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CROSS-REFERENCES 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/535,565, filed Sep. 16, 2011, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates to the general art of precast concrete bridge and culvert units, and to the particular field of four-sided bridge and culvert units. 
       BACKGROUND 
       [0003]    Overfilled bridge structures are frequently formed of precast reinforced four-sided concrete units commonly referred to as arch units, arch culverts, box units or box culverts. As used herein the terminology four-sided bridge unit encompasses all of such structures. The units are used in the case of bridges to support one pathway over a second pathway, which can be a waterway. Four-sided bridge units have a bottom wall structure that facilitates on-site placement with reduced need for foundation preparation. 
         [0004]    In the past, the four-sided bridge units of overfilled bridge structures have been constructed with bottom wall structures having a generally planar and continuous top surface and a generally uniform thickness. There is an increasing demand for construction efforts to provide more natural environments and/or to decrease impact on wildlife. 
         [0005]    A four-side bridge unit adapted to create a more natural environment through the pathway defined by the bridge units and/or adapted to reduce impact on fish migrations would be desirable. 
       SUMMARY 
       [0006]    In one aspect, a method of providing an environmentally appealing region for water flow along an surrounded pathway tunnel is provided. The method involves: providing a plurality of four-sided concrete bridge units in abutting relationship to create a surrounded pathway tunnel, one end of the tunnel located upstream along a water path and an opposite end of the tunnel located downstream along the water path; allowing water to flow through the surrounded pathway tunnel during a rain or other flow event; and providing a multiplicity of the four-sided bridge units with a corresponding bottom wall structure that interacts with the flowing water and earthen material in the flowing water such that capture and settling of the earthen material at locations along the tunnel occurs to produce a more natural water flow pathway along the tunnel. 
         [0007]    The bottom wall structure of each of the multiplicity of the four-sided bridge units may be provided with a plurality of through openings such that at least forty percent of the bottom wall structure is open. For example, at least fifty percent of the bottom wall structure of each of the multiplicity of the four-sided bridge units may be open. 
         [0008]    A lip structure may be provided at a top portion of at least some of the through openings, the lip structure facing upstream. 
         [0009]    The plurality of openings of each bottom wall structure may be arranged in rows that extend along a span of the respective four-sided bridge unit. 
         [0010]    The plurality of openings may be formed in the shape of elongated slots, each elongated slot defining a row, such that multiple beams are formed in the bottom wall structure and also extend along the span. At least one beam with a height that is greater than a height of another beam, the higher beam interacting with the flowing water and earthen material to reduce flow velocity and thereby enhance settling out of earthen material. By providing a lip structure along at least one beam, the lip structure extending in an upstream direction into an adjacent elongated slot, wash out of earthen material that has settled in the adjacent elongated slot can be limited. 
         [0011]    The plurality of openings may be provided as multiple series of openings, each series of openings forming a respective row. By staggering openings of adjacent rows, nesting of the openings is achieved. By providing upper lip structure along one or more edges of at least some of the openings, the lip structure extending into its respective opening, wash out can be limited. 
         [0012]    By providing the bottom wall structure of each of the multiplicity of the four-sided bridge units with a recessed portion, a low flow channel through which marine life can travel is created. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective view of one embodiment of a four-sided bridge unit; 
           [0014]      FIG. 2  is an end elevation of the bridge unit of  FIG. 1 ; 
           [0015]      FIG. 3  is a cross section along line  3 - 3  of  FIG. 2 ; 
           [0016]      FIG. 4  is bottom view of the bridge unit of  FIG. 1 ; 
           [0017]      FIG. 5  is a cross-sectional view of two bridge units of  FIG. 1  arranged edge to edge; 
           [0018]      FIG. 6  is an enlarged partial view of the cross-section of  FIG. 5 ; 
           [0019]      FIG. 7  shows a partial cross-section of an embodiment of a unit with both upstream and downstream facing lips; 
           [0020]      FIG. 8  shows a partial cross-section of an embodiment of a unit in which the beams all have a common height; 
           [0021]      FIGS. 9 and 10  show perspective views of another embodiment of a four-sided bridge unit in which continuous haunches are provided in the corners where the bottom wall meets the side walls; 
           [0022]      FIG. 11  is a perspective view of yet another embodiment of a four-sided bridge unit; 
           [0023]      FIG. 12  is an end elevation of the bridge unit of  FIG. 11 ; 
           [0024]      FIG. 13  is a cross section along line  13 - 13  of  FIG. 12 ; 
           [0025]      FIG. 14  is bottom view of the bridge unit of  FIG. 11 ; 
           [0026]      FIG. 14A  is a partial cross-section along line  14 A of  FIG. 14 ; 
           [0027]      FIG. 15  is a perspective view of still another embodiment of a four-sided bridge unit; 
           [0028]      FIG. 16  is an end elevation of the bridge unit of  FIG. 15 ; 
           [0029]      FIG. 17  is a cross section along line  17 - 17  of  FIG. 16 ; 
           [0030]      FIG. 18  is bottom view of the bridge unit of  FIG. 15 ; 
           [0031]      FIGS. 19A-B  show another embodiment of a bridge unit; 
           [0032]      FIG. 20A-C  show another embodiment of a bridge unit; 
           [0033]      FIG. 21A-C  show another embodiment of a bridge unit; 
           [0034]      FIG. 22  shows a plurality of four-sided units arranged along a water flow path; and 
           [0035]      FIG. 23  shows a schematic end elevation of the system of  FIG. 22  as buried. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    Referring to  FIGS. 1-4 , a four-sided precast concrete bridge unit  10  is shown. In the illustrated embodiment bridge unit  10  is formed by a generally horizontal extending bottom wall  12 , substantially vertically upward extending side walls  14  and  16  at the ends of the bottom wall and a top wall  18  having a generally arch-shaped configuration. However, four sided bridge units having top walls other than arch-shaped (e.g., flat top walls) are also contemplated. Likewise, side walls other than vertical are possible. As used herein, the terms “length” and “span” of an individual unit or portions of the unit refers to a horizontal dimension extending parallel with the direction of arrow  20  (which is substantially perpendicular to a horizontal through axis  22  of the unit) and the terms “width” and “depth” of the individual unit or portions of the unit refer to a horizontal dimension extending parallel to the through axis  22 . As used herein the term “arch” and “arch-shaped” when referring to the top of an arch unit means a curved shape (including constant radius curves, curves with multiple radii, curves with continuously varying radius) or any top wall shape that is higher in the middle of the top wall as opposed to where the top wall meets the side walls (e.g., an inverted V-shape or a combination of three or more planar segments angularly arranged with respect to each other to produce a vaulted top wall or a combination of curved segments and flat segments that produce a vaulted top wall). 
         [0037]    The bottom, top and side walls are preferably precast as a single monolithic structure in a single casting operation. However, in certain implementations, one or more walls may be cast separately and then connected together by suitable connecting structure (e.g., reinforcing bars or by casting one or more elements separately and then placing that cast element in the formwork that is used to cast the final structure). 
         [0038]    The bottom wall  12  of the unit  10  is shaped and configured to facilitate both sedimentation within and passage of marine life once the unit is installed. Specifically, the bottom wall  12  includes a plurality of elongated, spanwise extending through openings that extend completely through the thickness of the bottom wall  12 . As shown, each elongated opening  24  has a length L O  that is at least about sixty percent of the overall width of the unit L U  (e.g., L O  is at least about 70% of L U , such as for example, between 80% and 95% of L U ). However, other variations are possible. Intermediate beams  26  separate the elongated openings  24  and serve to maintain a rigid connection between the lower ends of the side walls  14  and  16 . Edge located beams  28  are also provided, thereby providing a continuous peripheral support surface at the lower side of the bottom wall. The lower surface of each beam  28  is preferably in common plane with the continuous peripheral support surface to provide added stability and distribution of loads. As shown, roughly about 40% to 60% (e.g., about 45% to 55%) of the lower side of the bottom wall makes up the support or resting surface of the bridge unit and the remainder (about 60% to 40%) is open via the openings  24 . However, other variations are possible. Lengthwise extending reinforcement may be provided in each of the beams for structural integrity, with some continuity provided between that reinforcement and the reinforcement of the vertical side walls. 
         [0039]    As seen in  FIG. 3 , where the anticipated water flow direction through the bridge unit is shown by arrow  30 , the combination of the beams  26 ,  28  and the openings  24  are configured to promote sedimentation at the bottom of the bridge unit. Specifically, the beams  26  and one of the beams  28  are formed with a lip structure  32  and  34  that overhangs the adjacent opening  24  and extends from the beam in an upstream direction. Also, one or more of the beams  28  has a thickness or height that exceeds that of the adjacent beams  26  and/or  28 . The effect of this configuration is best described with reference to  FIGS. 5 and 6 , where  FIG. 5  shows two units  10  in edge to edge relationship as such units would typically be installed on a job site and  FIG. 6  shows an enlarged partial view with a flow pattern. 
         [0040]    As seen in  FIG. 5 , the edge located beams  28 ″ (located at the upstream flow edge of the units) lack any upstream facing lip structure while the edge located beams  28 ′ (located at the downstream flow edge of the units) incorporates an upstream facing lip structure. In this manner, when two units  10  are installed edge to edge, there is no lip structure to interfere with the placement and the adjacent beams  28 ′ and  28 ″ combine to form effective beam that is similar in overall configuration and size to intermediate beam  26 ′. In this regard, the width of the beam structures  28 ′ and  28 ″ is preferably smaller than the width of beam structures  26 ′ and  26 ″ (e.g., on the order of about 50% to about 60% of the width of beam structures  26 ′ and  26 ″) so that the overall width of the effective beam is more consistent with the overall width of the beams  26 ′ and  26 ″. The height of beams  26 ″ is greater than the height of beams  26 ′,  28 ′ and  28 ″ as shown. Beams  26 ′,  28 ′ and  28 ″ have the same thickness or height and beams  26 ″ may have a thickness or height that is about 110% to about 140% greater (e.g., about 120% to about 130% greater). However, variations are possible. The width W L  of the lip structure may be on the order of about 10% to 20% of the overall width W O  of the opening  24 . In the illustrated embodiment, a tapered surface  36  connects the vertical side surface  38  of the beam with the protruding edge of the lip. 
         [0041]    Referring to  FIG. 6 , as water flows through the units the higher beams tend to reduce the velocity in the vicinity  40  of an opening  24  which tends to cause sediment to drop out of the flow and into the opening. The lip structure  32  helps prevent washout of any sediment that builds up in the openings  24 . The lip structures  32  and  34  of the shorter beams  26 ′ and  28 ′ also help prevent washout in respective openings and creates respective areas  42  and  44  of lower velocity that can promote sedimentation. 
         [0042]    In the illustrated embodiment, the connection of every other beam to the vertical side wall includes a haunch  46 , which may include reinforcement, to resist the moment loads in the corners. Placing the haunches in a spaced apart manner, rather than providing a continuous haunch, can also help promote sedimentation. However, continuous haunches are also contemplated for some applications, as reflected in the embodiment of  FIGS. 9 and 10 . In this embodiment, the relative length of the slotted openings  24  (as compared to overall length of the unit) is smaller than that shown in  FIG. 4  in order to accommodate the haunch  46 . Moreover,  FIGS. 9 and 10  show a four-sided bridge unit with a flat top wall structure rather than an arched top wall structure. 
         [0043]    While the embodiment of  FIGS. 1-6  contemplates upstream facing lips only, in an alternative embodiment downstream facing lips may also be provided on the beams as shown in  FIG. 7 . Likewise, embodiments in which all the beams have a common height are contemplated, as shown in  FIG. 8 . 
         [0044]    Referring again to  FIGS. 1 ,  2  and  4 , and regardless of the relative height of the plurality of beams, each of the beams may be formed with a section  48  of reduced thickness to create a low flow channel through the unit, making it easier for marine life (e.g., fish) to travel through the unit. The reduced thickness sections  48  may be formed without any lip structures. 
         [0045]    An alternative embodiment of a four-side bridge unit  50  adapted for sedimentation is shown in  FIGS. 11-14 . As shown, the bottom wall  52  of the bridge unit  50  includes a plurality of openings  54 . The openings are arranged in a plurality of lengthwise extending rows  56  and  58 , with the rows  56  and  58  arranged in an alternating and staggered relationship that provides some nesting of the openings of one row into the spaces between the openings of another row. The openings are distributed along a lengthwise extending mid-portion L O  of the bottom wall  52  that represents between about 50% to about 80% of the overall length L U  of the bottom wall of the unit. In this manner, the bottom wall lacks any openings in roughly about the first 10% to 25% of the extent of the bottom wall from its ends. Reinforcement  60  may be located in this area for structural integrity. Likewise, as the edges of the bottom wall are continuous, lengthwise reinforcement  62  may be included along such edges as well. About 75% to about 90% of the bottom wall in the mid-portion L O  may be open space, while only about 55% to about 70% of the overall area of the bottom wall (as viewed from the bottom) may be open space. As shown in  FIG. 14A , the openings  54  may include lip structure to promote sedimentation and reduce washout effects. The lip structure may be upstream facing lip structure  66 , downstream facing lip structure  64  and/or lengthwise facing lip structure  68 . 
         [0046]    A further embodiment of a four-sided bridge unit  70  is shown in  FIGS. 15-18 . In this embodiment the openings  74  of the unit actually include rows of partial openings along each edge. The partial openings  74 ′ are preferably about one half the size of a regular opening such that when one unit is abutted with another unit the partial openings combine to effectively form an opening similar in size and shape to the openings  74 . The mid-point arrangement of the openings along the length of the bottom wall  72  may be similar to that of the embodiment of  FIGS. 11-14 , with reinforcement  76  in the end areas of the bottom wall  72 . However, due to the edge openings  74 ′, no reinforcement is provided in the mid-section where the openings are located. The openings  74  of the unit  70  may also include lip structure as described relative to  FIG. 14A . 
         [0047]    It is to be clearly understood that the above description is intended by way of illustration and example only and is not intended to be taken by way of limitation, and that changes and modifications are possible. For example, other possible unit configurations are reflected in  FIGS. 19A-B ,  20 A-C and  21 A-C. For reference, the unit  90  of  FIGS. 19A-B  includes lengthwise extending openings  82  having ends adjacent the side walls  84 , alternatingly raised  86  and lowered  88  beams and upstream facing lips, with no haunches or gusseting between the bottom wall and the side walls. The unit  90  of  FIGS. 20A-C  is similar to that of  FIGS. 19A-B  but also includes reduced thickness sections in the beams to provide a low flow channel  92 . The unit  100  of  FIGS. 21A-C  includes beams and slots with ends spaced from the side walls, and no haunches or gussets, such that the corner areas between the bottom wall and the side walls form low flow areas. 
         [0048]      FIG. 22  shows a plurality of four-sided concrete bridge units, which could be any of the unit configurations previously described, in abutting relationship to create a surrounded pathway tunnel  110 . One end  112  of the tunnel is located upstream along a water path  114  and an opposite end  116  of the tunnel is located downstream along the water path  114 .  FIG. 23  shows the units in profile as buried in earthen material  118 .  FIG. 23  could also represent a series of buried units used for the purpose of storm water collection, with infiltration into the surrounding earth occurring through the openings in the bottom walls of the units. 
         [0049]    Other embodiments are contemplated and modifications and changes could be made without departing from the scope of this application.

Summary:
A method of providing an environmentally appealing region for water flow along an surrounded pathway tunnel involves providing a plurality of four-sided concrete bridge units in abutting relationship to create a surrounded pathway tunnel, one end of the tunnel located upstream along a water path and an opposite end of the tunnel located downstream along the water path; allowing water to flow through the surrounded pathway tunnel during a rain or other flow event; and providing a multiplicity of the four-sided bridge units with a corresponding bottom wall structure that interacts with the flowing water and earthen material in the flowing water such that capture and settling of the earthen material at locations along the tunnel occurs to produce a more natural water flow pathway along the tunnel.