Concrete pier installation

Two concrete systems do most of the underpinning work in North Texas, and they are less alike than the shared word "pier" suggests. A pressed piling is a stack of precast cylinders driven into the ground. A drilled bell-bottom pier is concrete poured in place in a hole with a flared base. The choice changes depth reached, installation sequence, and price by a factor of two or more.

Pressed concrete pilings

The crew excavates at the grade beam, sets a hydraulic ram against the underside of the foundation, and presses precast concrete segments — cylinders about a foot long — into the soil one after another, using the building's own weight and stiffness as the reaction force (Foundation Performance Association FPA-SC-08-1). Load transfers "primarily through skin friction along the length of the Pile, although some end-bearing load transfer also occurs." Segments may be interconnected by a central steel bar, rod or cable, or non-interconnected; the FPA notes non-interconnected piles without reinforcement "are typically less expensive," which is why bids differ by a few hundred dollars a pier.

The system's honest limitation is in the same document. Depth is not chosen; it is discovered. Each pile stops at refusal, "the point when the driving force exerted on the Piling exceeds the load to be supported," and depth "will vary from Pile to Pile" with tributary weight and structural stiffness. If a pile "cannot attain sufficient penetration into stable soils, then it may not be resistant to potential movements that occur due to swelling or shrinking of the soils in the movement active zone." That is the whole risk in one sentence. A light single-story house presses shallow because there is not enough weight above to drive further, and a shallow pile in Houston Black clay — which develops cracks "1.3 to 10 cm (1/2 to 4 in) wide at 30 cm (12 in) depths during dry periods" and runs 40 to 60 percent clay (USDA Official Series Description) — is sitting inside the moving zone rather than under it. DFW pressed piers are reported reaching 12 to 22 feet (Stratum Foundation Repair).

Drilled bell-bottom piers

Here a rig augers a shaft, then an underreamer cuts a flared bell at the bottom; steel is set and concrete placed. One Texas contractor publishes its standard geometry as 12 feet deep with a 9-inch shaft and a 22-inch bell (Dawson Foundation Repair). The bell gives real end bearing instead of relying mostly on friction, and depth is specified rather than driven to refusal. The trade-offs are practical: a rig has to reach the location, spoil leaves the yard, and concrete needs cure time before load transfer, so the lift happens on a later visit. Drilled piers are still poured against expansive clay, so uplift on the shaft during wet cycles is a design consideration — Texas practice treats expansive clay as soil with a weighted plasticity index above 15, and Blackland Prairie series routinely test 28 to 42 (Texas Section ASCE; USDA Soil Survey of Tarrant County).

Spacing and layout

Spacing is standardized, not negotiable. Exterior pilings are capped at 8 feet on center for a one-story wood or brick structure, 7 feet for a 1½-story or a two-story with brick on the first story only, and 6 feet for a two-story brick structure; three or more stories "should have an engineered underpinning design." Interior pilings go at beam intersections and along beams at a maximum of 10 feet on center in a stiffened slab, 8 feet under one-story walls and 7 feet under two-story walls (FPA-SC-08-1). Piles are expected within 12 inches of their location on the layout drawing, and piles driven simultaneously at least 25 feet apart.

Prices per pier

Pressed pilings across DFW price lists run $300–$700 per pier (The North Texas Home Guide), with one DFW contractor publishing $300–$600 (Stratum Foundation Repair) and a Texas firm publishing $300–$400 plain and $400–$700 with a steel insert (Dawson Foundation Repair, priced 2022). Drilled bell-bottom piers are quoted at $800–$1,200 per pier regionally, while two Texas contractors publish $650–$800 and $700–$800 for their own bell-bottom work (Dawson; Anchor Foundation Repair, priced 2021). Interior piers carry a premium of roughly 20 to 30 percent and appear in only 10–15 percent of Texas jobs (Anchor). Typical DFW jobs use 8–12 piers, 20 or more in severe cases.

What to ask, and what should worry you

Ask which system is being bid and why; ask for the specified minimum depth and what happens if a pile refuses shallow, since the FPA lists the legitimate options — terminate at refusal with engineer approval, replace with a smaller cross-section at least three pile diameters away, water-jet, or abandon and pre-drill. Ask whether segments are interconnected, and ask for the layout drawing. A bid with no depth language, spacing beyond the FPA maximums, a pressed-piling price far below the $300 floor in the regional data, or a "lifetime warranty" with no stated remedy and no engineer's seal deserves a second opinion.