Why do houses on Oklahoma hillsides slowly move downhill?

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

In short: Soil on a slope can move downhill very slowly, a process geologists call creep. Each wet-dry season can nudge clay a little farther downslope, and creeks that cut into the toe of a slope remove support. A house on or near that slope can be pulled outward and down on its downhill side.

What is slope creep?

Creep is an imperceptibly slow, steady downhill movement of soil or rock. It happens when the stress on a slope is large enough to deform the soil permanently but too small to cause a sudden slide ¹. The U.S. Geological Survey describes three kinds ¹:

• Seasonal creep: movement within the surface layer whose moisture and temperature change through the year.
• Continuous creep: the stress constantly exceeds the strength of the material.
• Progressive creep: the slope is approaching a larger failure such as a landslide.

Seasonal creep is limited to the soil layer affected by seasonal changes in moisture and temperature ¹. See shrink-swell and the active zone.

What drives slope creep?

For creek-side lots, see erosion, creeks and slopes and creeks and flood history.

What are the warning signs of slope creep?

Signs of creep include curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small ripples or ridges in the soil ¹². In a house, field notes link slope movement to tilted retaining walls, fences or trees, stair-step brick cracks wider at the top, sloping floors and diagonal drywall cracks at openings. Creep shows sideways displacement and tilting, while fill consolidation settlement moves mostly straight down.

How common is slope movement in Oklahoma?

The Oklahoma Geological Survey reports that most landslides occur in the eastern third of Oklahoma, where the wetter climate and steeper terrain make them more likely ³. Survey researchers mapping shallow landslides in eastern Oklahoma and western Arkansas counted about 25 events before 2000 and 62 during 2010-2016 in their inventory; they found a qualitative match with the rise in Oklahoma earthquakes but did not confirm that specific quakes triggered specific slides ⁴.

An Oklahoma Department of Transportation study identified and mapped 113 historic and current landslides statewide ⁵. At its monitored site near Idabel in McCurtain County, the slope was a high-plasticity clay with a plasticity index of 46 on a roughly 3:1 slope, and rainfall drove the sliding as the clay absorbed water ⁵. The study found slope, soil texture and land cover strongly affected landslide susceptibility ⁵.

What setbacks from slopes do codes require?

Building codes based on the International Residential Code (Section R403.1.7) set two common distances ⁶:

• Top of a slope: footings set back one-third of the slope height, but no more than 40 feet is required.
• Bottom of a slope: buildings set back one-half of the slope height, but no more than 15 feet is required.

Check local adoption with the city; see codes and permits.

How is slope movement repaired?

Slope movement is treated as both a slope problem and a foundation problem. Options matched to this mechanism include engineered slope and erosion stabilization, helical piers designed for sideways loads, vertical steel push piers combined with lateral design, and drainage solutions. On highway slopes, ODOT lists rock drains to remove water faster and laying slopes back to flatter angles as mitigation methods ⁵.

Related

• Erosion, creeks and slopes
• Tilted retaining walls, fences or trees
• Erosion, undermining and washout voids
• Slope and erosion stabilization
• Southeast (Ouachitas and Red River)
• Tulsa Metro