The invention relates to a concrete screw pile, having its soil-penetrating base provided with a helical flange mounted on a drive shaft.

Conventional reinforced concrete piles are manufactured by tying upright steel rebars with steel stirrup rebars for the reinforcement of a pile, and by casting the pile in a mold, generally for square shape. For installation, special pile driving machines have been developed. In these, a weight is dropped on the head of a pile, and the impact energy buries the pile deeper at every strike until the pile barely sinks further. Final strikes are used for

experimentally ensuring the load bearing capacity. Depending on the size of a pile, the ramming block may have a mass ranging from 50 kg to several thousand kilograms. A drawback with the driven pile is strike-generated intense noise, as well as strike-generated vibration in the vicinity. The pile is also dimensioned to withstand the impact-generated strike energy. Due to what has been described above, driven piles are abundantly oversized with regard to loads of the working process. The goal of this invention is to reduce the oversize of concrete piling, to eliminate the strike-generated noise and vibration. In order to achieve these goals, a pile according to the invention is characterized by what is presented in the characterizing clause of claim 1. Another goal is to increase the tensile strength of concrete piling.

In terms of its operation, the pile according to the invention is preferably based on a screw principle as described in the Applicant's earlier patent FI94885. The discussed screw pile operates in such a way that, while drilling into the ground, the helical flange loosens the soil in its working area. The soil loosened by the screw pipe is re-compressed by a conical segment whose volume is in the same order of magnitude as a cavity in the ground produced by the progressing screw. This pile method has proven highly useful, and a good tensile and compressive strength is achieved thereby, even with a short pile. Since the compression of soil proceeds sequentially, the power needed for screwing is also less than what is required by a majority of other screw piles.

The piles according to the above-cited patent consist of a steel pipe, carrying a compacting cone, a shaft, and a helical auger at its end. However, the steel pipe pile is often a less favorable solution than a concrete pile, the latter being better than steel pipe piles of the same price category in terms of corrosion resistance and buckling strength. The durability of concrete, for example in porous oxygen-pervious sandy soil, is better than that of a standard galvanized steel pipe.

A problem with the screw-driven use of a concrete pile is the poor tensile strength of concrete, and the shear strength of a concrete pile is

comparatively poor in conventional reinforcement solutions. According to a first embodiment of the invention, the concrete casting is provided with an internal reinforcement, which is a screw type helix and oppositely handed in its threading direction with respect to a helical flange pulling the pile into the ground, the reinforcement thus converting the rotation to a tensile stress, which results in a further compaction of concrete. This reinforcement can be provided in such a way that pile-directed lengthwise rebars are tied by means of spiral-shaped rebars. The number of both vertical rebars and tying rebars is preferably not less than three, for example 3 to 6 pieces.

Consequently, the shear strength of a pile is high in the direction in which the spiral-shaped rebars tighten in the driving process. The spiral rebars and lengthwise rebars are tapered at the screw end merged into a shaft of the screw. At the top end of a pile, the rebars are respectively attached to a flange provided with grippers for driving the pile. This flange may be simply a plate, to which the rebars are fastened by welding either directly or by way of holes, and which is provided with slots for a driving tool. It is an objective of the invention to provide reasonably priced concrete- framed screw pile options for currently employed steel screw piles, and to enable the cost-effective use of a screw pile, even when the steel screw pile does not provide a beneficial or durable solution.

One preferred embodiment of the invention will now be described with reference to the accompanying figure.

Fig. 1 shows a helically reinforced screw pile, in which the shaft of a helical flange is coiled from concrete rebars.

Fig. 2 shows the screw pile of fig. 1 in cross-section at the helical

flange. The joining of concrete rebars 5 and 6 to a shaft 3 of the helical flange can be effected by coiling the rebars for a braided configuration. Thus, the rebars themselves constitute a coiled shaft 3 passing through a base 7 of the pile casting mold, for example through a semi-spherical or conical base 7, which functions at the same time as a concrete mold in the casting process and as a soil compacting cone in the driving process. The shaft 3 has its end cut off at an angle for a sharp tip, and a helical flange 1 itself is fitted by means of a bushing 2 on the shaft 3, for example by welding. Hence, the flange need not be fixed until after casting or shipping. The helical flange 1 can have its shaft 3 fixed also with some other type of mounting. The coiled reinforcement is connected from both ends of the pile to a driving element and to a shaft of the helical flange, respectively. The shaft mounting can be implemented for example with a parallelopiped sleeve or rod capable of receiving a matching piece of the helical flange shaft. In this case, the rebars are welded or joined to the element left within the casting, and the flange, along with its shaft, can be fixed afterwards. The attachment of rebars can also be effected to a plate or the like, in which case the helical flange is fastened to the plate itself.

According to one embodiment, the end mounting of a concrete pipe is suitable for use with the above-described fasteners of steel piles, whereby the end of a concrete pile is provided e.g. with a weldable flange. This embodiment provides a benefit of being able to use various materials at various depths of one and the same pile, enabling the use of concrete as the uppermost pile sections in topsoil, or vice versa. This enables the use of a different material in topsoil which is corrosive to some other material.

The helical flange 1 of fig. 1 comprises two helix segments 1 on opposite sides of a shaft, fig. 2 showing a cross-section at the shaft so as to reveal a position of the flanges 1 with respect to the shaft. Each segment has a rotation of about 180 degrees. This reduces the volume of soil that is loosened over a single rotation and, respectively, there will be a lesser volume remaining to be compressed during the same rotation by a packing cone trailing the helical flange, whereby the cone can be made shorter. In addition, it is easier to manufacture and join two shorter helical flange blades than a single helix flange with a length of 360 or more than 360 degrees. The manufacture of a 360-degree long flange cannot be managed from a single flat sheet. The advantages of a twin-bladed flange are not dependable upon the actual material of a pile. The preferred configuration of a helical flange is also dependent on the type of soil, while, on the other hand, two parallel blades include two cutting leading edges with an inter- blade gap between the leading and trailing edge. In the process of drilling a large particle soil type, it may be preferable to use helix segment flanges of more than 180 degrees, or to use flanges whose leading edge is relieved more prominently than what is shown in the figure, whereby the flange displaces material also outward with the result that large particles do not become jammed between the leading and trailing edges of two flanges. The disturbance of soil taking place during a drilling process can also be influenced for example by bending the blades of a helical flange at the trailing edge for increasing the pitch, or by increasing a thickness of the trailing edge, thereby enhancing the soil working action performed by the flange during a single rotation, because of an increased volume left behind by the trailing edge of the flange. This enables a less stressful operation for packing cone, and hence enables pulling a larger diameter pile into the ground with a flange of the same size.

According to another embodiment, the shear-force receiving helix type reinforcement is made from a relatively thin metal sheet on the surface of a pile. The sheet can be provided as a spiral-seam pipe or, in case of using an external mold, possibly also as a seamless configuration. In any case, the seams themselves need not be strong, nor is it critical to have a strong joint between concrete and sheet metal, but the spiral metal sheets can be basically installed even upon a previously cast rod in a suitably prestressed condition to keep the same in attachment with the rod solely by means of a clamping force. The spiral-seam pipe may conveniently function as a casting mold, and the pipe can be manufactured at the time of a casting process. In this embodiment, from the standpoint of strength, the spiral metal sheets are only essential in the process of driving a screw pile into the ground, after which the rebars within the pile are responsible for the shear strength of the pile. The steel sheets can also be fastened to driving elements at the ends of a pile in such a way that the elements are able to move relative to the pile, in which case the steel sheets can be replaced with any high-tension- resistant material as thermal expansion is not a problem. In the event that a pile drilling process must be reversed, i.e. the pile is driven backward, it is necessary to provide driving elements for rotating also the actual concrete body of the pile. As a rule, reverse screwing requires considerably less force. The pile screwing head can be attached for example to a bolt or nut cast on the pile, whereby the flange can be tightened and clamped to the pile. The external sheet reinforcement may also be a non-spiraled pipe, which only deforms as late as in the driving process in such a way that it tightens and reshapes into an at least somewhat helical configuration while being rotated and at same time tightened against concrete both in its girth and length dimension. In this case, however, it is necessary to make sure that the pipe functions as intended without, for example, tearing off at the lengthwise seam in the driving process. In comparison with a concrete-filled steel pipe, the thus established outer skin can be remarkably thin, nor is the attachment to concrete critical. Preferably, however, the outer skin is spiral also in terms of its seams, whereby the seam is not subjected to a tensile or shearing force while the outer skin is taking shape.

The corrosion resistance of outer surface metal sheets or strips does not matter, and after the installation these can even be removed from the section above ground. Hence, for example a spiral-seam pipe can be used for providing an adequate shear strength for the duration of a pile driving process, and for providing a disposable casting mold, but the pipe material can be selected solely on the basis of price, workability, for example weldability and tensile strength, nor is it necessary to consider corrosion or attachment to concrete in dimensioning the material. This way, an adequate shear strength is provided by means of inexpensive steel pipe or merely by using helix-shaped sheet metal strips tightened on the surface, as in the ultimate usage of pile, its corrosion characteristics essential in terms of its strength are solely dictated by concrete and rebars remaining inside it. The spiraled outer shell is welded or otherwise fastened by both of its ends to the flanges, and in this embodiment it will be necessary to effect the attachment of a helical flange by using for example a separate brace, bracket, collar, or a pipe matching the pile in thickness, to which the metal sheet is welded or otherwise fastened, for example by a press joint. Steel sheet can also be replaced with some other material as the disparity in thermal expansion coefficients is not as important as in other solutions. The preferred material for a shear-force receiving external brace is any high- tension-resistant thin material, for example fiberglass cord or fiberglass- reinforced cord. The brace may also be a reticulated hose or stocking, and on top of it may be a protective sheath against the ripping or tearing action of soil. The protective sheath can be made for example in plastics, rubber, or metal. The required minimum number of cord type braces is two in order to not generate a pile twisting resultant force while tightening. On the other hand, a hose type shell can be more resistant in one of the screwing directions, whereby a majority of fiberglass strands or the like have a screwing handedness which is opposite to the screwing direction.

The external shell can also be constructed from a wire rope mesh, which tightens around a pile upon wrapping. At this time, at least a part of the mesh assumes a helix shape while tightening. It is also possible that a wire rope mesh, a cable stocking, or a helix of sheet metal not be threaded or wrapped around several extensible piles until in the process of driving the same into the ground, whereby the driving device itself is provided with mesh gripping elements and the mesh may have a length equal to the extended multi-section pile, nor is it necessary to construct the pile extensions in a way to withstand the shearing force.