Patent Publication Number: US-2002008328-A1

Title: Pile formation

Description:
[0001] This invention relates to the formation of piles for providing structural foundations for buildings or infra structures, in particular to the formation of concrete piles.  
       [0002] There are several different ways of forming a concrete pile in the ground. One way is to drill a bore hole into the ground with an auger and fill the hole with fluid concrete that is pumped through a fluid passage running down the length of the auger.  
       [0003] Another way is to create a bore hole by forcing a vibrating poker into the ground, withdrawing the poker and filling the resulting void with fluid concrete as the poker is being withdrawn. It is known to only partially fill the bore hole with concrete and then reinsert the vibrating poker in order to expand the cross sectional area of the lower part of the pile, before filling the remaining upper part of the bore with concrete. However, due to the substantial mass of the poker, the vibrations produced by this vibrating poker method can propagate some distance, making it unsuitable when there are buildings or other structures present nearby.  
       [0004] According to one aspect of the invention, there is provided a method of forming a concrete pile in the ground using an auger having a stem with a fluid channel communicating between an inlet towards the top end of the stem and an outlet located towards the step tip, the method comprising the steps of:  
       [0005] (a) creating a bore hole by screwing the auger into the ground in an axial direction;  
       [0006] (b) pumping fluid concrete into the bore hole through the fluid channel in the stem whilst retracting the auger from a lower portion of the bore, such that the lower portion of the bore becomes filled with a volume of fluid concrete;  
       [0007] (c) applying a pressure to the volume of fluid concrete by at least partly reinserting the auger into the lower portion of the bore, such that the applied pressure causes the volume of concrete to expand radially, thereby increasing the diameter of the lower portion of the bore; and,  
       [0008] (d) retracting the auger from the remainder of the bore whilst pumping further concrete into the bore, such that the bore fills with concrete and forms a concrete pile.  
       [0009] This will result in a generally cylindrical pile having an enlarged bulb like region at its lower end. Due to the increased cross sectional area at its lower end, the pile will have a greater load bearing capacity.  
       [0010] Because the bore hole is drilled with an auger, the bore hole can be formed in hard ground, and without significant vibration.  
       [0011] It will be appreciated that the auger need not be completely removed from the bore after having been partially reinserted into the lower portion of the bore a first time. Steps (b) and (c) may be repeated several times in a cyclic fashion, such that fluid concrete is pumped into the bore and compressed in stages until the lower portion of the bore has been expanded sufficiently.  
       [0012] It will be further appreciated that the term concrete includes cements and other self setting fluid building materials, and/or that the concrete may be of a granular nature.  
       [0013] Preferably, the auger will be reinserted into the lower portion of the bore by screwing the auger into the concrete, since the screwing motion will provide a mechanical advantage in applying a pressure to the concrete, with the result that pressure applied to the concrete is not solely due to the weight of the auger.  
       [0014] Preferably, pressure to pump the fluid concrete will be applied to the fluid concrete at the inlet of the fluid passage by a pump. The pumping pressure may be continuously applied with the -pump during steps (b) to (d). Preferably the pumping pressure will be in the range 0.3 bar to 3 bar. In particular, the pumping pressure will preferably be in the range 0.5 bar to 1 bar.  
       [0015] To retract the auger, the auger will preferably be rotated in the opposite sense to that in which it is rotated when it is inserted. However, the auger may be retracted by simply pulling the auger in an upward direction, without rotating it.  
       [0016] Concrete will preferably be continuously pumped into the bore as the auger is being retracted, so that the chance of a void being formed in the concrete is reduced. (It will be understood that as used herein, the term retraction also covers extraction).  
       [0017] To increase the pressure of the concrete in the lower portion of the bore, fluid concrete will preferably continue to be pumped as the auger is reinserted into the lower portion of the bore.  
       [0018] The stem of the auger will preferably have a cylindrical shaft portion and a tip portion, the tip portion having at least one upstanding flight arranged thereon in a helical fashion to make it easier to screw the auger into the ground and the fluid concrete. The flight(s) may continue some distance up the shaft portion beyond the tip portion, and/or the shaft portion may have an additional helical upstanding flight arranged on a section thereof.  
       [0019] In a preferred embodiment, the tip portion will have two flights rotationally displaced by 180 degrees from one another.  
       [0020] At least part of the tip portion will preferably taper in order to displace the ground material sideways as the auger penetrates the ground. The tip may taper to a point, but in a preferred embodiment the tip will taper to a region of reduced diameter.  
       [0021] To reduce the amount of ground material that is displaced in an upward direction when the auger is screwed into the ground, the ratio of the maximum radial extent of the upstanding flights as measured from the centre of the auger and the radius of the shaft portion will preferably be between 4:3 and 32:30. In a preferred embodiment, this ratio will be substantially 35:30.  
       [0022] According to another aspect of the invention there is provided an auger for drilling a bore hole in the ground, comprising a stem with a tapering tip portion, the tapering tip portion having at least one upstanding flight arranged thereon in a helical fashion, and the stem having fluid channel communicating between an inlet towards the top end of the stem and an outlet towards the step tip.  
       [0023] According to yet another aspect of the invention, there is provided a method of forming a concrete pile in a bore hole in the ground comprising the steps of:  
       [0024] (a) filling a lower portion of the bore hole with a volume of fluid concrete;  
       [0025] (b) applying a pressure to the volume of fluid concrete by at least partly inserting an auger into the lower portion of the bore in an axial direction, such that the applied pressure causes the volume of concrete to expand radially, thereby increasing the diameter of the lower portion of the bore;  
       [0026] (c) retracting the auger from the remainder of the bore and filling the remainder of the bore with concrete; and,  
       [0027] (d) allowing the concrete to cure. 
     
    
    
     [0028] The invention will now be further described by way of example with reference to the following drawings in which:  
     [0029] FIGS.  1  to  6  show a sequence of steps in the forming of a concrete pile in accordance with the invention; and  
     [0030]FIG. 7 is a side view of an auger for forming concrete piles as shown in FIG. 1. 
    
    
     [0031]FIG. 1 is a cross sectional view of part of an auger  10  which has penetrated some distance into the ground  12 , thereby forming a partially dug bore hole  14  of cylindrical shape. The auger has a shaft  13  at the end of which there is a tapered drilling tip  16  with helically arranged flights  18  such that rotating the auger causes the auger to penetrate the ground. The auger is connected to a drilling rig (not shown) in order that the auger can be rotated whilst being retained in a substantially vertical position.  
     [0032] In FIG. 2, the auger  10  has further penetrated the ground  12  and formed a bore hole  14  whose depth is approximately equal to the depth of the pile that is to be formed.  
     [0033] Retraction of the auger  10  is then initiated, and fluid concrete  24  is pumped into the lower part  20  of the bore hole  14  through a fluid channel  22  running down the centre of the shaft  13  of the auger  10  to an exit port  23  in the auger tip  16 . The concrete is pumped into the bore hole as the auger is being retracted. The rate of retraction and the pump rate of concrete are matched so that there is a reduced risk of voids being formed or collapsable soil falling into the concrete  24 . This is achieved by maintaining the pressure of the fluid concrete  24  in the fluid channel  22  as the auger is being retracted.  
     [0034] When the auger  10  is being retracted, the region of the bore hole  14  below the auger  10  fills with fluid concrete  24 . Once the auger  10  has been retracted from a lower portion  21  of the bore hole  14  as shown in FIG. 3, the auger  10  is then at least partially reintroduced in a downward direction into the fluid concrete  24  in the lower portion  21  of the bore hole  14 . The auger is reintroduced by rotating the auger  10 , which is forced downward due to the engagement of the flights  18  in the fluid concrete  24  as the tip  16  is rotated.  
     [0035]FIG. 4 shows the auger  10  after it has been partially reintroduced into the lower portion  21  of the bore hole. The introduction of the auger  10  increases the pressure of the fluid concrete  24  in the lower portion  21  of the bore hole  14 . Consequently, the side wall  26  of the lower portion of the bore hole expands outwardly, as shown in FIG. 4. The resulting bore has a wider horizontal cross section in the lower portion  21 . (The depth of the bore hole  14  will also increase due to the pressure applied to the concrete  24 , but this increase will be small in comparison to the depth of the bore hole  14 ).  
     [0036] The auger then is retracted to a level where the tip  16  of the auger  10  is above the expanded portion of the bore  14  as shown in FIG. 5. At this stage, the auger  10  can be retracted completely. Alternatively, the auger  10  can be reintroduced again in order to expand the bore hole  14  in the region immediately above the expanded lower bore portion  21 , or to further expand the lower bore hole portion  21 .  
     [0037] Once the lower region of the bore has been sufficiently expanded by the reintroduction of the auger  10 , the auger is withdrawn from the bore whilst pumping concrete into the bore  14 . When the auger  10  is completely withdrawn, the resulting bore  14  is completely filled with concrete. An elongate metal cage  28  is then inserted into the concrete-filled bore  14  for structural support and the concrete  24  is allowed to set, resulting in a pile as shown in FIG. 6.  
     [0038] The auger  10  of FIG. 1 used to drill the bore hole  14  is shown in more detail on FIG. 7. The tip  16  has a cylindrical portion  16   a  joined to the auger shaft  13 , followed by a tapering portion  16   b  whose narrow end forms the lower extremity of the auger  10 . Two upstanding helical flights  18   a  and  18   b  are disposed on the side surface  31  of the tip  16 , the two flights  18   a  and  18   b  being rotationally separated by 180 degrees. The flights  18   a  and  18   b  are inclined with respect to the side surface  32  of the tip  16  such that when the auger rotates about 180 degrees about a vertical axis  30 , the inclination of a flight in the radial direction will reverse, passing through a horizontal inclination after having been rotated through 90 degrees. When the auger  10  is rotated in the ground in one sense, the helically arranged flights will screw into the ground and force the auger to move in a downward direction. Rotating the auger in the opposite sense will cause the auger to move upwards.  
     [0039] The diameter of the shaft  13  and the cylindrical portion  16   a  of the tip  16  is approximately 300 mm. The tapering portion of the tip  16   b  tapers down to a diameter of about 100 mm at the extremity of the tip  16 .  
     [0040] The radial extent of the flights as measured from the surface of the tip diminishes as the tips widens. When measured from the central axis  30 , the radial extent of the flights is constant and about 175 mm. Therefore, in this example the ratio of the radial extent of the flights as measured from the central axis  30  and the radial extent of the tip varies between about 35:12 towards the narrow end of the tip and 35:30 when the tip is at its widest. Because the flights  18   a  and  18   b  are relatively short in a radial direction as compared to the diameter of the tip  16 , and because of the orientation of the flights, the auger  10  displaces earth predominantly in a sideways direction rather than in a vertical direction when it is screwed into the ground. This means that displaced earth is not brought up to the surface where it can be a nuisance.  
     [0041] The fluid channel  22  through which concrete is pumped, here a tubular passage running down the length of the shaft and to the tip  16 , is indicated by a dashed line. In this example, the long axis of the fluid channel  22  is co-axially aligned with the central axis  30  of the shaft  13 .  
     [0042] The exit port  23  in the fluid passage  22  is situated on the side surface of the tip  16 , such that concrete pumped through the auger  10  is injected into the bore hole  14  at an angle to the vertical direction. However, the exit port could alternatively be positioned in line with the long axis of the fluid channel  22 .  
     [0043] An expendable and releasable cap (not shown) placed on the exit port  23  is used to prevent soil being forced into the fluid channel and blocking it as the auger is drilling the bore hole  14 . After a hole  14  of the desired depth has been drilled with the auger  10 , concrete is pumped into the fluid passage. The pressure of this concrete will build up until it reaches 0.5 bar and 1 bar and is sufficient to release the expendable cap from the exit port  23 , thereby allowing fluid concrete to flow from the exit port  23  and into the bore  14 .  
     [0044] As it will be appreciated from the above description, this invention provides a simple way of forming concrete piles having an enlarged base, without producing excessive vibration, and allowing firm or hard soils to be penetrated.