If a foundation investigation by a registered structural engineer rules out remediation or minimization of foundation movement with the application of techniques designed to stabilize the supporting soil or improve/modify drainage and it is deemed necessary that foundation repair with the installation of underpinnings be implemented in order to restore slab elevations and floor slopes to reasonable magnitudes, several pier/piling systems are available. The most commonly installed systems include drilled reinforced concrete piers and steel or concrete pilings.
The drilled reinforced concrete pier has been the predominant system utilized for foundation underpinning and restoration for a number of decades in north Texas. A common diameter of the concrete pier is 12 inches but double 8-inch piers are also widely used. Some applications require 18-inch or larger diameter piers and under-reaming or belling the bottom of the pier is sometimes required. The engineer should specify the diameter and the required depth of the pier as well as the physical properties of the concrete and the necessary steel reinforcing based on the local soil profile and his experience in the immediate area. The recommended depth of the pier can vary widely depending on the location.
Several different steel piling systems are being installed in the north Texas area. These systems generally utilize 2 7/8" or 3 1/2" nominal (outside diameter) heavy-wall steel sections that are joined together to ensure adequate stiffness to facilitate being pushed vertically into the soil beneath the foundation with minimal lateral deflection (buckling). The pilings are pushed to "refusal", defined as the depth at which they are prevented from moving downward further due to encountering a soil formation with sufficient bearing capacity to elevate and support the foundation. Rather than being pressed directly beneath the grade beam, some popular systems are installed vertically adjacent to the grade beam with the beam supported on the piling by an L-shaped steel bracket. Some systems require the bracket to be anchored to the grade beam.
As a general rule, concrete pilings consist of 6-inch diameter concrete cylinders between 10 inches and one foot in length. As with the steel pilings, the concrete cylinders are pressed vertically into the soil directly beneath the foundation. As discussed in reference to the steel pilings, the concrete pilings are pushed to "refusal", at which point the foundation is elevated as necessary. Some of the concrete piling systems have a longitudinal hole in the center, through which a cable can be run or sections of rebar can be inserted to provide resistance to lateral forces and potential buckling as discussed previously regarding steel pilings. Ideally, two separate concrete piles should be pushed independently of the other at each recommended location for additional support. The depth at which refusal occurs can vary considerably in the north Texas area.
The bottom line is that all of these methods will work somewhere. None will work everywhere. The property owner, therefore, is charged with seeking out the most knowledgeable engineering expertise to advise as to the best approach to restoration and, then, to obtain competitive bids from several contractors who have been referred by trustworthy people or agencies.