Patent Application
20160251819
A method and apparatus for static load-bearing capacity testing of helical piles is provided.
Anchoring systems forming the foundations of buildings or other large structures are commonly used where adequate bearing capacity cannot be found to support the structural loads. Helical piles, which have a pile shaft with one or more spiral helical plates affixed thereto, can be rotated into the ground to support structures, providing a versatile and efficient alternative to conventional pile systems. It is well known that a pile's capacity is highly dependent upon the pile's configuration and the surrounding soil conditions, and that axial capacity of each pile in both tension and compression must be tested with significant accuracy prior to commencing construction of the supported structure. Such tests need to account for the impact of the number and size of plates upon the pile's capacity. Thus, there is a desire for precise and reliable apparatus and methodologies for testing load capacity of helical piles.
Conventional “top-down” load testing methods typically involve driving a test pile into the ground, incrementally applying pressure directly to the top of the pile (using a hydraulic jack or ram), and then measuring axial (e.g. upward and/or downward) movement of the pile. Conventional tests can be used to determine the maximum pressure required to pull the pile from the soil and the maximum load that can be supported by the pile without failure.
Some load testing methods incorporate the use of a “reaction pile system”, which comprises the positioning of two “reaction piles” adjacent to the test pile, each reaction pile supporting a cross-beam braced against the test pile to monitor the pressure supplied by the jack and the associated displacement of the test pile.
Conventional “top-down” testing systems, however, require substantial loads to be applied to the test the piles (e.g., thousands of tonnes), resulting in extremely dangerous, costly, and time-consuming testing procedures. Further, at least two separate tests must be performed to determine the upward resistance and end-bearing capacity for each test pile.
U.S. Pat. Nos. 4,614,110 and 5,576,494 describe the Osterberg Cell®, or O-cell®, which is a load-generating cell designed to reduce the loads required in load capacity testing. The O-cell® is a hydraulically-driven cell installed at or near the bottom of drilled shafts, bored piles, driven piles, or other similarly constructed pile foundations. When the cell is pressurized, it generates loads bi-directionally along the shaft of the pile, that is—it expands to simultaneously apply force both upwardly and downwardly. The upward “pullout” force from the top of the cell is resisted by the shaft of the pile (i.e. providing the “skin-friction”) and the downward force from the bottom of the cell is resisted by the interaction between the soil and the pile (i.e. determining the “end-bearing capacity” or “resistance”). Because the O-cell® is irretrievably instrumented into the shaft of the pile, however, each cell must be sacrificed and cannot be reused. Further, because of its positioning within the pile shaft, the overall strength of the pile shaft is reduced, preventing it from withstanding the higher torque necessary to rotate the piles into the soil. As such, the O-cell® is not readily appropriate for use with helical piles.
U.S. Pat. No. 8,517,640 describes an expandable bi-directional static load capacity testing system that is adapted for use with helical piles. The system involves dividing the helical pile into shaft and “toe” sections, and positioning a jack-like apparatus (e.g. such as the O-cell®) between the shaft and toe sections, and positioning first and second helical plates above the jack-like apparatus. This system, however, still requires that the jack-like apparatus be sacrificed and that two separate tests must be performed to obtain accurate shaft resistance and end bearing capacity. Further, where multiple helices are desired, the system requires that at least two helices are positioned above and one below the O-cell®.
There is a need for an improved testing system that can more accurately determine the load capacity of helical piles, and particularly of helical piles having a plurality of helical plates. It is desired that such a testing system could provide accurate resistance and end-capacity information for each plate(s). It is further desired that much of the system could be salvaged for reuse.
Embodiments of the present apparatus and methodologies may be used for testing the load capacity of helical piles having any size (e.g. area), shape (e.g. angle) or number of helical plates, and may provide the shaft resistance and end-bearing capacity measurements for the pile (e.g. taking into account each helical plate or “helix”). It is understood that the skin resistance and end-bearing capacity of helical test piles can depend upon the number and size (area) of the plates, thus impacting the total capacity of the pile.
Broadly speaking, an apparatus for testing the load capacity of a helical pile positioned in soil is provided wherein the helical pile is operably connected to a reaction system having a cross beam and at least two reaction piles positioned in the soil adjacent to the helical pile and the apparatus comprises a bi-directional load-generating device for simultaneously generating tensile and compressive load on the helical pile, the helical pile, having a shaft with an upper end and a lower end, at least one load-transferring pipe, positioned within the pile shaft, the load-transferring pipe having an upper end and a lower end, the lower end of the helical pile and the load-transferring pipe configured to provide a hub and Kelly-bar arrangement, wherein the tensile and compressive loads impart simultaneous upward forces to the pile and downward forces to the load-transferring pipe for determining the shaft resistance and end-bearing capacity of the pile.
Broadly speaking, a method of testing the load capacity of a helical pile is provided, the pile being operatively connected to a pile reaction system, and the method comprising providing a helical pile, having at least one helical plate, the pile comprising an inner load-transferring pipe telescopically received within an outer pile shaft, the lower end of the load-transferring pipe and pile adapted to provide a Kelly-bar coupled to a tubular hub, providing a load-generating device for producing a tensile load on the pile and a compressive load on the load-transferring pipe, and measuring the pile resistance and end-bearing capacity of the pile.
According to embodiments herein, apparatus and methodologies for testing load capacity of helical piles is provided. The present apparatus and methodologies aim to provide a more accurate determination of both pile shaft resistance (e.g. “skin-friction”) and end-bearing capacity of helical test piles, particularly where the piles comprise multiple helical plates (i.e. “helices” or “bearing plates”). It is desired that the present system 100 provide an instrumented helical test pile with sufficient strength to withstand the rotational forces required to install the helical piles having a plurality of helical plates, such instrumentation being used to measure and determine the shaft resistance and end-bearing capacity. The present system will now be described having regard to the enclosed
By way of background, helical piles, also known as screw piles or screw anchors, are deep foundation elements that have circular steel plates pressed into a spiral shape with a uniform pitch around the pile shaft. Helical piles are well known and commonly used to transfer the load from the shaft into the surrounding soil. Helical piles are rotated or screwed into the soil at their “lead” or “toe” ends by the application of rotational forces, or torque, and/or axial downward forces.
It is known that the installation of helical piles can be assisted by the addition of one or more helices to each pile. As such, piles with multiple helices must be manufactured to withstand the torque, friction, and/or axial forces required to rotate the shaft into the ground. The materials used to manufacture helical piles (e.g. galvanized steel) is generally dictated by many factors including the required length of the pile shaft, the geographic location of the structure being built, the surrounding soil characteristics, the size and number of helical plates, and the anticipated load. Helixes are commonly attached to the pile shaft in such a manner as to allow the plates to displace the soil, rather than to excavate the soil. Helixes are commonly positioned along the pile shaft near the lead end being positioned in the ground.
Embodiments of the present apparatus and methodologies may be used to test the load capacity of helical piles having any size (e.g. area), shape (e.g. angle) or number of helical plates, and may provide the resistance and end-bearing capacity measurements for the pile (e.g. taking into account each helix). It is understood that the skin resistance and end-bearing capacity of helical test piles can depend upon the number and size (area) of bearing plates, thus impacting the total capacity of the pile.
Having regard to
It is an aspect of the present system 100 that the pile-driving assembly 10a is positioned above ground and configured such that the assembly components can be easily replaced or reused, thereby minimizing costs associated with prior art systems where, for example, testing components are sacrificed with each use. The pile-driving assembly 10a may be operative to generate and apply bi-directional, self-repulsive tensile and compressive forces to the pile 12, simultaneously determining the resistance/skin-friction and end-bearing capacity of the helical pile 12.
The present pile-driving assembly 10a may comprise a load-generating device 2 (e.g. a hydraulic jack) operative to expand a bi-directional load cell 4 capable of imparting both tension and compression loads to the helical test pile 12. It is one aspect of the present system 100 that the size and capacity of the load-generating device 2 and load cell 4 are not limited by the size (e.g. inner diameter) of the helical pile 12. It also an aspect of the present system 100 that the above ground pile-driving assembly 10a (e.g. jack, load cell, etc.) and reaction system may be recovered, recycled and reused.
As will be described in more detail below, the tensile forces generated by the load cell 4 serve to “pull” the test pile 12 from the ground, the upward forces being resisted by the peripheral surface of the test pile 12 against the surrounding soil. It is an aspect of the present assembly 100 that such “resistance” of the pile 12 can be measured using a reaction system having a cross bar “C” and at least two reaction piles “R”. At the same time, the compressive forces generated by the load cell 4 serve to “push” the pile 12 into the ground, the downward forces being resisted by the at least one helical plates 14. It is an aspect of the present system 100 that such end-bearing capacity of the pile 12 can be measured by the load-testing assembly 10b.
The pile-driving assembly 10a will now be described in more detail having regard to
The present test pile 12 may comprise a hollow, or partially hollow central shaft having an upper (surface) 13 end and a lower (“toe” or “lead”) end 15. As would be known, a substantial portion of the pile 12 shaft may penetrate the soil to be tested, with a smaller portion of the pile 12 shaft exposed aboveground (e.g. upper end 13). Pile 12 may be any desired diameter and length, and may be constructed from any appropriate material, as would be appreciated by a person skilled in the art. The pile 12 may be constructed from galvanized steel. Further, lower end 15 of the pile 12 may be configured in any manner to facilitate rotational penetration into the soil. For example, lower end 15 may be conical in shape, or any other such appropriate shape for penetrating the soil. Each test pile 12 may comprise one or more helical or load-bearing plates 14 affixed at or near the lower end 15 of the pile shaft. It is appreciated that any size, shape or number of helical plates 14 may be positioned in longitudinal spaced relation along the shaft.
As above, the pile-driving assembly 10a comprises load-cell 4 for imparting bi-directional loads. Load-generating device 2 and load cell 4 may be positioned within a hanging system, or housing, comprising upper and lower plates 16,17, respectively positioned above and below the assembly 10a, and connected together via adjustable connection means, such as a plurality of threaded nut and bolt assemblies 18. Housing elements 16,17,18 may be operatively connected to the pile 12 so as to impart forces from the load cell 4 to the pile 12. It is an aspect of the present system that components of the pile-driving assembly 10a may be pre-assembled, or assembled on site, at least in part due to the adjustability of the housing elements 16,17,18. Housing may further comprise spacer 19 mounted to upper plate 16 to absorb movement of upper plate 16 as it contacts cross bar “C” of the reaction system. Spacer 19 may be manufactured from any applicable impact-absorbing material, such as rubber or foam. According to embodiments herein, the hanging system containing the pile-driving assembly 10a may be suspended from the cross bar “C”, enabling axial movement of the pile 12 within the soil.
In operation, as the load-generating device 2 causes load cell 4 to generate bi-directional force which initially pushes upper plate 16 upwardly, raising housing towards cross beam “C”. Via nut/bolt assemblies 18, upper plate 16 pulls lower plate 17, which is operatively connected to upper end 13 of the pile 12, upwardly to convey the tensile forces to the pile 12 and to pull it from the ground. The “pull” is resisted by the peripheral surface of the pile 12. The pile 12 may continue to move upwardly until spacer 19 contacts cross beam “C” and causes cross beam “C” to be pushed upwardly. As would be known, the upward movement of cross beam “C” is resisted by the at least two reaction piles “R” and can be measured to determine the skin friction of the pile 12.
As would be understood that the at least two reaction piles “R” may or may not also be helical piles having at least one helical plate, and are each driven into the soil to be tested to a predetermined depth and a predetermined distance from the test pile 12. Cross beam “C” may be fixedly mounted at each end onto the reaction piles “R”, provided that reaction piles “R” having sufficient strength to endure and/or withstand axial tensile forces and bending movement resulting from the load test.
As operation continues (i.e. at the same time as the load-generating device 2 causes load cell 4 to generate tensile forces), the bi-directional load cell 4 also generates equal and opposed compressive force (i.e. downwardly to the subsurface load-testing assembly 10b) on the pile 12. More specifically, as skin friction is being mobilized, the drive assembly 10a continues the load test by applying compressive force to the load-testing assembly 10b below ground to measure the end-bearing capacity of the pile 12. The load-testing assembly 10b will now be described in more detail having regard to
It is an aspect of the present system 100 that the load-testing assembly 10b is instrumented into, and integral with, the shaft of the helical pile 12, such that when the pile 12 is removed from the ground, the load-testing assembly 10 may subsequently be replaced or reused, minimizing costs associated with prior art systems where, for example, testing components are sacrificed with each use). Broadly speaking, the present load-testing assembly 10b comprises a load-transferring system having a Kelly-bar and hub type apparatus integral to the shaft of the pile 12 (described in more detail below).
More specifically, the present load-testing assembly 10b may generally comprise an inner load-transferring pipe 22 telescopically positioned within the outer helical test pile 12 and operative to receive compressive forces from the load cell 4. In embodiments herein, one or more load-transferring pipes 22 having an upper end 23 and a lower end 25 may be slidably received within, and coaxially aligned with, the shaft of the test pile 12. Each upper and lower end 23,25 of pipe 22 may be adapted to be connected end-to-end to additional piping 22 thereabove or therebelow, enabling the load-transferring pipe 22 to be any desired length within the pile 12. In embodiments herein, one or more load-testing pipe 22 may be directly connected end-to-end, or alternatively may be connected together via spacers. Compression forces imposed upon the load-transferring pipe 22 can cause the pile 12 to penetrate deeper into the soil.
Having regard to
Having regard to
As above, the Kelly-bar 26 is in telescopic arrangement with hub 28, such that Kelly-bar 26 extends in response to compressive forces. Hub 28 may comprise a tubular component having a diameter substantially similar to the diameter of pile 12. The diameter of hub 28 may correspond to that of the pile shaft, and hub 28 may be securely affixed to the lower end 15 of the shaft. Preferably, hub 28 is welded directly to the lower end 15 of the shaft. The diameter of kelly-bar 26 may be slightly less than the diameter of tubular hub 28, such that Kelly-bar 26 may be slidably received within hub 28. As would be understood, the inner surface of the hub 28 may be configured to substantially correspond with the external surface of the Kelly-bar 26, such that torqueing of the Kelly-bar 26 imparts rotational movement to the hub 28.
It is an aspect of the present load-testing assembly 10b that Kelly-bar 26 be configured so as to withstand the rotational torque required to install the helical test pile 12. In one embodiment, Kelly-bar 26 may comprise a hexagonal cross-sectional profile, as depicted in
Having regard to
According to embodiments herein, the present system 100 may be used to test the load capacity of helical piles 12 having a plurality of helical plates 14. It is contemplated that the plates 14 may be positioned on the shaft of the pile 12 and/or the lower portion of the Kelly-bar 26. The present system 100 may allow for the load testing of a helical pile 12 by instrumenting a load-testing system within the pile, whether the system is positioned above, below, or in between the helical plates 14.
Helical plates 14 may be affixed to the lower end 15 of the pile 12, and preferably at least one of the helical plates 14 may be positioned along the lower portion of Kelly-bar 26. It is understood that one or more helical plates 14 may be positioned longitudinally along the outer surface of the pile 12 and that the length of the pile 12 may be any such length desired to support helical plates 14. It is contemplated that one or more helical plates 14 may be spaced longitudinally along Kelly-bar 26, and that the length of Kelly-bar 26 may be any such length desired to support helical plates 14.
Having regard to the Kelly-bar 26, in embodiments herein, Kelly-bar 26 may comprise lower limiting plate 32, the plate 32 having a diameter similar or substantially similar to Kelly-bar 26 and a plurality of radial apertures for receiving threaded screws 33. Accordingly, the design of the present system provides a helical pile having an instrumented load-testing system integral therewith without limiting the strength of the pile shaft or its ability to withstand the rotational forces required to drive the pile into the ground.
Embodiments of the present system 100 will now be described with reference to the following examples.
By way of example, having regard to
The load test can begin with the activation of hydraulic jack 2 to impart bidirectional expansion of the load cell 4 to impart bidirectional expansion of the cell 4. Initially, tensile forces imparting on the pile 12 cause the pile to pull upwardly out of the ground until upper cap plate 16 (and spacer 19) of housing reaches the cross beam “C” of the reaction system. Beam “C” is pushed upwardly for a predetermined distance (e.g. approximately one inch) until movement of the cross beam “C” transfers to the anchored reaction piles “R”, preventing further movement of beam “C”. At this point, the peripheral resistance or “skin friction” of the pile 12 can be considered to be fully mobilized and can be measured.
Next, (albeit simultaneously), the hydraulic jack 2 and load cell 4 imparts compressive force on the load-transferring pipe 22, causing downward movement of the pipe 22 and thus of Kelly-bar 26, causing Kelly-bar 26 to extend from hub 28 into the soil below the pile 12. Displacement of the Kelly-bar 26 from hub 24 creates a gap therebetween, and enables measurement of the end-bearing capacity of the pile 12 and specifically of the at least one helical plates 14 affixed thereto. It would be understood that in order for the compressive load to push pile 12 downwardly, the required capacity of the reaction system must be greater than the end-bearing capacity of the pile 12, said reaction system capacity being calculated as: skin friction+total uplift capacity of the two reaction piles +self-weight of the reaction system.
By way of example, the present system 100 may provide for the pile capacity (Qtotal) from the skin friction of the shaft (Qshaft)+the end-bearing capacity (Qend), where Qshaft=Ashaft×fs (unit skin friction), and where Qend is Ahelix×qb (end-bearing pressure).
After testing is complete, the entire drive assembly 10a may be disassembled and removed for reuse.
While particular embodiments have been shown and described in the present Detailed Description, it is understood that changes and modifications can be made without departing from the present invention and the claims below.
This application claims priority to U.S. Patent Application No. 62/126,252 entitled “Method and Apparatus for Testing Helical Piles”, filed Feb. 27, 2015, incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62126252 | Feb 2015 | US |
