How does erosion leave voids under a foundation or slab?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: Erosion and washout happen when moving water carries soil away from beside or beneath a foundation, leaving an empty space (a void) where support used to be. The water can come from concentrated roof and yard runoff, flooding, or a leaking pipe under the slab. Once support is lost, the concrete above can crack or settle into the void.

How does water remove soil from under a foundation?

Water that is concentrated, such as at a downspout outlet, along a low spot against the house, or at a broken pipe, can pick up soil particles and move them. The Oklahoma Geological Survey describes sinkholes forming when water flowing underground erodes sediment and creates voids that the ground above eventually collapses into ¹.

After the heavy rains of April 2025, the OGS expected more sinkholes in the hardest-hit parts of Oklahoma, most of them small and located where water flowed beneath roads and driveways ¹. The same process can work under slab edges, footings, porches and flatwork.

Three common routes

• Surface runoff and scour. Concentrated flow erodes soil at slab edges and footings. Erosion channels are one of the warning signs inspectors look for in crawl spaces ⁸.
• Flooding. Flood exposure adds another source of moving water. In Oklahoma, flood-prone areas are mapped for about a 1-in-100 chance of flooding in any year, and major floods often occur in spring and fall ⁶.
• Plumbing leaks. A leaking supply or drain line under a slab releases water continuously into the soil. The EPA estimates household leaks can waste more than 10,000 gallons a year in an average home ³.

Which soils erode easily?

Some soils are far more erodible than others. USDA soil surveys rate surface erodibility with the K factor, which ranges from 0.02 to 0.69, with higher values meaning greater susceptibility to sheet and rill erosion; it depends mainly on silt, sand and organic matter content, structure and permeability ⁴.

A special case is dispersive clay, a clay whose particles break apart and wash away readily in water because sodium dominates its pore water ². In dispersive soils, a small leak through a crack can enlarge into a pipe or tunnel, and if the tunnel grows large enough its roof can collapse ². The USDA Natural Resources Conservation Service documented Oklahoma flood-control dams, including Owl Creek Site 13 and Upper Red Rock Site 20, that failed by this kind of tunnel erosion ². Standard index tests do not identify dispersive clays; specialized tests such as the crumb test, double hydrometer and pinhole test are used ².

What are the signs of erosion voids?

• an exposed slab edge or void under a footing
• visible slab cracks over an unsupported area
• heaved or settled flatwork such as sidewalks and patios
• a sudden depression or sinkhole in the yard or under a slab

How do erosion voids differ from karst sinkholes?

Karst sinkholes form when soluble bedrock such as limestone, gypsum or salt dissolves and the ground above collapses into the cavity ⁷. Erosion voids form in the soil itself, close to the surface, where water carries particles away ¹. The USGS notes that human changes to natural drainage can also contribute to sinkhole formation ⁷. See karst subsidence and sinkholes for the bedrock-driven version.

How are erosion voids typically addressed?

• Stop the water first. Leaks are found with testing; the EPA's meter test (read the meter, use no water for two hours, read again) checks supply lines ³, and Texas engineering guidelines call for testing supply and drain lines whenever a foundation will be lifted ⁵. See plumbing leak detection and repair.
• Control runoff. Where concentrated surface flow is the source, grading and downspout routing are the related topics; see surface drainage and grading.
• Fill the void. Texas guidelines list grouting and mudjacking among slab repair methods ⁵. See void and compaction grouting.

Related

• Karst Subsidence and Sinkholes
• Erosion, Creeks and Slopes
• Slabjacking and Polyurethane
• Drainage Solutions
• Plumbing Leaks Under Slab