Every so often a customer calls because a hole opened in the yard, a fence post dropped, or a new depression keeps swallowing topsoil no matter how much they fill it. In parts of Sussex and Warren County, that is not bad luck. It is the ground itself.
Karst country
The valleys of northwestern New Jersey, including the Kittatinny Valley that runs through Sussex County, sit on limestone and dolomite bedrock. Water moving through the ground slowly dissolves that rock, opening voids and channels underneath. Geologists call this karst terrain, and the New Jersey Geological and Water Survey maps karst susceptibility across our area for exactly this reason. When the soil above a void finally lets go, you get a sinkhole.
What it looks like on your property
Most of what we see is not dramatic. It is a circular depression that keeps sinking after you fill it, a soft spot that swallows mulch and stone year after year, cracks radiating around a low area, or a culvert or drain that suddenly loses water into the ground. Water often makes it worse, because a leaking pipe, a downspout, or concentrated runoff speeds up the process that opens the void.
What we do about it
First we figure out whether it is really karst or just poorly compacted fill, a rotted stump, or a failed pipe, and a quick dig usually tells the story. For a true karst feature, the repair is to excavate down to sound material, choke the throat of the hole with graded rock, cap it so soil cannot wash through, then rebuild the soil above and regrade so water moves away instead of feeding the hole. Bigger or structural cases may need an engineer, and we work alongside them with the excavation.
If something on your property keeps sinking, do not keep topping it off. Call us at 862-268-2510, and we will look at it, tell you what it actually is, and fix it so it stays fixed.
Sources: Geology of Sussex County in Brief (NJ Geological & Water Survey), Karst features of the NJ Kittatinny Valley (NJGWS map).