A method for pile-driving and a ram head for fixation of a hammer assembly in relation to a pile

Abstract: In a method for driving of a pile (2), where the pile has a top side and side faces extending substantially perpendicularly to the top side, a hammer assembly is held firmly in position in relation to the longitudinal axis of the pile and drives the pile into the ground by hammering on the top of the pile. According to the invention the hammering action is exerted within a defined area of the pile top, not extending to the edges of the pile, while the edge zones are held free of the hammering action. This may be carried out by means of a ram head (110) for fixation of the hammer assembly in relation to the pile, said ram head having a bottom part provided for transmitting the impact impulse to an intermediate plate or impact piece (100) which, in turn, transmits the impulse to the pile top, said bottom part of the ram head being provided with a projecting collar (126) embracing the impact piece to hold it within a defined area not extending to the edges of the pile top.

Description: The present invention relates to a method for pile-driving and to a ram head for fixation of a hammer assembly in relation to a pile to be driven by means of the hammer assembly. Pile-driving is a well-known technique for placing many different forms of foundations in the ground. Piles are today often made of concrete and are often produced under industrial conditions in a factory, and very effective machinery for pile-driving is available nowadays, so that the driving can be carried out quickly and effectively on site. Modern pile-drivers are provided with a hammer assembly, or ram, and a slide guide, called the leader, which is arranged along the pile driving direction, for example vertically or almost vertically. A modern and very effective hammer assembly comprises an elongate unit being placed on top of the pile in extension of the axis thereof, which unit by means of hydraulically supplied power throws an internal weight upwards and allows it to drop in order to perform the ramming. The hammer assembly follows the pile down-wards during the ramming, so that the drop height and frequency of the weight need not be dependent on the actual driving depth of the pile. The pile top must, of course, be guided in relation to the hammer assembly, so that the pile is hit squarely on the top, and this is suitably achieved by means of a sort of holding mechanism arranged in the form of a socalled ram head. Normally, an intermediate plate or impact piece is placed between the hammer mechanism and the pile top, said intermediate plate or impact piece being of a material such as for example wood or plastic which may, to some degree, cushion the blows. In the case of piles made of a material of some friability, such as concrete, there is a risk of impact damages on the top which will disfigure the appearance of the pile, although it is normally possible to hit the piles so gently that the damages will be of no importance for the load-bearing capacity or strength of the pile. For this reason, driving of concrete piles is at present not very often used at places where the pile top will remain visible, and if, nevertheless, this technique is used, it is anticipated that there will be a need for a refinishing of the pile top. US patent no. 2,931,186 discloses a ram head which may be adapted for the driving of different kinds of piles. This ram head is in the form of a cap or a cup turned so that the opening is oriented downwards. In order to adapt this ram head for the driving of concrete piles, a filler pad is placed inside the cup so as to provide a planar, smooth lower surface inside the cup and a cushion pad is interposed between the filler pad and the pile top. The edge of the cup projects downwards so as to enclose the filler and cushion pads and part of the pile top. Practical experience with ram heads of kinds similar to that of US patent no. 2,931,186 has revealed that in cases of heavy hammering through this type of ram head onto a large pile made of a material of some friability, such as concrete, damage to the pile top edges is generally unavoidable. The reason for the damage to the pile top edges is believed to be the comparatively lower structural strength of the pile top surface in regions near the edges combined with the fact that the impact piece is severely deformed or completely mashed, expanding transversely and adapting to any non-planeties of the surfaces enclosing it, so that, regardless of its initial size and shape, it will end up covering the entire area of the pile top surface distributing the impact force from further hammer blows evenly all over the pile top surface including the above-mentioned regions of lower strength. WO-A-88/05844 discloses a silencer for pile rammers comprising a drop load, an impact piece and a guide cylinder for maintaining the alignment of the drop load relative to the impact piece. The document is silent regarding how to maintain the alignment of the impact piece relative to the pile. Nor does this document mention anything about the relative sizes of the surfaces of contact between the impact piece and the pile top surface. DE-C-659274 discloses a hammer assembly for pile ramming with leaf springs for connecting the hammer assembly to the pile. The leaf springs have pivots in both ends, and the connection to the pile is established by pivotable fittings connected to a tensioning bolt extending across the pile through a transverse bore. The document does not mention anything about the relative sizes of the surfaces of contact between the hammer assembly and the pile top surface. The leaf springs are adapted to exert a tensioning force biasing the hammer assembly towards the pile top surface, while they can hardly be believed to be able to resist a transverse displacement, and the leaf springs can, therefore, not be expected to be effective in maintaining the alignment of the hammer assembly relative to the axis of the pile. The invention provides a ram head adapted for receiving impact pulses produced by a hammer assembly and for transmitting the impact pulses through an impact piece and into the top surface of a pile with the purpose of driving said pile into the ground or into a similar substance, said ram head comprising guiding means with surfaces adapted for facing side faces of the pile over contact areas on the pile sides in a distance from the top edges of the pile, said ram head comprising a bottom plate provided with a collar projecting axially from said bottom plate in the direction towards the intended position of the pile, said collar receiving a deformable impact piece for confining it within a defined area, said guiding means being adapted for maintaining the alignment of said ram head relative to the axis of the pile, said ram head being characterized in that said collar is enclosing the impact piece and is arranged in a manner, with relation to said guiding means, so that the impact piece is kept away from the edge zones of the pile top surface, when the pile is held by said guiding means. Hereby, the impact pulses act only upon a defined area of the pile top surface not extending to the edges, while the edge zones are kept free from the action of the hammer. The result is that piles of a relatively friable material, such as concrete, can be driven without any damages to the top edges of the pile. According to an advantageous embodiment of the invention, a ram head is provided as stated in Claim 2. It is hereby achieved that such a pile can be driven without damage to the side edges, even if the pile material is relatively friable. It should be mentioned that there are limits to the accuracy with which piles maybe placed so that certain disadvantages have to be accepted, such as slight displacements and angular deviations of the piles. Also, in the case of piles with longitudinal edges, f. ex. piles of rectangular shape, there is a risk of turning the pile during driving. The risk of displacement, angular deviation or turning furthermore has the effect that driven piles cannot readily be used for masts which must be positioned very accurately. If they were to be used for this purpose there would be a need for the possibility of adjusting or levelling by fastening of the mast which entails a further complication. The simplest way of mounting a mast in an adjustable manner is to provide the pile with upwardly projecting fixation elements, such as bolts, whereby the mast may be adjustably fastened. It is very difficult, however, to drive a pile if it is provided with bolts projecting from its top. There is an immediate risk of deforming or otherwise damaging the bolts. Experiments as to providing the ram head and impact piece with openings for projecting elements such as bolts, have for various reasons not been successful. One important reason is that the impact piece during ramming is severely deformed or crushed to such an extent that the impact piece must be renewed after each pile driven. It is, therefore, useless to provide openings in the impact piece as the latter is normally completely mashed, and thereby in practice forged onto the bolts, so that the ram head after ramming is stuck in the pile. It should be remarked that normally the ram head is of a relatively simple configuration, f. ex. having a plane impact surface and sides fitting around the pile top, as by more sophisticated configurations there will be a substantial risk that the ram head will crack or be broken during ramming. According to a preferred embodiment of the invention, a ram head is provided for use in connection with a pile having elements projecting from its top as stated in Claim 3. It is hereby achieved that such a pile may be driven without any risk of damaging the projecting elements and without any problems in connection with releasing the ram head from the pile top after driving. According to another preferred embodiment of the invention, a ram head is provided as stated in Claim 4. It is hereby achieved that a pile of f. ex. quadrangular cross section may be placed in a selected one among different angular orientations about its vertical axis and be driven under control of the orientation. This is suitable where there is a strong exigency of a very accurate pile orientation, such as for example by foundations for masts which must have a specific orientation and where the site for placing the pile driver cannot be freely chosen. This ram head is particularly suitable for pile drivers moving along and working from their position on rails. According to an advantageous embodiment the ram head is made of steel of low hardness and low strength. Practical experiments have surprisingly shown that ram heads of such material in practice are more durable than ram heads of high strength steel having a tendency to crack during hammering. The reason herefor is not quite clear but is is assumed that it may be due to the fact that low strength steel is relatively vibration-absorbent. The invention further provides a method as stated in Claim 7. It is hereby achieved that a pile may be driven without damages to the top edges, even when the pile material is relatively friable, and with the use of only a very simple and cheap impact piece. According to an advantageous embodiment of the invention, a method is provided for driving a pile having at least partially plane side faces and longitudinal edges as stated in Claim 9. It is hereby achieved that such a pile may be driven without any risk of damaging the side edges. The invention is more fully explained in the following with reference to preferred embodiments shown in the drawings, wherein FIG. 1 shows a vertical section through a pile top with a schematic view of a ram head according to the prior art, FIG. 2 shows a horizontal section through a pile top and the bottom part of the ram head according to Fig. 1, FIG. 3 shows a vertical section through a portion of a ram head according to the invention placed in position above a pile top, FIG. 4 shows a horizontal section through a pile top with elements of a ram head according to the invention, FIG. 5 shows a side view of a ram head according to another embodiment of the invention, FIG. 6 shows a side view corresponding to Fig. 5, but in direction perpendicular thereto, FIG. 7 shows a horizontal section through the top part of a ram head according to the invention, and FIG. 8 shows a horizontal section corresponding to Fig. 7, but at a lower position. Initially, the principle used in the prior art ram heads will be explained, reference being made to Figures 1 and 2. The ram head 106 is shown embracing a rectangular pile top, and it appears that the ram head has the form of a box with a bottom or hammer surface 107 lying above the pile and with side faces 108 extending downwardly along the pile sides for fixation of the pile. A usual impact piece or intermediate plate 100 is interposed between the hammer surface 107 and the pile top 101. Although it does not appear from Fig. 1, the intermediate plate is practically so dimensioned that it may be hammered in position in the ram head 106 and may stick thereto without falling out, while the pile and the ram head are hoisted in position and are brought together. With a ram head of this type the pile can be driven quickly and effectively, but in practice it has been found that in piles of a material of some brittleness or friability, such as concrete, there will be damages on the top edge 103 of the pile and on the longitudinal edges 104. From a cosmetic point of view these damages are very adverse, and upwardly open cracks may be formed which outdoors may give rise to problems, as water may seep down and cause frost bursting of the foundation. In the following, various embodiments of the invention will be discussed with reference to Fig. 3 and the following Figures. Fig. 3 shows a vertical sectional view of part of a ram head 110 according to an embodiment of the invention and in position above a pile 2 having upwardly projecting elements 105, f. ex. in the form of embedded bolts. This ram-head part has a bottom plate 125 facing an impact piece 100 placed between said bottom plate 125 and the pile top 101, said impact piece 100 being of f. ex. wood, preferably oak, or synthetic material, preferably nylon. According to the invention the bottom plate is provided with a downwardly extending collar 126 enclosing the impact piece 100. This collar serves the purpose not only of holding the intermediate plate in its undeformed shape, but also of holding the impact piece 100 within its limits, even in the case of a severe deformation thereof. It is hereby achieved that the impact impulses are only acting on the central area of the pile top, and not near the top edges 103. Experiments have shown that hereby the pile may be driven without any damage of the top edges, if only the impact piece is of a suitable thickness and the length of the collar is adapted thereto, for example half the thickness of the impact piece. The collar may have the form of any closed, or substantially closed, outline, but preferably it is rectangular or circular. Furthermore, the bottom plate 125 is provided with openings 127 leaving free space around the bolts 105. The hammer assembly acts on the impact plate 121 being firmly mounted on top of the horizontal top section 120 which transmits the impact impulse to the bottom plate 125 through the body 122 being in the form of a cylindrical tube. In the following, a practical embodiment of the ram head according to the invention is explained with reference to Figures 4, 5, 6, 7, and 8 of the drawings. Those parts of the ram head of Figure 3 which are also found in the present embodiment are provided with the same reference numerals. As clearly apparent from Figures 5 and 6, this ram head is provided with guiding beams 128 mounted on the body 122 of said ram head and strengthened by welding to the bottom plate 125, extending downwardly around the pile which is secured by the contact pieces 129, as clearly apparent from Figure 4. These lateral guides secure the pile so that it can neither be displaced laterally nor rotate about its longitudinal axis, said lateral guides acting on contact faces on the pile sides 102 in distance from the side edges of the pile and in distance from the top edges of the pile. Practical experiments have shown extremely good results with this construction, as damaging of the pile edges and the pile as a whole can be avoided during driving. The contact pieces 129 are secured to the guide beams 128 by means of simple bolts 130, so that the contact pieces may be removed and for example be replaced by contact pieces of other dimensions, so that the guide may be adjusted to different pile dimensions. The ram head of this embodiment is made in two parts, i.e. a stationary part 111 (see Figure 6) and a part 112 being rotatable about the vertical, central axis. The stationary part 111 is firmly secured in relation to the hammer assembly (not shown) by appropriate securing of a projecting collar 113. The stationary part continues from the collar 113 downwards into a cylindrical body 116 having a supporting plate 114 mounted below. The entire stationary part is constructed in such a manner that it is dividable in a plane containing the longitudinal axis for ease of mounting and dismantling, and the two parts are held together by means of fish-plates 115. The rotatable part 112 is guided by sliding rings 119 and is supported by the supporting plate 114. As mentioned in connection with the embodiment of Figure 3, the rotatable part also here comprises a top section 120 with an impact plate 121 on which the hammer acts. The top section is welded to the cylindrical body part 122. Just below the top section 120 a number of radially projecting fins 124 are provided (see Figure 7) and below these fins a horizontal supporting flange 123 is mounted by means of which the rotatable part is supported on the supporting plate 114. The fins which are welded to the top section 120 and to the supporting flange 123 serve the purpose of partly supporting these parts and partly fixing the rotary orientation of the rotatable part in relation to the stationary part 111. As shown in Figure 7, eight fins define between themselves seven grooves having equal intervals, shown here with a pitch of 15° between them. In the stationary part two bushings 117 are provided at a mutual positional distance of 22,5°. In these bushings a guide pin 118 with conical point may optionally be inserted and secured, said pin fitting into the interval between the fins. As appears from Figure 7, the rotatable part may thus by means of the two bushings 117 alternately be turned and fixed in a total of fourteen positions at intervals of each 7,5°, over an angular region of totally 105°. Of course, a further possibility of rotation can be achieved by raising the ram head from the pile and turn the rotatable part 90° in relation to the pile top and lower it again. The arrangement has in practice been found to be able to perfectly endure the impact actions, which is supposed to be due to the possibility of longitudinal displacement of the rotatable part in relation to the stationary part, so that the impact impulse is not transferred to the guide pin 118 or to the fins. The ram head is suitably made of steel of low hardness and preferably of steel type 37-2 according to DIN 17100 or ISO 630.

Claims: A ram head adapted for receiving impact pulses produced by a hammer assembly and for transmitting the impact pulses through an impact piece and into the top surface (101) of a pile with the purpose of driving said pile into the ground or into a similar substance, said ram head comprising guiding means (128,129) with surfaces adapted for facing side faces of the pile over contact areas on the pile sides in a distance frog the top edges of the pile, said ram head comprising a bottom plate (125) provided with a collar (126) projecting axially from said bottom plate in the direction towards the intended position of the pile, said collar receiving a deformable impact piece (100) for confining it within a defined area, said guiding means being adapted for maintaining the alignment of said ram head relative to the axis of the pile, CHARACTERIZED in that said collar (126) is enclosing the impact piece (100) and is arranged in a manner, with relation to said guiding means, so that the impact piece is kept away from the edge zones of the pile top surface (101), when the pile is held by said guiding means. A ram head according to Claim 1, CHARACTERIZED in that the surfaces of said guiding means are arranged to face the side faces of a pile having at least partially plane side surfaces and longitudinal edges (104), the surfaces of said guiding means being adapted to hold said pile by contacting the plane side surfaces of the pile at positions in a distance from the longitudinal edges. A ram head according to Claim 1 or Claim 2, adapted for use in connection with a pile comprising elements (105) projecting upwardly and above the pile top surface, CHARACTERIZED in that the ram head is provided with openings (127) in the underside to allow free space around said elements, and in that the axially projecting collar (126) is arranged to keep the impact piece (100) clear of the upwardly projecting elements, also while the impact piece is being deformed by the impacts from the hammer assembly. A ram head according to Claim 2 or Claim 3 for the driving of a pile into the ground or into a similar substance, while maintaining a selected rotary orientation about the longitudinal axis of the pile, CHARACTERIZED in that the ram head is divided into a first and a second part, said first part (111) being arranged above said second part (112) in the operating position, said parts being mutually rotatable about an axis extending parallel to the intended axis of driving the pile and through the middle of the second part and being adapted so that a mutually rotational orientation may be selected and fixed by the first part being provided with a removable and insertable guide pin (118), which may be arrested in the inserted position, an end portion of said guide pin being adapted so that it may fit into anyone among a number of longitudinal grooves defined between radially projecting fins (124) in the second part when the guide pin is in the inserted position, so that the engagement between the pin and the selected groove may fix a rotational orientation, while allowing mutual displacement of said parts in axial direction. A ram head according to Claim 4, CHARACTERIZED in that said second part is provided with seven grooves at angular spacings or intervals about the longitudinal axis of 15°, and in that said first part is adapted with a possibility of inserting and fixing the guide pin at either of two positions, said positions being arranged with an angular spacing of 22,5° about the longitudinal axis so as to achieve a total of fourteen possibilities of mutual orientation at angular intervals of 7,5°. A ram head according to any of the Claims 1-5, CHARACTERIZED in that it is made of steel of low hardness and low carbon content, such as steel type 37-2 according to DIN std. 17100 or ISO std. 630. A method for the driving of a pile (2) into the ground or into a similar substance, said pile having a top surface disposed substantially at right angles to its longitudinal axis, and side faces extending substantially parallel to its longitudinal axis, wherein a hammer assembly constricted to move in the direction of the longitudinal axis of the pile is used to drive the pile downwards by hammering onto a ram head (110) which transmits the hammer blows into the top surface of the pile, said ram head comprising guiding means (128,129) with surfaces adapted for facing side faces of the pile over contact areas on the pile sides in a distance from the top edges of the pile, said ram head comprising a bottom plate (125) provided with a collar (126) projecting axially from said bottom plate in the direction towards the intended position of the pile, said collar receiving a deformable impact piece (100) for confining it within a defined area, said guiding means being adapted for maintaining the alignment of said ram head relative to the axis of the pile, CHARACTERIZED in that said collar is enclosing the impact piece and is arranged in a manner, with relation to said guiding means, so that the impact piece is kept away from the edge zones of the pile top surface (101), when the pile is held by said guiding means. A method according to Claim 7, CHARACTERIZED in that said impact piece is made of a material adapted to cushion the hammer blows, at least to some degree. A method according to Claim 7 or 8 for the driving of a pile into the ground or into a similar substance, said pile having side faces being at least partially plane and with longitudinal side edges, CHARACTERIZED in that the pile is secured in relation to the hammer assembly by being held over contact areas of the side faces, said areas terminating short of the side edges and short of the upper edges, while the zones adjacent these edges are kept free.

Inventors: LUCASSEN OLE HOLM, RASMUSSEN FINN, LUCASSEN, OLE HOLM, RASMUSSEN, FINN
Applicants: AARSLEFF AS, PER AARSLEFF A/S
Application year: 1990
Main classification: E02D13
IPC codes: E02D13