Pressed Pile System

In accordance with several request, we have reviewed the literature you provided us on the ” pressed pile system” (PPS) which uses sixinch diameter by twelve inch long unconnected contcret cylinder with a large concrete block pile cap. We have evaluated this underpinning method in general and made comparisons to the foundation repair system utilizing drilling piers with underreams. The following discussion reflects our most prudent professional opinions based on our knowledge of and expansive experience in commercial and residential foundation design involving expansive clay soils. We certify that we made the study as independent professional engineers and have no interest, present in either the PPS or drilled pier system or anyone involved with these systems.

  • Live Loads Required By Building Codes
    It is stated in the PPS "Specifications for the Design and Installation of Underpinning for the Foundation of a Structure" Provided by you that the piles shall be designed to permit only an "accepable" amounts of settement in supporting a combination of all dead loads and live loads required by the applicable building codes. The dead load of the structure and only the actual live loads superimposed at the time of installation are used to resist the jacking force required to install the PPS. In most cases the actual load, at any giving time, is a very small percentage of the live load required by the building code.

    The PPS, 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 are used in the jacking force no factor of safety. If the live load is increased, until it reaches that which is required by the building codes, the structure could experience differential settlement, which would mostly be far beyond " acceptable" limits.

    On the other hand, the drilling pier with its larage bearing area provided by the underream has the capacity to resist additional loads. The Combination of skin friction on the larger shaft in low initial shear and the much larger bearing area of the underream results in low initial shear and bearing stresses on the building codes. The proper combination of pier spacing, shaft to prevent future settlement, which is consistent with good engineering practice.
    Characteristics Of The Driven Pile
    Piles pressed into clay deposits will be characterized as being either friction piles or point bearing piles. The character of the pile will depend on the type of clay and moisture content of the clay at the time of installation. If the pile encounters a soft saturated plastic clay it will take on the characteristics of a friction pile. Although the term friction pile implies that the shearing forces between pile and soil are derived from friction, they may also consist of surface adhesion. If a friction pile has parallel sides, as does the PPS, the load is transferred from the pile to the soil principally through shear. If piles are driven in soft saturated plastic clay the shear consists primarily of adhesion and very little resistance is derived from true friction. When seasonal drying occurs, it progresses from the surface downward and as the clay gives up moisture, the shear resistance ( friction due to adhesion) will be lost as the plastic clay shrinks away from the pile. To maintain the applied load capacity the lower levels of pile will have to develop increased resistance. For this to happen the lower portion of the non-tapered PPS wil have to settle to encounter clay soil of higher consolidation or less compressibility. Settlement must occur to encounter increased soil strength at greater depths to develop and maintain resistance to a given load. Most likily this settlement will not be equal throughout the structure will inherently fail (or fail again) due to the resulting distress.

    Futhermore, there is no assurance that the PPS 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 roct or tree root it may skew off to one side. This tip section may in turncause other sections of the pile to skew of as well. This, of course, could not be detected by the installer. Although a large number of sections may be jacket into the ground there is no guarantee that the pile is deep an that adequate long term capacity will be sustained.

    Conversely, if the pile encounters a stiff unsaturated plastic clay when pressed, it will take on the characteristics of a point bearing pile, which contradicts the inital assumption of the PPS. Piles installed in stiff, unsaturated plastic clay will typically be shallow because the stiff clay will be capable of developing possible high shearing ( skin fiction ) resistance, and considerable point bearing resistance. As seasonal changes occur, the clay soil at the tip of the shallow pile will begin to loose the high bearing strength it possessed in the unsaturated stated. The PPS has non tapered shaft and a small bearing area. Settlement will occur when the high compressive strength of the saturated soil.
    Conclusions
    The pressed pile system when compared to drilled piers with under reams has several inherent disadvantages. Settlement can occure 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. Settelment can also occure after the piles are pressed into unsaturated expansive clay soil because bearing resistance is lost when the soil gains moisture and loses its compressive strength.

    Also, the unconnected sections and the non integral pile cap of the PPS will not provide resistance to uplift, which will occur when the moisture content of the unsaturated clay soil increases. Uplift will cause material distress to the structure similar to settlement.

    The pier and underream system is not as susceptible to loss of load carrying and settlement due to the effects of change of moisture content because the primary load carrying element, the underream, is located at the depth where the moisture content of the soil is constant or changes very little. The underream on a reinforced drilled pier not only reduces compressive bearing stresses on the soil but also acts as an anchor to resist uplift forces on the upper portion of the pier. The underream prevents the pier from rising and lifting the structure above its original position. The reinforced drilled pier with underream hed definite advantages over the pressed pile systems for eliminating long term settlement and uplift on expansive clay soils.


    We trust these comments will be helpful to you. If you can be of further assistance please let us know.