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BACKGROUND OF THE INVENTION 
     The present invention relates to a method of leveling an existing concrete slab which has had portions settle into the ground so as to become uneven over time. More specifically, to a method of carefully raising a section of the sunken slab so as not to damage the section and allow for the injection of pressurized mason&#39;s sand into the cavity created between the bottom of the uneven slab and the settled ground. 
     Regardless of the care and skill used in the initial construction, concrete slabs tend to become misaligned over time due to different rates of settlement of the earth. Uplift from freeze/thaw cycles or tree root lifting are also common causes of slab misalignment. These problems cause cracks in the slab to develop and can also cause step-like structures to occur between sections of the slab. The end result of this condition is the creation of hazards to users and liability for those who are responsible for their care. Additionally, the uneven slabs are extremely difficult to clear of snow and ice during the winter months in the northern areas of the United States, thus creating further hazards and liabilities for their users and owners. 
     In the past, there was a number of ways these problems were solved. One of these was to completely remove the damaged section of the slab and then re-pour it. The problem with this method is that although it works very well, it is time consuming and very expensive. The re-pour method also results in a checkerboard looking slab as the new portions are often a very different color from the older weathered sections. Another method that has been used with the step formation problem is to construct concrete or tar ramps from the lower section of the slab to the upper. The problem with this method is that it still leaves uneven slab surfaces that are hazardous and difficult to maintain. 
     Finally, another method that is often used is mud jacking. In this repair method a hole is drilled through the uneven slab and wet mud is pumped under the slab until the slab becomes level. The main problem with this method is that it may be difficult to effectively level a slab as the mud will settle over time while drying. 
     Therefore, from the foregoing discussion it can be clearly seen that it would be desirable to provide a means of leveling existing large slabs in an inexpensive manner that is easily used. 
     SUMMARY OF THE INVENTION 
     It is the primary objective of the present invention to provide a method of repairing driveways or other similar concrete slabs that have become uneven and damaged due to settling or other changes in the elevation of the earth upon which they are built. 
     It is an additional objective of the present invention to provide such a method of repairing driveways or other similar slabs which is economical and efficient in operation. 
     It is a further objective of the present invention to provide such a method of repairing sidewalks that is highly transportable and that can be easily operated. 
     These objectives are accomplished by the use of a portable high volume air compressor that is connected to a sand storage tank by the use of a high pressure air line. The connection at the sand storage tank is accomplished through an air manifold which is primarily a splitter mechanism which allows the single line of air flow from the air compressor to be diverted to a number of possible different paths. One of these possible paths is through the manifold bleed line which extends up over the sand tank and serves the purpose of providing a mechanism by which any excess or unneeded air pressure existing at the manifold can be vented from the system. The manifold also provides the point of attachment for the manifold pressure gauge which allows the user to closely monitor and control the air pressure within the system. 
     The tank manifold also provides the point at which the fl compressed air is diverted to the remaining components of the present invention. The air travels from this point through the venturi line to the venturi chamber where it first comes into contact with the mason&#39;s sand that is exiting the storage tank through the sand outlet. This sand may typically be a well graded, dry, mason&#39;s sand so as to provide optimal compaction. The inflow of highly compressed air into the venturi chamber tends to swirl the sand around inside the mixing chamber which ensures that the flow of compressed air will carry its maximum volume of sand. From the mixing chamber, the mixture of sand and pressurized air passes through the mix chamber venturi which has the effect of increasing the speed and pressure exerted by the sand and air mixture entering the air/sand line. 
     From this point, the air and sand mixture travels the length of the air/sand line until it gets to the injector gun located at its terminus. The injector gun is the component of the present invention which directs the flow of air and sand into the desired location under the sidewalk slab and is equipped with an external gauge for monitoring pressure at this point and also a bleed off valve. Additionally, the injector gun is manufactured with easily replaceable components as the steady flow of the air and sand mixture through the gun is extremely abrasive which tends to wear the components quickly. 
     These components of the present invention function together to facilitate the repair of a damaged sidewalk in the following manner. First, once the targeted section of slab has been identified, the operator drills a hole through the slab in a position that is roughly in its center or strategically placed. Once this has been accomplished, the nozzle portion of the injector gun is forced into this hole which forms an air tight seal between the gun nozzle and the sidewalk slab. The operator then opens and closes the post manifold valve in quick successive bursts which forces corresponding bursts of air and sand into the space below the slab. The air pressure in the system is sufficient so that these short bursts will actually lift the sidewalk slab off of the ground which will slightly increase the size of the cavity between the lower surface of the slab and the settled ground. This cavity is then partially filled in by the sand being carried by the air before the slab drops back. The operator simply repeats this process until the settled slab is at the same level as the remaining sidewalk. Finally, the gun nozzle is removed from the hole which is then patched thus, completing the repair of the damaged slab. 
     For a better understanding of the present invention reference should be made to the drawings and the description in which there are illustrated and described preferred embodiments of the present invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the present invention which illustrates the manner in which its individual components are connected together. 
         FIG. 2  is a front elevation view of a the sand storage tank component of the present invention and illustrates the method of construction and orientation of the air manifold and the air/sand mixing venturi. 
         FIG. 3  is a front elevation cut-away view of the air/sand mixing venturi component of the present invention illustrating the manner in which pressurized air is introduced into the sand flow. 
         FIG. 4  is a side elevation view of the injector gun component of the present invention illustrating its manner of construction which feature easily replaced parts. 
         FIG. 5  is a top elevation view of the injector gun component of the present invention illustrating its manner of construction and the locations of the air pressure gauge and the pressure relief valve. 
         FIG. 6  is a side elevation cross sectional view of a section of typically damaged sidewalk in which one portion has settled into the underlying earth. 
         FIG. 7  is a side elevation cross sectional view of a section of typically damaged sidewalk illustrating a settled portion which has been prepared for repair by the addition of the gun nozzle hole through its body. 
         FIG. 8  is side elevation cross sectional view of a section of sidewalk to which is being repaired by the use of the injector gun component of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and more specifically to  FIG. 1 , slab leveling system  10  is made up of three primary components. The first of these is a high volume air compressor  12  which is typically a transportable device having independent wheels and a trailer tongue by which it is pulled to and from a work site. Additionally, the air compressor is most commonly powered by a small gasoline or diesel engine which allows it to be operated independently without the need for an outside power source. 
     The high volume air compressor  12  is connected to the second primary component of the invention, the sand storage tank  16 , by the compressor to tank air line  14  which is simply a length of high pressure air hose that is of an inside diameter that is sufficient to handle the volume of air that is required for the efficient operation of the invention. The connection at the sand storage tank  16  is facilitated by the use of the tank manifold  18  which is a threaded cross apparatus which allows the single compressor to tank line  14  to be split into a plurality of different applications. The primary direction the compressed air takes from the tank manifold  18  is down to the venturi chamber  22  which is located adjacent to the sand outlet  20  at the bottom of the sand storage tank  16 . 
     The venturi chamber serves to mix the sand from the sand storage tank  16  with the compressed air stream for its use in accordance with the theme of the present invention. From the venturi chamber  22 , the air and sand mixture is channeled into the air/sand line  24  which transports it to the injector gun  26 . The injector gun  26  is the component of the present invention which is employed by the operator to direct the flow of air and sand into the proper location that will effectuate the desired repairs. 
     The manner of construction of the tank manifold  18  and the venturi chamber  22  are further detailed in  FIGS. 2 and 3 . The compressor to tank air line  14  enters one of the plurality of ports of the tank manifold  18  through the pre-manifold shutoff valve  34 . The pre-manifold shutoff valve  34  allows an operator to close off the air pressure in the compressor to tank air line  14  from the rest of the components of the invention which allows maintenance to be performed without a complete system shut down. This component works partly in conjunction with the manifold bleed line  28  which functions to bleed off air pressure from the system through the bleed line shutoff line  30  located between the tank manifold  18  and the manifold bleed line  28 . Thus, this component is employed most commonly in maintenance situations when the air to the system has been closed off by the activation of the pre-manifold shutoff valve  34 . 
     The tank manifold  18  also contains an outlet port which provides the point of attachment for the manifold pressure gauge  32 . The manifold pressure gauge  32  provides a point at which the operator can monitor the pressure contained within the system to ensure both that the present invention is operating with enough air pressure so that it operates effectively and under the maximum pressure as prescribed by the design limitations of the invention. 
     Finally, the tank manifold  18  also provides an outlet port which provides the point of attachment for the manifold to venturi line  38  through the post-manifold shutoff valve  36 . The post-manifold shutoff valve  36  is important to the use of the invention as it is the component through which the operator controls the flow of air to the remaining components downline from it. Thus, by opening the post-manifold shutoff valve  36  compressed air will flow to the injector gun  26  which will in turn carry sand to the desired location. 
     From the post-manifold shutoff valve  36  the manifold to venturi line  38  extends downward until it connects to the venturi chamber  22 . The venturi chamber  22  is the component of the present invention in which sand is introduced into the compressed air flow within the invention. The venturi chamber  22  consists of an air injector  42  which connects the manifold to venturi line  38  to the injector chamber  46  of the venturi chamber  22 . The air injector  42  extends a short distance into the injector chamber  46  to a point within the injector chamber  46  that is beyond the sand feed tube  44  which directs the flow of sand from the sand outlet  20  to the venturi chamber  22 . The flow of the sand in the sand feed tube  44  is controlled by the opening and closing of the sand outlet valve  78  which is located just below the sand storage tank  16  in the sand outlet  20 . 
     The flow of highly compressed air into the injector chamber  46  at a point that is beyond the area at which the sand is introduced, creates a significant amount of negative pressure within the injector chamber  46  which serves to draw sand into the mix chamber  40  located just downstream from the venturi chamber  22 . The mix chamber  40  is a hollow cylindrical tube which is designed to throughly mix the air and sand prior to moving to the other components of the invention. Finally, the downstream end of the mix chamber  40  is equipped with the mix chamber venturi  80  which significantly decreases the inside diameter of the components through which the air and sand mixture is traveling which in turn increases the velocity at which it travels. This increase in velocity enhances the effectiveness of the present invention as it tends to pack the sand in a denser fashion at its intended point of deposit. 
     After leaving the mix chamber  40  through the mix chamber venturi  80 , the air and sand mixture travels through the length of the air/sand line  24  to the injector gun  26 . The injector gun  26  is the component of the present invention which is used to direct the flow of air and sand into the desired location. The injector gun  26  is generally made up of common pipe fittings which are easily replaceable and very inexpensive. The reason for this method of construction is the abrasive nature and the high velocity of the air and sand mixture will generally wear through them fairly quickly. Thus, the replaceable nature of the components of the injector gun  26  allows for its continuous operation even when a component wear through problem has occurred. 
     The flow of the air and sand mixture flows from the air/sand line  24  into the injector gun  26  until it is primarily diverted in a downward fashion by the nozzle tee  56 . The nozzle tee  56  has connected to its lower end the gun nozzle  54  which is the component of the invention inserted into the damaged area. This insertion is generally accomplished by placing the tip of the gun nozzle  54  into a pre-drilled hole and forcing it down by pounding on the top of the injector gun  26  with a hammer or other heavy tool. The problem with this is that it tends to damage the tip of the gun nozzle  54  and the easily replaceable nature of the gun nozzle  54  is an added advantage of the overall design of the injector gun  26 . 
     The upper end of the nozzle tee  56  serves as the point of attachment for the pressure read chamber  58  which provides a point of attachment for the gun pressure gauge  48 . The gun pressure gauge  48  allows the operator to monitor the air pressure within the injector gun  26  which is very important to the proper operation of the present invention. The pressure read chamber  58  is connected to the body of the injector gun  26  through the connector  62  attachment to the nozzle tee  56 . The connector  62  is then connected at its upper end to the pressure tee  60  which is plugged at its upper port by the use of the cap  64 . This configuration leaves the pressure tee  60  free for the attachment of the pressure read chamber  58 . Finally, the pressure read chamber  58  also contains a gun bleed off  50  which is operated through the bleed off valve  52  and allows an operator to relieve any excess or unwanted air pressure which may be present within the injector gun  26 . 
     The manner in which the present invention is employed to repair a slab  82  which has been damaged through ground settling is detailed in  FIGS. 6 ,  7 , and  8 .  FIG. 6  illustrates the typical situation in which a damaged slab  68  of a larger slab  82  exists which has in whole or in part settled into the underlying ground  70  below the normal position of a level cement slab  66  of the slab  82 . This situation creates a potentially dangerous variance in the heights of neighboring portions of the slab  82  which must be repaired. 
     The repair process of the present invention is commenced by drilling a gun nozzle hole  72  though % the damaged slab  68  as illustrated in  FIG. 7  which provides access to the ground  70  below the damaged slab  68 . Upon completion of this step, the gun nozzle  54  of the injector gun  26  is secured within the gun nozzle hole  72  and the operator engages the present invention by providing compressed air flow to the invention&#39;s delivery components, sharp blasts or at a steady pressure. The initial effect of these blasts is to momentarily lift the damaged slab  68  off of the underlying ground  70  which creates a settle cavity  74  between them. This settle cavity  74  is then partially filled in by the mason&#39;s sand  76  being carried by the compressed before it can drop back down. This process is simply repeated until the upper surface of the damaged slab  68  is at the same elevation as the level cement slab  66  of the slab  82 . With this accomplished, the injector gun  26  is removed from the gun nozzle hole  72  which is then filled in with the appropriate material to complete the sidewalk  82  repair process. 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.

Summary:
A system for leveling a slab by raising and filling in voids under the slab. A mixture of compressed air and well dried sand is pumped through hose to a gun nozzle. The slab to be leveled is supplied with at least one hole for receiving the gun nozzle in a fluid tight connection. The compressed air is used to lift the slab while the sand is used to fill the void and keep the slab in a level position. The operator may supply successive burst of pressure to lift the slab as needed, while adjusting the air and sand flow. Finally, the gun nozzle is removed from the hole which is then patched thus, completing the repair of the damaged slab.