The Principle in One Paragraph
The slab is currently supported by soil that has proven unreliable. Underpinning transfers that load onto piers that reach material capable of carrying it. Once the piers carry the load, seasonal movement in the surface clay no longer moves that part of the structure.
Everything below is the mechanics of doing that without damaging the house in the process. Our slab foundation repair page covers how this fits into a full project.
Step One: Placement
Pier locations come from two inputs: the elevation profile, which shows where the structure has dropped, and the load path, which shows where the grade beam is actually carrying weight.
Piers go beneath the grade beam, the thickened edge of the slab that carries the walls. That means placements follow the perimeter and, where interior load lines need support, follow those too.
Spacing is not a fixed dimension. It closes up under heavier loads, at corners, and where the profile shows greater deflection, and it opens along lighter runs. A proposal should show the placements on a plan, because a count alone does not tell you whether the support is going where the load is.
Step Two: Excavation and Bracket Attachment
A pit is excavated at each location to expose the grade beam. The pit needs to be large enough to work in, which is why perimeter clearance of roughly three feet is discussed before a project starts.
A steel bracket is then fixed to the grade beam. This is the component that transfers the structural load into the pier, and its positioning relative to the beam matters for how that load transfers.
Step Three: Driving to Refusal
The pier is advanced downward, one section at a time, using the weight of the structure as the reaction force. A hydraulic ram pushes against the bracket, and because the house above is heavier than the resistance the pier meets in soft clay, the pier goes down.
It continues until the gauge shows refusal pressure: the reading at which the pier stops advancing meaningfully, indicating it has met material capable of resisting the applied force.
Two consequences follow:
Depth is an outcome, not a choice. It is whatever the ground dictates at that location. Across the San Antonio area this varies considerably, and on the Hill Country side it can vary across a single foundation.
Reaction load limits the force. A push pier can only be driven as hard as the structure above it allows. On very light sections, this can become the binding constraint, which is one situation where a helical pier is the better system. Our guide on steel pier systems covers this in more detail.
Depth and pressure are recorded per pier and form the pier log.
Step Four: Load Transfer and the Lift
This is the stage that requires the most care.
Once all piers on a run are set, hydraulic jacks are placed at each bracket and connected so they can be operated together. The structure is then raised in controlled increments, with all jacks on the run moving in coordination.
Why synchronisation matters: a slab is rigid. Lifting one point substantially ahead of its neighbours does not raise the slab, it bends it, and concentrated bending in a rigid concrete element is exactly how a lift produces new cracking. Distributing the movement across the pier line keeps the slab moving as a unit.
Readings are taken between stages, and the lift is staged rather than completed in a single continuous push. That is progressive recovery: bringing the structure back toward level over a period rather than all at once.
What the Lift Realistically Achieves
The objective is meaningful reduction in the relative differential, with the foundation then supported on piers rather than on surface clay.
It is not a return to new-construction flatness, and a proposal promising that is worth questioning. A structure that has settled over years has adjusted around its position: framing has taken a set, finishes were fitted to the geometry as it was, and pushing past a reasonable target produces more damage than benefit.
What you will notice during the lift
Noise, crew movement around the perimeter, and in some cases perceptible movement inside. New hairline cracking above door and window openings is normal and expected. Doors that were binding may operate differently, sometimes better and sometimes needing adjustment afterwards. All of this should be discussed before the work starts.
Step Five: Locking Off and Backfill
Once the target is reached, the load is locked onto the piers through the bracket assembly and the jacks are removed. The piers now carry that portion of the structure permanently.
Pits are backfilled and the perimeter is regraded so the ground falls away from the foundation. Backfill and rough grading is the baseline restoration on most projects; anything beyond it, including turf, beds, and irrigation reinstatement, is scope that should be stated in writing.
Step Six: Documentation
Post-repair elevation documentation records the floor profile once the lift is complete. Together with the pier log, the written scope, and the warranty terms, it forms the project record.
Keep it. Years later, when a buyer’s inspector asks about the foundation, that folder is the difference between an answer and an open question.
What Underpinning Does Not Do
It does not support areas that were not piered. It does not address drainage, which continues to matter. It does not stop movement caused by an under-slab plumbing leak, which is why testing is recommended where the profile suggests heave. And it does not repair cosmetic damage, which is separate finishing work usually best done after the structure has settled.