EP0185435A1

EP0185435A1 - Pile cap for the transmission of force between a piling hammer and a pile

Info

Publication number: EP0185435A1
Application number: EP85202107A
Authority: EP
Inventors: Johannis De Wit; Henk Van Koten
Original assignee: FUGRO CONSULTANTS INTERNATIONAL BV
Current assignee: FUGRO CONSULTANTS INTERNATIONAL BV
Priority date: 1984-12-18
Filing date: 1985-12-18
Publication date: 1986-06-25
Also published as: NL186188B; NL186188C; NL8403839A

Abstract

A piling cap protects a piling hammer and a pile against stress peaks which are too high. In order to ensure that the striking energy transmitted by the piling hammer to the piling cap is passed on to the pile over a relatively long period essentially without peak, the shock absorption material in the piling cap consists of plastic rods (12) which are kept spaced apart through the fact that they project through the holes of at least one perforated plate (13, 14), while such free spaces exist between the rods that they can deform freely under stress.

Description

The invention relates to a piling cap for the transmission of force between a piling hammer and a pile, said piling cap comprising a quantity of shock absorption material to be stressed by the piling hammer, said shock absorption material being provided with a number of plastic rods whose centre line is parallel to the direction of the strokes to be carried out, as known from WO-A-8,101,262.
The purpose of a piling cap is to protect both piling hammer and pile against excessive stress peaks. It is known to use wood or plastic as shock absorption material in piling caps. Wood is in the initial stage too soft, so that a relatively large amount of energy is absorbed. During the pile-driving, the wood is compressed to an increasingly solid mass, which leads to peak stresses which are too high occurring and the head of the pile being damaged. Besides, the wood can catch fire. An attempt was made to eliminate these disadvantages by replacing the wood in the piling cap by a piece of polypenko plastic. It was, however, found that high internal stresses occur in the solid piece of plastic during the pile-driving, and that as a result of this the plastic ages quickly and the service life becomes unacceptably short.
An important improvement seemed to have been found in the so- called hydraulic block, _\\hich is a cap filled with oil and pressurized gas fastened under the piling hammer. The gas pressure is increased in such a way that a high pressurizing force (e.g. 1,000 kN) is produced, and through exceeding of this value during the pile driving stroke the gas is compressed. In this way the level of the force transmission can be limited and the duration thereof can be extended. The stresses occurring can be influenced by the pressurizing. Nevertheless, the stress curve first shows an inadmissible peak before the pressure levels off.
The object of the invention is to avoid the disadvantages_' of known piling caps and to produce a piling cap which can absorb a high peak stress without problems, while still exhibiting a low modulus of elasticity.
According to the invention, the piling cap is to this end characterized in that the rods are held at a distance from each other through the fact that they project through the holes of at least one perforated plate, while between the rods such free spaces exist that they can deform freely under stress.
Through the use of a number of relatively thin plastic rods instead of a solid plastic block, the internal friction in the plastic during pile-driving is limited to a minimum. The total volume of plastic material needed is theoretically calculated from, inter alia, the weight of the piling hammer, the height of fall thereof, the weight of the pile, and the ground resistance to be overcome. This volume is divided into a number of relatively thin rods made from a carefully selected material which can take high stress and exhibits a low modulus of elasticity. The preferred material is nylon. While a bad piling cap transmits a rapidly rising and falling energy peak to the pile when undergoing a stroke from the piling hammer, the piling cap according to the invention will pass on the stroke energy transmitted by the piling hammer over a relatively long period of time, essentially without any peak, to the pile, which is, as it were, pushed into the ground, instead of being driven. The plastic rods will in the process thicken between their clamping points and subsequently return to their initial shape. It can happen that apart from rods made of nylon (polyamide), rods made of a second type of plastic (for example, high-density polyethylene) have to be used in order to transmit the stroke energy to the pile without peak stresses.
The piling cap according to the above-mentioned publication WO-A-8 101 262 does not display the effect according to the invention: that the energy transmitted by the piling hammer is passed on to the pile essentially without peak stresses, because no free spaces exist between the rods in such a way that they can deform freely under stress.
In order to enclose the rods inside the cap in such a way that a piling hammer can transmit its energy to all rods without direct contact with the rods, the rods rest on a strike plate, onto which is welded a ring which is telescopically surrounded by a guard which rests on the top ends of the rods.
A bottom perforated plate preferably rests on short tubular pieces which are fastened to the strike plate, while a top perforated plate is fastened by longer tubular pieces to the bottom perforated plate.
A guide ring for the top of a pile can be attached under the strike plate.
The shield can be provided with two guides, into each of which a part projecting upwards from the strikeplate is conveyed.
The invention will now be explained with reference to the figures, in which an embodiment is illustrated.

Fig. 1 shows a view of a piling frame.
Fig. 2 shows a cross section, along the line II-II in Fig. 3, of the piling cap according to the invention.
Fig. 3 shows a longitudinal section of the piling cap, along the line III-III in Fig. 2, in the unstressed state.
Fig. 4 shows the same longitudinal section as that in Fig. 3, but in the stressed state.

Fig. 1 shows a common piling frame with two guides 1, 2, a piling hammer 3 which is conveyed between those guides and which can be drawn upwards by means of a winch cable 4 (piling strap), and a piling cap 6 which rests on a pile 5 and which transmits the stroke energy of the free-falling piling hammer to the pile.
The invention is concerned with the design of the piling cap;- other pile driving plants are also possible.
The piling cap 6 comprises a strike plate 7 with a downward- extending ring 8 which falls round the top of the piling plate 5, and an upward-extending ring 9 over which a shield 11 is disposed in telescopically slidable fashion. This shield 11 rests on a set of plastic rods 12 of equal length. The rods each project through a hole of two perforated plates 13, 14 and are thus held at a distance from one another. The bottom perforated plate 13 rests on three short tubular pieces 15 which are welded onto the strike plate. The top perforated plate 14 is attached to the bottom one by means of three longer tubular pieces 16.
In order to prevent rotation of the shield 11 relative to the ring 9, two upward-projecting parts 17 are welded onto the strike plate diagonally opposite each other, and are each passed between two lugs 18 of the shield 11.
The strike plate 7 has two projections 19 which are disposed diametrically opposite each other and are conveyed into recesses of the guides 2 of the piling frame.
It can be seen from Fig. 4 how the plastic rods are deformed when the piling hammer strikes the shield 11 in free fall. The rods become thicker between the points where the rods project through the perforated plates 13, 14. The deformed rods return to their initial cylindrical shape, while the energy is passed to the pile without peak stresses. Ideally, a practically constant force is exerted for a relatively long time on the pile.
The plastic from which the rods 12 are made must have a low modulus of elasticity and must be able to absorb a high peak stress and release it relatively slowly. Polyamide (nylon 6) is particularly suitable. There are, however, conceivable cases in which rods made of two different plastic materials, for example nylon and high-density polyethylene, are used in one and the same piling cap.
Due to the shape of the rod, the internal friction during deformation will be reduced to a minimum. In any case the rods must not buckle, and the energy from the piling hammer will have to be absorbed within the elastic range by the rods, without the admissible stress in the plastic being exceeded. The plastic must exhibit little or no creep under stress.
The diameter of the rods is, for example, 50 mm and the length 300 mm. These values can be varied, depending on requirements.
Essential for the invention is that the plastic filling of the piling cap does not consist of one single solid piece, but is divided into a number of rods which are arranged with their centre line according to the direction of striking parallel to each other and spaced from one another.
It has been found that nylon is particularly important as a material for the plastic rods, because this material hardly ages at all and lasts for a very long time. Rods made of this material essentially meet the theoretical requirement that a peak stress exerted upon them is gradually passed on to the pile without peak. Fewer strokes of the hammer are therefore needed to work a pile into the ground, and the height of the hammer is reduced.
Various modifications of the pile driving cap are possible within the scope of the invention.

Claims

1. Piling cap for the transmission of force between a piling hammer and a pile, said piling cap comprising a quantity of shock absorption material to be stressed by the piling hammer, said shock absorption material being provided with a number of plastic rods whose centre line is parallel to the direction of the strokes to be carried out, characterized in that the rods (12) are held at a distance from each other through the fact that they project through the holes of at least one perforated plate (13, 14), while between the rods such free spaces exist that they can deform freely under stress.

2. Piling cap according to Claim 1, characterized in that the rods rest on a strike plate (7 )onto which is welded a ring (9) which is telescopically surrounded by a shield (11) which rests on the top ends of the rods (12).

3. Piling cap according to Claim 2, characterized in that a bottom perforated plate (13) rests on short tubular pieces (15) which are fastened to the strike plate (7), and a top perforated plate (14) is fastened by longer tubular pieces (16) to the bottom perforated plate (13).

4. Piling cap according to one of the preceding claims, characterized in that the plastic rods (12) are made of nylon.

5. Piling cap according to one of Claims 1 to 3, characterized in that both plastic rods of nylon and plastic rods of high-density polyethylene are used.

6. Piling cap according to one of the preceding claims, characterized in that a guide ring (8) for the top of a pile is fastened under the strike plate (7).

7. Piling cap according to one of Claims 2 to 6, characterized in that the shield (11) is provided with two guides (18) into each of which a part (17) projecting upwards from the strike plate (7) is conveyed.