The Ground Under Overland Park: Clay, Shale and Creeks

In a 49-point USDA sample across the Overland Park area, 42 points sit on soils with a layer rated high or very high for shrink-swell, and at 11 points bedrock lies within about three feet. Beneath that, the Kansas Geological Survey describes about 500 feet of alternating shale and limestone.

The soils in the sample

• Chillicothe (16 of 49 points): very deep, formed in loess, or loess over material weathered from limestone or shale.
• Sharpsburg (9 points): very deep, formed in loess, with a frequently saturated zone 1.2 to 1.8 meters down in the wettest periods.
• Oska (6 points): moderately deep, formed in material weathered from limestone and shale, with limestone 51 to 102 centimeters down.
• Martin (4 points): deep, formed in material weathered from interbedded shales, limestone and clay beds.
• Sogn (4 points): shallow to bedrock.
• Others (10 points): Grundy, Chase, Kennebec, Ladoga, Woodson and Vinland.

Source: USDA NRCS soil survey data for Johnson County and official soil series descriptions. A few points near the eastern edge of the grid may fall just outside city limits.

What a shrink-swell rating means

USDA soil surveys rate shrink-swell by linear extensibility, the change in a soil's length between wet and dry. Under 3% is low, 3 to 6% moderate, 6 to 9% high and 9% or more very high, and USDA notes that moderate to very high ratings can damage buildings. In the sample, the most clay-rich layers of Chillicothe, Martin and Sharpsburg rate high, and Oska's rates very high.

Shale and limestone underneath

The Kansas Geological Survey's study of Johnson County describes its exposed rock as Pennsylvanian layers about 500 feet thick in all, chiefly alternating shale and limestone beds from 2 to 100 feet thick. Limestone makes a firm base where it's close to the surface, and the Oska and Sogn soils in the sample sit on it. Shale weathers into the clay that soils like Martin formed in. Two houses a few blocks apart can stand on quite different ground.

Rain and creeks

NOAA's 1991–2020 normals for the Olathe station show 37.74 inches a year, peaking in May and June. On July 27, 2017, after 5 to 7 inches of rain overnight, Indian Creek in Overland Park set a record crest of 17.97 feet; on Aug. 21–22 it topped that at 18.39 feet (National Weather Service Kansas City). Water soaking clay backfill is the common thread behind cracks, bowed walls and wet floors.

Putting it to use

• Keep downspouts extended and the soil sloping away from the house.
• Test the sump before May.
• Check walls and cracks after a wet spring and again after a dry August.

If a wall has started to lean, read about bowing basement wall repair; if water is the issue, see sump pump installation. Typical prices are in the foundation repair cost guide for Overland Park.