The present invention relates to an arrangement and a procedure for increasing the load carrying capacity of a preferably in the ground installed pile or similar.

Technical sphere

The most common way of installing piles in the ground has up to now been to hammer with a mechanical or hydraulic drop hammer. Abroad also diesel hammers or steam driven hammers are used. At hammering with a drop hammer a shock wave is brought onto the pile head aiming at overwinning ^' the resistance of the soil along the envelope surface of the pile and at the tip of the pile. There is no lasting mechanical connection between the hammer and the pile and after every beat the so called static friction is therefore once again established between the pile and the surrounding soil. Thus, this must be overcome over and over again. Hammering with a drop hammer also causes disturbances, for instance in the form of noice and shakings of the ground and because of this standards of many countries restrict the use of drop hammers .

Lately powerful vibrators have been developed now making it possible to vibrate down piles in the ground. For these vibrators the frequency of vibrations can be continuously variated during the hammering process whereby a better pene¬ tration of the pile in the ground is achieved. In this way

the pile installation can be done in a way which is more adapted to the environment. The pile is rigidly connected to the vibrator and the pile is during the whole hammering process in a vertical oscillation motion where the static friction between the pile and the soil cannot develop along the envelope surface of the pile. The largest difficulties concerning the installation of the pile in the ground with vibrating arise when it comes to getting the pile to enter firm layers of soil with gravel and sand. This is especially true with piles with large tip area where the compact soil must be forced out of the way for the pile to be able to penetrate deeper. Because of this the driving with vibrations has been utilized mainly for hammering of open tubes and rabbets with a small tip area. A drawback is, however, that the load carrying capacity of piles with a small tip area is bad because the main part of the load is transferred to the soil along the envelope surface of the pile. These piles are called envelope carrying piles.

To increase the load carrying capacity of a vibrated pile vibration has in certain cases been combined with hammering with a drop hammer. This can either be done by using a special drop hammer in connection with the terminating hammering (stop or post hammering) . Alternatively the grip between the vibrator and pile head can be opened during the vibrating whereby the vibrator functions as a fast beating hammer. This method is, however, very inconvenient since the impact load can lead to costly damages within the vibrator.

Another way of increasing the load carrying capacity of hammered or vibrated piles is to inject a cement mortar.

This can be done during hammering to diminish the envelope resistance between the pile and the surrounding soil . This

method is common in connection with hammering with a drop hammer but is not needed in connection with vibrations since the envelope resistance then commonly is low. The load carrying capacity of the pile can also be increased by injection of a cement mortar after installation of the pile.

Prior art

GB,A, 2241272 shows an arrangement for base injection (point injection) of a displacement pile, that is a massive pile of concrete or wood. The arrangement is in the form of a metallic pile shoe comprising means for locating the pile shoe con¬ sidering the lower end of the pile. The arrangement further shows an injection supply passage extending downwards from the means of location to receive injection material feeded through the pile or through a tube being fixedly arranged with reference to the pile or similar. The pile shoe is provided with a plurality angularly spaced apart wings extending radially from the injection supply passage. The wings are provided with oversized edges creating weakness zones within the penetrated soil. The wings are surrounded by a ring with the aim to stiffen the wings and to prevent the cement mortar to be uncontrollably spread from the tip of the pile. Injection makes the installation more expensive.

Derwents Abstract No. 93-099031/12 of SU,A1, 1726652 shows a pile that demands a reduced driving force and that shows an increased load capacity. The shaft has a lower open-bottomed cavity in which vertical partitions are secured. The shaft further has a downwardly narrowing projection in its cavity made in one piece with the upper solid cross section of the shaft. Even if the energy consumption at the installation of the described pile is reduced as compared with the use

of conventional piles at the same time as the load capacity is increased, the described construction is not at optimum in any of these respects.

Summary of the invention

The present invention solves the problem described above. The invention comprises an arrangement for increasing the load carrying capacity of a preferably in the ground installed pile or the like. At least one with the pile mainly concentrical tube is attached to the pile with at least one rib extending between the pile and the tube. The pile extends throughout the tube. At least one with the pile mainly concentrical additional tube with a larger diameter than the first tube may be present. The additional tube is attached to the first tube with at least one between the tube and the additional tube extending rib. The tube and/or the additional tube can be of a cylindrical cross section or of a cross section/cross sections in the form of a polygon/plygons. The tube and/or the additional tube may be slidably attached to the pile. The pile may be provided with at least one shoulder with a play cooperating with at least one slit situated within the tube. In this case every rib which is extending between the pile and the tube and attached to these may be deleted.

The invention also comprises a procedure for increasing the load carrying capacity of a preferably in the ground installed pile or similar provided with the arrangement according to the above. According to the procedure a vibrator is connected to the preferably vertical pile in its upper end, the pile with the arrangement is vibrated down in the ground after which the frequency of the vibrator is lowered so that the arrangement

is exposed to the resonance frequency/frequencies from the surrounding layers of earth.

The measures according to the invention considerably improve the load carrying capacity of a pile or the like installed in the ground. Injection of cement mortar or similar is not needed. The design of the arrangement does not make the driving down of the pile into the ground more difficult.

The characteristics of the present invention are evident from the following patent claims.

Embodiments of the invention will be further described referring to the attached drawings.

List of drawings

Figure la shows in cross section a pile with an embodiment of the arrangement according to the invention during driving down in the ground with the aid of vibrations.

Figure lb shows in a view from below the pile and the arrange¬ ment in Figure la.

Figure 2a shows in cross section a pile with a second embodi- ment of the arrangement according to the invention during driving down in the ground with vibrations.

Figure 2b shows in a view from below the pile and the arrange¬ ment in Figure 2a.

Description of the embodiments

From Figure la it is evident how a vibrator 1 with the aid of a grip arrangement 2 has been attached to a pile 3 and how the pile 3 together with an arrangement 4 according to the invention intended to increase the load carrying capacity is driven down in the ground. The arrangement in Figures la and lb partly comprises a with the pile mainly concentrical tube 5 attached to a pile with eight between the pile 3 and the tube 5 extending ribs 6 and partly a with the tube 5 mainly concentrical additional tube 7 with larger diameter than the first tube 5. The additional tube 7 is attached to the tube 5 with sixteen ribs 6 extending between the tube 5 and the additional tube 7. The pile 3 extends throughout the tube 5 and a bit further. The tube 5 as well as the tube 7 have a cylindrical cross section.

The arrangement 4, popularly denoted "vibro tip", thus presents a plurality of spaces or cells upwards and downwards open. Thus, the walls of the cells are constituted by ribs 6 and either parts of the pile 3 and the tube 5 or parts of the tube

5 and the additional tube 7. For the arrangement 4 together with the pile 3 to give the persued stability when the pile and the arrangement have been installed in the ground, the height of a preferably vertically installed cell must be at least as large as the diameter or the largest diagonal of the base or bottom area of the cell. In this way a pile with belonging arrangement 4 is obtained which at the installation with vibrating shows a low envelope resistance but in the installed state at static load shows a high load carrying capacity. This is due to the so called "silo effect" where the earth at static load is trapped in between the cells

and creates a stiff tip. The whole arrangement 4 with therein included soil or the like can be said to function as a stiff plate. It can be mentioned that the described arrangement does not function satisfactory when the pile is hammered by a drop hammer since the static friction after every beat leads to the formation of plugs, that is that the soil is stuck within the high cells. The number, geometry and size of the cells are adapted to the relations of the soil, type of pile and necessary load carrying capacity. One or more of the walls of the cells may after the installation be somewhat inclined in relation to the pile 3 so that the diameter or diagonal of the cells is somewhat smaller at the upper edge than at the lower edge. One or more similar arrangements 4 may be placed along a pile depending on the geotechnical prerequisite conditions.

In a special shaping a sliding connection is created between the pile shaft and the arrangement 4 with cells, see Figures 2a and 2b. Here every rib 6 between the pile 3 and the inner tube 5 is missing. The pile instead presents four shoulders 8 with play cooperating with the same number of in the tube 5 situated slits 9. There are two layers of additional tubes 7 each with larger diameter than the first tube 5 and where also the outer additional tube 7 presents a larger diameter than the inner additional tube 7. Eight ribs 6 connect the tube 5 with the inner additional tube 7 and the same number of ribs 6 connect the inner additional tube 7 with the outer additional tube 7.

When the pile shaft according to Figures 2a and 2b is set into oscillation movement, the oscillation movement of the pile is in shock load transferred to the sliding arrangement 4 with

cells. Thereby the vibration driving of the pile shaft can be combined with a shock load for the arrangement 4 with cells. The size of the impact amplitude can be regulated by choice of the magnitude of the loose between the shoulder 8 of the pile and the slit 9 of the arrangement.

To further improve the load carrying capacity mainly in friction soils a special vibration procedure according to the application can be utilized. The vibrator is connected to the preferably vertical pile in its upper end and the pile with connected arrangement according to the invention is vibrated down in the soil whereafter the frequency of the vibrator is changed (lowered) so that the arrangement now filled with soil or similar is exposed to resonance frequeny/frequencies from the surrounding layers of earth. Hereby the packing effect is increased for the soil around and within the arrangement 4 with cells. At conventional piling the oscillation frequency of the vibrator is normally above 20 Hertz. To create resonance packing of the earth around the arrangement 4 a lower frequency must be used. This is achieved by using a vibrator with a variable frequency (and preferably also a variable movement amplitude) . The vibrator can be controlled by a controlling device which in turn is connected to an analyzer with sensors installed in the ground. It is sufficient to expose the pile tip- with the arrangement 4 to a few resonance impacts, normally 2-5 impacts.

The advantage with the described arrangement 4 according to the invention is that it presents a small tip area but has a large envelope surface. Therefore the resistance at vibration down in the ground or similar is small and the pile can thus be installed fastly and economically also in compact soils. After

terminated vibration down static friction is created between the soil and the walls of the cells.

The arrangement 4 according to the invention is preferably cylindrical but it may also consist of a triangularly, quadrangularly or in any other way formed construction. It may have a smaller diameter or diagonal in one end than in the other. (Diagonals at comparable places are compared, for example the largest diagonals.) Existing ribs 6 are preferably aimed so that they not unnecessarily slow the installation movement down in the soil . The arrangement is made of a strong material such as steel or similar. It does not need to be attached in the lower end of a preferably vertically installed pile but can also be present in another convenient location along the pile. The arrangement 4 is not exclusively used for preferably vertical piles but also for inclined piles and tie bars of different shapes for instance for inclined ties such as earth ties or other constructive arrangements in the ground or similar. Ties are firstly used for diminishing the earth pressure against vertical stiffeners such as rabbets, slit walls or other vertical supporting arrangements. The arrangement 4 according to the invention can be utilized for compressive as well as tension load. The pile 3 can either be a massive bar, an open or at the lower edge closed tube, a steel profile, a beam or any other convenient arrangement.

The invention is not restricted to the embodiments shown here but may be varied within the framework of the following patent claims .