Patent Publication Number: US-8992131-B2

Title: Retaining wall

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/387,222 filed Sep. 28, 2010, and U.S. Patent Application No. 61/420,890 filed Dec. 8, 2010, both of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally directed toward retaining walls and freestanding walls. In particular, the invention is directed to modular retaining walls and to components of such walls. 
     BACKGROUND OF THE INVENTION 
     Retaining walls are used in landscaping around residential or commercial buildings. Retaining walls can be made of various materials, but for reasons of durability are most often either concrete structures cast in situ or walls formed of stacked courses of natural stone or masonry blocks. Concrete masonry blocks have become the most popular retaining wall components, due to their ease of manufacture, transport and handling. The blocks are stacked either manually or with the aid of machinery. 
     Conventional concrete masonry blocks are either wet cast or dry cast. In the dry cast process, a concrete mixture is filled into a mold box and compressed to generate a pre-consolidated block. This pre-block is removed from the mold box and transported to a setting location at which the block is stored for setting of the concrete mixture. Several methods have been developed to provide hollow dry cast blocks with a textured front surface. Molding a slab including several blocks and subsequently braking the slab into individual blocks allows for the creation of an irregular, rough front surface similar to the surface of a split natural stone. Such blocks are generally referred to as split face or hardsplit blocks. Alternatively, the smooth front surface of a finished molded block can be subjected to a percussive treatment, which brakes up and roughens the front surface. Finally, a three dimensional surface structure can be embossed into the front surface of the block during compression of the concrete mixture in the mold. 
     A retaining wall is also known from WO2008092237, which system includes base or wall blocks forming the actual retaining wall and decorative facing blocks or panels, which are mounted onto the wall blocks to form a decorative facing on the retaining wall. In that system, the wall blocks are of sufficient size and mass to perform the retaining function. They may even be able to support the facing blocks or panels. Although that system is very flexible, since the retaining wall can be provided with many different facing surfaces, which can even be exchanged without dismantling the wall, the base blocks suffer from the same drawbacks as other known retaining wall blocks. 
     The performance of retaining walls or freestanding walls is generally determined by the height of the wall, the overall mass of the wall and the width or thickness of the wall at the base, with the mass being the most critical. Local building code requirements dictate the forces such walls must be able to withstand, which in turn limit the design possibilities in terms of maximum wall heights for a given width and mass of a wall. Generally, the larger the mass and the width of the wall at the base, the base width, the higher the retaining capacity or resistance to tipping of the wall. More generally, the higher the mass, the higher the retaining capacity of the wall. This must be taken into consideration when building retaining walls of stacked blocks. In a conventional retaining wall of monolithic, stacked blocks, the wall blocks themselves must have a sufficient width to provide the minimum base width and mass required for the retaining wall. This in turn limits the maximum length and height of retaining wall blocks useful for manual installation. It also limits the overall retaining capacity achievable with conventional, manually installed, stacked block walls. As a result, retaining walls of higher retaining capacity are either cast in situ or made of large blocks which must be handled with often specialized machinery. The exposed length and height of an installed retaining wall block are normally referred to as the length and height of the block, while the remaining dimension of the block is referred to as the width of the block. To address the problem of excessive weight of conventional retaining wall blocks, hollow retaining wall blocks have been developed in an effort to reduce block weight and to thereby expand the size range of manually installed blocks. However, using hollow blocks reduces the overall mass of the stacked retaining wall and, thus, limits the retaining capacity of the wall achievable with hollow blocks. Thus, the height and retaining capacity of retaining walls made of conventional monolithic blocks for manual installation is limited, even if the blocks are sized for maximum retaining performance (optimum width) and maximum coverage (maximum length and/or height). 
     Conventional retaining wall blocks are often tapered towards the back to allow a curved placement of the blocks for the assembly of curved walls. In walls with convex curvature, the blocks then touch at the tapered sides, while in a straight line installation or in walls of concave curvature the blocks only touch at their front edges and comparatively large triangular gaps or spaces are defined between the blocks at the back. Those gaps are disadvantageous, since they reduce the overall mass of the wall and therefore the retaining capacity of the wall. 
     Modular retaining wall systems made of interconnected facing blocks and buried, spaced apart backer blocks are known from U.S. Pat. No. 4,068,482, U.S. Pat. No. 5,350,256, U.S. Pat. No. 5,468,098, U.S. Pat. No. 5,688,078, U.S. Pat. No. 7,503,729 and US200910041552. In those conventional walls, the wall of stacked facing blocks principally function as the principle material retaining component of the retaining wall, while the backer blocks have an anchoring function to reduce the tendency for tipping of the wall. The backer blocks are generally spaced apart and buried within the material to be retained and, thus, do not contribute to the mass and width of the wall. 
     Retaining wall systems including stacked blocks with interlocking projections for forming a hollow wall with front and back partial walls and intermediate connectors are disclosed in U.S. Pat. No. 4,490,075, U.S. Pat. No. 5,403,127 and DE 2549162. However, the connectors in those systems interlock with the blocks in the front partial wall in such a way that the ends of the connectors/spacers between the front and back partial walls are visible in the installed condition, giving the wall an artificial rather than natural appearance. 
     Thus, a modular retaining wall system which overcomes at least one of these disadvantages is desired. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the invention to provide an improved modular wall system for manually installed retaining walls. 
     This object is achieved by a retaining wall system including backer blocks, facing blocks, and connecting spacers or connectors for connecting the backer and facing blocks at a preselected spacing. In one embodiment, a modular retaining wall system is provided for the construction of a retaining wall having a preselected total mass, which system includes backer blocks for engagement with material to be retained and stackable into a continuous rear wall portion of a first mass; facing blocks to be exposed in the installed condition of the wall system and stackable into a continuous front wall portion of a second mass, a sum of the first and second mass being less than the total mass; and connectors for connecting each facing block with at least one backer block in a spaced apart back to back arrangement, the connectors having a length for forming between the front and back wall portions an intermediate hollow space for filling with a filler material of a third mass constituting at least the remainder of the total mass. 
     In another embodiment, a modular retaining wall system is provided for the construction of a retaining wall having a preselected height and total mass, the system including backer blocks for engagement with material to be retained; facing blocks to be exposed in the installed condition of the wall system; and connectors for connecting each facing block with at least one backer block in a spaced apart back to back arrangement, the backer blocks and facing blocks when connected by the connectors being respectively stackable into a continuous rear wall portion of the preselected height and a first mass and a continuous front wall portion of the preselected height and a second mass, a sum of the first and second mass being less than the total mass; and the connectors having a length for forming between the continuous front and back wall portions an intermediate hollow space for filling with a filler material of a third mass constituting at least the remainder of the total mass. 
     The backer and facing blocks are stackable into respective front and rear wall portions of the retaining wall, when connected by the connectors. In one embodiment, each wall portion has an insufficient width to function as a retaining wall itself. In another embodiment, the facing and backer blocks are even of insufficient width to respectively allow stacking into a front or rear wall portion of the selected height of the retaining wall. During assembly of the wall, the intermediate space between the backer and facing blocks is filled with loose filler material, such as earth, sand gravel, crushed stone, or the like to achieve a wall of a preselected mass. 
     The present inventors have surprisingly discovered that a reliable and effective retaining wall structure can be constructed using blocks, which are of insufficient width and mass to function as retaining wall or freestanding wall themselves and providing the remaining mass by way of a loose filler material in between front and back wall portions. Despite the filler material being loose, to enable filling of the intermediate space between the front and back wall portions, the inventors have surprising discovered that the finished retaining wall has the same retaining capacity as a solid wall of equal mass per unit length. The backer and facing blocks according to the invention have a small width and, thus, are much thinner and lighter than conventional retaining wall blocks of equal coverage (length×height). As a result, the wall blocks are much easier to handle and install manually. Of course, backer and facing blocks which are comparable in weight to conventional retaining wall blocks can be produced, which will then provide a much larger coverage than conventional blocks. 
     All blocks of the wall system of this application have a width, which is insufficient for the blocks to function as retaining wall blocks. Thus, to make the system functional as a retaining wall, two wall portions, made of the facing and backer blocks respectively, must be assembled and connected back to back by the connectors in order to form a hollow wall and the intermediate space must be filled with filler material to create a retaining wall of sufficient mass to reliably function as a retaining wall. The retaining wall system of this application is easily adapted to different building code requirements with respect to width and mass of the wall, without any changes to the backer or facing blocks being necessary. The base width of the wall can be adjusted by selecting connectors of different length. The mass of the wall consists of the combined mass of the wall portions and the mass of the filler material. The required total mass of the wall for a given retaining capacity is achieved by selecting a connector length which generates sufficient spacing between the front and rear wall portions so that, for a filler material of given density, the mass of the filler material makes up the at least the difference between the total mass and the combined mass. In order to allow filling of the hollow wall and avoid loss of the loose filler material from the wall, each partial wall must be continuous and free of gaps. That means the facing and backer blocks are stacked end-to-end in the front and rear wall portions to avoid a leaking of the filler material. 
     The backer and facing blocks are preferably cast concrete blocks, such as wet cast or dry cast concrete blocks. In this description, the terms cast concrete block, or cast block, are intended to include both wet cast and dry cast concrete blocks. In one embodiment, the facing blocks are cast blocks with a patterned, decorative surface. In another embodiment, the facing blocks are dry cast concrete blocks with an embossed decorative front surface, more preferably with an embossed, patterned decorative front surface. The facing blocks may also be constructed as cast concrete blocks with a veneer of natural stone attached thereto. 
     The facing blocks and backer blocks each have a front and back surface and are stacked in a back to back orientation in the form of first and second walls which are spaced apart connected by way of the connectors to form an overall hollow wall assembly. The connectors are preferably removably connectable to the back surface of the backer and/or facing blocks. Preferably, every facing block in the first wall is connected with at least one backer block in the second wall. The hollow wall assembly is then filled with a filler material of desired weight or density to achieve a retaining wall of a desired mass. 
     In another aspect, the invention provides a wall kit including at least a facing block having a back surface and a decorative front surface, at least a backer block having a back surface and a front surface, and a connector for connecting the facing and backer blocks in a back-to-back arrangement. 
     Preferably, each facing block and backer block has at least one retaining structure on its back surface, either in the form of a retaining recess in the back surface or a retaining protrusion protruding from the back surface and the connector has at least a pair of interlocking members each for engaging the retaining structure in one of the facing or backer blocks respectively, to connect the blocks in a back to back arrangement. The retaining recesses may be keyhole slots or dovetail slots and the connector preferably has a central web or rod with opposite, terminally positioned enlarged portions forming the first and second interlocking members respectively. Each interlocking member is preferably shaped and constructed for interlocking engagement with a retaining recess. In one embodiment, the retaining protrusions are dovetail shaped protrusions with an undercut for engagement by an interlocking member on the connector. However, any other construction of the retaining structures and interlocking members is possible which ensures reliable permanent or releasable interlocking of the interlocking members with the retaining structures. 
     In yet another aspect, the invention provides a modular wall system including individual stackable wall components in the form of the facing and backer blocks discussed above and connectors for connecting the wall components in a back to back arrangement. The facing and backer blocks may be of equal or different thickness and may have different lengths and widths. The facing and backer blocks preferably have the same base height or a multiple of the base height. The blocks of the preferred wall system all have graduated lengths, each length being a multiple of a base length or pitch which is preferably equal to a thickness or base width W of the facing blocks. Thus, the blocks may have lengths of 2W, 3W, 4W, 5W, 6W . . . . To facilitate the formation of walls with corners or ends, such as right angled corners, the back-to-back arrangement preferably has an overall thickness which is equal to a multiple of W. 
     The blocks of the modular wall system are stackable in rows and each include at least one retaining recess in a back surface and each connector preferably has a body and opposing first and second interlocking members for respectively engaging the retaining recess in one of the blocks for interconnecting the blocks in the back-to-back arrangement. The resulting hollow wall assembly is then converted into a retaining wall by filling the intermediate space between the back-to-back first and second walls with loose filler material such as crushed stone, gravel or soil, or setting materials, such as concrete. The retaining grooves in the facing and backer blocks are preferably spaced apart by 1W to facilitate connection of the blocks at a corner and for providing a preselected breaking point for the block at intervals of 1W. A special corner assembly can be used to reinforce the corner connection, or special corner connectors can be used. 
     The inventors of the present application have surprisingly discovered that providing a hollow wall made of relatively thin facing and backer blocks and filled with filler material can result in a retaining wall of much improved integrity and retaining capacity compared to walls made of stacked rows of full width blocks, since the filler material, especially crushed stone, not only provides added mass, but provides additional interlocking between the stacked rows of facing and backer blocks, which counteracts the problem of row displacement observed in retaining walls of stacked rows of monolithic blocks. 
     In an alternate embodiment, the length of the connectors is variable to permit the selection of a desired spacing between the first and second partial walls and, thus, of the overall wall width and mass. In another embodiment the spacing of the retaining recesses in the facing and/or backer blocks is selected to be less than W, to permit placement of fixed length connectors at an angle other than 90° to the wall and the blocks. 
     The wall in accordance with the invention can be built in situ, and preferably uses only the facing and backer blocks as wall components and their connectors. The connectors are preferably constructed with multiple connecting ends to engage at least a pair of blocks in a back-to-back arrangement. The connecting ends can be joined by interconnecting webs. The connectors are dimensioned to occupy as little as possible of the space between the back-to-back block walls to thereby maximize the amount of fill which can be placed in the space between the back-to-back blocks. The connectors are preferably constructed of a material which provides sufficient flexibility for interlocking engagement of the connectors with the blocks, even when the connector is not perfectly aligned with the complementary retaining structure in the block, while resisting longitudinal extension. Thus, the connectors are preferably flexible but non-extendible. 
     The wall of the present application can be assembled straight or curved. Curved hollow walls made of a pair of spaced apart parallel wall portions, provide the additional challenge that due to the curvature of the wall, the outer portion wall is longer than the inner portion wall, which leads to a mismatching of the blocks in the inner and outer portion wall of the curved hollow wall. Moreover, maintaining the inner and outer portion wall continuous is important for avoiding loss of the loose fill. Misalignment of the facing and backer wall portions in a curved wall also creates challenges with interconnecting the facing and backer wall portions, since the retaining structures in respectively opposing blocks are no longer aligned. This problem is addressed by providing one of the facing and backer blocks with retaining structures spaced apart by one pitch (1W), to allow for the assembly of a wall end or corner, and the other of the facing and backer blocks with retaining structures spaced apart by less than 1W, or by making the connectors of a dimensionally stable, but flexible material, or both. Dimensionally stable yet flexible means the connectors are flexible, to allow interconnection with retaining structures on the facing or backer blocks which retaining structures are not perfectly aligned with the connector, while maintaining a fixed length. In other words, the connectors are flexible but not extendible in length. The backer blocks may have rounded ends to ensure an end-to-end engagement of the backer blocks without intermediate gaps, even in curved installations. The backer and facing blocks may also have a T-shaped horizontal cross-section in order to facilitate the stacking of the facing and backer blocks in a curved arrangement. In a preferred embodiment, the facing blocks have vertical retaining grooves in their rear surface which are spaced apart by 1W and the backer blocks have retaining grooves which are spaced apart by ½W. Alternatively, all blocks can have retaining structures in the form of vertical grooves spaced apart by ½W. 
     In still another embodiment, the invention provides a kit for forming a wall. The kit includes a number of facing blocks with a total coverage area of X and a number of backer blocks having the same coverage area. Preferably, the kit includes X facing blocks, and an equal number of backer blocks and connectors for connecting the facing and backer blocks in a back-to-back arrangement. The facing and backer blocks when connected by the connectors are stackable into front and back wall portions, respectively. The blocks of each kit may be molded in a single mold frame to facilitate manufacture, packaging and transport. 
     In one embodiment, the modular retaining wall system further includes cap blocks for covering a top of the retaining wall. In this embodiment, the modular retaining wall system preferably further includes coping support connectors between a top row of the facing blocks and a top row of the backer blocks in the wall, the coping support connectors in addition to the pair of connecting ends including a support for supporting at least one of the cap blocks in the installed condition. 
     In another embodiment, the invention provides a method for assembling a retaining wall having a preselected height and total mass per unit length, comprising the steps of obtaining a plurality of facing blocks respectively having a back surface and a front surface, the facing blocks being cast concrete blocks with a patterned decorative front surface and a known mass; obtaining a plurality of backer blocks respectively having a back surface and a front surface, the backer blocks being cast concrete blocks having a known mass; determining a first mass per unit length of a wall of stacked facing blocks of the selected height; determining a second mass per unit length of a wall of stacked backer blocks of the selected height; determining a required volume of a filler material of known density needed per until length of the wall to provide a mass of filler material equal to at least a difference between the total mass per unit length and the sum of the first and second mass per unit length; stacking the facing and backer blocks in a back-to-back orientation to form a continuous front wall portion of facing blocks and having the preselected height and a continuous rear wall portion of backer blocks having the preselected height; during stacking of the facing and backer blocks, connecting the back surface of each facing block in the front wall portion with the back surface of a least one backer block in the rear wall portion with a connector for connecting the front and rear wall portions in the back-to-back orientation and forming an interior space for receiving the filler material, a length of the connectors being selected for the interior space having a volume at least equal to the required volume; and filling the interior space with the required volume of the filler material to form the retaining wall of the preselected total mass. 
     In another embodiment, the invention provides a method for forming a corner of first and second intersecting retaining walls in accordance with the invention. The method includes the steps of placing at least one of the backer blocks of the first wall at the corner within the interior space of the second wall. Preferably, the method includes the further step of placing at least one of the backer blocks of the second wall at the corner within the interior space of the first wall. Most preferably, the steps of placing the at least one backer block of the first wall and placing the at least one backer block of the second wall are carried out for each horizontal row of backer blocks. In a variant of the method, at the corner and in each row of backer blocks, the row of backer blocks of one of the first and second walls is continuous with the backer block placed within the interior space of the other of the first and second walls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be further described by way of example only and with reference to the attached drawings, wherein 
         FIG. 1  is a schematic top view of a modular wall as disclosed, including facing and backer blocks connected back-to-back to form a hollow retaining wall; 
         FIG. 2  is a perspective view of facing and backer blocks connected with a connector for use in a wall as disclosed; 
         FIGS. 3   a  and  3   b  are perspective views of the decorative wall of  FIG. 1  with facing and backer blocks connected in a back-to-back arrangement, and filled with gravel; 
         FIGS. 4   a  and  4   b  are perspective views of a different exemplary modular wall including different connectors, whereby  FIG. 4   b  shows the wall filled with gravel; 
         FIGS. 5   a  and  5   b  are front and rear views of the wall of  FIG. 4   a ; and  FIG. 5   c  is an end view of the wall of  FIG. 3   a.    
         FIGS. 6   a  to  6   e  illustrate different steps in the assembly of a modular wall as disclosed; 
         FIG. 7  shows a rod type connector for use in a modular wall as disclosed; 
         FIG. 8  shows a web type connector for use in a modular retaining wall as disclosed; 
         FIGS. 9   a  to  9   b  show different web type connectors and corner connectors for use in a modular wall as disclosed; 
         FIG. 10  shows a block with dovetail shaped retaining protrusions and a spring steel connector with clip shaped interlocking members for elastic and removable engagement with the retaining protrusions; 
         FIGS. 11   a  to  11   c  are front and rear perspective views of different backer blocks as disclosed; 
         FIGS. 12   a  to  12   d  are front and rear perspective views of embossed face and split face facing blocks as disclosed; 
         FIGS. 13   a  to  13   c  are a schematic illustrations of a mold frame arrangement for the molding of the facing and backer blocks for a wall kit; 
         FIGS. 14   a  to  14   f  show different retaining walls as disclosed including structures to create a setback for consecutive rows; 
         FIGS. 15   a  to  15   c  illustrate an end-to-end connection of the backer blocks; 
         FIGS. 16   a  and  16   b  illustrate the principle of vertically interlocking or connecting successive rows of facing or backer blocks; 
         FIGS. 17   a  to  17   c  illustrate the principle of supporting a coping or wall cap having a depth smaller than the wall assembly, using a specialized connector; 
         FIGS. 18   a  and  18   b  illustrate a specialized facing block and its incorporation into a wall as disclosed; 
         FIGS. 19   a  to  19   c  illustrate a decorative freestanding wall made with hardsplit facing blocks; 
         FIGS. 20   a  and  20   b  illustrate a wall system with facing and backer blocks of different sizes; 
         FIGS. 21   a  to  21   e  illustrate different orientations of the interlocking between the connectors and the blocks; 
         FIGS. 22   a  and  22   b  illustrate schematically the relationship between total mass of the retaining wall and the length of the connectors; 
         FIGS. 23   a  to  23   d  illustrate schematically a corner assembly for the retaining wall of the invention; and 
         FIGS. 24   a  to  24   d  illustrate a retaining wall with setback. 
     
    
    
     DETAILED DESCRIPTION 
     Before explaining the present invention in detail, it is to be understood that the invention is not limited to the preferred embodiments contained herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein are for the purpose of description and not of limitation. 
       FIG. 1  and  FIGS. 6   a  to  6   e  illustrate the method in accordance with the invention of constructing a modular wall  100 , such as a retaining wall, by connecting pairs of wall blocks, namely facing blocks  200  and backer blocks  300  in a back-to-back arrangement with an intermediate space filled with a filler material  500 . The facing blocks  200  have a decorative surface  210 , in the illustrated embodiment. Each facing block  200  is connected by way of connectors  120 , with at least one backer block. The facing blocks  200  and backer blocks  300  in the illustrated embodiment have rear faces  214  and  314  which are provided with a plurality of retaining structures, in this embodiment keyhole slots  102  for engagement by interlocking members of the connectors  120 . The preferred connectors  120 , which are discussed in more detail with reference to  FIGS. 8 and 9   a - 9   c  have at least a pair of spaced apart parallel, interlocking members  122  interconnected by an intermediate rod or web  124 . The interlocking members  122  each engage and are reliably held in a keyhole slot  102  provided in the rear face  214  or  314  of the wall blocks. The wall is preferably made of stacked wall blocks as illustrated in the attached Figures. For ease of use, the connectors  120  are preferably symmetrical, which means the interlocking members  122  are identical in cross-section and size, but non-symmetrical variants with interlocking members  122  of different diameter and cross-sectional shape can also be used. 
       FIGS. 12   a  and  12   b  illustrate an exemplary facing block  200  for use in a wall in accordance with the invention. The facing block  200  is a cast concrete block, preferably a dry cast block, which was compressed in the top to bottom direction during manufacture and has a front surface  212  and a back surface  214 . However, the facing block  200  can also have a split face front surface  212 , or an embossed decorative surface  212 , more preferably an embossed, patterned surface. In a facing block  200  provided with an embossed or patterned front surface  212 , the front surface is the top surface during molding. The facing block  200  has multiple spaced apart parallel keyhole slots  102 , in its back surface  214  (bottom surface during molding of a dry cast block). Each keyhole slot  102  has a slot portion  202  penetrating the back surface  214  of the facing block  200  and a cylindrical bore portion  206  connected thereto. The interlocking members  122  of the connectors  120  are respectively inserted into the keyhole slot bore portion  206  to mount the facing blocks  200  in a back-to-back arrangement with the backer blocks  300  (see  FIGS. 1 and 2 ). The facing block  200  is preferably sized and shaped to permit stacking into a continuous wall portion. However, the width of the facing blocks  200  is insufficient for the stacked facing blocks to function as a retaining wall. The width may even be so small that stacking the facing blocks into any wall is difficult without connecting them to backer blocks. The facing blocks  200  preferably all have a base width W and the keyhole slots  102  are preferably spaced apart by W or a multiple of W. 
       FIGS. 11   a  to  11   c  illustrate exemplary backer blocks  300  which may be used in a wall in accordance with the invention. In this example, the backer block  300  is a cast concrete block, preferably a dry cast concrete block, which was compressed in the top to bottom direction during manufacture and has a front surface  312  and a back surface  314 . Other types of cast concrete blocks may also be used, which may be manufactured in a standard mold frame or a big board mold. The backer block  300  of  FIGS. 11   a  and  11   b  has in its back surface  314  multiple spaced apart parallel retaining structures, in this embodiment keyhole slots  102 . However, retaining structures in the form of keyhole shaped recesses or keyhole slots  102  can be provided on the front and back surfaces  312 ,  314  of the backer block, as well as in the end surface  315 . Each keyhole slot  102  has a slot portion  202  penetrating the back surface  314  of the backer block  300  and a cylindrical bore portion  206  connected thereto (see  FIG. 11   a ). The interlocking members  122  of the connectors  120  are respectively inserted into the bore portion  206  to mount the backer blocks  300  in a back-to-back arrangement with the facing blocks  200  (see  FIGS. 11   c  and  1  and  2 ). The backer block  300  is preferably sized and shaped to permit stacking into a continuous wall portion. However, the width of the backer blocks  300  is insufficient for the stacked backer blocks to function as a retaining wall. The width may even be so small that stacking the backer blocks into any wall is difficult without connecting them to the facing blocks. 
     To facilitate the construction of curved walls, the backer block  300  preferably has shaped ends, such as rounded ends  310 , or stepped ends, which allow placement of the backer blocks  300  end to end and at an angle to one another without any spacing between the ends  310 . This means a curved wall made with the modular wall system of this application has a continuous back surface and no spaces or gaps, as in conventional retaining walls, which increases the overall mass and, thus, the retaining capacity and stability of the wall. In order to ensure that the backer blocks  300  can always be stacked to form a continuous wall and still each be connected to the facing blocks  200  by at least two connectors  120 , the backer blocks  300  preferably have a larger number of keyhole slots  102  than the facing blocks  200 . The spacing of the keyhole slots  102  in the backer blocks  300  may be less than the base width W of the facing blocks to facilitate the assembly of curved, continuous backer block walls. The spacing of the keyhole slots  102  in the backer blocks  300  may be ½W or less. This facilitates the stacking of the backer blocks  300  into a wall with no intermediate gaps or spaces, even in curved walls. Alternatively, the keyhole slots  102  in the backer blocks  300  may be spaced at W, or a multiple thereof, with the keyhole slots  102  and the facing blocks  200  being spaced at less than W, or ½W. In still another alternative, all keyhole slots  102  in all blocks are spaced at ½W. 
     The backer block  300  in its front surface  312  also preferably includes a set of vertical notches  330  to facilitate breaking of the block into smaller parts without the need for cutting equipment. As seen in  FIGS. 11   a  and  11   b , the notches  330  are preferably placed at ¼, ½ and ⅔ of the length of the block. Of course, the notches  330  can be placed at any desired location in the front surface  312 . The backer block  300  is preferably sized and shaped to permit stacking into a continuous wall portion. However, the width of the backer blocks  300  is insufficient for the stacked backer blocks to function as a retaining wall. 
       FIGS. 3   a ,  3   b ,  4   a ,  4   b ,  5   a  to  5   c  and  6   a  to  6   e  illustrate modular walls in accordance with this application and their method of assembly. The decorative facing blocks  200  and the backer blocks  300  are arranged spaced apart parallel with their back surfaces  214  and  314  facing one another. Connectors  120  are then inserted into the keyhole slots  102  to connect the facing and backer blocks in the back-to-back orientation. Each facing block  200 , preferably a facing block intended for providing a decorative finish on a wall or wall block, is provided with a decorative facing surface. The modular wall  100  is preferably made of a multitude of backer blocks  300  stacked in rows to form a rear wall portion  301  and a multitude of facing blocks  200  stacked in rows to form a front wall portion  201 , which wall portions are spaced apart parallel and connected in a back-to-back orientation by the intermediate connectors  120 . All of the backer blocks  300  and facing blocks  200  are of a width insufficient for the first or second portions wall to individually function as a retaining wall. The facing blocks  200  have a base width W and multiple parallel keyhole slots  102  which are spaced apart by W, whereas the keyhole slots  102  in the backer blocks  300  may be spaced apart by less than W. Preferably, for the facing blocks  200 , the spacing is W or a multiple of W and the spacing of the keyhole slots  102  in the backer blocks  300  is less than W preferably ½W. Keyhole slots  102  may also be spaced at ½W in both the facing and backer blocks  200 ,  300 . 
     In one embodiment, the invention provides a kit for forming a retaining wall. The kit includes X facing blocks  200  and an equal number of backer blocks  300  and connectors  120  for connecting the facing and backer blocks in a back-to-back arrangement, for forming a hollow retaining wall. The facing and backer blocks are all stackable for forming a wall portion, but are of insufficient width for the wall portion to form a retaining wall. The blocks of each kit may be molded in a single mold frame  400  as shown in  FIG. 13 , to facilitate manufacture, packaging and transport. Molding an equal number of facing and backer blocks in the same mold frame allows the stacking of the blocks produced from each frame as consecutive layers on a pallet, thereby giving the installer of the blocks always access to the right number of facing and backer blocks at all times during installation. Preferably, the facing blocks  200  are split face blocks and are molded in pairs and subsequently split. This allows the casting of 8 blocks in each standard frame  400 , two back-to-back facing block pairs  200   a  and four separate backer blocks  300 , while otherwise only 7 blocks of 7 cm thickness could be cast. 
     The interconnection of the back-to-back facing and backer blocks is preferably carried out on a row by row basis, as each row of facing and backer blocks is finished, so that the connectors need not be forced through the keyhole slots of more than one block. In the alternative, only the insertion of the connectors into one partial wall is done on a row by row basis. However, this will require moving facing blocks for the other partial wall along several connectors, which may increase the time required for installation of the complete wall. 
     Facing blocks of different sizes can be used in the same wall as shown in  FIGS. 20   a  to  20   c . As will be apparent from the drawings, in order to facilitate the close fitting of facing blocks of different sizes, the height of all facing blocks is a multiple of a base height H, normally the height of the smallest blocks. The length of the facing blocks is a multiple of the base width W of the facing blocks, in order to ensure a close fit of all blocks in corners or at ends of the wall. The base width and length of the backer blocks preferably follows the same rules. 
     Jumper blocks can be included in the wall, which are larger in size than the remaining blocks and possibly rotated by 90°. When jumper blocks of the same principle construction as the surrounding blocks are used, which are rotated by 90°, the facing block back-to-back thereto is preferably installed immediately after placement of the jumper block and before the rows of blocks around the jumper block are finished. Sliding of the facing block onto the connector in the jumper block may no longer be possible once the connectors of the adjoining blocks are installed, due to their orientation perpendicular thereto. However, where jumper blocks are used which have keyhole slots oriented 90° to those of regular blocks, installation of the facing block back-to-back onto the jumper block can be carried out in the ordinary course of installation since the slots in the jumper block are then parallel to those in the surrounding blocks. In addition, connectors can be used which have a pair of connecting members oriented at 90° to one another, which assists in connecting blocks that are rotated by 90° or blocks which have vertical and horizontal connecting recesses. For added stability of the decorative wall, the connectors can be inserted into the keyhole slots so that they each engage a pair of facing blocks in vertically adjacent rows of facing blocks and thereby not only connect the first and second walls, but also the stacked rows. The alignment of consecutive horizontal rows of blocks can be offset to the back in order to create a slightly backwardly slanted retaining wall. This can be achieved with the setback structures or connectors shown in  FIGS. 14   a  to  14   f , or  FIGS. 24   a  to  24   d.    
     The wall in accordance with the invention can be built in situ, and preferably uses only the facing blocks  200 , the backer blocks  300 , the connectors  120  and the filler material  500 . Connectors of different construction are illustrated in  FIGS. 7 ,  8 ,  9   a  and  9   b , and  24   a  to  24   d . The connectors  120  preferably all have the same basic construction with at least a pair of interlocking members  122  to engage at least a pair of blocks in a back-to-back arrangement and an intermediate connector body  124  in the form of a web or rod. The connectors can include multiple connecting members joined by multiple intermediate connector bodies  124 , such as interconnecting webs, for example oriented in a crossing arrangement to provide lateral stability to the back-to-back arrangement. The connectors  120  can be made of any material sufficiently strong to reliably connect the facing and backer blocks  200 ,  300  of the partial walls. The connectors are preferably made of any material which will be resistant to deterioration upon exposure to the elements, soil, gravel and the like. The most preferred material is plastic, although non-corroding metal alloys or metal connectors with a non-corroding surface finish can also be used. The exact construction of the connectors  120  and their connecting ends  122  can vary widely and can be achieved through machining of materials (such as bending and welding) or with molding techniques (such as injection molding or extruding). Although the form or shape of the connecting ends  122  can vary widely, they must be of sufficient size and/or of an appropriate shape to allow insertion into the bore portion  206  of the keyhole slot  102 , while preventing pulling of the connecting end  122  through the slot portion  202  of the keyhole slot  102 . For the assembly of curved walls, the connectors also are preferably constructed of a material which allows lateral flexibility of the connectors so that a misaligned insertion of the connectors into the retaining structures of the facing and backer blocks is possible, while ensuring longitudinal dimensional stability. In other words, the connectors are preferably flexible, but non-extensible. 
       FIG. 8  illustrates a rod type connector  120  in accordance with the application. The rod type connector includes a pair of connecting ends  122 , made of a bent rod with two or more undulations, welded to a rod shaped interconnecting body  124 . 
       FIGS. 9   a - 9   c  illustrate embodiments of an injection molded type connector  120  in accordance with the application, which is preferably of symmetrical construction to facilitate its use in the decorative wall of the invention in different orientations. The connector  120  includes a planar web  124  with opposite ends  125 ,  126  and a stem portion  122  at each of the ends. The stem portion  122  is preferably cylindrical, for interfacing with the keyhole slots  102  in the facing or backer blocks, but can be of any shape with allows engagement with the retaining recess in a facing or backer block and prevents the connector being pulled out of the retaining recess. Although the connectors  120  shown in  FIGS. 9   a  to  9   c  include interlocking members  122  in the form of generally cylindrical stems intended for being mounted to the facing blocks  200  by sliding them along the keyhole slots  102 , connectors with stems of different cross-section can also be used, the only requirement being that the stems have a shape and thickness which prevents the connection being pulled through the slot portion  202  of the keyhole slot in which it is engaged. Reinforcing flanges  128  are preferably provided on the web  124  and the interconnecting members  122  preferably have flexible or spring biased locking members  129  which lock the stems in the bore portion  206  of the keyhole slot  102  to maintain the connectors  120  stationary in the blocks until the hollow wall  100  is filled with the loose filler material. 
     Different types of injection molded or extruded corner connectors  127  are shown in  FIGS. 8 ,  9   a  and  9   b . The extruded corner connectors  127  are especially economically manufactured. All corner connectors  127  have at least two interconnecting members  122  and an interconnecting body  124  which may include multiple webs  130  and reinforcing flanges  128 . Furthermore, connector and retaining groove combinations other than those particularly exemplified can be used without deviating from the present invention. For example connectors of the snap in type can be used (see  FIG. 10 ). Although corners can be formed in the modular retaining wall of this application by using these corner connectors, a different corner assembly method, which does not involve the use of specialized connectors is also part of this invention and will be described further below with reference to  FIGS. 23   a  to  23   d.    
     The keyhole slots  102  in the facing and backer blocks  200 ,  300  will now be discussed in more detail with reference to  FIGS. 1 ,  11   a  to  11   c  and  FIGS. 12   a  and  12   b . Each keyhole slot  102  has a slot portion  202  penetrating the rear surface  214 , of a facing block  200  or the rear, front or end surface  314 ,  312  or  315  of a backer block  300  and a cylindrical bore portion  206  connected thereto. The cylindrical bore portion  206  is sized and shaped for receiving one of the interconnecting members  122  of the connectors. The slot portion  202  is sized and shaped for receiving the interconnecting body  124  of the connector  120 , the width of the slot portion  202  being less than the size (diameter) of the connecting end  122  in order to prevent the connector  120  being pulled out of the keyhole slot  102  through the slot portion  202 . For maximum flexibility in connecting the facing and backer blocks  200 ,  300  to one another, the blocks  200 ,  300  preferably have at least a pair of keyhole slots  102  in the rear surface  214 ,  314 . When multiple keyhole slots  102  are provided, the slots are preferably parallel and equidistantly spaced on the rear surface  214 ,  314  of the facing and backer blocks  200 ,  300  or the front surface and end surface  312 ,  315  of the backer blocks. The slots are preferably oriented vertically or horizontally and centered on the blocks when in the installed condition. Although other orientations of the slots are possible those orientations may make assembly of the decorative wall more challenging. The keyhole slots  102  preferably extend completely across the rear surface  214 ,  314  of the facing and backer blocks  200 ,  300  or the front or end surface  312 ,  315  of the backer blocks. Facing and backer blocks  200 ,  300  with retaining structures in the form of recesses or keyhole slots  102  which extend vertically in the installed condition of the blocks are shown in  FIGS. 12   a  to  12   d . Backer blocks  300  and facing blocks with retaining recesses extending horizontally in the installed condition of the blocks are shown in  FIGS. 11   d  and  12   e  respectively. 
     Of course, it will be readily apparent to the art skilled person that a retaining structure other than keyhole slots can be provided in the blocks  200 ,  300  as long as a reliable interlocking engagement between the retaining structure and the connectors respectively used is ensured. For example, the retaining structure can be in the form of a slot or bore and the connector can be a compressible/expandable connector, which is insertable into the slot or bore and locks in the slot or bore when fully inserted in order to reliably retain the connector in the slot. Alternatively, the retaining structures can be dovetail shaped slots and the connectors can have complementary connecting ends, or vice versa. In yet another alternative, the retaining structure is a protrusion  150  on the rear surface  214 ,  314  of a facing or backer block  200 ,  300  as schematically illustrated in  FIG. 10 . The illustrated protrusion  150  is dovetail shaped for engagement with connector  120  provided with clip shaped connecting ends  122 . 
       FIG. 1  schematically illustrates an exemplary corner arrangement of a modular wall in accordance with the invention, wherein an end of the wall is formed with facing blocks  200 . As is apparent, the facing blocks  200  are stacked to form the corner and special corner connectors  125  are used. The corner connectors  125  can extend diagonally as shown in  FIG. 1  or be L-shaped and extend along the corner as shown in  FIGS. 6   a  to  6   i.    
     The facing blocks  200  are preferably provided with a bevel or step at their lateral ends in order to allow for a closer fit of the facing blocks in curved wall applications (see FIG.  1 ). The curvature of the wall can then be adjusted by using facing blocks of different length, longer blocks being used in the outer partial wall of the decorative wall. However, the same effect can be generated with stepped ends, or blocks with a T-shaped cross-section in horizontal cross-section. Generally, the shorter the blocks, the tighter the radius that can be created. 
       FIGS. 14   a  to  14   f  show different retaining walls as disclosed including structures to create a setback for consecutive rows.  FIGS. 14   c  to  14   f  illustrate the use of setback plugs  132  which are inserted into the keyhole slots  102  of the consecutive rows of backer blocks  300  to create a backward setback  135  (see  FIGS. 14   b  and  14   d ) of consecutive rows. This setback  135  is achieved in the embodiment of  FIGS. 14   a  and  14   b  by providing each backer block  300  with a downwardly extending setback nose  320  at a bottom edge of the front surface  312  of the block. The setback  135  is achieved in the embodiment of  FIGS. 14   c  to  14   f  with a setback plug  132  having a Z shaped body  133  having a first leg  137  for engagement of the rear surface  314  of a first backer block  300   a  and an offset second leg  138  for engagement of the rear surface  314  of a second backer block  300   b  stacked on top of the first backer block  300   a  and a central web  139  creating the offset between the legs  137 ,  138  and, thus, the offset  135  between successive rows of backer blocks  300 . The setback plug  132  further includes an interlocking member  134  for engagement of the keyhole slot  102  and may also include a stiffening web  136  for support of the second leg  138 . 
     To ensure a proper end-to-end placement of the backer blocks  300  and to reliably form a continuous rear wall portion of backer blocks, the ends of the backer blocks  300  can be connected by end connectors  140  inserted into keyhole slots  103  provided in the end surfaces  315  of the backer blocks  300 , as illustrated in  FIGS. 15   a  to  15   c . The term continuous wall portion as used here refers to a wall portion made with stacked blocks (facing or backer blocks) which are stacked end-to-end with little or no intermediate spacing so that loss of the loose filler material in the intermediate space between the front and rear portion walls is prevented. It is understood that the finer the filler material the tighter the required end-to-end fit of the blocks. 
       FIGS. 16   a  and  16   b  illustrate different principles of vertically interlocking or connecting successive rows of facing or backer blocks. Connecting studs  160  can be used which have a generally cylindrical body  162  for insertion into the bore portion  206  of the keyhole slots  102  of vertically adjacent blocks  200 ,  300 . A central flange  164  on the body  162  is sandwiched between the vertically adjacent blocks in the installed condition, which prevents sliding of the connecting stud  160  in the bore portion  206 . 
       FIGS. 17   a  to  17   c  illustrate the principle of supporting a coping or wall cap  360  having a depth smaller than the wall assembly, using a specialized connector  340 . 
       FIGS. 18   a  to  18   b  illustrate special facing blocks  200   b  and  200   c  for use in a wall in accordance with this application. The facing block  200   b  can be of natural or synthetic material, such as wood, steel, stone, etc., but is preferably a slab of natural stone which has a front surface  212   a  and a back surface  214   a . The facing block  200   b  has multiple spaced apart parallel dovetail shaped retaining slots  102   a  cut into its back surface  214   a . Each retaining slot  102   a  receives a connector  180  with a dovetail shaped protrusion  182  to engage the retaining slot  102   a  and a keyhole slot  102  for receiving the connecting portion  122  of a connector  120 . The keyhole slot has a slot portion  202  and a cylindrical bore portion  206  connected thereto. The facing block  200   b  is preferably sized and shaped to permit stacking into a continuous wall. However, the width of the facing blocks  200   b  is insufficient for the stacked facing blocks to function as a retaining wall. 
       FIGS. 19   a  to  19   c  illustrate a decorative freestanding wall made with hardsplit facing blocks. 
       FIGS. 20   a  and  20   b  illustrate a concrete panel wall system with facing and backer blocks of different sizes. 
       FIGS. 21   a  to  21   e  illustrate different orientations of the interlocking between the connectors  120  and the blocks  200 ,  300 , wherein the connectors can have interlocking members  122  at opposite ends of the connector body  124  which are oriented at 90° to one another. The keyhole slots  102  in the facing blocks  200  and/or the backer blocks  300  can be extending in horizontal or vertical direction in the installed condition of the blocks. 
     The invention also provides an assembly method for assembling a modular retaining wall in accordance with the invention the wall a preselected height H and total mass per unit length.  FIGS. 22   a  and  22   b  respectively show retaining walls of different height and mass, made of identical facing blocks  200  and backer blocks  300 , but using connectors  120  of different length and different amounts of the same filler material. The mass of the different walls is determined solely by the length of the connectors and the amount of the filler material. The method includes the steps of obtaining a plurality of the facing blocks  200 , each having a known mass, obtaining a plurality of the backer blocks  300 , each having a known mass, stacking the facing and backer blocks in a back-to-back orientation to form a continuous front wall portion  201  of facing blocks and having the preselected height H and a continuous rear wall portion  301  of backer blocks having the preselected height H, connecting the front and rear wall portions  201 ,  301  during stacking of the facing and backer blocks by connecting the back surface  214  of each facing block  200  in the front wall portion  201  with the back surface  314  of a least one backer block  300  in the rear wall portion  301  with a connector  120  for connecting the front and rear wall portions in the back-to-back orientation for forming an interior space for receiving a filler material  500  of known density, and filling the interior space with the filler material. The filler material is a loose filler material loose filler material, such as earth, sand gravel, crushed stone, or the like, which can be easily poured into the intermediate space and have a known density. Most preferred are free-running materials, such as gravel, crushed stone, or the like to reliably and completely fill the intermediate space. 
     In order to achieve a preselected total mass, the method of the invention includes the further steps of determining a first mass per unit length of the front wall portion  201 , determining a second mass per unit length of the rear wall portion  301 , determining a required volume of the filler material  500  needed per until length of the wall to provide a mass of filler material equal to at least a difference between the total mass per unit length and the sum of the first mass and second mass per unit length, and selecting the length of the connectors so that the interior space has a volume at least equal to the required volume. With this method, retaining walls of any desired height and mass can be achieved, always using the identical facing and backer blocks components which can be installed manually. More importantly, this method allows the construction of retaining walls of a height and mass previously not possible with manually installed monolithic retaining wall blocks, whether solid or hollow. 
     In one embodiment of the method, facing blocks are used which are cast concrete blocks with a back surface and a patterned decorative front surface, preferably dry cast concrete blocks with an embossed, patterned decorative front surface. In this embodiment, the backer blocks are also cast concrete blocks, preferably dry cast concrete blocks. 
     In another embodiment, the invention provides a method for forming a corner assembly in a modular retaining wall in accordance with the invention, as will be discussed in more detail in the following with reference to  FIGS. 23   a  to  23   d . The term corner in this context defines an area of intersection or overlap between a pair of first and second intersecting walls, which meet at a point. In the illustrated corner assembly which includes the first and second intersecting walls  100   a  and  100   b , each intersecting wall is built in accordance with the invention and has facing blocks  200 , backer blocks  300  and interconnecting connectors  120  to define an intermediate space I for filling with loose filler material (not shown for illustration purposes). For the formation of the corner assembly, the method includes the steps of placing, at the corner, at least one of the backer blocks of the first intersecting wall within the intermediate space of the second wall. Preferably, the method further includes the step of placing, at the corner, at least one of the backer blocks of the second wall within the intermediate space of the first wall. 
     In one embodiment of the corner assembly method, the steps of placing the at least one backer block of the first wall and placing the at least one backer block of the second wall are carried out for each horizontal row of backer blocks. 
     In another embodiment of the corner assembly method, in each row of backer blocks, the row of backer blocks of one of the first and second walls is continuous with the backer block placed within the intermediate space of the other of the first and second walls at the corner. This is illustrated in  FIGS. 23   a  and  23   b , wherein one of the intersecting walls has a continuous row of backer blocks (circled area) which extends all the way to the back surface of the facing block row in the other intersecting wall. The row of backer blocks which is continuous at the corner is preferably alternated between the first and second intersecting walls for consecutive horizontal rows of backer blocks, as illustrated in  FIGS. 23   c  and  23   d . In order to avoid special interference between the intersecting connectors  120  from the first and second intersecting walls at the corner, the connectors are either offset in height so that the connecting ends  122  respectively engage consecutive rows of facing and backer blocks, or special connectors  120   b  are used which can be broken in half. Such a connector  120   b  is shown in  FIG. 24   b , which connector can be split by bending along the connecting tabs  120   a.    
       FIGS. 24   a  to  24   d  illustrate a retaining wall with setback, wherein the setback is achieved similar to the manner illustrated in  FIGS. 14   a  to  14   f , except that the setback or offset between consecutive rows of facing and backer blocks is achieved not with a separate setback plug, but with a connector  120  including a setback leg  129  integrated into that end of the connector intended to interlock with the backer block. In the installed condition of the connector as illustrated in  FIGS. 24   c  and  24   d , the connector is interlocked with a first backer block  300   a  and the setback leg  129  engages the rear surface  314  of a second backer block  300   b  stacked on top of the first backer block  300   a.    
     While the invention has been described with a certain degree of particularity, it is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification, but is to be limited only by the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled. 
     The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.