Why is the soil pulling away from my foundation?
In short: A gap between the soil and the foundation, often with a network of cracks in the yard, means the clay along the house has dried and shrunk. It is a moisture indicator, not proof of damage, but the same drying under the slab edge is what lets the perimeter of a house settle.
What does a soil gap look like?
The soil has separated from the foundation beam or stem wall, leaving a gap you can put a finger or a probe into. The ground nearby may be broken into many-sided blocks by cracks. These are dry-season features of clay soil 1. The sign shows up on slab-on-grade, stem wall, crawl space and manufactured homes.
Why does clay pull away?
Expansive clay swells when water enters it and shrinks as it dries out 3. Most of Oklahoma has these soils 2. In the dry season they form deep cracks in roughly polygonal patterns; one documented crack was at least 32 inches deep 1.
The active zone, meaning the depth of soil whose moisture changes with the seasons, can run from about 3 to 40 feet deep, and successive droughts have shown it can reach deeper than once believed 1. In one documented case, piers 6 feet deep handled normal yearly cycles but not a two-year drought followed by an extremely wet year 1. See shrink-swell and the active zone and drought and wet cycles.
Oklahoma State University researchers have used Oklahoma Mesonet climate data to map equilibrium soil suction, a moisture-related measure, across the state 3.
What can it mean for the house?
| Observation | What it suggests |
|---|---|
| Gap along most of the perimeter after a dry spell | General drying of perimeter soil 1 |
| Gap widest near a tree or large shrub | Root moisture withdrawal 4 |
| Gap plus cracks and floors sloping toward that wall | Slab edge dropping as soil shrinks 2 |
| Gap at a pier-supported house | Soil can pull away from piers and reduce their skin friction (side grip) 1 |
The slab-edge pattern works in both directions: the perimeter drops when the soil shrinks and rises when it swells 2. It is covered in perimeter drying settlement.
Trees
Trees and large shrubs withdraw water from soil, which can cause downward foundation movement 4. Repair guidance notes that pruning may reduce future movement but will not likely restore elevation, and that root barriers or periodic root pruning may reduce vegetation effects, with an arborist consulted because barriers can harm the tree 5. See tree-root desiccation settlement.
How is it documented?
- Gap size. Record the width and the depth a probe reaches, and where along the house the gap is largest.
- Timing. Note the season and recent rainfall, since the gap reflects current moisture.
- Floor survey. Compare with an elevation survey to see whether the floor is low on the same side.
How do moisture and grading around the house matter?
Uneven moisture from lawn watering is one cause of differential movement 1. Federal building guidance calls for the ground next to a house to slope away, typically about 6 inches of fall over the first 10 feet, and for the grade near the foundation to be kept from settling 6. Moisture control options are described in root barriers and moisture control.
Related
- Perimeter Drying Settlement (Edge Drop)
- Tree-Root Desiccation Settlement
- Shrink-Swell and Active Zone
- Cracks That Open and Close Seasonally
- Trees and Roots
Common questions
Why is there a gap between the soil and my foundation?
Expansive clay shrinks as it dries, so in dry weather it pulls back from the foundation and splits into cracks. The gap is a sign that the soil along the house has lost moisture.
How deep do clay shrinkage cracks go?
Dry-season cracks in expansive clay form rough polygon patterns, and one documented crack was at least 32 inches deep. The zone of seasonal moisture change can extend from about 3 to 40 feet deep depending on the site.
Does soil pulling away mean my foundation is settling?
Not necessarily, but it is a drying indicator. When clay under the slab edge shrinks, the perimeter of the slab can drop, which can show up as wall and brick cracks and skewed door and window frames.
Do trees make the gap worse?
Trees and large shrubs withdraw water from the soil and can cause downward foundation movement. Gaps and cracks that are worst near a tree fit that pattern.
Sources
- Damage to Foundations from Expansive Soils Missouri University of Science and Technology
- The Mystery of Expansive Soils STRUCTURE magazine
- Residential House Foundations on Expansive Soils in Changing Climates U.S. Department of Housing and Urban Development, Cityscape
- Recommended Practice for the Design of Residential Foundations, Version 2 Texas Section, American Society of Civil Engineers
- Guidelines for the Evaluation and Repair of Residential Foundations, Version 3 Texas Section, American Society of Civil Engineers
- A Professional's Guide to Durable Home Design: Durability by Design, 2nd Edition U.S. Department of Housing and Urban Development
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