Abstract:
A walkway adjacent trees is made up of modular sections of perforated plates supported on hinged frames that are supported on spaced piers. Retaining edges of biodegradable material serve to maintain a shallow trench beneath the perforated plates into which leaves or mulch are periodically added. This provides a root-friendly zone under the walkway for tree roots that are watered through the perforated plates and may grow without damaging the walkway. A fringe of plantings maintains the trench level when the biodegradable material disintegrates.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to landscape architecture and to a structure and method for designing walkways and vehicle traffic areas that promote the growth of trees and other vegetation in close proximity thereto so as to enhance their overall utility and beauty. 
   Trees and adjacent hardscape surfaces are mutually incompatible. The hardscape surfaces, defined here as load-bearing, generally impervious, surfaces, such as concrete sidewalks, parking lots, and driveways that are necessary for the convenience of humans are usually supported on compacted soil or fill. Trees and similar vegetation, on the other hand, require nutrients and water, and send forth an extensive root structure in search of these elements that are essential to growth of the tree. The structural root system is destructive to the hardscape surface over time. The compacted soil beneath the hardscape stunts the growth of the root system and the growth of the tree. 
   There have been many proposals to solve the problem, which are unfriendly to tree root growth. These comprise “root barrier” systems that place an impenetrable, permanent shield between the tree roots and the subsurface beneath the hardscape. Exemplary of these “root barrier” systems are U.S. Pat. No. 4,019,279 issued Apr. 26, 1977 to Moorman and U.S. Pat. No. 5,070,642 issued Dec. 10, 1991 to Albrecht, which are cited to show well-known permanent root barrier or deflector systems protecting an adjacent, non-permeable hardscape. 
   A system that seeks to promote the growth of tree roots beneath an adjacent hardscape is seen in U.S. Pat. No. 7,080,480 issued Jul. 25, 2006 to Urban, et al. Urban discloses a structural cell system comprising a matrix of load-bearing cells of concrete composites or plastic stacked in horizontal layers with permeable barriers separating the layers into three groups. The cells are of an open design to allow water, soil and/or roots to penetrate. The center group has soil-filled cells while upper and lower groups are for short term and long-term water storage respectively. Tree root growth takes place in the center group filled with low compacted soil. The system is designed to capture storm water and pipe it to the upper and lower groups of stacked structural cells which serve to perform a load-bearing function to support the hardscape above. The system is designed to be installed under impervious hardscape such as sidewalk, parking or roadway pavement but may also be used under permeable hardscapes such as turf, planting beds or compacted soil. In either case, the hardscape is a permanent surface structure, requiring inspection ports, overflow pipes and soil injection pipes in order to reach the layers of structural cells below. Many layers of structural cells stacked on top of one another are required when using the aforementioned Urban system and there is no convenient way to augment the contents of the soil-filled root growing cells except through one or more soil injection ports. 
   A modified root barrier system is shown in U.S. Pat. No. 5,383,302 issued Jan. 24, 1995 to Hill, which describes a combination tree root disbursement system and tree root barrier system. An inner cylindrical structure with openings surrounds the root ball and an outer barrier structure is spaced from the inner structure to protect an adjacent concrete hardscape. The openings are designed to promote some horizontal growth through the openings. However, a permanent barrier prevents horizontal root growth beneath the adjacent non-permeable hardscape. 
   It would be desirable to have an inexpensive method and structure for controlling and enabling tree root growth adjacent a walkway or similar load-bearing surface. It would also be desirable to have a convenient way to periodically nourish the root growth zone with mulch, nutrients, loose loam, water and other elements necessary to healthy tree root growth. Furthermore, it would be desirable to plant trees adjacent a hardscape with an inexpensive root-friendly substructure beneath it that allows tree roots to gradually establish themselves over time, extending roots into the substructure. 
   Accordingly, one object of the present invention is to provide an improved method and structure for enabling tree root growth beneath adjacent load-bearing surfaces. 
   Another object of the invention is to provide a structure and method for periodically nourishing a tree root growth zone with elements necessary to healthy tree root growth. 
   Another object of the invention is provide a structure and method for laying out trees, walkways or parking areas with an inexpensive root-friendly substructure beneath the walkways or parking areas. 
   Another object of the invention is to provide a walkway with a root-friendly substructure to be located adjacent to trees that allows the tree roots to gradually establish themselves over time, extending roots into the root-friendly substructure. 
   SUMMARY OF THE INVENTION 
   Briefly stated, the invention in its simplest form comprises a structure for enabling tree root growth beneath a walkway adjacent to at least one tree having roots planted in soil, comprising a first row of spaced support piers disposed near said tree within reach of tree root growth and defining one side of the walkway, a second row of spaced support piers defining the other side of the walkway, a shallow trench extending along the walkway between the rows of piers, the shallow trench having sidewalls extending to a selected depth below the level of said soil, a retaining edge temporarily maintaining the selected depth, a cover member supported on selected piers and having a load-bearing surface covering a portion of the shallow trench, said load-bearing surface being permeable so as to allow water to percolate therethrough upon the soil, the cover member being movable so as to temporarily allow access for adding nourishing elements to the soil. 
   The invention also comprises the method of periodically removing the load bearing cover and adding a composting layer of leaves or mulch. 
   A preferred embodiment of the invention utilizes a walkway comprising a series of load-bearing, water pervious covers comprising perforated sheets supported by frames resting on the support piers. The frames are hinged so that they can be opened to receive leaves or mulch maintained at a selected level in a shallow trench beneath the perforated sheet. Biodegradable retaining edges of wood or similar material that will disintegrate over time serve as temporary sidewalls for the trench and maintain the level of the trench below ground level at the side of the walkway. Stabilizing plants line the walkway. After the roots of the stabilizing plants become established they maintain the difference in level between the trench and the surrounding soil. The tree roots grow into the space under the trench and are nourished from above by periodically opening the hinged covers and adding the leaves or mulch. Water is supplied by rainwater entering through the perforated sheets. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, both as to organization and method of practice, together with further objects and advantages thereof, will best be understood by reference to the following description, taken in connection with the accompanying drawings, in which: 
       FIG. 1  is an elevation view, in cross-section, of a prior art integrated tree root and storm water system, 
       FIG. 2  is a perspective elevation view, partly in cross-section of a prior art apparatus and method for growing strongly-rooted trees and preventing root damage to hardscapes, 
       FIGS. 3   a  and  3   b  are elevation views, partly in cross-section, of a tree adjacent a walkway constructed in accordance with the present invention, shown at earlier and later time periods, respectively, 
       FIG. 4  is a plan view of the walkway of  FIG. 3   a  according to the present invention, showing portions of connecting modular units, and 
       FIG. 5  is a perspective view of a modular walkway unit with soil omitted and with its hinged cover shown partially open, and 
       FIG. 6  is a perspective view of a modified modular walkway unit viewed from the underside of a modified cover member. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 and 2  of the drawing, prior art systems for controlling and enhancing root growth adjacent to a hardscape are shown, as taken from the Urban et al and Hill patents referenced above. 
   In  FIG. 1 , a structural cell system is disclosed for supporting hardscape areas in a way that enables tree root growth and accommodates filtering, retention, storage and infiltration of storm water while preventing hardscape damage. The system includes a plurality of structural cells (S) capable of being positioned below a hardscape (H). The structural cells are designed to have openings that are sized to accept tree roots. The system has one or more permeable barriers around the structural cells with a way for water to flow into the plurality of structural cells and to flow out of the structural cells. 
   In  FIG. 2 , a root disbursement element (D) is provided around the root ball of a tree, being adapted to direct some of the roots from the root ball downwardly while permitting others to grow outwardly. In those numerous situations where there is a hardscape near the tree, a root-control barrier (B) is provided between the root disbursement element and the hardscape, in spaced relationship from the root disbursement element. The root-control barrier directs downwardly those roots that emanate outwardly from the disbursement element, to a depth sufficiently great that damage to the hardscape is prevented. 
   Referring now to  FIG. 3   a  of the drawing a newly planted tree  10   a  is shown closely adjacent to a newly constructed walkway indicated generally at  20 . 
   The tree  10   a  is transplanted from a tree nursery and having a root ball  12  that is supported in surrounding compacted soil  14 . Newly established roots are shown at  16   a  extending beyond the root ball. 
   Walkway  20  is constructed by providing a first row  22  of support piers  24  disposed near tree  10   a  within reach of tree root growth and defining one side of walkway  20 , and a second row  26  of spaced support piers  24  defining the other side of the walkway. The rows  22 ,  26  are spaced laterally apart from one another and may run straight to be connected with curved sections or follow a sinuous path as desired to accommodate the functional or aesthetic architectural landscape plan desired. 
   The support piers  24  may be of any conventional construction suitable for bearing a load in compression. They may be of poured concrete as depicted with footings  25 , or alternatively of steel pipe set in footings, or alternatively can be steel pilings driven into the soil. The type of construction of the piers  24  is not material to the present invention. 
   Disposed on top of the piers  24  is a cover module, shown generally at  30 . The cover module is made up of a lower frame  32  supported on and attached to selected piers  24  by anchor bolts  33 , and a permeable cover member, shown generally at  34 . The permeable cover member  34  is, in turn, comprised of an upper frame  36  attached to a permeable load-bearing sheet  38 . The upper and lower frames  32 ,  36  are connected on one edge by one or more hinges  40 , so as to allow the permeable cover member  34  to open, as seen in dotted line position  42 . 
   Permeable load-bearing sheet  38  may be of any conventional material suitable for a walking surface, such as steel, aluminum, plastic, or wood treated to be weather resistant. The thickness of sheet  38  is chosen with regard to the construction of the underlying frames  32 ,  36 , so that it will carry the expected load, be it casual walkers or vehicular traffic. Sheet  38  is provided with spaced perforations  44 , which make the sheet permeable to air and water. 
   In accordance with the present invention, a shallow trench  46  is dug to extend along the walkway  20  between first and second rows  22 ,  26  of piers. Trench  46  has sidewalls between piers  24  that extend to a selected depth below the level of the soil on either side of walkway  20 . The preferred depth is on the order of 6 inches, but may range between 4 and 8 inches or more depending on the type of soil  14  below the trench and depending upon how often material is added, as will be later explained. 
   Lastly, retaining edges  48   a  and  50   a  are placed so as to extend along walkway  20  on the outer sides of rows  22 ,  26  respectively. The retaining edges are of a vertical height so as to extend into the soil  14  to a greater depth than that of the shallow trench  46  so that they will serve as temporary sidewalls for trench  46 . The retaining edges are of a thickness and of a material selected to be biodegradable over a pre-selected time period of 3 to 8 years, or about 5 years. A suitable material is conventional untreated (i.e., non-pressure-treated) lumber, such as fir or pine. Another suitable material is untreated interior plywood, particleboard or even multiple layers of corrugated cardboard. Also suitable are some plastics such as homoplastic material that will disintegrate over time. 
   The tops of piers are constructed so as to be substantially at ground level, so that when the cover member  34  is in place, there is a slight downward grade from the upper surface of the sheet  38  to the level of the surrounding soil  14 . In this area are planted varieties of stabilizing plants  52   a  that run along either side of walkway  20  to stabilize the soil near the retaining edges  48   a ,  50   a.    
   In accordance with the present invention, elements to nourish the soil  14  are periodically added into the shallow trench  46  beneath the perforated sheet  38 . Preferably such elements simply comprise leaves  51  from adjoining trees. This accomplishes a dual purpose of providing the rotting leaves as mulch for the underlying soil and also ridding the surrounding area of leaves. However, other nourishing elements added may comprise wood chips, leaf mulch or organic or inorganic fertilizers. 
   Referring now to  FIG. 3   b  of the drawing the same structures as in  FIG. 3   a  are shown, and which have the same reference numbers as in  FIG. 3   a , with the exception of those indicated below. A time period of about 5 years has passed. The retaining edges, now indicated as  48   b ,  50   b  have partially or totally disintegrated. Their function of maintaining a difference in level between that of trench  46  and surrounding soil has been replaced by the roots of stabilizing plants  52   b.    
   The tree is shown at  10   b  and has grown in size so that its roots  16   b  extend through the spaces between piers  24  and into the soil beneath the walkway  20 . The roots are nourished by the leaf mulch and by air and periodic rainwater percolating through the perforations in the sheet  38 . 
   Referring to the plan view of  FIG. 4 , the piers  24  are spaced along the trench so that piers support the corners of cover members  34 , as well as intermediate portions of the lower frame  32 . Portions of adjacent cover members are seen at  54  and  56 . In order to provide economical construction the cover members are preferably all of the same length, Similarly, if the walkway is curved, the cover members preferably have the same length and curvature. 
   Portions of the sheet  38  have been cut away to show that the upper frame  36  consists of longitudinal members  58  and cross members  60 . A recessed handle  62  is attached to a longitudinal member to assist in raising the sheet and upper frame on the hinges  40 . 
   Spanning the trench  46  and supported on piers  24  is the lower frame  32  comprising load-bearing cross members  64  and load-bearing longitudinal members  66  (see  FIG. 3   b ). Support frame  36  is preferably of structural steel, but could also be made of plastic or wood. 
   The cover member  34  is provided with a handle  62  that is attached to one of the upper frame longitudinal members  58  and recessed into the perforated plate. 
     FIG. 5  of the drawing is a perspective view of a single module  30  supported on  4  piers  24  with soil omitted so as not to obscure the construction. Cover member  34  is shown partly open, by using handle  62  to pivot the cover on hinges  40 . Temporary retaining edges are indicated at  48  and  50 . The dotted line  70  indicates the difference in level between the surrounding soil and the bottom of the shallow trench that is maintained by retaining edges  48  and  50 . 
   While the preferred embodiment employs hinges attached between upper and lower frame members, the invention also contemplates a cover member that is not attached to the lower frame member, but having projections received within pockets in the lower frame, the upper frame being maintained in place by its weight as it is raised. In such a case, the cover member may be completely removed and set to one side, if desired. 
   Modification 
   While the preferred embodiment of the invention uses a perforated sheet supported by a separate upper frame for the cover member  34 , it is also within the scope of the invention to substitute a plastic molding with a perforated plate and integral strengthening ribs beneath the sheet that perform the function of the upper frame.  FIG. 6  shows such a modified cover member partially opened along a walkway  74 . Walkway  74  is defined between a first row of spaced piers  76  and second row of spaced piers  78 . Longitudinal support beams  80 ,  82  are attached to the tops of the piers  76 ,  78  respectively. Between the piers is a shallow trench  84  dug at a lower level than the surrounding soil. Biodegradable retaining edges  86 ,  88  are seen, which maintain the level of the bottom of the trench below that of the surrounding soil. 
   Cover member  72  is preferably a plastic molding having perforated flat sections  90  reinforced by strengthening ribs  92  on the underside. The plastic material provides a suitable walking surface on the flat upper side when the cover  72  is lowered on hinges  94 . Adjacent module cover members are partially seen at  96 ,  98 . Plants  100  line the walkway and their root system maintains the intended difference in ground level between shallow trench and surrounding soil after the retaining edge biodegrades. 
   While the term “walkway” has been used throughout, the invention contemplates a broader interpretation of the term “walkway” to include a roadway or a parking area for vehicular traffic as well as for pedestrians. It will be apparent to those skilled in the art that this merely involves a more substantial structure for the cover member and supporting piers. 
   While there has been described what is considered to be the preferred embodiment of the invention, other modifications will occur to those skilled in the art. It is desired to secure in the appended claims all such modifications as fall within the true spirit and scope of the invention.