How are interior slab piers installed by tunneling or coring?

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

In short: Interior piers support the inside of a slab foundation, under interior beams or settled floor areas, instead of the outside edge. Crews reach the soil by tunneling under the slab from outside or by coring a hole through the floor. They address interior settlement only; they cannot pull down a center that has heaved upward.

How do crews reach the soil?

Coring means cutting a cylinder out of concrete with a diamond-tipped drill. A tunnel here is a crawl-height excavation under the slab.

Where do interior piers go?

Slab foundations can have interior footings under bearing walls, cast with the slab; in Seismic Design Categories D0 to D2 the code requires those footings to extend at least 12 in below the top of the slab ². Interior piers are typically set under these thickened beams or under areas of the slab that have settled, such as over soft or poorly compacted fill. Oklahoma's 2024 code adoption also sets limits on fill under slabs: compaction in 8 to 12 in lifts, with clean sand or gravel fill up to 48 in and earth fill up to 8 in ⁵.

What do Oklahoma install records show?

Steel push piers rely on the weight above them to drive the pier down. Records from Oklahoma steel push pier installations show a clear difference between piers at the slab and at the perimeter footing ¹:

The slab readings fall below the 6,000 psi minimum used as the field standard in the same records ¹. One explanation consistent with these numbers is that a slab provides less weight to push against than a loaded perimeter footing, so the pier may stop because the slab lifts rather than because the pier has reached firm bearing. The records alone do not separate those two causes, so low-pressure interior piers are flagged on the installation log.

What can interior piers not do?

Interior piers lift or hold up; they cannot pull a slab down. That matters because a raised center is common on clay:

• On a uniformly loaded rectangular slab, corners press on the soil with about one-fourth the pressure of the center, and corners tend to rise relative to the middle when clay swells ³.
• Research on slabs over expansive soil models a center-lift condition after a long dry period, and the effect of a water line leak under the slab center ⁴.

When the center is high and the edges are low, the movement is either center heave or perimeter drying settlement, and an elevation survey with heave vs settlement analysis is used to tell them apart. Where plumbing runs under the high area, leak testing is part of that analysis; see plumbing leaks under slab.

How do floating slabs and interior walls affect them?

Engineering guidance for swelling soils notes that floating slabs are kept isolated from bearing walls, columns, stairs and utilities and that some cracking should be expected ⁶.

Can interior piers be used in post-tensioned slabs?

Oklahoma's 2024 code adoption requires post-tensioned slabs on grade to carry a label on the electrical panel door ⁵. Post-tensioned slabs contain tendons, steel cables stressed after the pour, so the slab type is identified before coring; see post-tensioned slabs.

Related

• Slab on Grade
• Steel Push Piers
• Center Heave (Moisture Build-up Under Slab)
• Fill Consolidation Settlement
• Slabjacking and Polyurethane