Cementitious foundation cap with post-tensioned helical anchors and method of making the same
A post-tensioned concrete cap foundation has helical anchors with pipes having several helical discs welded around the pipe perimeter to spin drill deep into subsurface soils or other soft materials with holes in the pipe for high pressure-grouting in place. The helical anchor pipes include a tensioning element for pulling and post-tensioning the helical anchor. The helical anchors are tension anchors which can be converted to compression anchors. The helical anchors in tension serve to pull the foundation cap down to compress the underlying soil while the compression anchors limit the maximum settlement of the concrete foundation cap. The foundation also includes perimeter-forming and interior corrugated metal pipes with upper and lower sleeved horizontally extending radial bolts that are secured to the pipes and post-tensioned to provide lateral foundation compression.
1. A method for forming a post-tensioned concrete foundation with helical anchors and a concrete foundation cap for supporting on its upper surface a structure subject to high upset and dynamic forces comprising the steps of:
a) Preparing a ground surface for said foundation;
b) Drilling a plurality of helical anchors to depth, each helical anchor including a helical anchor pipe that extends into the ground and a tensioning element atop said anchor pipe;
c) Setting sleeves over said tensioning elements to enable post-tensioning of said helical anchors;
d) Pouring a concrete/slurry leveling course encasing electrical, communication, and grounding trench with conduits if conduits are routed under the foundation;
e) After concrete/slurry cures, pouring the concrete foundation cap with cementitious material;
f) Allowing said cementitious material in said concrete foundation cap to cure and solidify around, without bonding to, said tensioning elements; and
g) Post tensioning the helical anchors from above the concrete foundation cap using the tensioning elements.
2. The method of claim 1 , further comprising, before the step of drilling, the step of coupling a plurality of linearly aligned hollow bars end to end with couplers to form a hollow bar helical anchor pipe, said hollow bar helical anchor pipe having helical discs and said couplers having grout holes formed therein.
3. The method of claim 2 , further comprising the step of pressure grouting the hollow bar helical anchor pipes to force grout out through said grout holes and around said helical discs for ground improvement around the hollow bar helical anchor pipe and helical discs to improve the soil strength, increase the anchor size and improve the bond between the helical anchor pipe and the soil to increase the anchor pullout or downward load resistance thus increasing the foundation loading capacity and stiffness.
4. The method of claim 3 , further including the step of isolating certain zones of the hollow bar helical anchor pipe for pumping measured grout quantities and pressure to specific zones using a packer.
5. The method of claim 1 , further comprising the steps of:
positioning corrugated pipes interior to and around a perimeter of said concrete foundation cap; and
placing sleeved radial bolts or tendons horizontally across the foundation and securing the radial bolts to the corrugated pipes.
6. The method of claim 5 , further comprising the step of tensioning the sleeved horizontally extending radial bolts or tendons from outside the perimeter corrugated metal pipe.
7. The method of claim 1 , further comprising adding a steel plate topped with a compressible material below the foundation cap to provide compression anchor capabilities to some of the helical anchors to limit the maximum settlement of the concrete foundation cap, said compressible material allowing the concrete foundation cap to be pulled down so the steel plate contacts the bottom of the concrete foundation cap, limiting additional concrete foundation cap settlement.
8. The method of claim 1 , further comprising the step of pressure grouting each of the helical anchor pipes through grout holes formed in the pipes.
9. A method for forming a post-tensioned concrete foundation with helical anchors and a concrete foundation cap for supporting on its upper surface a structure subject to high upset and dynamic forces comprising the steps of:
drilling a plurality of helical anchors to depth in a ground surface, each helical anchor including a helical anchor pipe that extends into the ground and a tensioning element atop said anchor pipe;
pouring the concrete foundation cap with cementitious material over the plurality of helical anchors, said tensioning elements extending upwardly through said concrete foundation cap;
allowing said cementitious material in said concrete foundation cap to cure and solidify around, without bonding to, said tensioning elements; and
post tensioning the helical anchors from above the concrete foundation cap using the tensioning elements.
10. The method of claim 9 , further comprising the step of setting sleeves over said tensioning elements before said step of pouring the concrete foundation cap.
11. The method of claim 10 , further comprising the step of coupling a plurality of linearly aligned hollow bars end to end with couplers to form a hollow bar helical anchor pipe, said hollow bar helical anchor pipe having helical discs and said couplers having grout holes formed therein.
12. The method of claim 11 , further comprising, after the step of setting sleeves over said tensioning elements and before pouring the cap, the step of pressure grouting the hollow bar helical anchor pipes to force grout out through said grout holes and around said helical discs for ground improvement around the hollow bar helical anchor pipe and helical discs to improve the soil strength, increase the anchor size and improve the bond between the helical anchor pipe and the soil to increase the anchor pullout or downward load resistance thus increasing the foundation loading capacity and stiffness.
13. The method of claim 12 , further including the step of isolating certain zones of the hollow bar helical anchor pipes and pumping grout in measured quantities and pressure to specific zones in said pipes using a packer.
14. The method of claim 11 , further comprising, before the step of pouring the cap, the step of placing a steel plate topped with a compressible material on the hollow bar at or above ground level, the compressible material providing compression anchor capabilities to some of the helical anchors to limit the maximum settlement of the concrete foundation cap, said compressible material allowing the concrete foundation cap to be pulled down so the steel plate contacts the bottom of the concrete foundation cap, limiting additional concrete foundation cap settlement.
15. The method of claim 11 , further comprising, after the step of pouring the cap, the step of pressure grouting the hollow bar helical anchor pipes to force grout out through said grout holes and around said helical discs for ground improvement around the hollow bar helical anchor pipe and helical discs to improve the soil strength, increase the anchor size and improve the bond between the helical anchor pipe and the soil to increase the anchor pullout or downward load resistance thus increasing the foundation loading capacity and stiffness.
16. The method of claim 15 , further including the step of isolating certain zones of the hollow bar helical anchor pipes and pumping grout in measured quantities and pressure to specific zones in said pipes using a packer.
17. The method of claim 9 , further comprising, before the step of pouring the cap, the steps of:
positioning corrugated pipes interior to and around a perimeter of said concrete foundation cap; and
placing radial bolts or tendons horizontally across the foundation and securing the radial bolts to the corrugated pipes.
18. The method of claim 17 , further comprising the step of placing sleeves over said radial bolts or tendons before placing said radial bolts or tendons horizontally across the foundation.
19. The method of claim 18 , wherein the step of post tensioning the helical anchors using the tensioning elements includes the step of tensioning the sleeved horizontally extending radial bolts or tendons from outside the perimeter corrugated metal pipe.
20. The method of claim 9 , wherein the step of drilling includes the steps of:
pressure grouting each of the helical anchor pipes through grout holes formed in the pipes; and
attaching the tensioning elements to upper ends of the helical anchor pipes.