Method for the vibration-free construction of a soil displacing foundation pile

Abstract

 A method for the vibration-free production of a foundation pile, in which method a hollow tube (1), closed at the lower end thereof by a pile base (4) and provided on the outside thereof with a helix (3), is screwed into the ground, an amount of grout (5) is introduced into the hollow tube and a prefabricated pile (7) having a diameter smaller than the internal diameter of the tube is introduced into the hollow tube approximately as far as the pile base and the space between the pile and the tube (8) is filled entirely with grout, whereafter the hollow tube is retracted from the ground, leaving the pile base behind, while during the retraction of the tube from at least the part of the ground that exhibits positive adhesion, grout is pumped under pressure into said space.

Description

[0001] This invention relates to a method for the vibration-free construction of a soil displacing foundation pile, in which method a hollow tube, closed at the lower end thereof by a pile base and provided on the outside thereof with a helix, is screwed into the ground, an amount of self-hardening, liquid pile material is introduced into the hollow tube and a pile having a diameter smaller than the internal diameter of the tube is arranged in the hollow tube, whereafter the hollow tube is retracted from the ground, leaving the pile base behind.

[0002] Such a method is disclosed in Dutch patent application 88.02318. In the known method, as a self-hardening, liquid pile material, so-called swelling concrete is employed. In that specification it is not further explained what swelling concrete is. It can be inferred from the text, however, that it is a type of concrete which swells during hardening. This swelling appeared to have presented problems hitherto and in that document measures are proposed to remove those problems.

[0003] The object of the present invention is to provide a method by means of which a good soil displacing foundation pile can be constructed without vibration, without using swelling concrete as the self-hardening, liquid pile material.

[0004] The object contemplated is realized, according to the invention, with a method of the type described in the preamble, in which, as pile material, grout is used and as a pile of smaller diameter, a prefabricated pile is used, which is introduced into the hollow tube approximately as far as the pile base, whilst the space between the pile and the tube is filled with grout and during the retraction of the tube from at least the part of the ground where positive adhesion is developed, the grout in the space mentioned is kept under pressure.

[0005] If a conventional self-hardening material, such as grout, is used for the construction of a foundation pile utilizing a hollow tube, which, prior to hardening, is retracted or unscrewed from the ground again, the soil stress at the location of the pile to be formed will be disturbed relatively drastically, i.e., the stress of the soil will tend to decrease. If, in accordance with the invention, the grout in the space between the hollow tube and the prefabricated pile is kept under high pressure as the hollow tube is being retracted from at least that part of the ground where positive adhesion is developed, any relaxation of the ground at the point where the hollow tube has been pulled out is prevented, so that the original soil stress over the path of positive adhesion is maintained as much as possible. As a result, the bearing capacity of the pile to be formed will be optimal. The hollow tube may be rotated during retraction, but it is commonly pulled from the ground without being rotated. As a result, a "skin" or "peel" of adjacent soil is removed along with the tube. If, in accordance with the invention, the grout is kept under pressure during the retraction of the hollow tube, the grout will occupy the position of the skin of soil that has been pulled out along with the tube, with the result that the surrounding soil cannot relax. Even when the tube is retracted without rotation, an optimum bearing pile is constructed. If necessary, further grout can be supplied while the grout is being kept under pressure.

[0006] According to the invention, during the retraction of the tube from the part of the ground where positive adhesion is developed, preferably a pressure of at least 8 bar is exerted on the grout. At a lower pressure, the prevention of relaxation of the surrounding soil cannot be guaranteed under all circumstances, certainly not if the tube is retracted without rotation.

[0007] In a preferred embodiment of the method according to the invention, after the hollow tube has been retracted over a part of the length thereof, the tube is rotated, without being pulled, in a direction such that the soil located in the helix is forced from the helix in downward direction, while during such rotation the pressure on the grout in the tube is maintained. By reversely rotating the hollow tube in stationary position in this manner at a given time, the soil is discharged from the helix in downward direction, so that the pressure on the soil surrounding the lowermost part of the pile to be formed is further increased. An additional advantage is that soil and grout are mixed at that point, so that a covering layer of good quality is formed around the pile.

[0008] In accordance with a further preference, the hollow tube is screwed into the ground at so high a pressure that the tube, in one revolution, penetrates the ground over a length equal to the pitch of the helix. This occurs at a turning moment of about 25 ton/meter. The advantage of arranging for the tube to penetrate the ground over exactly one pitch in one revolution is that thereby practically no soil will be conveyed upwards, so that, again, the soil stress is left unchanged by the introduction of the hollow tube.

[0009] The invention will be explained with reference to the drawings, in which:

[0010] Figs. 1-6 are schematic representations of a pile during different stages of the construction thereof, utilizing an embodiment of the method according to the invention.

[0011] In the drawings, similar or corresponding parts are indicated by the same reference numerals.

[0012] Fig. 1 shows a hollow tube 1 as it is being screwed into the ground or soil 2. The tube 1 is provided at the circumference thereof with a helical strip or collar 3 directed more or less perpendicularly to the surface of the tube. By virtue of the helix 3, the tube 1 draws itself into the ground if the tube is rotated under some pressure, quite like a screw penetrating into a supporting surface under the pressure of a screwdriver. The lower end of the hollow tube 1 is closed by a pile base or pile tip 4. The pile base 4 is connected to the hollow tube 1 in such a manner that, upon removal of the hollow tube, the pile base 4 is left behind in the soil 2. Such a manner of connecting tube 1 and pile base 4 is known to those skilled in the art.

[0013] Fig. 2 schematically shows that an amount of grout 5 has been introduced via a supply duct 6 from the top into the hollow tube 1 after it has been screwed into the soil 2 to the desired depth. The grout 5 fills the hollow tube 1 from the pile base 4 up to a suitable height, partly depending on the size of the prefabricated pile to be subsequently introduced into the tube. It is observed that, for the sake of clarity, Figs. 2-5 do not show the entire strip-shaped helix 3.

[0014] Fig. 3 shows how, after the hollow tube 1 has been filled with grout 5 from the pile base 4 up to a certain height, a prefabricated, for instance concrete, pile 7 is lowered into the hollow tube 1, until the pile 7 rests on the pile base 4. The diameter of the prefabricated pile 7 is smaller than the internal diameter of the hollow tube 1, leaving a space 8 between pile 7 and tube 1. The condition wherein the pile 7 rests on the pile base 4 is shown in Fig. 4, where it is further shown that the entire space between the prefabricated pile 7 and the inside wall of the hollow tube 1 is filled with grout. The reinforcement bars 9 projecting from the pile 7 at the top thereof can serve as gripping elements for keeping the pile 7 in position as long as the grout 5 has not hardened yet. Instead of reinforcement bars 9, gripping elements of a different type can be used, for instance a screwed sleeve or an anchor incorporated into the pile head. It will be clear that the condition shown in Fig. 4 can also be achieved by first lowering the prefabricated pile 7 into the tube 1 and subsequently introducing grout 5 into the space 8 between pile 7 and tube 1. Further, it is not always required that the entire space 8 be filled with grout 5 up to the very top. This, of course, depends partly on the condition of the surrounding soil.

[0015] Fig. 5 shows how, in accordance with the invention, the hollow tube 1 is subsequently retracted from the soil 2 over a part of its length, while simultaneously the grout 5 in the space 8 between the pile 7 and the hollow tube 1 is kept under a high pressure (for instance 8 bar or more). By keeping the grout 5 under a high pressure, the space that threatens to be formed between the prefabricated pile 7 and the surrounding soil 2 at the point where the hollow tube 1 has been retracted - due to the fact that a part of the soil 2 disposed between the windings of the helix 3 is removed along with the tube 1 - is filled entirely with grout, so that the space 10 filled with grout cannot be filled up with soil 2 which might otherwise be forced towards the pile 7. This prevents the soil 2 from relaxing and the original stress profile of the soil 2 is maintained or even improved.

[0016] When the hollow tube 1 has been retracted to the extent where the lower end thereof is located at the upper limit of the layers of soil exhibiting positive adhesion, the retraction of the tube 1 is preferably discontinued for some time and subsequently the tube is rotated in the opposite screwing direction, so that the soil present in the helix 3 is discharged in downward direction and, partly as a result thereof, the grout 5 is kept under pressure. Finally, the retraction of the hollow tube 1 is resumed and completed, while the pressure on the grout is maintained. Thus, the final result is the condition shown in Fig. 6, where the prefabricated pile 7 resting on the pile base 4 is surrounded at the lower end thereof by a grout jacket 10 and, further upwards, by a jacket 11 of grout partly mixed with soil. After hardening, a foundation pile of eminent bearing capacity is obtained.

Claims

1. A method for the vibration-free construction of a soil displacing foundation pile, in which method a hollow tube, closed at the lower end thereof by a pile base and provided on the outside thereof with a helix, is screwed into the ground, an amount of self-hardening, liquid pile material is introduced into the hollow tube and a pile having a diameter smaller than the internal diameter of the tube is arranged in the hollow tube, whereafter the hollow tube is retracted from the ground, leaving the pile base behind, characterized in that, as pile material, grout is used and as a pile of smaller diameter, a prefabricated pile is used, which is introduced into the hollow tube approximately as far as the pile base, whilst the space between the pile and the tube is filled with grout and during the retraction of the tube from at least the part of the ground where positive adhesion is developed, the grout in said space is kept under pressure.

2. A method according to claim 1, characterized in that during the retraction of the tube from the part of the ground where positive adhesion is developed, a pressure of at least 8 bar is exerted on the grout.

3. A method according to claims 1-2, characterized in that, after the hollow tube has been retracted over a part of the length thereof, the tube is rotated, without being pulled, in a direction such that the soil located in the helix is forced from the helix in downward direction, while during such rotation the pressure on the grout in the tube is maintained.

4. A method according to any one of claims 1-3, characterized in that the hollow tube is screwed into the ground at so high a pressure that in one revolution the tube penetrates the ground over a length exactly equal to the pitch of the helix.

Drawing