Hydraulic and earth engineering – Foundation – Columnar structure
Reexamination Certificate
1991-01-02
2002-12-17
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Foundation
Columnar structure
C405S250000, C403S300000, C403S306000
Reexamination Certificate
active
06494644
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to piling and pile driving and more particularly to an improved piling connector and a unique method of connecting pilings. It is equally adaptable to both new construction and for repairing existing construction, and has particular utility in the repair of concrete slab foundations on pilings. Similarly, it is applicable in all terrain conditions in which pilings are used but has particular utility in the most difficult conditions such as saturated soils and terrains in which the water content exceeds complete saturation.
The problems which the present invention overcomes are long-standing and have been known for decades if not longer. For example, friction pilings of the type and size for which this invention is expected to be frequently employed typically might have a ten ton maximum rating. That is to say, such pilings typically are not able to resist a total downward force in excess of twenty thousand pounds. If the environment in which such pilings are to be used is expected to produce a downdrag force of two tons, the pilings may be analyzed as consisting of a two-ton downdrag portion and only an eight-ton frictional resistance portion. The conservative designer will then subtract the two tons of downdrag force from the eight tons of frictional resistive force to obtain a net maximum of six tons per piling and then, in order to have a reasonable margin of safety, use one-half that number, or three tons, as the design capacity of such pilings. Knowing the maximum load which the particular foundation must support, the designer would then calculate the number of pilings needed and distribute that number about the foundation.
The difficulty with super-saturated soils, and even in many soils that are less than saturated but near saturation, is that such soils usually will not remain uniformly wet. When dry, or even when only partially dry, such soils experience enormous contractions, and as they settle, extremely strong downward forces are created. When the downdrag exceeds the maximum resistive force of the friction pilings, failure results.
Due to the difficulty of access, repair of such a failed foundation is typically quite expensive. For reasons of economy, most friction pilings are wooden poles or, literally, de-barked trees. To prevent decay, and subsequent foundation failure as a result therefrom, wooden pilings are commonly treated with preservatives. However, full-length treated pilings typically cost from twice as much as untreated pilings of the same length and diameter, up to three times as much.
Generally, the deeper a piling is set, the greater is its capacity to resist downward forces. In fact, it is not at all uncommon for the resistive force or resistive capacity of such pilings to increase in a non-linear manner with depth. A typical soil profile in which pilings are normally used may provide three tons of resistive capacity at thirty feet of piling length, four tons at forty feet, but perhaps eight tons at sixty feet. Thus it is apparent that the deeper the designer places the pilings, the greater the capacity, perhaps non-linearly greater, and the fewer the number of pilings needed. Offsetting this advantage, however, is the fact that the longer the one-piece piling, the greater is the cost—also a non-linear function. If the installed cost of a treated thirty-foot residental or light commercial piling (e.g., a Modified Class Five piling) in a particular locale is fifty-two dollars, for example, the cost for a forty-foot piling might be seventy-five dollars, and the nearest comparable sixty-foot piling, three hundred thirty dollars.
The dramatic increase in costs for exceeding forty feet is due to several factors, one of which is that the piling material itself must be of a larger class in order to achieve the desired length; this necessitates a non-linear increase in the cost of the material employed. In addition, small “house rigs” can be used to drive pilings up to forty feet; the costs for driving piles with such equipment is typically as low as fifty cents per foot. Going beyond the 40-foot limit, however, exceeds the capacity of such small equipment; much larger driving rigs must be used, the cost of which may be as much as five dollars per foot. Combined with the non-linear cost-of-material increase, the final, installed cost of a sixty-foot piling might typically be as much as four or five times the final, installed cost of a forty-foot piling.
The cost for treating extra-long pilings also increases non-linearly because of the more expensive equipment needed to treat such pilings. It is known that a piling need not be treated along its entire length in order to preserve it; only the portion above the lowest water table need be treated. However, since most treatment means call for the preservatives to be forced into the wood pores under high pressure, and since the non-uniformity of the raw materials makes consistent sealing around the circumference of the work pieces difficult to achieve, equipment which will pressure-treat only an end of a piling is typically either not available or so expensive as to not afford any savings.
The prior art has therefore looked to various means of connecting shorter pilings, i.e., each of forty feet or less, so as to make an effective and economically affordable longer piling. One such early attempt is that of U.S. Pat. No. 1,073,614, “Pile Splice”, to W. A. McDearmid. McDearmid employs a specially-cast tubular body with an integral transverse partition dividing the body into two chambers of equal diameters. The device is placed over a snugly fitting lower pile, a short pin is driven longitudinally into the lower pile with one end protruding, the upper pile is then dropped into the upper chamber onto the pin, and a bolt is then passed horizontally through each chamber and secured by a nut on the distal end thereof. Several disadvantages are presented by this approach, however. One such disadvantage, if the holes in the pilings are pre-drilled, is the difficulty of precisely aligning the holes in the environment intended, i.e., under water or in semi-watery mud. If the holes are not pre-drilled in the pilings, it is virtually certain that a bolt secured through the piling in that environment would often not meet the opposite hole in the chamber.
Perhaps a greater disadvantage of the McDearmid splice, however, is the necessity to adapt or pre-prepare the ends of the pilings to be received in the connector. Not only is this step an additional expense, but if the pilings do not fit quite snugly within both chambers, there will be a tendency for the splice to act not like a rigid connection but pin-like about one or both horizontal bolts until further rotation is prohibited by the walls of the chambers. At this point an eccentricity—perhaps a destabilizing eccentricity—will already have been introduced into the system. The amount of resistance which the small, vertical pin would provide to such a moment is expected to be negligible.
Another approach is that of U.S. Pat. No. 4,525,102, “Timber Pile Connection System”, to Gerard J. Gillen, which also discloses a number of other prior approaches to this problem. Gillen appears to call for a hollow splice to be driven internal to each piling with a confined levelling material therebetween to avoid point or edge stresses and to distribute the forces at the interface more widely. Such an internal splice is of course at least partially destructive of the piling material. In addition, the piling itself becomes the “weakest link” in that only a small fraction of the piling material remains exterior to the splice to hold the splice in place. A small error in aligning the splice along the longitudinal axis could easily cause failure during subsequent driving.
Further, it is apparent that the technique of Gillen will not produce a rigid mechanical joint. The joint will be held together only by the force of friction between a piling end and the connector, and once that resistive force is exceeded, the joint will be
Cenac Michael Allen
Scandaliato Samuel Zengal
Field Milton M.
Foundation Systems of Louisiana, Inc.
Lee Jong-Suk
Shackelford Heather
LandOfFree
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