GB2154630A - Construction method for foundation piling - Google Patents

Abstract

A method for foundation piling utilizes an excavator comprising a rotatable casing 6 with a spiral rib on its outer surface and a lower end cutter 10a, and a screw 8 introduced into the casing 6 and having a spiral vane 13 with a translation component having a direction opposite to that of the rib 7 and also having a lower end cutter 10b. The method comprises the threading the casing 6 and the screw 8 into the subsurface 14 with the casing going ahead while they are rotated in opposite directions, into a bearing layer 15 to a desired depth, whereupon the bearing layer is cut by the screw 8 to eliminate the depth difference; withdrawing the screw 8, with or without rotation thereof, to discharge soil surrounding it out of a pile hole; erecting a pile 16 within the casing 6; then withdrawing the casing 6 while being rotated in the direction such as to direct the translation component of the spiral rib 7 downwards to compact soil around the pile 16. <IMAGE>

Description

SPECIFICATION Construction method for foundation piling The present invention relates to a construction method for foundation piling by which a precast pile, such as an H-section steel pile or a PCC precast concrete pile, is embedded into a subsurface in civil engineering construction works.

In the conventional erection of precast piles, use of a pile-driver which strikes the pile head with a hammer or the like has caused intolerable noise and vibration during the pile driving operation, and the surrounding districts adjacent to the pile driving site have been seriously affected by this environmental pollution.

It has been proposed to overcome this problem by a procedure in which the subsurface is initially excavated to form a hole in which the pile is erected, whereby the operation is performed without noise and vibration. However, another problem occurs when a commonly used excavator such as an earth auger is used for the initial excavation, this problem being that the side wall of a vertical hole thus excavated falls-in during the course of operation.

A casing may be used to avoid-the side wall falling-in. However, an initial excavation down to a level approximately 10 to 20 cm deeper than the front end of the casing has usually been employed to reduce the collapsing resistance encountered by the front end of the casing.

With this method of excavation, falling-in of the wall of the hole can certainly be avoided to some extent but, when subsoil water is present, such subsoil water invades the casing and is then discharged together with the excavated soil out of the pile hole. Such a method may seriously affect existing structures in the neighbourhood in the form of land subsidence-or the like, and therefore it is not perfect.

The object of the present invention is to provide a construction method for foundation piling, in view of various problems encountered by the prior art, which permits a mass of subsurface surrounding the foundation pile to be compacted tightly enough to prevent the side wall of an excavated vertical hole from falling-in, permitting even a deep and hard bearing layer to be smoothly and reliably excavated and enabling a reliable high pile-supporting force to be obtained.

This object is achieved, in accordance with the present invention, by a construction method for foundation piling utilizing an excavator comprising a rotatable casing having a spiral rib on its outer peripheral surface and provided at its lower end with a cutter, and a screw adapted to be introduced into said casing, said screw being provided the-rearound with a spiral vane having a translation component with a direction opposite to that of said rib and provided at its lower end with a cutter: said construction method comprising the steps of forcibly threading said casing and said screw into the subsurface with said casing going ahead of said screw while said casing and said screw are rotated in mutually opposite directions; threading the casing into a bearing layer to a desired depth before the screw reaches this depth whereupon said bearing layer is cut by the screw so as to eliminate the depth difference between the casing and the screw; withdrawing the screw with or without rotation thereof in the direction such as to direct the translation component in the spiral defined by the vane thereof upwards to discharge a mass of soil surrounding said screw out of a pile hole; erecting a pile into a resultant space defined within the casing; and then withdrawing the casing while the latter is rotated in the direction such as to direct the translation component in the spiral defined by the rib thereof downwards in order to compact a mass of soil around the pile.

The invention will now be further described with reference to the accompanying drawings which illustrate, by way of example, an excavator for use in a construction method in accordance with the invention and the method itself. In the drawings: Figure 1 is a side view showing an excavator for use in the construction method of the invention; Figure 2 is a front view, partially broken away, of an important part thereof; Figure 3 is a bottom view of the same part; Figures 4 to 9 are vertical sections sequentially showing the construction method of the invention; and Figure 10 is an enlarged vertical section showing partially and in a perspective view an important part of the excavating step as shown by Fig. 5.

First of all, an excavator used to execute the construction method of the present invention will be described with reference to Fig. 1 showing, in side view, the excavator as a whole and Fig. 2 showing an important part of this excavator in side view and partially broken away. A construction vehicle 1 is provided with a leader mast 2 shown in derrick mode, and separate drive means 4, 5 are suspended from a top sheave 3 of the leader mast 2 in such manner that said drive means can be vertically displaced.

These drive means 4,5 may be hydraulic motors, reduction gears or the like and the drive means 4 is connected through an associated chuck to a casing 6 which will be described in detail later while the other drive means 5 is connected through an associated chuck to a screw 8, to rotate the casing and screw, respectively.

The casing 6 has a spiral rib 7 on its outer peripheral surface and is provided at its lower end with a cutter 10a.

The reference numeral 8 designates the screw shown introduced into the casing 6, and the screw has a cutter head 9 detachably mounted on its lower end. A cutter 10b projects from the cutter head 9. The screw 8 is hollow and the cutter head 9 is provided with a cement milk discharging port 12 which communicates with a cement milk feeding port provided within the screw 8. The screw 8 further includes a spiral vane 13 on its outer periphery.

It should be noted here that the rib 7 of the casing 6 and the vane 13 of the screw 8 have transla tion components of mutually opposite directions, respectively, and the cutters 10a, 10b have their knife-edges appropriately oppositely directed.

Now an embodiment of the method of the invention utilizing the illustrated excavator will be described with reference to Figs. 4 to 9.

With the drive means 4,5 located at relatively high levels with respect to the leader mast 2, the casing 6 and the screw 8 are rotationally driven in mutually opposite directions.

Excavation is started with the screw 8 housed in casing 6 into the subsurface 14 and, in this case, the casing 6 goes a little ahead of the screw 8.

If the casing 6 and the screw 8 were rotated in the same direction, a surrounding mass of the subsurface 14 would be rotated by the casing 6 rotating outside the screw 8 in the same direction in which the screw 8 rotates and would thus make it impossible to obtain the desired cutting action of the screw 8. Rotation of the casing 6 and the screw 8 in mutually opposite directions as mentioned above-eliminates such inconvenience and is advantageous also in that reaction forces which are generated during excavation and transmitted to the drive means cancel each other, facilitating the support of the drive means.

In such excavation of the subsurface 14, events after the casing 6 has reached a bearing layer 15, it still travels ahead of the screw 8 as seen in Fig. 5.

Only after the casing 6 has cut into the bearing layer 15 to a predetermined depth is the screw 8 advanced with respect to the casing 6 so that it reaches the same level as that of the casing.

During excavation of the bearing layer 15, the screw 8 excavates a portion of the bearing layer 15 which has been separated by the casing 6 from the surrounding portion of the bearing layer 15 so that, even when the bearing layer is extremely hard, reliable excavation is achieved.

In another embodiment, the distance of the leading end of the screw 8 from that of the casing 6 is progressively reduced during cutting of the bearing layer 15 with the casing 6, until these both components while simultaneously advanced reach a desired depth at which the screw 8 finally coincides with the casing 6 in respect of their levels.

Now the screw 8 is withdrawn from the excavated space with or without rotation, leaving the casing 6 in the subsurface as shown by Fig. 7. In the case of withdrawal with rotation, the screw 8 is rotated in such a direction that the translation component in the spiral vane 13 is directed upwards and a mass of soil surrounding the screw 8, i.e., within the casing 6, is excavated upwards by said spiral vane. In the case of withdrawal without rotation, on the other hand, the screw 8 is withdrawn from the excavated space carrying soil clinging thereto. In such excavation, the mass of soil around the casing 6 is held thereby against falling into the space excavated by the screw 8.In the course of the withdrawal of the screw 8, a feeding device (not shown) pours a quantity of cement milk through the cement milk feeding port arranged within the screw 8 into this screw 8 and the quantity of cement milk thus poured into the screw 8 flows through the cement milk discharging port 12 down to the bottom of the casing 6.

Then a precast pile, such as precast concrete pile or an H-section steel pile, is erected in the space excavated by the screw 8, as seen in Fig. 8. A gap around such precast pile is filled with said quantity of cement milk which is, as will be described later, hardened and holds the mass of soil surrounding the pile 16 after the casing 6 has been withdrawn.

The casing 6 is now lifted with rotation and withdrawn while the mass of soil surrounding the foundation pile 16 is tightly compacted from the bottom layer. Thus the foundation pile 16 is supported not only by its point resistance but also by a high peripheral friction. The compacting action of the casing 6 is achieved by the rotation of the casing 6, while it is axially held, in the direction such as to direct the translation component in the spiral rib 7 downwards, or by the rotation of the casing 6 in the direction such as to direct said translation component upwards with the weight of the excavating mechanism and the casing 6 being exerted on the subsurface, while simultaneously the casing 6 is withdrawn.In the case of compaction with the casing 6 rotated in the direction such as to direct said translation component downwards, withdrawal of the casing 6 approximately by 10 cm while said casing 6 is rotated in such direction as to direct the translation component in the spiral rib 7 upwards and compaction, after said withdrawal operation has been temporarily stopped, with rotation of the casing 6 in such direction as to direct said translation- component downwards are alternately repeated.

As will be apparent from the foregoing description, the inner screw and the outer casing are forcibly threaded into the subsurface and excavation is accomplished by the inner screw while the surrounding soil is held by said outer casing against falling-in the construction method according to the present invention, so that the excavation can be performed smoothly, without noise and vibration.

During excavation, the casing goes ahead of the screw and, as seen in Fig. 10, muck of bedrock or the like produced by the cutter 10a falls into a gap defined inside the front end of the casing, i.e. between a mass of bearing layer 15 cut by said front end of the casing 6 and the latter. In such situation, the casing 6 is being rotated and, therefore, said muck 17 also follows this rotation of the casing 6.

Thus, the muck serves as frictional material and generates heat. In consequence, the bearing layer 15 is inwardly compressed as indicated by a broken line- and particles constituting the layer 15 become fine so that the bearing layer 15 itself becomes rocky.

The bearing layer 15 which has thus become rocky can completely prevent subsoil water from intruding from the outside into the inside of the casing 6 and thereby avoid any significant change occurring in the subsoil water such as would cause land subsidence. Furthermore, the invention provides the subsurface compaction by the outer casing which permits an adequate supporting force to be achieved without hammering, as has been re quired by the foundation piling of prior art, and permits even a hard bearing layer to be reliably excavated.

Claims (4)

1. A construction method for foundation piling utilizing an excavator comprising a rotatable casing having a spiral rib on its outer peripheral surface and provided at its lower end with a cutter, and a screw adapted to be introduced into said casing, said screw being provided therearound with a spiral vane having a translation component with a direction opposite to that of said rib and provided at its lower end with a cutter: said construction method comprising the steps of forcibly threading said casing and said screw into the subsurface with said casing going ahead of said screw while said casing and said screw are rotated in mutually opposite directions; threading said casing into a bearing layer to a- desired depth before the screw reaches this depth, whereupon said bearing layer is cut by the screw so as to eliminate the depth difference between the casing and the screw; withdrawing the screw with or without rotation thereof in the direction such as to direct the translation component in the spiral defined by the vane thereof upwards to discharge a mass of soil surrounding said screw out of a pile hole; erecting a pile into a resultant space defined within said casing; and then withdrawing the casing while the latter is rotated in the direction such as to direct the translation component in the spiral defined by the rib thereof downwards in order to compact a mass of soil around the pile.

2. Apparatus for foundation piling according to claim 1, comprising said excavator including said rotatable casing and said rotatable screw.

3. A method of foundation piling substantially as herein particularly described with reference to the accompanying drawings.

4. Apparatus for foundation piling substantially as herein particularly described with reference to Figures 1 to 3 of the accompanying drawings.