Points of Interest from The Witherspoon Report
(Published in the International Association of Foundation Drilling, May 2003)
- The first shortcoming of the pressed concrete or steel piles is that they can seldom be set below the zone of seasonal moisture variation.
- The second flaw of this system is that in expansive clay soils the shear strength and density of the stiff clay in this region cannot be penetrated below the active zone, especially in dry seasons. At the house witnessed by this writer, the repair contractor was only able to drive the piles to an average depth of 8′ prior to lifting the house at an individual piling. Remember, a house provides limited weight to push a piling and if the contractor attempts to push beyond the first life of the house, damage to the grade beam and walls will surely occur. Confirmation of this has been confirmed with reports that the pressed concrete piles could only be driven to depths of 4′ and 5′ if attempted during a dry hot summer.
- Consider that a typical house weighs approximately 1,000 pounds per foot at the perimeter and that is the only weight that can be engaged for installation.
- Even if the piles can be driven below the active zone, a closer look at the system shows that even when the targeted depth is obtained, support is not advanced below the active zone because they are not connected.
- Recent investigations, however, have revealed cases where these piles have heaved significantly and lifted the house perimeter to cause measurable damage to the veneers, framing, and door function.
- In the case of the concrete pressed pile the cable is seldom stressed/locked at the top, which allows each pile member to act more like cylindrical footings than a driven pile. In fact, the piles that rest in the active zone will lift and push up the house without engaging any of the piles sections that may have been driven below this area of seasonal moisture change.
- To further degrade the value of these piles is the fact that during heaving there will be a remolding of active clay below the bottom heaved pile and between piles that do not act as quickly that will remain between the piles when the active zone become dry. As a result, the piles will never be completely restored to the original integrity at the time of installation.
- Therefore, unless the cable was truly tensioned to a sizable load, a few inches of movement could occur, which would allow some upheaval in the active zone.
- With concrete piling, the problems would appear to be greater. First there is no good way to tension the cable without causing damage to the concrete cylinder. These cylinders are just not poured with design considerations for post tensioning.
- Therefore, it would appear that the foundation repair contractors of this type of piling have two choices. They can work to develop systems that will provide protection against upheaval potential, or they can go back to the more sophisticated, high-tech and tested system of drilled concrete piers. (also known as Bell Bottom Piers)
- The method of underpinning is very popular because of its quick installation (no concrete cure time is required on the concrete) and with an installation price that is normally much cheaper because of the great reduction in labor and material costs.
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More Points from M. Lewis Coody, P.E. and Ronald J. Kruhl, P.E.
Engineers at Texas A&M University
- The Press Pile System, therefore, will not be capable of supporting the maximum combination of dead load and live load as stated in the specifications without excessive settlement.
- Furthermore, since only the dead load and existing live load at the time of installation are used in resisting the jacking force no factor of safety can be incorporated.
- Settlement can occur in varying amounts after the piles are pressed into saturated expansive clay soil because friction resistance will be lost when the soil loses moisture and pulls away from the pile.
- Furthermore, there is no assurance that the Press Pile System will be driven straight or what the effective length of the pile will be in soft saturated plastic clays. If one portion of the tip section encounters a foreign object such as a rock or tree root it may skew off to one side. This tip section may in turn cause other sections of the pile to skew off as well. This, of course, could not be detected by the installer. Although a large number of sections may be jacked into the ground there is no guarantee that the pile is deep and that adequate long term capacity will be sustained.
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