Karst, Sinkholes and Clay Soil in the Roanoke Valley

What karst means for your foundation

Karst is ground formed over soluble rock. Virginia's Department of Conservation and Recreation describes how karst in the Roanoke and New River valleys formed as mildly acidic water dissolved ancient limestone and dolomite over very long periods. The result is bedrock with cracks, channels and, in places, open voids.

When soil above those voids slowly washes down into them, the ground can settle. Most of the time this is gradual, a dip in the yard or a slow drop in one corner of a house, rather than a sudden collapse. Virginia's own geology maps note that many more sinkholes exist than have ever been mapped.

Clay soils that swell and shrink

Much of the soil over that bedrock is clay. Clay swells when it gets wet and shrinks as it dries, so a long wet spring followed by a dry summer moves the ground under a footing up and down. Over the years, that movement shows up as stair-step cracks in brick or block, doors that stick in one season and not the next, and basement walls pushed inward by wet, heavy soil.

Hillside lots

Roanoke is surrounded by ridges, and many neighborhoods are built on slopes. On a hillside, the uphill basement wall holds back more soil and more water, which is why bowing walls are so often on the uphill side. Fill soil placed to level a lot can also settle more than the undisturbed ground around it.

Warning signs specific to this kind of ground

Watch for a crack that keeps getting wider, one corner of the house dropping, a new dip or hole in the yard near the foundation, water that suddenly disappears into the ground, or floors that start to slope. Any of these is worth an inspection. A structural engineer's report can confirm what's happening before you commit to a repair.

How these problems are fixed

Settling caused by soft or washed-out soil is usually fixed with steel push piers or helical piers driven down to stable soil or rock, which carry the weight of the house and can lift it back toward level.

Bowing walls are stabilized with wall anchors, steel braces or carbon-fiber straps. And on almost every job, drainage matters: gutters, downspouts that carry water well away from the house, and grading that slopes away from the foundation keep clay soil from swelling against the walls.