If you live in eastern Chula Vista, you have probably heard someone say “it’s the clay.” They are often right, but it helps to know what that means. Chula Vista Structural evaluates and repairs homes affected by expansive clay (arcilla expansiva) across the city. When a fix needs a soils report or engineered design, we coordinate with licensed engineers as needed.
What expansive clay is
Some clay minerals pull water into their structure. When they get wet, they swell. When they dry out, they shrink. Bentonite is one of the strongest swelling clays.
The California Geological Survey describes the Otay Formation, which lies under large parts of eastern Chula Vista, as sandstone mixed with siltstone and claystone. It notes that much of the claystone is light-gray bentonite in beds up to about one meter thick. Local geotechnical reports use strong words for it. A 2019 Ninyo & Moore report in City records says bentonite lenses in the upper Otay Formation have high expansion potential and poor strength when wet. A City of San Diego pipeline report for a route through Bonita and Chula Vista calls clayey parts of the Otay and Sweetwater formations highly expansive, with a “waxlike” feel.
That is the science. The practical version: some soil in Chula Vista can grow and shrink enough to lift or drop part of a house.
How clay moves a house
A house does not sit evenly on clay. The soil under the middle of a slab stays fairly steady because the slab covers it. The soil at the edges gets rain, sprinkler water, and sun. So the edges go up and down more than the middle.
When the edges swell more, the house can end up high at the perimeter. When the edges dry and shrink, the house can end up high in the middle, sometimes called center lift or “doming.” Either way, the frame above gets twisted a little. You see it as:
- Diagonal cracks at the corners of doors and windows
- Doors that stick in one season and swing free in another (see sticking doors and windows)
- Cracks in stucco that open and close
- Gaps between the house and a patio, walkway, or pool deck
- Floors that feel slightly tilted (see uneven floors)
Drought years followed by heavy rain can make this swing bigger. That makes sense physically, but nobody has published a Chula Vista study measuring drought damage, so we will not put numbers on it.
What the City’s grading rules did
Chula Vista wrote clay into its grading code. Under Municipal Code §15.04.055, if expansive soil is found within 2 feet of finished grade in a cut, or 3 feet in a fill, in a building area, it has to be removed and replaced with compacted non-expansive soil. The exception is when the City Engineer accepts a soils engineer’s alternative foundation or slab design. Code §15.04.270 also lets the City require a soil investigation that addresses expansive soil treatment.
In plain terms: many east-side tracts were graded under these rules, so most newer homes did not simply get set on raw bentonite. Some sit on a cap of better soil. Others sit on slabs designed to handle clay, often post-tension slabs. A County-filed geotechnical report for an Otay Ranch village recommended considering post-tension slabs designed to industry (PTI) criteria for expansive soil and uneven fill settlement.
Clay below that cap still reacts to water. So do yards where later owners regraded or added planters against the house.
Is clay a big risk in Chula Vista?
It depends how you measure. The City’s 2023 hazard mitigation plan rates expansive soil as “Extensive” in how much ground it covers, but “Weak” in magnitude and “Low” in overall significance as a disaster. Earthquake and wildfire rank high in that plan. Flood is medium.
That matches how clay works. It is not going to knock a house down. It is a slow, chronic problem that shows up as cracks, sticking doors, and sloping floors. It is also widespread, which is what the plan’s “Extensive” rating reflects, and that is why it gets its own page.
What you can control
Water is the lever. Clay moves when its moisture changes, so the goal is steady, moderate moisture near the foundation.
- Point downspouts away from the house, several feet out, onto ground that slopes away.
- Check the grade. Soil should fall away from the foundation, not toward it. Planters built against the wall often hold water right where you do not want it.
- Adjust sprinklers. Heads spraying on the stucco or pooling next to the slab are a common trigger. So are broken lines nobody noticed.
- Avoid big swings. Shutting off all watering in a drought and then soaking the yard later can make movement worse.
- Watch for leaks. A slow leak under or near the slab can swell clay in one spot.
Our foundation drainage page covers this in more detail.
Repair options for clay-related movement
Drainage and moisture control often come first. Beyond that, the fix depends on how much the house moved and whether it is still moving:
- Sealing or bonding stable cracks, after the cause is under control
- Slab repair, with extra care on post-tension slabs
- Piers or other underpinning, designed for clay by an engineer, when surface soil cannot be relied on
- Cosmetic repair of drywall and stucco once the movement has settled down
Lifting a house on clay without fixing the water problem usually just resets the clock.
Questions about clay
How do I know if my lot has expansive clay?
The original grading or soils report for your tract is the best source. Some owners get them from the City or their HOA. Without that, a soils engineer can test the soil.
Can clay damage start years after the house was built?
Yes. A new pool, a patio, changed landscaping, or a leak can change moisture under the house long after construction.
Should I water my foundation?
Not with a hose aimed at the slab. The goal is steady moisture, not soaking. Normal, even irrigation of plantings a few feet from the house is usually better than none at all or too much.
Talk to us about clay movement
Tell us what you are seeing and when it started. We will come look and tell you whether clay is the likely cause.
Sources for local facts
- California Geological Survey, Geologic Map of the San Diego 30’×60’ Quadrangle, pamphlet (Kennedy & Tan, 2008)
- Ninyo & Moore report, City of Chula Vista Clerk WebLink (2019); City of San Diego Otay 2nd Pipeline Phase 4 geotechnical report
- County PDS, Otay Ranch Village 13 Area B geotechnical report (2014)
- Chula Vista Municipal Code §15.04.055 and §15.04.270
- County of San Diego 2023 Hazard Mitigation Plan, City of Chula Vista Annex