Patent Publication Number: US-9416513-B2

Title: Helical screw pile and soil displacement device with curved blades

Description:
FIELD OF THE INVENTION 
     The invention relates to foundation systems, in particular, helical pile foundation systems, which use a screw to pull a shaft and a soil displacement device through the ground. 
     BACKGROUND OF THE INVENTION 
     Piles are used to support structures where surface soil is weak by penetrating the ground to a depth where a competent load-bearing stratum is found. Helical (screw) piles represent a cost-effective alternative to conventional piles because of their speed and ease of installation and relatively low cost. They have an added advantage with regard to their efficiency and reliability for underpinning and repair. A helical pile typically is made of relatively small galvanized steel shafts sequentially joined together, with a lead section having helical plates. The pile is installed by applying torque to the shaft at the pile head, which causes the plates to screw into the ground with minimal soil disruption. 
     The main drawbacks of helical piles are poor resistance to both buckling and lateral movement. Greater pile stability can be achieved by incorporating a portland-cement-based grout column around the pile shaft. See, for example, U.S. Pat. No. 6,264,402 to Vickars (incorporated by reference herein in its entirety), which discloses both cased and uncased grouted screw piles and methods for installing them. The grout column is formed by creating a void in the ground as the shaft descends and pouring or pumping a flowable grout into the void to surround and encapsulate the shaft. The void is formed by a soil displacement disk attached to the shaft above the helical plate(s). The grout column may be reinforced with lengths of steel rebar and/or polypropylene fibers. A strengthening casing or sleeve (steel or PVC pipe) can also contain the grout column. However, because the casing segments are rotated as the screw and the shaft advance through the soil, substantial torque and energy are required to overcome frictional forces generated by contact with the surrounding soil. More effective compaction of the surrounding soil would reduce skin friction during installation and lessen damage to the casing. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a soil displacement device for penetrating and forming a void in the ground when rotated about a central longitudinal axis by a helix-bearing shaft. The device comprises a disk having a periphery, a top, a bottom and a central opening for receiving a shaft. At least two blades are disposed below the top of the disk. Each blade projects substantially axially from the bottom of the disk to a free distal end and curves outward from near the opening to at least the periphery of the disk. The blades preferably extend beyond the disk periphery, and the radius of curvature of each blade preferably is non-uniform. Each blade preferably tapers toward its distal end, and the bottom of the disk preferably tapers toward its periphery. The top of the disk may carry an axially extending adapter ring that defines an annular seat on the disk for centering a tubular casing. 
     Another aspect of the invention is a helical screw pile for penetrating the ground and forming a support. The screw pile comprises a shaft having a longitudinal axis and a bottom end, at least one helical plate on the shaft near the bottom end and a soil displacement device, as described above, on the shaft above the helical plate. Each blade of the soil displacement device preferably has an axial height that is greater than the axial pitch of the helical plate(s) divided by the number of blades. The shaft may comprise sequentially connected segments including a lead shaft and extension shafts, the lead shaft carrying at least the helical plate(s). The soil displacement device is carried by either the lead shaft or one of the extension shafts, and an extension displacement plate may be located above the soil displacement device, the extension displacement plate having oppositely facing annular seats for centering tubular casings surrounding the extension shafts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Embodiments of the disclosed invention, which include the best mode for carrying out the invention, are described in detail below, purely by way of example, with reference to the accompanying drawing, in which: 
         FIG. 1  is a perspective view of an assembled helical pile according to the invention shown without a surrounding grout column or casing; 
         FIG. 2  is a perspective view of a soil displacement device according to the invention used in the pile of  FIG. 1 ; 
         FIG. 3  is a perspective view of an extension displacement plate according to the invention used in the pile of  FIG. 1 ; 
         FIG. 4  is an exploded perspective view of the soil displacement device of  FIG. 2  shown with an optional insert; 
         FIG. 5  is a bottom perspective view of the soil displacement device and insert of  FIG. 4  assembled together; 
         FIG. 6  is a top plan view of the assembly of  FIG. 5 ; 
         FIG. 7  is a bottom plan view of the assembly of  FIG. 5 ; 
         FIG. 8  is right side view of the assembly of  FIG. 7 ; 
         FIG. 9  is a sectional view taken along line  9 - 9  in  FIG. 8 ; 
         FIG. 10  is an exploded perspective view of the extension displacement plate of  FIG. 3  shown with an optional insert; 
         FIG. 11  is a bottom perspective view of the extension displacement plate and insert of  FIG. 10  assembled together; 
         FIG. 12  is a top plan view of the assembly of  FIG. 11 ; 
         FIG. 13  is a bottom plan view of the assembly of  FIG. 11 ; 
         FIG. 14  is a right side view of the assembly of  FIG. 13 ; and 
         FIG. 15  is a sectional view taken along line  15 - 15  in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a helical pile according to the invention has a central screw pier  10  comprising 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, 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 can be installed above those shown in like manner until a competent load-bearing stratum is reached. 
     A conventional lead shaft  12  at the lower end of the pile carries helical plates  14   a ,  14   b  that advance through the soil when rotated, pulling the pile downward. In the illustrated example, the soil displacement device (lead displacement plate)  20  is attached to lead shaft  12  above helical plate  14   b  together with a first extension shaft  16 . A second extension shaft  18  is joined to first extension shaft  16  with an interposed extension displacement plate  50 , and so on with additional extension shafts and extension displacement plates  50  to the top of the pile. Lead displacement plate  20  preferably is located at a position such that it will encounter and ultimately come to rest in or near relatively loose soil. Thus, depending on the soil conditions in the various strata, lead displacement plate  20  could be carried by one of the extension shafts  16 ,  18 , etc. instead of by lead shaft  12 . Furthermore, additional lead displacement plates  20  could be used instead of extension displacement plates  50  along all or part of the length of the pile. 
     Referring to  FIGS. 4-9 , lead displacement plate  20  is made of steel and comprises a disk  22  having a circular periphery  24  and a square central through-opening  26  for receiving a close-fitting shaft or, optionally, a close-fitting insert  70 , which has a smaller square through-opening  72  for receiving a smaller shaft. An integral adapter ring  28  extends axially from the top of disk  22  inboard of the disk periphery  24 , thus defining an annular seat  30  for centering an optional tubular casing (used for forming a cased pile), which fits over the adapter ring. As seen in  FIGS. 8 and 9 , the distal portion of the outer face  32  of adapter ring  28  is tapered to facilitate mating with a range of casing sizes. 
     Two integral, identical, curved blades  34  project axially from the bottom  36  of disk  22  to their free distal edges  35 . The blades are symmetrically positioned about the central axis of the disk, 180° apart. The disk may be provided with a greater number of blades, and all should be identical and symmetrically positioned about the central axis. As best seen in  FIG. 8 , the distal edges  35  of the blades are substantially coplanar and substantially normal to the disk&#39;s central axis X. To minimize soil-to-disk friction from downward installation forces, the axial height of the blades should be greater than the axial pitch of the helical plate(s) divided by the number of blades. The curvature of the blades increases the strength of the disk and reduces the jerk observed with straight-bladed disks during installation through soil transitions and impurities. 
     Each blade  34  has a leading (convex) face  38  and a trailing (concave) face  40 . As best seen in  FIGS. 7 and 8 , the leading faces  38  are substantially parallel to the disk&#39;s central axis X. As viewed in  FIG. 7 , the direction of rotation R of the lead displacement plate is counterclockwise whereby the leading blade faces  38  push soil outward. Each blade preferably is tapered on its trailing (concave) face  40  (see  FIGS. 5, 7 and 9 ), which facilitates manufacture and locates more material at and near the blade root, where higher reaction forces are required. As best seen in  FIG. 7 , the curvature of each blade preferably is non-uniform; specifically, the blade&#39;s radius of curvature preferably is larger near the central opening  26  and near the disk&#39;s periphery  24  than its radius of curvature in the intermediate portion. The blades preferably extend beyond the disk&#39;s periphery  24 , where a portion of each blade preferably is substantially normal to a radius of the disk, thus tending to smooth the cavity wall as the disk rotates. This arrangement also enhances blade-to-disk strength, adds stability and enhances soil packing to make for a solid cavity wall and reduced friction when installing casing. 
     Disk  22  is thicker in its central region, its bottom  36  tapering uniformly from near central opening  26  toward its periphery  24  (see  FIGS. 8 and 9 ). The thicker central region enables greater torque transfer from the shaft to the disk and enhances disk stability as it rotates with the shaft (disk stability is important in forming and maintaining a solid cavity wall). As the shaft rotates it moves the disk deeper, so soil is moved from the lower (innermost) blade area to the upper (outermost) portion of the blade and the underside of the disk. The tapered bottom  36  increases soil penetration per normal force unit and allows for shorter blades while displacing the same amount of soil per revolution, reducing installation torque by reducing friction. Reduced installation torque results in increased tension and compression capacity of the installed pile under load. 
     Referring to  FIGS. 10-15 , extension displacement plate  50  is made of steel and comprises a central disk  52  having a circular periphery  54  and a square central opening  56  for receiving a close-fitting shaft or, optionally, a close-fitting insert  70 , which has a smaller square opening  72  for receiving a smaller shaft. Two integral adapter rings  58  extend axially from the disk  52  in opposite directions inboard of the disk periphery  54 , thus defining annular, oppositely facing seats  60  for centering optional tubular casings (used for forming a cased pile), which fit over the adapter rings. As best seen in  FIGS. 14 and 15 , the distal portion of the outer face  62  of each adapter ring  58  is tapered to facilitate mating with a range of casing sizes. Four holes  64  in the disk allow grout to flow through the disk and fill any voids on the other side. 
     Inserts allow for different styles of shafts to be used with lead displacement plate  20  and extension displacement plates  50 . In the illustrated embodiment, each insert  70  has a square opening  72  for mating with a square shaft. Four lips  74  surround the opening at one end and form disk-engaging shoulders. Nubs  76 , one on each of two opposite sides of the insert near its other end, retain the insert in position after it is forced into a central disk opening  26  or  56 . 
     While preferred embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined by the appended claims.