Sinkholes are subsidences or collapses in the earth's surface, usually caused by natural geological processes, but also sometimes caused or hastened by human activity. Sinkholes can cause structures and roadways to be damaged or to collapse or be swallowed up entirely. Sinkhole causes include the gradual dissolution of limestone by underground water flow through cracks and caverns. When sinkholes threaten homes, other buildings or infrastructure, engineers may attempt some methods to halt and repair them.
Grouting is a sinkhole repair method in which workers pump concrete into the sinkhole in an attempt to fill it and to stabilize the land and any structures above. This is the least expensive method of sinkhole repair, but its effectiveness will vary widely, depending on what caused the sinkhole in the first place, how large it really is, and whether it still is active.
Because it is difficult to determine how limited or vast a sinkhole really is, there is little chance of knowing how much concrete the grouting process will require. A major sinkhole could require enormous amounts of concrete to stabilize the structure above, and there would still be no guarantee that the void is stabilized. For this reason, engineers do not recommend grouting alone as a repair method, and most companies will not guarantee results.
Underpinning is an effective method of sinkhole repair in which workers drive metal support beams into the bedrock below a structure. The supports are then attached by brackets to a home or other structure. The underpins are placed strategically around the perimeter of a structure to support it. They also may be used under a building's load-bearing walls to help prevent shifting and cracking caused by sinkhole activity. Underpinning is a very expensive process, and is often combined with grouting for extra protection against structural damage.
While grouting and underpinning are the two main types of repair methods used when sinkholes threaten structures, other repair methods are used for less threatening sinkholes. The ground can give away at any place where underlying earth has been removed. It may simply be a small hole in the backyard caused by storm runoff. In this event, repairing the sinkhole may be as easy as using a shovel to fill the hole with dirt or other material.
According to the Pennsylvania Department of Conservation and Natural Resources website, a wide variety of materials can be used to manually fill a small sinkhole, depending on the area and needs. A simple, small lawn sinkhole may be adequately filled in with soil from the surrounding area. Meanwhile, places where water drainage may continue to be a problem may require more porous material such as crushed stone. Concrete is easily mixed at home and can be shoveled into the small sinkhole for a small type of grouting job.