Hydraulic and earth engineering – Foundation – Columnar structure
Reexamination Certificate
1999-11-08
2002-04-16
Will, Thomas B. (Department: 3673)
Hydraulic and earth engineering
Foundation
Columnar structure
C405S248000, C405S256000, C073S785000, C073S786000
Reexamination Certificate
active
06371698
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to techniques for increasing the load bearing capacity of structural foundation piers, and more particularly to the use of structures or devices placed beneath or within piers to enhance load bearing.
BACKGROUND OF THE INVENTION
Drilled shafts, or piers, are often used in the deep foundation industry because they provide an economical alternative to other types of deep foundations. Drilled piers are typically formed by excavating a cylindrical borehole in the ground and then placing reinforcing steel and fluid concrete in the borehole. The excavation may be assisted by the use of drilling fluids, casements or the like. When the concrete hardens, a structural pier suitable for load bearing results. These piers may be several feet in diameter and 50 feet or more deep. They are typically designed to support axial and tensile compressive loads.
A finished structural pier has an axial load bearing capacity which is conventionally characterized by components of end bearing (q
b
) and side bearing, which is a function of skin friction (f
s
). Loads applied at the top end of the pier are transmitted to the sidewalls of the pier and to the bottom of the pier at the distal end of the shaft. The end bearing capacity is a measure of the maximum load that can be supported there, and it will depend on numerous factors including the diameter of the pier and the composition of the geomaterial (soil, rock, etc.) at the bottom of the shaft. The side bearing capacity is a measure of the amount of load capable of being borne by the skin friction developed between the side of the pier and the geomaterial. It depends on numerous factors, including the composition of the pier and the geomaterial forming the side of the pier, which may vary with length (depth). The sum of the end bearing and side bearing capacities generally represents the total load that can be supported by the pier without sinking or slippage, which could cause destructive movements for a finished building or bridge atop the pier.
Although it is desirable to know the maximum end bearing and side bearing for a particular pier, it is difficult to make such measurements with a high degree of confidence. Foundation engineering principles account for these difficulties by assigning end bearing and load bearing capacities to a pier based on its diameter and depth, the geomaterial at the end of the pier and along its side, and other factors. A safety factor is then typically applied to the calculated end bearing and side bearing capacities. These safety factors are chosen to account for the large number of unknown factors that may adversely affect side bearing and end bearing, including geomaterial stress states and properties, borehole roughness generated by the drilling process, geomaterial degradation at the borehole-shaft interface during drilling, length of time the borehole remains open prior to the placement of concrete, residual effects of drilling fluids, borehole wall stresses produced by concrete placement, and other construction-related details. For example, it is common to apply a safety factor of 2 to the side bearing so as to reduce by half the amount calculated to be borne by skin friction. Likewise, a safety factor of 3 is often applied to the calculated end bearing capacity, reflecting the foregoing design uncertainties and others.
The use of safety factors, although judiciously accounting for many of the uncertainties in drilled shaft pier construction, often results in piers being assigned safe load capacities that are too conservative. To compensate, builders construct larger, deeper, and/or more piers than are necessary to safely support a structural load, unnecessarily increasing the time, effort and expense of constructing a suitable foundation.
As a partial solution, it has been known to directly measure the end bearing capacity and skin friction of a drilled-shaft pier. Osterberg (U.S. Pat. No. 4,614,110) discloses a parallel-plate bellows placed in the bottom of the shaft before the concrete pier is poured. The bellows are pressured up with fluid communicated through a pipe coaxial with the pier. Skin friction is determined by measuring the vertical displacement of the pier (corresponding to the movement of the upper bellows plate) as a function of pressure in the bellows. Likewise, end bearing is determined by measuring pressure against the downward movement of the lower bellows plate, as indicated by a rod affixed thereto and extending above the surface through the fluid pipe. Upon completion of the load test, the bellows are depressurized. The bellows may then be abandoned or filled with cement grout, and in the latter case becomes in essence an extension of the lower end of the pier.
The method of Osterberg most often serves only the purpose of load testing. In practice, most often a drilled shaft employing the “Osterberg cell” is abandoned after testing in favor of nearby shafts that do not contain a non-functioning testing cell at their base.
Other methods have been developed for enhancing the load bearing capacity of drilled shaft piers by permanently pressuring up the base, but they lack the testing capabilities of the Osterberg cell. For example, it is known to inject pressurized cement grout under the base of concrete piers to enhance load bearing. In post-grouting, the pressurized grout increases end bearing, but neither the resultant increase nor the absolute end bearing capacity can be determined from the pressure or volume of the grout. In some soils, skin friction may also be increased by allowing the pressurized grout to flow up around the sides of the shaft, but this side bearing capacity, too, is not determinable with this technique.
SUMMARY OF THE INVENTION
It is therefore desirable to enhance the load bearing capacity of a drilled shaft foundation pier in a manner that permits direct measurement of the resultant end bearing and side bearing capacities of the pier.
Accordingly, an object of the present invention is to provide a simple and convenient technique for directly measuring the end bearing and side bearing capacities of a foundation pier.
Another object of the present invention is to allow a reduction in the safety factors in determining the load bearing capacity of a pier.
Another object of the present invention is to increase the end bearing and side bearing capacities of a foundation pier in a known amount.
Another object of the present invention is to use the same device to aid in measuring the load bearing capacity of a pier and increase its load bearing capacity.
In satisfaction of these and other objects, the invention preferably includes a bladder, cell, or other supporting enclosure placed at the base or within the length of a pier for receiving pressurized grout. The enclosure is filled with pressurized grout to stress the base of the pier. The known pressure of the grout can be used to calculate end bearing and side bearing capacities of the pier. Upon hardening under pressure, the supporting enclosure permanently contributes to increased end bearing and side bearing in a known amount. In the resulting pier assembly, the supporting enclosure in essence becomes an extension forming the lower end of the pier. The post-base-stressed pier assembly has end bearing and side bearing capacities that are enhanced, and are determinable by direct measurement, thus reducing the safety factor used in the pier load bearing capacity calculation.
In one embodiment, the supporting enclosure is a bladder made of a strong material such as thick rubber. The bladder is filled with pressurized grout via a conduit extending axially down the concrete pier to be post-base-stressed. The grout hardens under pressure, and the actual end bearing capacity is calculated from the pressure and the area of the bottom of the shaft. Pressurization of the bladder pushes upward on the concrete pier portion, resulting in additional opposing skin friction in a known amount. Subsequent downward load is opposed by the end bearing, the original skin friction, an
Beck, III August H.
King Philip G.
A. H. Beck Foundation Company, Inc.
Akin Gump Strauss Hauer & Feld L.L.P.
Mayo Tara L.
Will Thomas B.
LandOfFree
Post stressed pier does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Post stressed pier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Post stressed pier will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2851558