An underdrain filter block for use in draining and backwashing a filtering media in a filter bed includes a top wall, a pair of side walls extending from the top wall, a bottom wall extending between the pair of side walls, and a transverse wall extending from one of the pair of side walls to the other of the pair of side walls. The top wall, the pair of side walls, and the bottom wall define an upper portion and a lower portion. The lower portion includes a grout chamber positioned between the bottom wall and the transverse wall and at least one of the bottom wall and said side walls contains an opening.

CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a National Stage of International Application No. PCT/US2011/021522 filed Jan. 18, 2011, which is a Continuation-In-Part of U.S. application Ser. No. 12/983,465 filed Jan. 3, 2011, now U.S. Pat. No. 8,889,003, which is a non-provisional of U.S. Provisional Application No. 61/295,891 filed Jan. 18, 2010, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an underdrain filter block and, more particularly, to a filter block used in draining and backwashing a filtering media in a filter bed and a method of securing the filter block to the filter floor. 
     2. Description of Related Art 
     Underdrain filter blocks provide a filter bottom by assembling a number of filter blocks together to provide an upper surface for supporting a filter media. The filter bottom upper surface is provided with openings to allow the flow of filtered liquid from the filter media to pass into the filter bottom where conduits carry the filtered liquid from the filter. The conduits also serve to provide backwashing fluids, such as gas or liquid or both, to the filter media for cleaning. U.S. Pat. No. 5,108,627 to Berkebile et al. discloses a filter underdrain block and is hereby incorporated by reference in its entirety. 
     Generally, conventional underdrain filter blocks are susceptible to uplift failure due to an internal pressure caused by headloss through the underdrain. The headloss or internal pressure of the conventional underdrain blocks tends to force the block off the filter floor as a bottom section of the underdrain block deflects or deforms from internal pressure. 
     U.S. Pat. No. 3,956,134 to Sturgill discloses, as shown in  FIGS. 5 ,  6 , and  13 , an underdrain unit with a bottom wall that extends outwardly beyond sidewalls to define a flange. Perforations in the bottom wall help to secure the underdrain to the floor of the filter basin by engaging with grouting material provided under the underdrain units. Referring to  FIG. 6 , an end wall of the filter unit is provided with a plurality of perforations that provide mechanical engagement with grouting material provided around the units. 
     U.S. Pat. No. 6,110,366 to Hunkele et al. discloses an anchor concept for securing an underdrain block to a filter floor. In particular, Hunkele discloses the use of anchors that comprise one or more widenings extending outwardly from a rib so as to form ledges that are embedded in grouting on a filter floor. 
     U.S. Pat. No. 3,110,667 to Stuppy and U.S. Pat. No. 4,619,765 to Roberts disclose filter block constructions in which a bottom wall of the filter block is defined with ridges or grooves. 
     SUMMARY OF THE INVENTION 
     In one embodiment, an underdrain filter block for use in draining and backwashing a filtering media in a filter bed includes a top wall, a pair of side walls extending from the top wall, a bottom wall extending between the pair of side walls, and a transverse wall extending from one of the pair of side walls to the other of the pair of side walls. The top wall, the pair of side walls, and the bottom wall define an upper portion and a lower portion. The lower portion includes a grout chamber positioned between the bottom wall and the transverse wall and at least one of the bottom wall and one of the pair of side walls contains an opening. 
     The bottom wall and the pair of side walls may define a plurality of openings in the lower portion. The upper portion may include a primary chamber and at least one secondary chamber defined by a pair of angularly extending walls. The top wall, the transverse wall, and a portion of said side walls may comprise a first piece, and the bottom wall and a portion of said side walls may comprise a second piece, where the first piece is secured to the second piece. The underdrain filter block may further include a plurality of ribs generally extending circumferentially along the pair of side walls and the bottom wall. The grout chamber may extend the full length of the underdrain filter block or may only extend a portion of the full length of the underdrain filter block. The grout chamber may be recessed inwardly relative to an outermost portion of the side walls. The upper portion may include a primary chamber and at least one secondary chamber defined by an arched wall. The plurality of openings may be circular-shaped. 
     In a further embodiment, an underdrain filter block for use in draining and backwashing a filtering media in a filter bed includes a top wall, a pair of side walls extending from the top wall, and a bottom Wall extending between the pair of side walls. The top wall, the pair of side walls, and the bottom wall define an upper portion and a lower portion. The lower portion includes a plurality of grout chambers defined by a grid-like structure positioned on a bottom surface of the bottom wall. The bottom surface of the block contains a plurality of air relief openings that extend into each of the plurality of grout chambers. 
     In another embodiment, an underdrain filter block for use in draining and backwashing a filtering media in a filter bed includes a top wall, a pair of side walls extending from the top wall, a bottom wall extending between the pair of side walls, a plurality of ribs extending circumferentially along the pair of side walls and the bottom wall, and a plurality of wall segments extending between the plurality of ribs to define a plurality of grout chambers. The plurality of grout chambers may be positioned below the bottom wall. The plurality of wall segments may each define a plurality of openings. The underdrain filter block may further include a primary chamber and at least one second chamber defined by an arched wall. 
     In a further embodiment, a method of installing an underdrain filter block includes positioning an underdrain filter block adjacent to a filter floor. The underdrain filter block has a top wall, a pair of side walls extending from the top wall, a bottom wall extending between the pair of side walls, and a transverse wall extending from one of the pair of side to the other of the pair of side walls. The underdrain filter block also including a grout chamber positioned between the bottom wall and the transverse wall. The grout chamber includes at least one opening defined by at least one of the bottom wall and one of the pair of side walls. The method further includes providing grout along the pair of side walls and the bottom wall such that grout extends into the at least one opening. 
     The grout may be provided along the bottom wall of the block by positioning a section of leveling grout on the filter floor. A plurality of openings may be defined by the bottom wall and the pair of side walls of the block with the openings in the bottom wall and the pair of side walls each receiving grout. The underdrain filter block may be positioned onto the section of leveling grout on the filter floor. The grout may be provided along the pair of side walls of the underdrain filter block after positioning the underdrain filter block onto the section of leveling grout. 
     In yet another embodiment, a method of assembling an underdrain filter block includes providing a first piece including a top wall, a pair of side walls extending from the top wall, and a bottom wall extending between the pair of side walls. The method further includes providing a second piece that defines interior walls, inserting the second piece into the first piece, and securing the first piece to the second piece. 
     The interior walls of the second piece may include a transverse wall and an arched wall. The method may further include engaging grooves defined by the side walls of the first piece with the transverse wall of the second piece. The method may also include spacing the transverse wall of the second piece from the bottom wall of the first piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of an underdrain filter block according to one embodiment of the present invention; 
         FIG. 2  is a bottom view of the filter block of  FIG. 1 ; 
         FIG. 3  is an enlarged cross-sectional view of the filter block along the line  3 - 3  shown in  FIG. 1 ; 
         FIG. 4  is a front view of the filter block of  FIG. 1 , showing the filter block being laid on the base grout; 
         FIG. 5  is an enlarged cross-sectional view of a plurality of filter blocks shown in  FIG. 1 , showing the filter blocks installed; 
         FIG. 6  is a perspective view of an underdrain filter block according to a second embodiment of the present invention; 
         FIG. 7  is an enlarged cross-sectional view of the filter block along the line  7 - 7  shown in  FIG. 6 ; 
         FIG. 8  is a perspective view of an underdrain filter block according to a third embodiment of the present invention; 
         FIG. 9  is a perspective view of an underdrain filter block according to a fourth embodiment of the present invention; 
         FIG. 10  is a perspective view of an underdrain filter block according to a fifth embodiment of the present invention; 
         FIG. 11  is a perspective view of an underdrain filter block according to a sixth embodiment of the present invention; 
         FIG. 12  is an exploded perspective view of an underdrain filter block according to a seventh embodiment of the present invention; 
         FIG. 13  is an exploded perspective view of an underdrain filter block according to an eight embodiment of the present invention; 
         FIG. 14  is a perspective view of an underdrain filter block according to a ninth embodiment of the present invention; and 
         FIG. 15  is an exploded perspective view of an underdrain filter block according to a tenth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For purposes of the description hereinafter, spatial orientation terms, if used, shall relate to the referenced embodiment as it is oriented in the accompanying drawing figures or otherwise described in the following detailed description. However, it is to be understood that the embodiments described hereinafter may assume many alternative variations and embodiments. It is also to be understood that the specific devices illustrated in the accompanying drawing figures and described herein are simply exemplary and should not be considered as limiting. 
     Referring to  FIGS. 1-3 , one embodiment of an underdrain filter block  10  includes a top wall  15 , a pair of side walls  17 , and a bottom wall  19 , which define a top surface  21 , side surfaces  23 , and a bottom surface  25 , respectively. The block  10  includes a plurality of reinforcing ribs  27  on the exterior surface of the walls  15 ,  17 ,  19 . The ribs  27  are generally oriented circumferentially around the block  10  rather than along a longitudinal axis of the block, although other rib orientations and configurations may be utilized. The ribs  27  on the top wall  15  are separate or discontinuous from the ribs positioned on the bottom wall  19 , although a single rib that extends circumferentially around the block  10  may be utilized. As shown in  FIG. 3 , a transverse interior wall  29  extends from one side wall  17  to the other side wall  17  to divide the interior of block  10  into an upper portion  31  and a lower portion  33 . The top wall  15  defines a recessed portion that extends along a longitudinal axis of the block  10 . The side walls  17  are generally planar, although the side walls may include recessed portions such that the width of the block  10  in cross-section is narrower in certain areas. The ends of the block  10  are configured to receive respective ends of adjacent blocks so that a plurality of underdrain filter blocks may be secured end-to-end. 
     The lower portion  33  of the block  10  includes a grout chamber  35  defined by the transverse wall  29 , side walls  17 , and bottom wall  19 . The grout chamber  35  is open at each end of the block  10 , although the chamber  35  may be closed at one or both ends. The grout chamber  35  extends longitudinally along the full length of the block  10 , although the chamber  35  may have a length shorter than the full length of the block  10 . The upper portion  31  of the block  10  includes a primary chamber  37  and a pair of secondary chambers  40 ,  41  defined by a pair of angularly extending walls  43 . Rather than providing the substantially planar angularly extending walls  43 , the interior walls that define the primary chamber  37  and the secondary chambers  40 ,  41  may be arched or curved such as those shown, for example, in  FIGS. 6 and 7  discussed below. The top wall  15  and the angularly extending walls  43  define a plurality of orifices (not shown) to provide for the distribution of liquid and gas as known in conventional underdrain filter blocks. 
     The bottom wall  19  of the filter block  10  defines a plurality of openings  45  that extend into the grout chamber  35 . Each of the side walls  17  also define a plurality of openings  47  in the lower portion  33  of the block  10  with the openings  47  extending into the grout chamber  35 . The openings  47  in the side walls  17  are not provided for the full length of the block  10 , but may be provided for the full length of the block  10  in other configurations. Although the side walls  17  and bottom wall  19  include openings  45 ,  47 , the side walls  17  or bottom wall  19  may alternatively not include openings. Further, the side walls  17  and the bottom wall  19  may not be provided with openings that extend into the grout chamber  35 . In such an arrangement, the block  10  may be provided with a grout introduction scoop (not shown) that is in fluid communication with the grout chamber  35 . Furthermore, the openings  45 ,  47  are substantially circular-shaped, but may have other configurations such as being oblong-shaped, square-shaped, or any other suitable shape. The grout chamber  35  may be formed integrally with the block  10  to define a unitary, one-piece construction. The grout chamber  35  may also be formed from multiple pieces. In particular, the grout chamber  35  may be formed separately and secured to the rest of the block  10  through a fastening arrangement, such as by bolts or welding. 
     Referring to  FIGS. 4 and 5 , during the installation process, the block  10  is secured in place by grout  50 ,  52  which flows into the grout chamber  35  through the openings  45 ,  47  in the side walls  17  and bottom wall  19 . The grout  50 ,  52  also flows around the grout chamber  35  and block  10  up toward the top of the side walls  17  of the block  10 . 
     In one embodiment, the underdrain filter block  10  is installed as generally shown in  FIGS. 4 and 5 . In particular, as shown in  FIG. 4 , the block  10  is positioned or set in leveling grout  50  provided on a filter floor  54 . The leveling grout  50  protrudes into the grout chamber  35  slightly via the openings  45  in the bottom wall  19 . Once the leveling grout  50  has set up enough, the side grout  52  is applied. During the installation of the side grout  52 , the grout chamber  35  on the lower portion  33  of the block  10  is at least partially filled with grout. As shown in  FIG. 5 , the entire grout chamber  35  may be filled with grout after installation. The protrusion of the grout from the openings  45 ,  47  in the grout chamber  35  holds the block  10  securely in place, thereby overcoming the buoyant forces acting on the block  10 . Furthermore, the underdrain filter block  10 , particularly the bottom wall  19  of the block  10 , is stiffened by the grout  50 ,  52  that enters the grout chamber  35 . The stiffening of the bottom wall  19  and the block  10  reduces deflection or deformation of the block  10  due to the internal pressure. Thus, the grout chamber  35  and openings  45 ,  47  prevent uplift failure due to the internal pressure caused by headloss through the block  10  and securely hold the filter block  10  on the filter floor  54 . In a further embodiment, the filter block  10  is installed by securely holding the block  10  with top and bottom supports (not shown) and installing grout between the bottom wall  19  of the block  10  and the filter floor  54  and along the side walls  17  of the block  10  within a short time frame. 
     Referring to  FIGS. 6 and 7 , an underdrain filter block  75  according to a second embodiment is shown. The underdrain filter block  75  is similar to the underdrain block  10  as shown in  FIGS. 1-5  and discussed above. Like reference numerals are used for like parts. The underdrain block  75  of the present embodiment also includes a top wall  15 , a pair of side walls  17 , and a bottom wall  19 , which define a top surface  21 , side surfaces  23 , and a bottom surface  25 , respectively. The block includes a plurality of reinforcing ribs  27  on the exterior surface of the top wall  15 , side walls  17 , and bottom wall  19 . The ribs  27  generally extend circumferentially around the block with the ribs  27  on the top wall  15  being discontinuous as they extend from the top wall  15  to the side wall  17 . The ribs  27 , however, extend continuously from the side walls  17  to the bottom wall  19 . A transverse wall  29  extends from one side wall  17  to the other side wall  17  to divide the interior of block  75  into an upper portion  31  and a lower portion  33 . The lower portion  33  of the block  75  includes a grout chamber  35  defined by the transverse wall  29 , side walls  17 , and bottom wall  19 . The grout chamber  35  only extends a portion of the full length of the block  75 . Further, each of the side walls  17  are recessed inwardly where the transverse wall begins. Thus, the grout chamber  35  is recessed compared to the outermost portion of the side walls  17 . 
     Referring again to  FIGS. 6 and 7 , the upper portion  31  of the block  75  includes a primary chamber  37  and a pair of secondary chambers  40 ,  41  defined by an interior arched wall  77  extending from the transverse wall  29  and connecting to the top wall  15 . A gas conduit  79  is defined by the arched wall  77  and a second transverse wall  81 . The top wall  15 , arched wall  77 , and second transverse wall  81  define a plurality of orifices (not shown) to provide for the distribution of liquid and gas. The bottom wall  19  of the filter block  10  defines a plurality of openings  45  that extend into the grout chamber  35 . The side walls  17  also define a plurality of openings  47  in the lower portion  33  of the block  75  with the openings  47  extending into the grout chamber  35 . As shown in  FIG. 6 , the block  75  includes an end connector portion  83  that defines a plurality of openings  85 . The openings  85  extend through the end connector portion  83  in a direction that is parallel to a longitudinal axis of the block  75 . The end connector portion  83  may be utilized to secure a plurality of blocks  75  end-to-end as discussed above in connection with the block  10  shown in  FIGS. 1-5 . The block  75  may be installed in a similar manner as described above in connection with the block  10  shown in  FIGS. 1-5 . 
     Referring to  FIG. 8 , an underdrain filter block  90  according to a third embodiment is shown. The underdrain filter block  90  is similar to the underdrain block  10  as shown in  FIGS. 1-5  and discussed above. Like reference numerals are used for like parts. The underdrain filter block  90  includes a plurality of grout chambers  92  defined by wall segments  94  positioned between ribs  96  that extend from a bottom surface of the block  90 . The wall segments  94  extend longitudinally between the ribs  96 . Thus, the grout chambers  92  are defined by the ribs  96  and the wall segments  94 . The wall segments  94  each define a plurality of openings  98  for receiving grout as discussed above in connection with the openings  45  of the underdrain filter block  10  shown in  FIGS. 1-5 . The interior  101  of the block  90  may be similar to the upper portions  31  of the underdrain filter blocks  10 ,  75  discussed above having angularly extending walls or an arched wall to define primary and secondary conduits. The block  90  may be installed in a similar manner as described above in connection with the block  10  shown in  FIGS. 1-5  with the open sides of the grout chambers  92  receiving grout rather than the openings  47  of the block  10 . 
     Referring to  FIG. 9 , an underdrain filter block  110  according to a fourth embodiment is shown. The underdrain filter block  110  is similar to the underdrain block  10  as shown in  FIGS. 1-5  and discussed above. Like reference numerals are used for like parts. The underdrain filter block  110  includes openings  45  in a bottom wall  19  and openings  47  in each of the side walls  17  similarly to the underdrain filter block  10  of  FIGS. 1-5 . The openings  47  of the underdrain filter block  110 , however, are positioned at a location where the side walls  17  join the bottom wall  19  such that the openings  47  are non-circular. In particular, the openings  47  of the underdrain filter block  110  have an upper circular portion with a generally-planar bottom portion. The block  110  may be installed in a similar manner as described above in connection with the block  10  shown in  FIGS. 1-5 . 
     Referring to  FIG. 10 , an underdrain filter block  120  according to a fifth embodiment is shown. The underdrain filter block  120  is similar to the underdrain block  10  as shown in  FIGS. 1-5  and discussed above. Like reference numerals are used for like parts. The underdrain filter block  120  includes a grout chamber  35  that does not extend the full length of the block  120  thereby defining a cutout  122  adjacent to an end of the block  120 . The openings  47  in the side walls  17  of the underdrain filter block  120  are similar to those described above in connection with the underdrain filter block  110  shown in  FIG. 9 . The cutout  122  is configured to receive a support structure (not shown). For example, the cutout  122  may receive a uni-strut. The block  120  may be installed in a similar manner as described above in connection with the block  10  shown in  FIGS. 1-5 . 
     Referring to  FIG. 11 , an underdrain filter block  130  according to a sixth embodiment is shown. The underdrain filter block  130  is similar to the underdrain block  10  as shown in  FIGS. 1-5  and discussed above. Like reference numerals are used for like parts. The underdrain filter block  130  includes a plurality of grout chambers  132  defined by a grid-like structure  134  positioned on a bottom surface  136  of the underdrain filter block  130 . The bottom surface  136  of the block  130  defines a plurality of air relief openings  138  that extend into each of the grout chambers  132 . The grid-like structure  134  strengthens the block  130  to limit deflection or deformation of the bottom surface  136 . Further, the grout chambers  132  each receive grout in a similar manner as described above in connection with the grout chamber  35  of the underdrain filter block  10  shown in  FIGS. 1-5 . The air relief openings  138  allow air to evacuate to ensure that the grout chambers  132  fill with grout, which further stiffens and strengthens the block  130 . The block  130  may be installed in a similar manner as described above in connection with the block  10  shown in  FIGS. 1-5 . 
     Referring to  FIG. 12 , an underdrain filter block  150  according to a seventh embodiment is shown. The underdrain filter block  150  is similar to the underdrain block  110  shown in  FIG. 9 . The filter block  150  is formed from separate first and second pieces  152 ,  154 . The first piece  152  has a rectangle-shaped body formed by a top wall  156 , side walls  158 , and a bottom wall  160  with ribs  162  extending in a circumferential direction around the first piece  152 . The first piece  152  has a hollow interior and includes openings  164 ,  166  defined by the side walls  158 , and bottom wall  160 . The second piece  154  defines interior walls including a transverse wall  168  and an arched wall  170 . The arched wall  170  is secured to the transverse wall  168 . Although not shown, the second piece  154  (particularly, the arched wall  170 ) may include orifices for the flow of fluids as generally known in the art. The first and second pieces  152 ,  154  are formed integrally as single, unitary structures, although the first and second pieces  152 ,  154  may also be individually assembled from multiple pieces. 
     As shown in  FIG. 12 , the underdrain filter block  150  is assembled by inserting the second piece  154  into the first piece  152  with outer edges of the transverse wall  168  engaging grooves  174  defined by the inner surface of the side walls  158 . The second piece  154  is further secured to the first piece  152  via fasteners (not shown) that extend through the top wall  156  of the first piece  152  and engage bosses  172  on the top of the arched wall  170  of the second piece  154 . After the first and second pieces  152 ,  154  are assembled, the transverse wall  168  of the second piece  154 , the bottom of the side walls  158 , and the bottom wall  160  define a grout chamber as discussed above, for example, in connection with the underdrain filter block  10  shown in  FIGS. 1-5 . The transverse wall  168  and the arched wall  170  of the second piece  154  define a primary conduit. Similarly, the first piece  152 , the transverse wall  168 , and the arched wall  170  define secondary conduits. 
     Referring to  FIG. 13 , an underdrain filter block  180  according to an eighth embodiment is shown. The underdrain filter block  180  is similar to the underdrain block shown in  FIGS. 6 and 7 . The underdrain filter block  180 , however, is formed from separate first and second pieces  182 ,  184  to define a grout chamber. The first piece  182  has a rectangle-shaped body formed by a top wall  186 , side walls  188 , and a bottom wall  190 . The first piece  182  includes ribs  192  that extend in a circumferential direction around the first piece  182 . The first piece  182  also includes an interior arched wall  194  and an interior transverse wall  196  to define primary and secondary conduits. The second piece  184  includes a pair of rectangle-shaped side walls  198  secured to a rectangle-shaped bottom wall  201 . The second piece  184  defines openings  203  in the side walls  198  and openings  205  in the bottom wall  201 . 
     As shown in  FIG. 13 , the underdrain filter block  180  is assembled by attaching the second piece  184  to the first piece  182 . More specifically, the second piece  184  includes bosses  207  on the side wall  198  and is secured to the bottom wall  190  of the first piece  182  via fasteners  209  that extend through the bosses  207 . The side walls  198  of the second piece  184  define notches  211  to receive the ribs  192  of the first piece  182 . The bottom wall  190  of the first piece  182  may be pre-drilled or self-tapping fasteners may be utilized. After the first and second pieces  182 ,  184  are assembled, the bottom wall  190  of the first piece  182 , the side walls  198  of the second piece  184 , and the bottom wall  201  of the second piece  184  define a grout chamber as discussed above, for example, in connection with the underdrain filter block  10  shown in  FIGS. 1-5 . The first piece  182  defines primary and secondary conduits as discussed above in connection with the underdrain filter block  75  shown in  FIGS. 6 and 7 . 
     Referring to  FIG. 14 , an underdrain filter block  220  according to a ninth embodiment is shown. The underdrain filter block  220  is similar to the underdrain block  180  shown in  FIG. 13 . The underdrain block  220  is formed from separate first and second pieces  222 ,  224  to define a grout chamber. The first piece  222  has a rectangle-shaped body formed by a top wall  226 , side walls  228 , and a bottom wall  230 . The first piece  222  includes an interior arched wall  232  and an interior traverse wall  234  to define primary and secondary conduits. The second piece  224  includes a pair of rectangle-shaped side walls  236  secured to a rectangle-shaped bottom wall  238 . The second piece defines openings  240  in the side walls  236  and openings  242  in the bottom wall  238 . 
     As shown in  FIG. 14 , the underdrain filter block  220  is assembled by inserting the second piece  224  into the first piece  222  and attaching the second piece  224  to the first piece  222 . More specifically, the second piece  224  includes fastener holes  244  in the side walls  236  and is secured to the side walls  228  of the first piece  222  via fasteners  246  that extend though the sidewall  228 ,  236  of the first and second pieces  222 ,  224 . After the first and second pieces  222 ,  224  are assembled, the bottom wall  230  of the first piece  222 , the side walls  236  of the second piece  224 , and the bottom wall  238  of the second piece  224  define a grout chamber as discussed above, for example, in connection with the underdrain filter block  10  shown in  FIGS. 1-5 . The first piece  222  defines primary and secondary conduits as discussed above in connection with the underdrain filter block  75  shown in  FIGS. 6 and 7 . 
     Referring to  FIG. 15 , an underdrain filter block  260  according to a tenth embodiment is shown. The underdrain filter block  260  is similar to the underdrain block  90  shown in  FIG. 8 . The underdrain filter block  260  is formed from separate first and second pieces  262 ,  264  to define a plurality of grout chambers, which are discussed above in connection with the block  90  shown in  FIG. 8 . The first piece  262  is generally rectangle-shaped and is defined by a top wall  266 , side walls  268 , and a bottom wall  270 . The first piece  262  includes ribs  272  that extend in a circumferential direction around the first piece  262  and also extend from the bottom wall  270 . The second piece  264  is a plate-like rectangle-shaped body defining a plurality of openings  274  and a plurality of fastener holes  276 . As shown in  FIG. 15 , the underdrain filter block  260  is assembled by securing the second piece  264  to the bottom of the ribs  272  of the first piece  262  via fasteners  278 . After the first and second pieces  262 ,  264  are assembled, the bottom wall  270  of the first piece  262 , the ribs  272 , and the second piece  264  define a plurality of grout chambers as discussed above, for example, in connection with the underdrain filter block  90  shown in  FIG. 8 . 
     This invention has been described with reference to the preferred embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.