Seismic restraint helical pile systems and method and apparatus for forming same

Abstract

A reinforced helical pile system suitable for use in seismically active areas incorporates steel fibers in the grout and a fiber reinforced polymer sleeve (casing). A low-friction driving assembly and low-friction sleeve couplings enable the sleeve to be drawn into the soil substantially without rotation, reducing power consumption and preserving the integrity of the casing.

Claims

1. A cased helical pile installed in soil, comprising: a segmented shaft having a screw near a lower end thereof; a radially outwardly projecting soil displacing member on the shaft near the screw; a segmented casing including a plurality of serially arranged, cylindrical sleeves surrounding the shaft, the lowest one of the sleeves disposed adjacent the soil displacing member; and at least one cylindrical sleeve coupling, each sleeve coupling surrounding the shaft and joining a pair of adjacent sleeves, each sleeve coupling including two axially opposed, annular coupling seats, each of the coupling seats abutting an end of one of the pair of adjacent sleeves, and wherein each sleeve coupling includes a center wall dividing the sleeve coupling into two oppositely facing recesses, each recess bounded by an annular outer side wall and forming the coupling seats, and wherein each of the annular coupling seats includes a self-lubricating washer abutting the center wall and a metallic washer abutting the self-lubricating washer and abutting an end of one of the sleeves.

2. The cased helical pile according to claim 1, wherein each of the self-lubricating washers is made of Teflon, and each of the metallic washers is made of steel.

3. The cased helical pile according to claim 1, further comprising grout substantially filling the interior of the casing and encasing the shaft, and wherein the grout is reinforced with steel fibers mixed into the grout.

4. The cased helical pile according to claim 3, wherein all of the sleeves are made of a fiber-reinforced polymer.

5. The cased helical pile according to claim 4, wherein the fiber-reinforced polymer includes continuous glass fibers wound in a matrix of aromatic amine cured by epoxy resin in a dual angle pattern.

6. The cased helical pile according to claim 3, wherein the grout is a Portland cement based and shrinkage compensated grout, and wherein the steel fibers account for about 1% of the grout mix by weight and are about 0.7 mm in diameter and about 30 mm long.

7. The cased helical pile according to claim 1, wherein the soil displacing member has a bottom seat facing axially away from the screw and abutting an end of the lowest one of the sleeves.

8. The cased helical pile according to claim 1, wherein each of the self-lubricating washers is made of Teflon.

9. The cased helical pile according to claim 1, wherein each of the metallic washers is made of steel.

10. The cased helical pile according to claim 1, wherein each sleeve coupling has an opening receiving the shaft.

11. The cased helical pile according to claim 1, wherein each of the sleeve couplings is rotatable relative to the sleeves.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain embodiments are described in detail below, purely by way of example, with reference to the accompanying drawing, in which:

(2) FIG. 1 is a schematic view in longitudinal section of the lower sections of a cased, grouted helical pile according to one embodiment;

(3) FIG. 2 is a perspective view in longitudinal section of a soil displacing coupling and pile shaft segment of the pile of FIG. 1;

(4) FIG. 3 is an exploded perspective view of a driving assembly usable to install the pile of FIG. 1;

(5) FIG. 4 is an exploded perspective view in longitudinal section of the driving assembly of FIG. 3; and

(6) FIG. 5 is a longitudinal sectional view of the assembled driving assembly taken along line 5-5 in FIG. 3.

DETAILED DESCRIPTION

(7) Referring to FIG. 1, a helical pile has a central screw pier 10 including a series of conventional steel shaft sections with mating male and female ends that are bolted together sequentially as the pile is installed, in a manner well known in the art. The shaft cross-section preferably is square, but any polygonal cross-section or a round cross-section, or a combination of cross-sections, may be used. The bottom three shaft sections are shown in FIG. 1, it being understood that additional shaft sections are installed above those shown in like manner. A conventional lead shaft 12 at the lower end of the pile carries helical flights 14 that advance through the soil when rotated, pulling the pier downward. A first extension shaft 16 is joined to lead shaft 12 within a soil displacing coupling 20, a second extension shaft 18 is joined to first extension shaft 16, and so on to the top of the pile. Casing sleeve sections 22, 24, etc. surround the shaft sections 16, 18, etc. above soil displacing coupling 20, each pair of adjacent sleeves being joined by a sleeve coupling 30, which also functions as a centralizer for the shaft. Grout G completely fills the casing to encase the screw pier.

(8) Referring to FIG. 2, soil displacing coupling 20 is made of steel and comprises a tapered central body 26, a bottom square elevation tube 28 and a top cup-shaped recess 32 formed by a cylindrical wall 34 and an annular inner web 36, which has a square hole 38 for passage of and rotational engagement with extension shaft 16. A bolt 40 through elevation tube 28, extension shaft 16 and lead shaft 12 (not shown) secures those three parts together. Cup-shaped recess 32 forms a seat for the end of sleeve 22. The seat optionally may have a low-friction insert including a self-lubricating (e.g., Teflon) washer 42, which abuts inner web 36, and a metallic (e.g., steel) washer 44, which is sandwiched between self-lubricating washer 42 and sleeve 22. Central body 26 optionally may be provided with one or more helical plates 42, which provide additional thrust when rotated to help advance the pier through the soil. The location of the bolt hole along elevation tube 28 is selected to properly position helical plate(s) 42 relative to the helical flights 14 on lead shaft 12.

(9) Enhanced strength and durability of the pile, especially for seismically active locations, is afforded by selecting the proper grout formulation, by uniformly including certain reinforcing elements in the grout mix at a certain concentration, and by using a certain type of reinforced casing material, which increases bending resistance. The grout preferably is high performance, Portland cement based and shrinkage compensated. A preferred grout is PT Precision Grout, manufactured by King Packaged Materials Company, Burlington, Ontario, Canada. Another suitable grout is MASTERFLOW 1341, manufactured by BASF Construction Chemicals, LLC, Shakopee, Minn. The grout reinforcing elements preferably are round-shaft cold drawn steel wire fibers, preferably on the order of 0.7 mm in diameter and 30 mm long, and preferably having flat ends that anchor well within the grout mix. A suitable example is NOVOCON FE 0730 steel fibers, manufactured by SI Concrete Systems, Chattanooga, Tenn., which conform to ASTM A820/A820M Type 1. The preferred grout mix contains about 1.00% of steel fibers by volume. The casing material (sleeve) is a fiber reinforced polymer (FRP), preferably constructed on continuous glass fibers wound in a matrix of aromatic amine cured epoxy resin in a dual angle pattern that takes optimum advantage of the tensile strength of the filaments. A suitable example is BONDSTRAND 3000A fiberglass pipe manufactured by Ameron International Fiberglass Pipe Group, Burkburnett, Tex., in accordance with ASTM 02996 Specification for RTRP. Such a pipe sized for use in helical piles would have a wall thickness on the order of about 2.0 to 3.0 mm. Greater bending resistance would be afforded by using custom-manufactured pipe as the casing.

(10) Testing of sample piles that combined FRP sleeves with the specified steel fiber reinforced grout as described in the preceding paragraph demonstrated assured integrity of the pile system during and after cyclic loading, allowing the pile system to sustain its axial capacity. See Y. Abdelghany and M. El Naggar, Full-Scale Experimental and Numerical Analysis of Instrumented Helical Screw Piles Under Axial and Lateral Monotonic and Cyclic LoadingsA Promising Solution for Seismic Retrofitting, presented Jun. 28, 2010 at the Sixth International Engineering and Construction Conference in Cairo, Egypt (incorporated by reference herein in its entirety). This testing demonstrated the above-described pile system as appropriate for highly seismic areas as it will maintain serviceability after severe lateral loading events.

(11) A pile driving assembly, usable to install a pile, will now be described with reference to FIGS. 3-5. Driving assembly 50 is shown interfaced with a generic pier shaft section X and generic sleeve sections Y, which are the particular shaft and sleeve sections being driven at any given state of pile installation. The same pertains to sleeve coupling and centralizer 30. A driving cap 52 has an annular end wall 54 and a depending annular side wall 56. An annular low-friction drive seat is formed in driving cap 52 by a self-lubricating (e.g., Teflon) washer 58, which abuts end wall 54, and a metallic (e.g., steel) washer 60, which is sandwiched between self-lubricating washer 52 and an end of upper sleeve section Y. The upper sleeve section Y may optionally be a short length of sleeve material or other pipe repeatedly used as a tool as successive shafts and sleeve sections are installed. Sleeve coupling 30 essentially resembles two driving caps 52 placed back-to-back, except that there is only a single annular central wall 62 that divides the coupling into two oppositely facing recesses bounded by annular side wall 64. Each recess has an annular low-friction drive seat similarly formed by a self-lubricating (e.g., Teflon) washer 66, which abuts central wall 62, and a metallic (e.g., steel) washer 68, which is sandwiched between self-lubricating washer 66 and an end of the adjacent sleeve section Y. A conventional square drive shaft tool 70, shown pinned to shaft X in FIG. 5, is adapted to be coupled to a conventional rotary tool head (not shown).

(12) Pile installation using the above driving assembly proceeds as follows. Lead shaft section 12 is screwed almost completely into the soil by a rotary tool head coupled to drive shaft tool 70. (Alternatively, initial soil penetration can be done with lead screw 12, soil displacement coupling 20 and sleeve 22 preassembled as shown in FIG. 1.) Tool 70 is then uncoupled, and first extension shaft 16 and soil displacing coupling 20 are bolted at 40 to the protruding upper end of lead shaft 12. A sleeve section 22 is then placed around extension shaft 16 and seated in cup-shaped recess 32 of the soil displacing coupling. (Sleeve section 22 should be short enough so as not to hamper connection of the next extension shaft 18.) Driving cap 52 is then placed over the upper end of sleeve section 22 and tool 70 is connected to shaft extension 16 and rotated to advance the pier and the sleeve into the soil as the soil displacing coupling creates a cylindrical void in its wake. Tool 70 is then uncoupled and the next extension shaft 18 is coupled to the upper end of the first extension shaft 16. A sleeve coupling 30 is then placed over the upper end of sleeve 22 followed by extension sleeve 24, which is seated in the opposite side of coupling 30. Driving cap 52 is then placed over the upper end of sleeve section 24 and tool 70 is connected to shaft extension 18 and rotated to advance the assembly into the soil. The process is repeated with subsequent shaft extensions, sleeves and sleeve couplings until a competent load-bearing stratum is reached. Grout is poured or pumped into the casing, preferably after all the sleeves are installed. Alternatively, the grout may be placed in the casing in batches: one batch after each sleeve section is installed.

(13) Whenever a sleeve section is placed in an annular low-friction seat, the seat interfaces preferably are lubricated with grease or other suitable lubricant to enhance the slipperiness of the interfaces. The low-friction characteristics of the annular seats may be provided by arrangements other than Teflon and steel washers, such as roller thrust bearings. The ability of the driving cap 52 and the sleeve couplings 30 to substantially freely rotate relative to the sleeve sections during pile installation advantageously enables the sleeve sections to be drawn into the soil by the lead screw (and pushed by the drive head, if necessary) substantially without rotation of the sleeve sections. This avoids the otherwise high frictional forces generated by constant rotational sleeve contact with the surrounding soil, reducing the amount of torque and energy needed for shaft rotation and minimizing abrasion of the sleeve.

(14) While preferred embodiments have been described and illustrated above, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope as defined by the appended claims.