Why slabs move here
Most Texas houses sit on slabs. ASCE’s Texas Section says most foundations built in the state are slab-on-ground. In San Antonio, whether a slab moves depends on what is under it:
- East, south and middle of the county: cracking clay with high shrink-swell potential. The slab rides the clay up and down with moisture.
- North side: alkaline soils over limestone. Less clay, but houses on hillsides can sit half on rock and half on fill.
The Texas Master Naturalist chapter’s guide to Bexar County soils says Houston Black is the dominant soil across the middle of the county; the NRCS soil map says this is a soil of Blackland Prairies and eastern Grand Prairies.
Settlement or heave: which way did it move?
Settlement is the slab dropping, usually at an edge after a long dry spell. Heave is it lifting, often after prolonged rain or a leak that soaks the clay. The two need different thinking. Piers help with settlement. As the FPA puts it, they will not prevent foundation uplift unless they are designed to do so. A plan that doesn’t say which way the house moved isn’t a plan yet.
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The methods compared
| Method | How it works | The caveat |
|---|---|---|
| Pressed concrete or steel pilings | Pushed down under the grade beam by hydraulics until they resist | The FPA says pressed piles are held by skin friction along the side of the pile; a steel pile that isn’t pushed to refusal may move with moisture |
| Drilled (bell-bottom) piers | A shaft drilled and filled with concrete | Bearing comes from the bottom of the pier |
| Helical piers | Screwed in | They are driven to the required torque for capacity |
| Mudjacking or foam | Material injected to fill voids and lift | The FPA calls it a shallow repair system affected by moving soil |
Each can be right. The question for a San Antonio quote is how deep the piers go and what they bear on. Ask for the depth in writing.
Exterior and interior piers
Piers go along the outside edge where there’s room to dig. Interior piers need tunnels under the slab or cuts through it. The more of the slab that sits on piers, the more even the support. ASCE’s Texas Section says repair documents should state that underpinning will not improve the performance of the foundation in non-underpinned areas, which is why a partial job can leave the rest still moving.
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What the repair involves, start to finish
- Inspection and elevation readings.
- A written plan from the contractor, and an engineer’s design, since San Antonio requires repairs to be designed by or under the guidance of a licensed professional engineer.
- Permit application: the engineer of record’s letter goes in with it.
- Excavation at each pier location, then the piers, then the lift in stages.
- Plumbing test if the plan calls for one.
- Engineer’s inspection letter to the city; city inspection.
- Elevations after the repair. ASCE’s Texas Section says they should be measured after the repair, with a copy to the homeowner.
After the lift
Doors that were jammed may close easily. Cracks may narrow but rarely vanish. ASCE’s Texas Section says an engineer retained to do so should give a letter of substantial completion saying the work generally conforms to the plan. And soil moisture still matters: the FPA lists continued movement as the usual result of poor drainage around a repaired slab. The drainage page covers the fixes.
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What slab repair costs
HomeGuide lists slab repair at $1,000–$10,000, Angi puts a typical job at 10 piers, and San Antonio contractors publish ranges such as $5,000–$7,000. The cost page goes through the line items.