Engineering Concerns about Pressed Piles - Witherspoon Engineering Report
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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