
         FIG. 14  is an end view in partial cross section, showing how a stirrup of one half chamber mates with the stirrup of another half chamber. 
         FIG. 15  is a cross section view of a locking rod which has an oblong cross section. 
         FIG. 15A  is a perspective view of a locking rod having oblong cross section segments which are offset from each other. 
         FIG. 16  is a view like  FIG. 14 , showing the rod of  FIG. 15  after it has been put in place an rotated so the large axis of the oblong cross section forces the stirrups apart. 
         FIG. 17  is a view like  FIG. 12 , showing a joint feature where one locking rod is above the peaks, on the outside of the chamber, and the other locking rod is below the valleys, on the inside of the chamber. 
         FIG. 18  is a vertical cross section view similar to that of  FIG. 4 , showing how a nub of one half chamber passes through an opening in the flange of a mating half chamber, so that a locking rod may hold the nub within the opening. 
     
    
    
     DESCRIPTION 
     Embodiments of chambers of the present invention are preferably made of injection molded thermoplastic, preferably a polyolefin such as polyethylene or polypropylene. Optionally, other known methods of plastic forming may be used, including rotational molding, thermoforming and the like. Exemplary chambers are comprised of half chambers which join to each other by coupling means at a joint proximate the top of the chamber. 
     Patent application Ser. No. 14/025,773, now U.S. Pat. No. 9,233,775 describes a chamber comprised of two half chambers which are hinged at a top joint, optionally having a locking rod. Patent application Ser. No. 14/025,782, now U.S. Pat. No. 9,016,979, describes a chamber comprised of two half chambers which are mated and joined together by various means at a lengthwise top joint. Those applications respectively claim benefit of provisional patent application Ser. Nos. 61/700,315 and 61/700,313, both filed Sep. 12, 2012. The disclosures of all the foregoing applications are hereby incorporated by reference. 
       FIG. 1  is a perspective view and  FIG. 2  is an end elevation view of a chamber  18  which is comprised of two mated chamber halves  22 ,  24  which have a joint  20  and opposing side base flanges  36  which lie in base plane HP. Chamber  18  has a length axis LL and a vertical lengthwise center plane CP which contains a vertical axis CC, used as a reference in other Figures. 
       FIG. 3  is an exploded view showing how the upper portions of exemplary half chambers  22 ,  24  couple together.  FIGS. 4, 5, 6 and 9  are partial transverse cross section views showing features of joints of alternative embodiment chambers. An exemplary thermoplastic chamber will have a width WW of about 100 inches, a height H of about 60 inches, and a length of about 52 inches (so the effective length is 48 inches when the chambers are end to end overlapped). Alternating peak corrugations  48 ,  50  and valley corrugations  38 ,  40  run transverse to the length of the chamber. The corrugations provide cross section area for vertical load transfer and section modulus which imparts bending strength to the walls. Below, the terms “valley” and “peak” are shorthand references to the valley corrugations and peak corrugations. 
     Stormwater chambers and their use have been described in the art. In particular, reference may be made to commonly owned U.S. Pat. No. 7,118,306 of Kruger et al., entitled “Stormwater Management System” and U.S. Pat. No. 6,991,734 of Smith et al, entitled “Solids Retention in Stormwater System.” The disclosures of the foregoing patents are hereby incorporated by reference. When a chamber is buried within crushed stone or other soil material the arch shape of the chamber cross section maintains the integrity of the interior cavity of the chamber. Simply stated, the vertical forces of the stone, soil, and anything on the surface of the soil, are transferred along the curve of the arch to the base flanges. As is characteristic of arches (for instance, arches comprised of stone or brick pieces), there need not be significant capacity to bear shear forces within the curved structure in order to maintain the integrity of the structure once it is in place and subjected to foregoing kind of vertical loads. Thus, it will be appreciated that in the present invention the joint  20  at the top of the chamber need not have strength to resist high shear load. However, a joint will desirably have sufficient shear strength to enable lifting and other handling of a chamber, to move it from the point of assembly to its position within a to-be-filled cavity in the earth or in such other water permeable substance as may be desired. 
     In one embodiment, half chambers are mechanically interlocked and optionally welded or otherwise secured at coupling features in vicinity of the joint. When the joint is planar it is preferably in the vertical center plane CP. When the joint is non-planar, the joint will be in proximity of the vertical center plane, with parts of the joint somewhat offset from the plane. Thus, it will be understood that each half chamber comprises about half of the whole of the arch shape wall of a chamber which runs from one base flange, to the top of the chamber, to the opposing side base flange. 
       FIG. 3  shows the top portions of the two half chambers  22 ,  24 , as they appear when spaced apart and rotated away from each other to reveal the coupling features. Arrows A, B show how the half chambers  22 ,  24  mate with each other when the coupling surfaces are brought together to form a joint. The top of half chamber  22 , on the left, has a serpentine lip  30  and a discontinuous vertical top flange  26 . The flange  26  is comprised of a plurality of flat plates which close the ends of the valleys  38  that are between adjacent peak corrugations  48 . The lip  30  is shaped to mate with the serpentine interior surface  29  of the half chamber  24 , on the right in  FIG. 3 . Half chamber  24  has a top flange  28  lying in or close to the vertical plane CP which contains a vertical reference axis CC. See  FIG. 1 . Flanges  26 ,  28  may be discontinuous as shown, comprising a plurality of flat plates closing off the ends of the valleys. In an alternative embodiment, flanges  26 ,  28  are continuous along the length of the top of each half chamber and close the underside cavity of the peak corrugations. See  FIG. 7 . 
     When half chamber  22  and half chamber  24  are engaged with each other, the upper end of half chamber  24  rests on the surface  29  of the serpentine lip  30 . Flange  28  has a multiplicity of horizontally extending pins  34  which fit into the female cavities of sockets  32  on the flange  26 , to help align the coupling features with each other. The pin-socket engagements provide some shear strength to the joint. 
       FIG. 4  is a transverse vertical cross section showing portions of half chambers  22 ,  24  when they are mated to form a chamber  18  as shown in  FIG. 2 . The parts are shown as they are ready for welding, as by ultrasonic, heat gun, hot plate, or other known means, for example at points  70 ,  20 A. Note how typical valley  40  of half  24  rests on lip  30  of half  22 . The engagement of the vertical flange  26  on one half chamber with the vertical flange  28  on a mating half chamber is helped by pins  34  and sockets  32 , or by means of substitutional functional equivalent features. The pins and socket features help locate the mating half chambers with respect to each other and also provide some vertical direction strength to the joint. 
     Optionally, as shown in  FIG. 4 , lip  30  (which has the ability to deflect elastically downwardly) may have a lengthwise ridge or a series of upward projecting protuberances  60  which are received in mating recesses  68  on the undersides of typical valleys  40 , to hold modestly the half chambers together at the joint prior to welding. 
     The welding process mentioned above may be carried out by placement in the joint region of a fusion element such as the commercial product known as PowerCore Welding Rod (PowerCore International Ltd., Ottawa, Ontario, Canada). See also U.S. Pat. Nos. 5,407,514 and 5,407,520, the disclosures of which are hereby incorporated by reference. Alternatively, the fusion element may be the preform which is supplied as an element of the commercial Emabond electromagnetic welding system (Emabond Solutions Co., Norwood, N.J., U.S.) As described in Lamarca U.S. Pat. No. 7,984,738 (the disclosure of which is hereby incorporated by reference) the fusion element preform may be a structure comprised of plastic and magnetic particles which is energized by a high frequency induction coil to effect a weld. 
     While welding is preferred with the  FIG. 4  joint design, it is within contemplation that chambers having joints like those shown in  FIG. 4  may be useful for assembling chambers at the point of installation without adding the welding step. Other fastening or securing means may be used with the  FIG. 4  embodiment, as described below. 
       FIG. 4  also shows one of several optional stiffeners  42  in phantom, which may be molded into the center one or more valleys  40  of typical half chamber part  24 . Like stiffeners may be used on the other half  22 , as well. 
       FIG. 5  shows a portion of an alternate another embodiment of the invention, chamber  318 , where a bolted joint  20  is formed between half chamber  322  and half chamber  324 —which half chambers have configurations largely like chambers  22 ,  24 . A multiplicity of exemplary threaded fasteners  27 A,  27 B inserted in holes, and associated nuts  333 ,  335 , are used to join vertical flanges  326 ,  328  to each other and to join lip  330  with valley  40 . Preferably, a multiplicity of fasteners will be spaced apart along the length of the joint. 
       FIG. 6  is a view like the view of  FIGS. 4 and 5 , showing a portion of another embodiment of the invention, chamber  118 . Mating half chambers  122 ,  124  have respective peak corrugations  148 ,  150  and valley corrugations  138 ,  140 . The half chambers  122 ,  124  meet at lengthwise flanges  126 ,  128 .  FIG. 7  is a partial view of the upper end of a half chamber  122 , showing that flange  126  is preferably continuous, as is flange  128 . In variations on this embodiment, the flanges may be intermittent as shown in connection with  FIG. 3 . Referring again to  FIG. 6 , lengthwise vertical lips  58 ,  60  run along the undersides of valleys  138 ,  140 . The lips  58 ,  54  and the upper ends of flanges  126 ,  128  are respectively clamped together by channels  52 ,  54 .  FIG. 8  is a perspective view of typical channel  52  which is preferably made of a metal or fiber reinforced plastic. The width of channel  52  is dimensioned so that there is an elastic force in the channel when the clamp is forcibly engaged (as with a rubber hammer) with the mated flanges  126 ,  128 . Channel  54  is similarly dimensioned with respect to the vertical lips  58 ,  60 . 
       FIG. 6A  is a detail of a portion of an alternative embodiment of the joint shown in  FIG. 6 . Flanges  326 ,  328  correspond with flanges  126 ,  128 . The lengthwise vertical lips  358 ,  360  are L shape in cross section, so that when mated as shown they present as a T shape cross section. Alternate embodiment channel  354  is C shape in cross section, so it is vertically captured in place by the T shape cross section. 
       FIG. 6B  shows another variation which may be used with the  FIG. 6  embodiment and other embodiments. Vertical flange  226  has a recess and vertical flange  228  has a protuberance  62  which fits in the recess. The recess and protuberance may be round as shown in  FIG. 3 , or may comprise lengthwise running portions. In the chamber  118  and in other embodiments of the invention, the mating flanges may have even more contoured and interlocking features than have been shown by example. 
       FIG. 9  is a vertical cross section like the view of  FIG. 6 , showing a portion of a chamber  218  comprised of half chambers  222 ,  224  having mating respective lengthwise vertical top flanges  226 ,  228 , intermittent in valleys  238 ,  240 . Alternately, the flanges are continuous. In the locations of the valleys, flange  226  has a top portion  64  shaped to create a pocket  74  within which is received the upper edge of flange  228 . The upper part of flange  226  may be characterized as a J shape channel (which defines the pocket  74 ). 
     An optional way of securing two half chambers to each other comprises the use of locking rods. Reference may be made to the aforementioned application Ser. No. 14/025,773, which shows a longitudinal-running locking rod that is used to hold together half chambers that are connected to each other by a hinge top joint. The preferred half chambers of the present invention are mirror shape components, each of the other, as are the preferred half chambers of the Ser. No. 14/025,773 application. That makes the resultant chambers well-suited for assembly as overlapping end-to-end strings of chambers. The foregoing related application describes ways of making a multiplicity of identical half chamber precursors, which precursors are then modified, as by cutting, to form the desired mirror shape half chambers. Such methods for making mirror half chambers, which substantially reduce the cost of molds, may be used in the present invention. Alternatively, separate molds, or molds with moving parts, as also described in the related application, may be used. 
     As shown in the perspective view of  FIG. 11 , chamber  418  has opposing side base flanges  436  and comprises two mated half chambers  422 A,  422 B that meet at lengthwise joint  420 . Mating vertical flanges  440 , which comprise stirrup portions  441 , described below, abut joint  420 . A plurality of spaced apart ribs  426  run within the inner concavity of each half chamber to provide strength to each vertical flange  440  by connecting the flange to the underside of the arch curve of the half chamber. The half chambers are secured to each other by means of lengthwise-running locking rods  442 T and  442 L which run through the stirrups. In  FIG. 11 , the rods are shown as they are about to be respectively inserted into lengthwise openings  460 T,  460 L. While examples of chambers having dual passageways and associated locking rods are pictured here, in the generality of the invention only one passageway may be present and one locking rod will be used. In the exemplary chamber of  FIG. 11  and  FIG. 12 , the passageways and locking rods are positioned below the elevation of the lower surfaces of the valley corrugations  432  at the top portion of the chamber, which surfaces face downwardly toward the base of the chamber. 
       FIG. 12  and  FIG. 13  are different views of fragments of the mating surfaces of the half chambers  422 A,  422 B. Portions  451  of the vertical flanges  440  extend upwardly into the concavity of a peak corrugation  434 ; other portions  452  extend downwardly below the elevation of the valley corrugations  432 . There is a plurality of essentially semi-circular cylindrical arches, called stirrups  441 . Stirrups  441  are spaced apart from each other by slots  443 , along the length of the half chamber, and in particular, along the length of flange  440 . Slots  443  of one half chamber vertical flange are shaped to receive the stirrups  441  of the mating half chamber. 
       FIG. 14  is a schematic end view of the stirrup portions of the vertical flanges  440  of two mated half chambers, illustrating how the stirrups  441  of each half chamber form one of a plurality of circular openings, which openings are spaced apart along the length of the chamber and aggregate to define representative passageway  460 L. As illustrated in  FIG. 11  and  FIG. 13 , a locking rod  442 T,  442 L is slidable lengthwise along the length of each passageway  460 T,  460 L, thereby to effect locking of the mated stirrups  441  and associated flanges  440 . The half chambers are thus prevented from separating at the joint. The opening has at vertical dimension D 1  and a horizontal dimension D 2  which are equal when the opening is circular, or which may be somewhat different when the opening is non-circular. 
     In one embodiment of the invention, the locking rod, which may be made of corrosion resisting metal or of sufficiently strong plastic material, may be round. In another embodiment of the invention, the rod may have a non-round (e.g., oblong) shape cross section.  FIG. 15  shows a cross section of a portion of locking rod  742  which is elliptical, having a major diameter DD 1  and a minor diameter DD 2 . In the  FIGS. 15, 15A, and 16 , the extent of non-roundness of the locking rod is greatly exaggerated for purpose of illustration.  FIG. 16  is like  FIG. 14 , showing the addition of rod  742  which has been placed within the opening  460 L that is defined by the two adjacent stirrups  441 . With reference to  FIG. 14-15 , in the  FIG. 16  embodiment, the dimension D 2  of the opening  460 L is by design larger than the dimension D 1 ; and the dimension DD 1  of the rod is slightly larger than dimension DD 2  of the rod. The rod dimension DD 1  slip fits into the opening having dimension D 2 . After having been slipped lengthwise into the opening, the locking rod  742  is rotated so the greater axis DD 1  is made horizontal, as pictured in  FIG. 16 . Thus, stirrups  441  have been thrust apart, and that has drawn the flanges  422 A,  422 B toward one another. A resultant small spaces  103  are adjacent the top and bottom of the locking rod cross section, as can be seen in  FIG. 16 . 
       FIG. 15A  is a perspective view of a portion of a locking rod  742 , showing how there is a first set of segments  745  that are offset in one direction and a second alternated set of segments  743  that are offset in the opposite direction. The alternating offsets of the segments  743 ,  745  accommodates that the stirrups on one half chamber alternate with the stirrups on the other half chamber, and that each half chamber&#39;s stirrups are to be moved in the opposite direction to those of the other half chamber. It will be appreciated that, relative to an imaginary lengthwise central axis of rotation of a rod  742 , each segment will be offset, and thus could be called a cam. 
     The above-described rotation of the rod can be called “camming” the rod. A locking rod may be configured with means such as a pin or latch, not shown, to prevent the locking rod from rotating out of its desired cammed position. Alternative shapes of rod may be employed to effect camming within the general principle of this aspect of the invention. 
       FIG. 17  shows the upper portion of alternative embodiment half chamber  522 A, which is shaped to mate with a mirror half chamber. A first set of alternating stirrups  541  and slots  543  run along the length of flange  540  and are configured to mate with the stirrups and slots of a mirror half chamber as described above. The first set of stirrups form a first passageway  560 L which is lower in elevation than is the elevation of the lowermost portions or surfaces of peaks  534  or valleys  532  at the top of the chamber. (The reference point for elevation is the plane of the base flanges, not shown.) A second set of stirrups  541  and slots  543  forms a second passageway  560 T. The stirrups  541  and slots  543  are arranged along the length of flange  540  at an elevation that is higher than that of the top (surfaces) of the peaks  534  of the half chamber  522 A. 
     In the embodiments of  FIGS. 12 and 13 , the vertical flanges  440  are continuous along the length of the half chambers and flange portions  451  extend upwardly into the space which is at the upper end of each peak corrugation  434 . In an alternative embodiment half chamber, a multiplicity of smaller discontinuous stirrup plates may be spaced apart along the length of the half chamber. In another alternative embodiment of the invention, only one locking rod may be used, for instance only the rod  442 T may be present in the half chamber embodiment shown in  FIG. 13 . In still another alternative embodiment half chamber, there may be more than two passageways defined by stirrups, with associated more than two locking rods. In the exemplary chambers described thus far, locking rods are centered on the joint between the half chambers. 
       FIG. 18  shows portions of two half chambers  634 , as they mate at the joint  620 . Each half chamber has a multiplicity of tabs  693  which alternate in valleys along the length of the half chamber with a multiplicity of slots  685 , at the vertical flange  640 . Each tab  693  is received in a slot  685  of the mating half chamber. Each tab has a horizontal opening  660  through which a pin (locking rod, not shown) may be passed, to keep the tab from withdrawing through the slot, thus to hold the vertical flanges  640  in intimacy. Each locking rod may be short, so it can be inserted within the tab in a single-valley tab. In an alternative embodiment, not shown, each peak corrugation has a hole so that a locking rod may be passed along the whole length of the chamber, to engage a plurality of tabs  693 . As is evident, the locking rod is not centered on the joint in this embodiment. 
     While a single locking rod having the nominal length of the chamber is preferably inserted in each lengthwise passageway that is defined by a plurality of stirrups in vicinity of the joint region, in an alternate embodiment the invention, a locking rod having a length which is half that of a chamber may be inserted from each end of the chamber. 
     While the joint between the above described half chambers having locking rods is formed at the vertical center plane of the exemplary chambers thus far described, in alternate embodiments of the invention the vertical flanges and the associated joint (and passageway and locking rod, when present) may be offset transversely somewhat from the center plane. 
     Half chambers may have other features that are used in combination with stirrups, passageways, and locking rod, to enhance the quality of the joint between half chambers. For example, there may be at the joint region one or more of (i) a plurality of fasteners, (ii) a plurality of C shape cross section clips, (iii) a plurality of mating bosses and recesses, and combinations thereof. 
     With reference to  FIG. 10 , in a method of making, shipping and installing chambers, half chambers (with or without a locking rod feature) may be shipped on a pallet  80  as a nested stack  82  as shown (for representative half chambers  24 ). The term “pallet” shall comprehend functionally equivalent devices. Typically, a pallet will be carried by a semi-trailer connected to a motor vehicle truck tractor, more generally, a transport vehicle, to a point of assembly which may be a job site or a location remote from the machine where the half chambers are molded, including within the same factory. 
     The following more completely recites the process. A method of manufacturing and transporting injection molded plastic corrugated chambers, for receiving water when buried beneath the surface of the earth, comprises the following. Each chamber has a length, opposing side base flanges running lengthwise and lying in a base plane, an arch shape wall running upwardly to a chamber top from the opposing side base flanges. The wall defines an arch shape cross section chamber interior and is characterized by alternating peak corrugations and valley corrugations running transverse to the chamber length. There is a lengthwise vertical center plane running intersecting the chamber top. The process comprises:
         (a) molding, preferably by injection, a multiplicity of first half chambers and second half chambers. Each half chamber comprises one of said base flanges, about half of said arch shape wall, and a coupling portion connected to the half wall in vicinity of the top of the to-be-formed chamber. The coupling portions are shaped for mating first half chambers with second half chambers at a joint, to form whole chambers. Preferably the mated portions of the half chambers are configured to define a passageway, for receiving one or more locking rods;   (b) Placing the first half chambers and the second half chambers in nested fashion on a pallet or directly on a transport vehicle, transporting the pallets containing the half chambers on a transport vehicle, for example a truck, to a point of assembly.   (c) Removing the half chambers from the pallet or vehicle at the point of assembly and de-nesting the half chambers.   (d) Providing at least one locking rod for each two half chambers when the two mated half chambers are configured to define one or more lengthwise passageways.   (e) Mating the coupling portion of each first half chamber with and to the coupling portion of each second half chamber, thereby to form a multiplicity of whole chambers.   (f) Securing the mated half chambers to each other, by means which optionally include one or more of welding, fastening, and inserting of one or more lengthwise locking rods.   (g) Placing each whole chamber within a cavity in the soil or other water permeable natural or artificial substance, for use.       

     The invention enables more compact, and therefore more economic, storage and shipping of unassembled half chambers, compared to whole chambers. The invention also enables fabrication of large chambers which are beyond the plastic-weight molding capacity of, or the platen size of, a particular injection molding press. 
     The invention, with explicit and implicit variations and advantages, has been described and illustrated with respect to several embodiments. Those embodiments should be considered illustrative and not restrictive. Any use of words which relate to the orientation of an article pictured in space are for facilitating comprehension and should not be limiting should an article be oriented differently. Any use of words such as “preferred” and variations thereof suggest a feature or combination which is desirable but which is not necessarily mandatory. Thus embodiments lacking any such preferred feature 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 of the invention embodiments which are described, without departing from the spirit and scope of the claimed invention.

Summary:
A molded plastic arch shape cross section stormwater chamber having a corrugated wall comprises separately molded half chambers which are connected by coupling features at a joint region at the top of the chamber, which coupling feature optionally includes at least one locking rod running lengthwise in proximity to the joint region. Preferably, the half chambers are made in the same mold and modified so they are mirror images of each other. The half chambers may be compactly stored and transported in nested condition. Near the point of use, the chambers may be assembled.