Post-tension pile anchor foundation and method therefor

• US 7,618,217 B2
• Filed: 05/07/2007
• Issued: 11/17/2009
• Est. Priority Date: 12/15/2003
• Status: Expired due to Term

• Current Assignee: Terracon Consultants Incorporated
• Original Assignee: Allan P. Henderson
• Inventors: Henderson, Allan P.
• Primary Examiner(s): MAYO-PINNOCK, TARA LEIGH
• Application Number:
  US11/797,772
• Publication Number:
  US 20070269273A1
• Time in Patent Office: 925 Days
• Field of Search: 522/234, 52294-296, 527/411, 527/411.4, 527/411.5, 405/233, 405/236, 405/239, 405/249, 405/251, 405/256
• US Class Current: 405/255
• CPC Class Codes: E02D 27/12 Pile foundations
• CPC Class Codes: E04H 12/085 Details of flanges for tubular masts

Abstract

A post-tensioned pile anchor foundation and method therefor creates a void or highly compressible region between a drilled or driven pile anchor surrounding a post-tensioned anchor bolt or tendon and the bottom of a foundation cap. The void or compressible region can be formed by compressible spacers or void forming elements placed between the top of the pile anchor and the concrete cap. As the anchor bolt or tendon is post-tensioned against the cap, the void forming element is compressed or crushed, allowing the cap to be pulled downwardly and the pile anchor to be pulled upwardly toward the cap against the resistance of the surrounding soil.

Claims

1. A post-tensioned concrete foundation for supporting on its upper surface a tower or other structure subject to high upset forces which comprises:

a cementious foundation cap for supporting a tower or other structure from an upper surface and having a lower surface;

a plurality of cementious pile anchors spaced from one another and extending downwardly from said foundation cap lower surface into surrounding soil underneath said foundation cap;

a plurality of tensioning elements each having a lower end adjacent and bonded to a bottom portion of a respective one of said pile anchors, and extending upwardly within an elongated sleeve through each said pile anchor and said foundation cap, and terminating at an upper end above said foundation cap upper surface; and

a void forming element for creating a void space or highly compressible region between a top of each of said pile anchors and an adjacent portion of said foundation cap lower surface which, upon tensioning of said tensioning elements, allows said foundation cap to move downwardly against soil adjacent said foundation cap lower surface and said pile anchors to move upwardly into said void space or compressible region to at least partly reduce a size of said void forming element.

2. The concrete foundation of claim 1, wherein said void forming element is a molded plastic form or a compressible disc.

3. The concrete foundation of claim 1, wherein said elongated sleeve is made from PVC tubing.

4. The concrete foundation of claim 1, wherein said tensioning elements are solid tendons or bolts.

5. The concrete foundation of claim 1, further comprising an anchor plate surrounding the upper end of each tensioning element and a tension nut threaded onto said upper end to retain elongation of said pile anchor bolts upon tensioning.

6. The concrete foundation of claim 1, wherein said foundation cap is generally cylindrical and said cementious pile anchors with tensioning elements therein are circumferentially spaced around a central axis of said foundation cap.

7. The concrete foundation of claim 1, further comprising a plurality of tower post-tensioning elements each extending between an embedment ring in a lower portion of the foundation cap upwardly through said foundation cap to an upper end projecting above the cap upper surface for engaging a base plate of a tower or other structure to be supported on said foundation cap.

8. The concrete foundation of claim 7, wherein each of said tower post-tensioning elements is encased in an elongated sleeve from said embedment ring to adjacent said respective upper end to permit free elongation of said tower post-tensioning element through said foundation cap upon post-tensioning.

9. The concrete foundation of claim 1, wherein said foundation cap includes reinforcing steel rebar to resist bending forces in the foundation cap.

10. The concrete foundation of claim 1, wherein said void forming element is selected from the group consisting of an open bottom disk, a disk made of expanded polyurethane or styrofoam, an inflatable or pressurized bladder, and any other material that is compressible or crushable under 1 psi pressure.

11. A method for forming a post-tensioned pile anchor foundation for supporting on its upper surface a tower or other structure subject to high upset forces which comprises the steps of:

(a) excavating a hole in a ground surface;

(b) forming a plurality of pile holes in soil below said excavated hole;

(c) suspending an elongated post-tensioning element centrally in each pile hole, said post-tensioning element extending from a bottom portion of said pile hole upwardly above said ground surface;

(d) filling each of said pile holes with a cementious material;

(e) allowing said cementious material in each pile hole to cure and solidify to form pile anchors which bond only with a lower end of a post-tensioning element associated with said pile anchor;

(f) inserting a void forming element on top of said pile anchor cementious material and filling said excavated hole in said ground surface with cementious material so that said void forming element forms a highly compressible region above a top of each pile anchor and below an adjacent bottom portion of said cementious material filling said excavated hole;

(g) allowing said cementious material in said excavated hole to cure and solidify around, without bonding to, said post-tensioning element to form a foundation cap while maintaining said highly compressible region; and

(h) tensioning said post-tensioning elements from above said foundation cap to elongate said post-tensioning elements above said lower end and cause said foundation cap to compact downwardly against soil at a bottom surface of said excavated hole and said pile anchors to move upwardly into said highly compressible region to at least partly reduce a size of said void forming element.

12. The method of claim 11, wherein the pile hole is filled with cementious material before the elongated post-tensioning elements are inserted into the pile hole.

13. The method of claim 11, further comprising the step of filling any remaining void space with grout or other cementious material after step (h).

14. The method of claim 11, wherein said excavated hole is circular or annular.

15. The method of claim 11, further comprising the step of suspending tower post-tensioning elements and an embedment ring in said excavated hole after step (e) and before step (f).

16. The method of claim 15, further comprising the step of placing reinforcement rebar in said excavated hole after step (e) and before step (f).

17. A post-tensioned concrete foundation for supporting on its upper surface a tower or other structure subject to high upset forces which comprises:

a generally cylindrical cementious foundation cap having an upper surface and a lower surface;

an embedment ring in a lower portion of said foundation cap;

a plurality of tower post-tensioning elements attached at their lower end to said embedment ring and extending upwardly through said foundation cap to above the cap upper surface for engaging a base plate of a tower or other structure to be supported thereon;

a plurality of cementious pile anchors spaced from one another and extending downwardly from said foundation cap lower surface into soil underneath said foundation cap, each of said pile anchors being individually surrounded by adjacent soil;

a plurality of tensioning elements each having a lower end adjacent and bonded to a bottom portion of a respective one of said pile anchors, and extending upwardly through an upper portion of each said pile anchor and said foundation cap, and terminating at an upper end above said foundation cap upper surface; and

said tensioning elements being post-tensioned to cause said foundation cap to compress downwardly against soil adjacent said foundation cap lower surface and cause said pile anchors to respectively pull upwardly and develop skin friction resistance against said adjacent soil.

18. The concrete foundation of claim 17, wherein said tensioning elements are solid tendons or bolts.

19. The concrete foundation of claim 17, wherein said foundation cap is generally cylindrical, with or without a central bottom opening.

20. The concrete foundation of claim 17, further comprising a plurality of void forming elements placed on top of said plurality of pile anchors, respectively, for creating highly compressible regions between a top of each of said pile anchors and an adjacent portion of said foundation cap lower surface, said highly compressible regions, upon tensioning of said tensioning elements, allowing said foundation cap to move downwardly against soil adjacent said foundation cap lower surface and allowing said pile anchors to move upwardly into said respective compressible regions to at least partly reduce a size of said void forming elements and thereby develop skin friction resistance with said adjacent soil.

21. A post-tensioned concrete foundation for supporting on its upper surface a tower or other structure subject to high upset forces which comprises:

a cementious foundation cap for supporting a tower or other structure from an upper surface and having a lower surface;

a plurality of individual cementious pile anchors spaced from one another, each extending downwardly from said foundation cap lower surface into surrounding soil underneath said foundation cap;

a plurality of elongated post-tensioning elements each having a lower end adjacent and bonded to a bottom portion of each of said pile anchors, and extending upwardly through an upper portion of said pile anchor and said foundation cap, and terminating at an upper end above said foundation cap upper surface; and

a pile anchor base plate embedded in said foundation cap upper surface for post-tensioning said post-tensioning elements.