IMPROVED SURFACE FOR SPORTS AND OTHER USES

Abstract: An improved playing surface (100) including a synthetic turf base (104) positioned atop a foundation (102) is provided. The synthetic turf base includes synthetic fibers (110) secured to a knitted backing (800). The synthetic turf base also includes a surface layer of growth medium (118) applied on top of the knitted backing to reach a predetermined depth. Natural grass (120) is planted in the surface layer of growth medium. The knitted backing includes openings (116) sufficient in number and size to allow roots (124) of the natural grass and water to pass through so that the natural grass will grow to form a playing surface.

Description: IMPROVED SURFACE FOR SPORTS AND OTHER USES Field of the Invention The present invention is directed to an improved surface for sports and other uses and, more particularly, to a combination of artificial and natural surfaces supported by a knitted backing, and a method of making the same. Background of the Invention For years natural turf surfaces were used for most outdoor sports, for example, soccer, football, field hockey, cricket, rugby, etc. Natural turf surfaces are surfaces constructed with a grass grown in soil, or some other surface layer of growth medium (e.g., sand and organic mixes, etc.), that is constructed upon a suitable foundation. A natural turf surface is generally preferred for its comfort, feel, grip, and appearance. However, under heavy use and/or poor weather conditions, natural turf surfaces deteriorate rapidly and maintenance is costly. Intense activity on the turf destroys the grass and its root system, leaving mud and/or dirt on the playing surface. Prior to reestablishment of the turf, the surface is unsightly and often pockmarked, uneven, and possibly hazardous to use. Another problem associated with natural turf surfaces is the use of painted- on yardage and boundary lines. Typically, such boundary lines are formed by painting the playing surface. For aesthetic reasons, such lines are generally painted just prior to each official game played on the playing surface. The repeated application of paint to the surface of the playing surface tends to kill the grass that is located under the painted surface. In addition, over time the multiple layers of paint build up forming a surface that is substantially harder than the surrounding natural grass playing surface. -2- Thus, the painted areas of the playing surface can create slippery spots within the playing surface, possibly leading to slips and falls by athletes using the playing surface. In addition, the painted areas can increase the possibility of injury to the athlete and create hard spots that can be uncomfortable or painful to an athlete who falls upon the painted area. Due to the needs of sports programs, even after destruction of portions of the turf, play usually continues on the playing surface, even when the surface is badly damaged, until the sport's season is over, when the turf can be reestablished. Thus, the playing conditions on the playing surface continually decline over the season. At the end of the season, the natural turf surfaces are reseeded, the divots leveled and filled, etc. The natural turf surfaces are not usable during this reestablishment period because use defeats the reestablishment of the turf. The reestablishment period typically takes at least four months, or longer, under ideal weather conditions, during which the natural turf surface should not be used. Recently, synthetic surfaces have been used as an alternative to natural turf surfaces. Synthetic surfaces generally come in two types, i.e., conventional and sand filled. Conventional synthetic is a dense synthetic material that has the appearance of dense grass blades. Sand-filled synthetic is a synthetic material similar to conventional synthetic turf, but with greater spacing between the blades, to accommodate a silica sand filling. Both the conventional and sand-filled synthetics are placed indoors or outdoors, upon a foundation that may include an asphalt, concrete, wood, or other supporting subsurface along with cushioning mats, water drainage, and water irrigation. Although synthetic turf surfaces are more durable than well-established natural turf surfaces, they are only moderately successful for sports and other uses. The most notable disadvantages of synthetic turf surfaces are the discomfort for the players and -3- an increased number of injuries. Additionally, synthetic turf surfaces are generally expensive to put in place and have a life expectancy of 8 to 15 years, if properly maintained, and less if poorly maintained. Outdoor synthetic turf surfaces also remove large areas from the ecosystem, reducing natural processes including ground water recharge, oxygen and carbon monoxide balance, temperature modulation, and dust filtration. For these and other reasons, a number of synthetic turf surfaces are currently being converted back to natural turf surfaces. As can be seen from the above discussion, there exists a need for an improved surface for sports and other uses, and a method of making the improved surface, wherein the surface provides improved comfort and fewer injuries to the users, while being durable under heavy use and in poor weather conditions. The present invention is directed to fulfilling this need. Summary of the Invention The present invention provides an improved surface for sports fields and for other high-traffic uses. In one embodiment, the surface includes a foundation and a synthetic turf base that is positioned atop the foundation. The synthetic turf base is filled with a layer of growth medium, and includes synthetic fibers mounted in a permeable backing material. The synthetic fibers are constructed of a flexible, synthetic material and extend generally vertically upward from the backing material. Natural grass is planted in the layer of growth medium. The backing material is formed of a woven material. The woven material is provided with some in-plane stability by having the strands of the woven material fused along the edges of the synthetic turf base. The woven material may be formed of nonbiodegradable strands, or may be partially or completely formed of biodegradable strands. -4- In accordance with still other aspects of the invention, the backing material is formed of a knitted or multi-ply knitted material. One, two, or more layers of knitted material may be used together to provide in-plane stability, while at the same time providing a sufficiently porous backing to allow passage of water and grass roots. A secondary backing may be applied to the underside of the knitted backing to further secure attachment of the synthetic fibers to the knitted backing. A secondary backing may be formed of, for example, a needle-punched biodegradable material or a light application of a biodegradable or nonbiodegradable latex material. In one embodiment of the invention, the backing is a mesh having the synthetic fibers tufted into the backing. The backing may be formed of a nonbiodegradable material or may be partially or completely formed of biodegradable material. In various embodiments, the layer of growth medium may be filled to a level that is below, at, or above the top of a first or second group of fibers of varied lengths. Natural grass is planted in the growth medium so that the roots grow downward through the growth medium, through the backing material, and into the foundation. The blades of the grass grow upward from the top of the growth medium to form a playing surface. The playing surface of the present invention improves the durability and visual appearance of natural grass playing surfaces. The playing surface of the invention helps to protect the crowns of the natural grass plants, thus helping to prevent damage to the natural grass. This allows the present invention to produce a playing surface that recovers more quickly than natural grass playing surfaces. At the same time, the playing surface of the present invention is formed substantially of natural grass blades, thus providing the comfort and appearance of a natural grass playing surface. -5- The present inventor improves upon the invention described in applicant's prior patent applications PCT US93/05963 and PCT/US97/20095, both of which are incorporated herein in this entirety. Brief Description of the Drawings The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1A is a cross-sectional view of an embodiment of an improved surface of the invention, FIGURE IB is an enlarged cross-sectional view of a portion of the surface of FIGURE 1A; FIGURE 2 is a partial cutaway view showing the construction of the surface of FIGURE 1A; FIGURE 4 is a cross-sectional view of another embodiment of the invention, illustrating synthetic fibers having varying lengths; FIGURE 3 is a cross-sectional view of still another embodiment of the invention, incorporating a knitted backing; and FIGURE 4 is a top plan view of a knitted backing suitable for use in the embodiment of FIGURE 3. Detailed Description of the Preferred Embodiment An improved surface 100 according to the invention is illustrated in FIGURES 1 A and IB. The improved surface 100 includes a foundation 102 upon which is mounted a synthetic turf base 104. The foundation 102 is constructed of a subgrade 106 and a subbase 108. The subgrade 106 is constructed for providing a structural base. The subgrade 106 may -6- be formed of earth and rock existing on the site as is known in the art. Further, piping (not shown) may be placed in or on the subbase 108 to provide drainage and/or irrigation to the subbase 108. A subgrade for use with the invention may be readily provided by those skilled in the art. The subbase 108 is positioned atop the subgrade 106 and is constructed, among other things, to provide sufficient drainage of water from the improved surface 100 to the subgrade 106. The subbase 108 is further constructed to provide structural support to the synthetic turf base 104 and a growth medium, as will be discussed more fully below. The subbase 108 may be constructed of any combination of materials known to those skilled in the art such as, for example, sand, rubber, rock, and other organic and/or inorganic materials. Like the subgrade 106, the subbase 108 may be readily constructed by those skilled in the art. Referring additionally to FIGURE 2, a first filter (optional) 109 is positioned intermediate the subgrade 106 and the subbase 108 and a second filter (optional) 111 is positioned intermediate the synthetic turf base 104 and the subbase 108. The first and second filters 109 and 111 may each comprise a fabric or other material constructed to permit the flow of water from the synthetic turf base 104 to the subbase 108 and the subgrade 106, respectively. The first and second filters 109 and 111 are further constructed to restrict the flow of other substances (such as mud and other contaminants) between the synthetic turf base 104, the subbase 108, and the subgrade 106, i.e., to prevent such substances from moving downward or from migrating upward through the areas protected by the filters 109 and 111. Adequate filters 109 and 111 may readily be selected by those skilled in the art. It will be apparent to those skilled in the art that either the first or the second perforated -7- filter 109 or 111, or both, may be eliminated in some applications without departing from the spirit of the present invention, The present invention is intended, among other uses, for use as a sports playing field, in which case the subbase 108 atop the subgrade 106 will most likely be desired to ensure adequate support and drainage to the improved surface 100. As will become apparent to those skilled in the art, the present invention is also suitable for use as other surfaces, such as, for example, fire access and parking medians, home yards, parks, and virtually anywhere that a natural or artificial turf surface is desired. In such applications, the subgrade 106 and subbase 108 may be modified to suit the particular use to which the invention is being put. In some applications, it may be desirable to eliminate the subbase 108 altogether. However, it is generally desirable to provide some subbase 108 (or alternatively, a modified subgrade 106) upon which the synthetic turf base 104 can be supported. The synthetic turf base 104 includes a multiplicity of synthetic fibers 110 that are tufted, or otherwise secured, to a flexible, porous backing 112 having a plurality of openings 116. The synthetic fibers 110 are constructed of a synthetic material that is substantially flexible. The synthetic fibers 110 each extend generally upward from one side of the flexible backing 112 and are generally perpendicular to the flexible backing 112. The synthetic grass fibers 110 are of sufficient length that the top portions 114 are spaced upward from the flexible backing 112. In the embodiment illustrated in FIGURE 1 A, synthetic fibers that are approximately 28 millimeters in length and that have a density of approximately 12 synthetic fibers per 10 centimeters are preferred for some turf applications. However, longer or shorter fibers could be used with greater or lesser density depending on the contemplated use and site conditions. As an example, one alternative embodiment uses -8- synthetic fibers that are 33 millimeters in length and have a density of 13 synthetic grass fibers per 10 centimeters, and another uses synthetic grass fibers that are 15 millimeters in length and have a density of 8 to 10 synthetic grass fibers per 10 centimeters. In one embodiment, the synthetic turf base 104 is of a type similar to that used for sand-filled synthetic turf surfaces. However, it is preferable that the backing material have sufficient openings, and that the density of the synthetic fibers be such as to allow a dense growth of natural grass over the surface and through the backing material. The flexible backing 112 is provided for positioning the synthetic fibers 110 during installation, and maintaining the synthetic fibers 110 in position during use of the improved surface 100. Further, the flexible backing 112 provides structural support to the improved surface 100 by distributing the force of impact upon the improved surface, thereby helping to prevent compaction of the subbase 108. The flexible backing 112 includes a plurality of openings 116 through which grass roots and water may pass. The openings 116 may be holes punched or otherwise formed in the flexible backing 112, or may be provided to the flexible backing 112 by forming the same with a knitted material, as more fully discussed below. The synthetic turf base 104 further includes a surface layer of growth medium 118 positioned atop the flexible backing 112. In the embodiment of FIGURE 1 A, the surface layer of growth medium 118 fills the synthetic turf base 104 from the flexible backing 112 to a point proximate the top portions 114 of the synthetic fibers 110. However, after filling the synthetic turf base 104, the surface layer of growth medium 118 may settle slightly so that the top portions 114 of the synthetic fibers 110 extend slightly outward beyond the surface layer of growth medium 118. -9- The improved surface 100 further includes natural grass 120 that is planted in the surface layer of growth medium 118. The natural grass 120 includes a multiplicity of grass blades 122, each having a crown 123 (FIGURE IB) and roots 124 associated therewith. The natural grass 120 is planted in the surface layer of growth therewith. The natural grass 120 is planted in the surface layer of growth medium 118 so that the crown 123 is positioned just below the top of the surface layer of growth medium 118, as is known in the art. Preferably, natural grass seeds are sown in the surface layer of growth medium at a point where the crown will be located after the seeds germinate and the surface is established. Generally, it is desirable to position the crown 123 approximately one eighth to one quarter of one inch below the top of the surface layer of growth medium 118. However, those skilled in the art will appreciate that the crown 123 may be positioned at varying distances from the top of the surface layer of growth medium depending upon a variety of factors, e.g., the type of natural grass 120 and the composition of the surface layer of growth medium 118. Those skilled in the art will further recognize that, when planting varieties of natural grass that propagate by stolons, e.g., Zoysia, bentgrass, etc., sprigs or plugs of natural grass will be inserted into the surface layer of growth medium 118 so that the crowns 123 are positioned just below the top of the surface layer of growth medium 118. Constructed in this manner, the synthetic fibers 110 provide protection to the crowns 123, which is particularly advantageous during periods of heavy use and/or poor weather conditions. It will be further appreciated by those skilled in the art that, in the embodiments of the invention discussed herein, the synthetic fibers 110 help to protect the crown 123 and the roots 124 of the natural grass 120, thereby minimizing the time required for the grass blades 122 to regrow at times when the improved surface 100 is -10- being reestablished. The synthetic fibers 110 also provide a surface with a smoother, grass-like appearance, which may be played on during reestablishment. The presence of the natural grass 120 gives the improved surface 100 the comfort, feel, grip, and appearance of a conventional natural turf surface, while the presence of the synthetic fibers 110 protects the crown 123 and roots 124 to protect the improved surface 100 from deterioration. Referring to FIGURE 2, a description of a method of constructing one embodiment of an improved surface 100 according to the invention is described. Initially, the subgrade 106 is prepared by forming trenches 400 and adding drainage piping 482. Thereafter, the first perforated filter 109 is added atop the subgrade 106, if desired, and is followed by formation of the subbase 108. If the second perforated filter 111 is desired, it is placed upon the subbase 108 followed by the synthetic turf base 104, including a surface layer of growth medium 118. It should be noted that the filters 109 and 111 are not considered to be within the scope of the invention. Those skilled in the art will readily appreciate that the filters 109 and 111 are optional and one or the other or both may be omitted, depending upon the application. After the synthetic turf base 104 has been installed, the natural grass 120 (FIGURE 1 A) is planted so that the crowns 123 and roots 124 can form, as described above. As mentioned above, the subbase 108 is constructed to provide structural support to the synthetic turf base 104, as with conventional or sand-filled synthetic turf. Unlike conventional or sand-filled synthetic turf, however, the subbase 108 is further constructed to provide a growing medium for the roots 124 of the natural grass 120. The surface layer of growth medium 118 may comprise a variety of materials for supporting the natural grass 120. In one embodiment of the invention, the -11- surface layer of growth medium 118 is a mixture of sand and rubber particles. However, it will be apparent to those skilled in the art that the surface layer of growth medium may comprise a variety of materials for supporting the natural grass 120. Further, it is desirable to provide the surface layer of growth medium 118 in sufficient quantity to assist in stabilizing the synthetic turf base 104 and the synthetic grass blades 110. Providing approximately five pounds of growth medium for each square foot of the improved surface 100 has been found satisfactory for this purpose. However, more or less material may be provided in different applications. Also, it is desired to provide the surface layer of growth medium 118 in sufficient quantity to extend from about one half to one inch from the flexible backing 112, to provide a sufficient growing medium for the roots 124 of the natural grass 120 above the flexible backing 112. Due to the improved construction of the synthetic turf base 104 in combination with the natural grass 120, the selection of adequate materials for the surface layer of growth medium 118 is not as limited as with sand-filled synthetic turf surfaces. As an example, it is generally accepted that only rounded silica sand, a somewhat scarce and expensive sand, is suited for use with sand-filled synthetic turf surfaces. This is because regular sand is abrasive against the synthetic grass blades, and also because it tends to compact, reducing surface drainage and creating a hard surface that is less comfortable for the user. However, in the invention, the roots 124 of the natural grass 120 hold the surface layer of growth medium 118 in place, and, thus, regular sand may be used without increasing abrasion against the synthetic grass blades 110. Furthermore, since the roots 124 of the natural grass 120 provide some resiliency to movement in the synthetic turf base 104, compaction is reduced. Still -12- further, the grass blades 122 of the natural grass 120 protect the user from abrasion against the surface layer of growth medium 118. In some applications where natural grass characteristics are desired, it is advantageous that the top portions 114 do not extend substantially beyond the surface layer of growth medium 118. In such applications, the synthetic fibers protect the crown 123 and roots 124 of the natural grass. Since the root system of the natural grass 120 is protected by the synthetic blades, even when the blades 122 of the natural grass 120 are destroyed during play, they regrow quickly without the maintenance required for fully natural surfaces; replanting may not be required since the established roots 124 and crown 123 will regenerate new blades 122. Another advantage of the invention is that the grass blades 122 of the natural grass 120 shield sunlight from the synthetic fibers 110 of the synthetic turf base 104, substantially reducing the breakdown of the synthetic fibers 110 due to ultraviolet light. Further, the incorporation of the natural grass 120 with the synthetic turf base 104 reduces wear of the synthetic turf base 104, since the roots 124 of the natural grass 120 reduce the grinding action of the sand on the synthetic turf. This reduction in wear and ultraviolet light breakdown increases the expected life of the improved surface 100, thereby reducing the long-term cost of the surface. Referring to FIGURE 1 A the roots 124 of the natural grass 120 extend downward through the surface layer of growth medium 118, through the blade openings 508 of the synthetic blades 110, and through the openings 116 of the flexible backing 112. Providing blade openings 508 through which the roots 124 extend permits the roots 124 to assist in integrating the natural grass 120 with the surface layer of growth medium 118 and the synthetic grass fibers 110. Providing backing openings 116 through which -13- the roots 124 can extend permits the roots 124 to assist in integrating the synthetic turf base 104 with the foundation 102. It is desirable, therefore, to provide blade openings 508 and backing openings 116 adequate in size and number to permit the roots 124 to penetrate from the surface layer of growth medium 118 to the subbase 108. One method of providing backing openings 116 of adequate size and number is to form the backing from knitted material. The size and number of openings can also be increased by using biodegradable fibers to form such material so that, after the biodegradable fibers have deteriorated, the size and numbers of openings in the backing will increase to permit additional penetration of the roots 124 from the surface layer of growth medium 118 to the subbase 108. Those skilled in the art will appreciate that, by permitting the roots 124 to penetrate the backing openings 116 of the flexible backing 112, the roots 124 will provide integration of the synthetic turf base 104 to the subbase 108 beyond any conventional method for fixing the synthetic turf base 104 to the subbase 108. The roots 124 will interact with the openings to firmly fix the synthetic turf base 104 to the subbase 108, thereby preventing any movement of the turf base 104 across the subbase 108 or lifting of the turf base 104 at the edges. The flexible backing 112 is constructed from a knitted or multi-ply knitted material, and, as illustrated in FIGURE 3, the synthetic fibers 110 are tufted into a knitted backing 800. FIGURE 4 is a top view of one example of a knitted backing material 802 to be used in this embodiment. The knitted backing material 802 is knitted from fibers or threads, i.e., formed from intertwined fibers or threads in a series of connected loops. The knitted backing material 802 includes a plurality of openings 116 that are sufficient in number and size, to permit the ready passing of the natural grass roots and water. The rate of water passing through the porous knitted backing material 802 is greater by a predetermined amount than for woven backings. -14- One example of a suitable knitted backing material is sold commercially for use as knitted shade fabrics manufactured by Baycor Products Group of Nicolson Company in Norcross, Georgia. It should be understood, however, that knitted backing materials suitable for use in this embodiment are not limited to this example. Various other knitted backing materials may be used, as long as they are porous enough to allow the grass roots and water to pass through and, yet, are sufficiently strong to allow synthetic fibers to be tufted into or otherwise attached to the knitted backing. As illustrated in FIGURE 4, the knitted backing material 802 may suitably be formed of transverse single-strand fibers 804 knitted to longitudinal multistrand looped fibers 806. It should be understood, however, that a knitted backing material suitable for use in the present embodiment may be formed of single-strand fibers only, or of multistrand fibers only, or of a mixture of both, depending on the particular application. Furthermore, it may be advantageous to form a knitted backing material at least partially from biodegradable fibers, as discussed above, so that the knitted backing 800 becomes even more porous over time. Thus, the knitted backing material 802 may include all or a portion of biodegradable threads or fibers, or a biodegradable secondary backing as described below. The knitted backing of the present invention may utilize one or more layers of knitted material. Referring to FIGURE 3, a preferred embodiment of a knitted backing 800 utilizes two layers or plies of knitted backing material. A first knitted ply 802 overlies a second knitted ply 808. The two layers are used together in the preferred embodiment to provide sufficient structural strength to the knitted backing 800 to support the synthetic turf fibers 110 with a strong tuft lock, i.e., resistance to removal, as well as increased dimensional stability in the horizontal plane. Use of two or more layers of knitted backing material 802, 808 may be advantageous where one layer of knitted backing material is too -15- flexible to provide sufficient support for the synthetic turf base 104. To further provide in- plane stability to the knitted backing 800, one or more layers of knitted backing material 802, 808 may be fused or calendered using a heated roller adjusted in degree of temperature and pressure as desired, or the fibers could be alternately stabilized such as by adhesive or knotting, prior to or after assembly with other layers. When calendered, a knitted backing material becomes difficult to warp or deform during tufting from its original shape and, thus, the knitted backing 800 itself becomes difficult to warp or deform during tufting. This makes it easier to install and maintain the synthetic turf base 104 that is supported by the knitted backing 800. The present invention includes the use of a single ply that is fused or unfused (i.e., calendered or uncalendered), or two or more layers of fused or unfused material, or combinations thereof. When using two or more layers of knitted backing material to form a knitted backing, it may be advantageous to combine layers of knitted backing material having different degrees of porosity. While more porous knitted backing material provides sufficient openings to allow passage of the natural grass roots and water, less porous knitted backing material provides greater structural strength and possibly more tuft lock. Thus, a combination of two layers having different degrees of porosity suitably provides both of these advantages. For example, a combination of a layer having a fiber-to-opening area ratio of 6:4 with a layer having a fiber-to-opening area ratio of 4:6 has been found to be satisfactory in simultaneously achieving these two goals. To add further in-plane stability, one or more of the layers to be combined may be calendered prior to or after combination, as discussed above. For another application, it has been found that use of a single calendered layer having a fiber-to-opening area ratio of 7:3, or 8:2, is satisfactory in providing both enough porosity and sufficient strength. Those skilled in the art can easily find the best degree of porosity, or the best combination of various layers of knitted backing -16- material having the same or different degrees of porosity, depending on the particular application. After a knitted backing 800 is thus optimally formed of one or more layers of knitted backing material, synthetic fibers 110 are tufted into, or otherwise attached to, the knitted backing 800 during installation. In FIGURE, 3, synthetic fibers 110, which preferably are fibrillated, are illustrated to be tufted into two layers of knitted backing material 802, 808. In order to further secure the tufting, or attachment, of the synthetic fibers 110 to the knitted backing 800, i.e., to enhance tuft-lock, a secondary backing (not shown) may be applied on the underside of the knitted backing 800. The secondary backing helps to prevent the synthetic fibers 110 from falling out of the knitted backing 800 during installation. In addition, the secondary backing provides further structural support to the knitted backing 800. One example of a material suitable for forming a secondary backing is biodegradable latex material. For example, a material made with a medium molecular weight styrene-butadiene rubber (SBR) including 20% starch, 20% urea (nitrogen), and 20% calcium carbonate has been found to be satisfactory for this purpose. One such material is BSD-132 manufactured by Act Technologies, Inc. The starch and urea render this secondary backing at least partially biodegradable. A latex material, such as one described above, is sprayed, coated, or otherwise applied to the underside of the knitted backing 800 after the synthetic fibers 110 are tufted into the knitted backing 800. Specifically, it has been found that application of such material at a rate of 2 to 2.5 ounces per square foot over the underside of the knitted backing 800 is satisfactory in forming a secondary backing in accordance with this embodiment. The application of the secondary backing in this manner cooperates with the -17- knitted backing 800 to assist in positioning the tufted synthetic fibers 110 in place during installation and maintenance of the synthetic turf base 104. Further, the application of a secondary backing material tends to adhere two or more layers of knitted backing material together, to provide additional structural strength to the knitted backing 800. In addition, use of a biodegradable material to form a secondary backing helps the secondary backing to degrade over time, thus allowing more water passage and more dense growth of the natural grass roots through the knitted backing 800, and the secondary backing, over time. It should be understood that materials suitable for use as a secondary backing are not limited to the example described above. Specifically, a secondary backing may be formed of nonbiodegradable material, or may be partially or completely formed of biodegradable material. Materials other than latex material or other coatings may also be used to form a secondary backing. For example, a needle-punched material may be used as a secondary backing to underlie the knitted backing 800. In this case, a secondary backing formed of a woven material or a mesh or fleece is applied to the underside of the knitted backing 800 before the synthetic fibers 110 are tufted into, or otherwise attached to, the knitted backing 800. In order to further secure the synthetic fibers 110 to the knitted backing 800 and to the woven secondary backing, fleece may be needle punched through the woven backing prior to the tufting of the synthetic fibers 110, as discussed above. Furthermore, a needle-punched material may be partially or completely formed of biodegradable material, so that the needle-punched material may degrade over time to allow more water and the grass roots to pass through the secondary backing. In addition, a needle-punched material may be formed of different biodegradable materials that degrade at different rates to produce a greater degree of porosity in the secondary backing material over time. It should be understood that materials other than a needle-punched material, or -18- a latex material, may also be used to form a secondary backing, all of which serve to further secure the attachment of synthetic fibers 110 to the knitted backing 800. In all embodiments, the roots 124 of the natural grass 120 grow down into the surface layer of growth medium 118 and through openings in the flexible backing 112 into the subbase 108. One advantageous way of installing the improved surface 100 is to fabricate panels of sod that have been preplanted with natural grass as described above. The synthetic turf base may be manufactured in panels of a size readily stored, transported, and installed, e.g., of approximately 16 inches by 48 inches. The synthetic turf base may also be fabricated at a sod farm, cut, rolled, transported to site, and installed. During manufacturing, the synthetic turf base is filled with a surface layer of growth medium that is seeded, sprigged, or sodded to create a preplanted panel. While the preferred embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims: -19- The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. A playing surface comprising: (a) a foundation; (b) a synthetic turf located on top of the foundation including a knitted backing and a plurality of generally upright synthetic fibers mounted in the knitted backing; (c) a layer of growth medium disposed on top of the knitted backing of the synthetic turf to at least partially fill the fibers; and (d) natural grass plants having grass blades, crowns, and roots disposed in the growth medium, the roots extending downward through the growth medium, the knitted backing, and into the foundation, and the blades extending upward above the top of the layer of growth medium and above the tops of the synthetic fibers to form a playing surface. 2. The playing surface of Claim 1, wherein the knitted backing comprises two or more layers of knitted material. 3. The playing surface of Claim 2, wherein the two or more layers of knitted material have different degrees of porosity. 4. The playing surface of Claim 3, wherein one layer of knitted material defines a fiber-to-opening area ratio of approximately 6:4, and another layer of knitted material defines a fiber-to-opening area ratio of approximately 4:6, are used. 5. The playing surface of Claim 1, wherein the knitted backing defines a fiber-to-opening ratio of approximately 4:6 to 8:2. 6. The playing surface of Claim 1, wherein the knitted backing comprises a fused knitted material. -20- 7. The playing surface of Claim 6, wherein the knitted backing comprises a calendered knitted material. 8. The playing surface of Claim 1, wherein the knitted backing comprises a knitted material formed of interlocked loops of fiber. 9. The playing surface of Claim 1, wherein the knitted backing comprises a knitted material that is at least partially formed of biodegradable material. 10. The playing surface of Claim 1, wherein the synthetic fibers are tufted into the knitted backing. 11. The playing surface of Claim 1, further comprising a secondary backing applied on the underside of the knitted backing. 12. The playing surface of Claim 11, wherein the secondary backing at least partially comprises a biodegradable material. 13. The playing surface of Claim 12, wherein the biodegradable material forming the secondary backing comprises a latex-based coating. 14. The playing surface of Claim 11, wherein the secondary backing is formed of a needle-punched fleece material. 15. The playing surface of Claim 14, wherein the needle-punched fleece material forming the secondary backing comprises different biodegradable materials that degrade at different rates to produce a greater degree of porosity in the secondary backing over time. 16. A playing surface comprising: (a) a foundation; (b) a synthetic turf located on top of the foundation including a knitted backing and a plurality of generally upright synthetic fibers mounted in the knitted backing; -21- (c) a layer of growth medium disposed on top of the knitted backing of the synthetic turf to at least partially fill the fibers; and (d) natural grass plants having grass blades, crowns, and roots disposed in the growth medium, the roots extending downward through the growth medium, the knitted backing, and into the foundation, and the blades extending upward above the top of the layer of growth medium to form a playing surface. 17. A method of forming a playing surface, comprising the steps of: (a) providing a foundation; (b) placing a synthetic turf on top of the foundation, the synthetic turf including a knitted backing and a plurality of generally upright synthetic fibers mounted in the knitted backing; (c) applying a layer of growth medium on top of the knitted backing of the synthetic turf to at least partially fill the fibers; and (d) growing natural grass plants within the layer of growth medium, the natural grass plants having grass blades, crowns, and roots disposed in the growth medium, the roots extending downward through the growth medium, the knitted backing, and into the foundation, and the blades extending upward above the top of the layer of growth medium to form a playing surface. 18. The method of Claim 17, further comprising the step of applying a secondary backing on the underside of the knitted backing. 19. The method of Claim 18, wherein the secondary backing at least partially comprises a biodegradable material.

Inventors: BERGEVIN JERRY G, BERGEVIN, JERRY, G.
Applicants: TURF STABILIZATION TECHNOLOGIE, TURF STABILIZATION TECHNOLOGIES, INC.
Application year: 1999
Main classification: A01C1
IPC codes: A01G1