Is That a Sinkhole? Holes and Soft Spots Under a Patio in Northwest Arkansas

Probably not, but in parts of Northwest Arkansas it can be, and that is why it is worth checking properly. The USGS advice starts in the same place: some holes are caused by leaky underground pipes, and some are old construction pits or buried material that has settled. Rule those out first.

Around a patio, the usual suspects are:

• A broken or disconnected downspout pipe or drain line, washing soil away underground.
• A leaking water or irrigation line.
• Buried debris, stumps or roots from before the house was built, rotting and collapsing.
• Loose backfill around the foundation, utility trenches or an old pool or tank settling.
• Animal burrows, which can undermine a patio edge.

Where none of those fits, and the ground is on the Springfield Plateau, karst is a real possibility.

Because of the rock. North of Fayetteville, from Springdale up through Rogers, Bentonville, Cave Springs, Centerton and Bella Vista, the bedrock is the Boone Formation of the Springfield Plateau: limestone interbedded with chert. Limestone dissolves slowly in water, and geologists describe caves and sinkholes as common in the limestones of the Springfield Plateau.

Above the rock sits red clay full of chert fragments, left behind where the limestone has dissolved. When a void opens in the rock below, the clay can hold as a bridge over it for a long time, then give way. The town of Cave Springs is named for exactly this kind of ground, and its drainage manual maps a recharge area where runoff has a direct route into the cave system.

South of there, around Fayetteville and into the Boston Mountains, the surface rock is sandstone and shale, and karst sinkholes are far less of a concern. Holes there are much more likely to be pipes, backfill or erosion.

Some can. The USGS distinguishes two main kinds of karst sinkhole in covered ground. Cover-subsidence sinkholes develop gradually where the covering sediment is permeable and sandy. Cover-collapse sinkholes occur where the cover contains a lot of clay, and they can develop abruptly, over hours.

Clay cover is what Northwest Arkansas has. That does not make every hole dangerous, but it is why an unexplained, steep-sided hole on the plateau should be treated with respect until someone knows what it is.

The USGS also notes that new sinkholes can form when human activity changes natural drainage in karst areas. Concentrating roof water, patio runoff or a pool overflow into one spot of ground is exactly that kind of change.

• Keep people, pets and vehicles away from it, and do not stand at the edge.
• Stop sending water toward it: redirect downspouts, turn off irrigation.
• If gas, water, sewer or electric lines could be involved, contact the utility.
• Take photos with something for scale, and note how fast it changes.
• Check your homeowner's insurance; the USGS notes most policies do not cover natural sinkholes, which varies by state.
• If it looks natural, the USGS suggests contacting your state geological survey, and for mitigation, a geologist or geotechnical engineer.

Because the soil is going somewhere. If a pipe is washing it away, fill disappears down the same route and the hole comes back. If it is karst, dumping soil into the throat of a sinkhole can plug it for a season, then let go again, and it does nothing about the water that started it.

Proper repairs start with the cause. For a broken pipe, that means repairing the pipe, then filling and compacting in lifts. For a genuine sinkhole, repairs are engineered, often building up graded stone and filter layers and dealing with the surface water, and they belong with a geotechnical professional.

A void often shows itself before a hole does. Watch for a paver or slab that sounds hollow when tapped, a small dip that keeps coming back after it has been reset, joints that open around one spot, or sand from the joints disappearing down into the base. On concrete, a crack that forms a rough circle or a corner that drops suddenly is worth investigating.

Any of those, especially near a downspout, a drain line or the pool, is a reason to lift a section and look, rather than reset the surface and hope.

Pools produce their own holes. A leak in the plumbing or the shell sends water into the ground beside the pool continuously, and it carries soil away with it. Backfill around the shell that was not compacted properly settles for years after the build. Both show up as dips, voids and cracks in the pool deck.

If you are topping the pool up more than evaporation explains, or the deck is dropping near the plumbing runs, get the pool checked for leaks before anything is done to the deck. Fixing the deck over a leak is fixing it twice.

Buried downspout pipes are one of the most common causes of holes beside a patio, and they fail in predictable ways. Joints separate as the ground moves, roots get in, or a pipe crushed during the original build leaks from day one. Every storm then pushes roof water into the soil at that point, and the soil washes along the outside of the pipe toward wherever it can escape.

The test is simple: run a hose into the downspout and watch where the water comes out, and where the ground goes soft. A pipe that should discharge at the far side of the garden but produces nothing there is leaking somewhere in between.

On the Springfield Plateau, karst features tend to cluster where water already goes: low ground, natural drainage lines, near springs and losing streams, and places where surface water has been concentrated by roads, roofs or paving. The Cave Springs drainage manual, for instance, maps losing stream segments where surface water sinks into the groundwater system.

A hole in a low corner of the garden where runoff has always gathered deserves more suspicion than one at the top of a slope next to a downspout pipe. The second is almost certainly the pipe.

Once the cause is dealt with, the patio above is repaired like any other failed section: lifted back to sound ground, the void filled with suitable material compacted in lifts, the base rebuilt and the surface relaid. On pavers, the same units usually go back. On a slab, the affected panel is cut out and replaced.

The one thing not to do is skip straight to this step. A patio repaired over an unfixed leak or an active void is a patio you will be repairing again.

"Any hole in NWA is a sinkhole." Most are pipes, backfill or buried debris. Real sinkholes are a Springfield Plateau phenomenon, and even there they are the less common explanation.

"Filling it solves it." Filling hides it. The soil is going somewhere, and until that stops, so will the fill.

"Concrete in the hole makes it permanent." Plugging the throat of a karst sinkhole with concrete can divert water sideways and open another one nearby. Repairs on genuine sinkholes should be designed.

"It is only a problem if it is big." Small, growing holes are the ones to catch. They are cheapest to deal with and tell you where the water is going.

• Have you ruled out the drain lines, downspout pipes and water lines?
• Is there anything buried here, from before the house or from the build?
• Is this ground karst, and does anything suggest a void in the rock?
• What will you do about the water feeding it?
• Does this need a geotechnical engineer?

A good answer to the fourth question matters most. Whatever the cause, water usually started it.

Most patios on the plateau are built without drama, and the ground is generally sound limestone under clay. What karst changes is how carefully the water is handled and how seriously the excavation is read.

When we dig, a void, a soft pocket or a pipe of red clay dropping into rock is a reason to stop and assess, not to backfill and carry on. Downspouts and drains go to designed outlets rather than being dumped into the nearest low spot. And on larger projects, particularly pools, a geotechnical opinion is money well spent where the ground gives any reason for doubt.

If you are in the Cave Springs recharge area, the city's drainage manual is worth a read. It explains why runoff and fine sediment matter to the cave and the threatened Ozark cavefish that live in it.