GB1561126A - Pile driving - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO PILE DRIVING (71) I, THE SECRETARY OF STATE FOR INDUSTRY, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to methods of pile driving and more particularly, but not exclusively, concerns methods of pile driving whereby a hollow pile may be driven for a considerable distance through a material such as, for example, densely compacted sand by a succession of percussive blows.

Certain materials, including some densely compacted sands, are too soft for piles to be drilled and grouted into them, and too hard to be penetrated by conventional purely percussive means in which the cutting end of a hollow pile is forced into the material by percussive blows on its upper or "anvil" end since material round the cutting end of the pile tends to become progressively more compacted and impenetrable. A further difficulty arises when materials such as sand and associated silt are forced upwardly within the pile during pile driving and tend to jam completely and be unable to move further. Such jamming is called "arching".

Arching effects increase rapidly after their first occurrence and a limit is quickly reached after which further penetration into the material by the pile is prevented.

It is possible to prevent arching by removing continuously all penetrated material from the centre of the pile, for example by pumping, but large power is usually required and discharge of the sand from the top of the pile complicates the operation of the driving means imparting the impulsive blows. Driving of piles by percussive means can be facilitated by a process called "jetting". In jetting, large volumes of liquid are pumped to the cutting end of the pile where they loosen the material into which the pile is being driven. The liquid then carries the loosened material in the pile upwardly along the length of the pile and out of the pile at the top thereof. The weight of liquid used in jetting is commonly heavier by a factor of two or three than the material within the pile into which the pile has cut.

This invention aims to provide a method of driving a hollow pile into a particulate medium by a succession of impulsive blows, which method ameliorates the problems of arching and of compaction beneath the cutting end of the pile in a manner which need not intrude on the operation of the driving means.

This invention therefore provides a method of driving into a particulate material a hollow pile having an anvil end which is adapted to receive impulsive blows and an open cutting end the method comprising the steps of a. imparting a succession of impulsive blows to the anvil end of the pile to drive the cutting end into the material, b.

generating a supply of fluid, and c. introducing said supply to a volume within the hollow pile and adjacent the open cutting end thereof; wherein the fluid is supplied as a series of pulses and at a rate which is sufficiently small that no more than a small proportion of the particulate material located within the hollow pile is conveyed by the fluid to the anvil end of the pile while driving of the pile is taking place.

The fluid to be introduced may be gas or liquid; as specific fluids air or water are suitable in particular circumstances. The pulses of fluid are preferably harmonised with, though not necessarily occurring at the same time as, the impulsive blows applied to the anvil end of the pile. Tests on small hollow piles have shown that while pulses applied at the same time as the blows can be effective, it is particularly effective to apply the pulses at the same frequency as the blows but out of phase with the blows.

Pulses applied in this way between blows are thought to reduce briefly the pressure of the cutting edge of the pile on the material beneath it and thus to allow the material to change its position in some way, so that the next following impulsive blow is able to move the material further and thus to drive the cutting edge further into the material.

The pulses of fluid can tend to reduce the likelihood of arching, and the tendency for arching can be further reduced by injecting fluid into the volume within the pile at intervals along its length.

In one form of pile suitable for carrying out the method of the invention, the cutting edge is provided with one or more inwardly facing recesses located in its inner surface and communicating through ducts with a source of pressurised fluid. The recess or recesses may take the form of annular grooves extending completely round the cutting member. One or more ducts communicating with the same source of pressurised fluid or an additional source can be provided, each with a plurality of orifices located at intervals along its length, whereby fluid may be injected into the interior of the pile at points some distance from the cutting member.

The weight of the fluid introduced into the hollow pile in a given time is generally only a small proportion of the weight of penetrated material which passes into the volume within the pile in the same time. In any case, the amount of fluid introduced is insufficient to convey more than a small proportion of the material to the anvil end of the pile. This is in contrast with known methods which sweep the bulk of this material out of the pile at the anvil end.

By way of example, one embodiment of the invention will now be described with reference to the drawings accompanying the provisional specification, of which Figure 1 is a diametral section of the cutting end of a pile suitable for use in a method according to the invention, and Figure 2 is a section aldong the line II-II of figure 1.

A pile, to be driven for example into compacted sand on the seabee comprises a tube 1 around the cutting end of which is welded an annular cutting member 2. The cutting member has an outer diameter equal to that of the tube 1, so that a single outer smooth surface is presented to the sand as the pile is driven into it.

The inner surface of the cutting member 2 is inclined outwardly and downwardly from a thick portion 3 to a relativelv narrow cutting edge 4. The thick portion 3 is provided with an annular groove 5 extending inwardly and upwardly to its mouth and around the complete circumference of the member 2, the mouth of the groove opening into the inner surface of the thick portion 3.

At three locations equally spaced around the circumference of the member 2 three bores, of which one is referenced as 6, extend upwardly from the groove 5 and parallel to the axis of the cutting member 2 to its upper surface 12.

Three steel tubular ducts, of which two are referenced as 8 and 9, are fixed to the inner surface of the tube 1 by brackets 10.

The three ducts extend the complete length of the tube in an axial direction and are equally spaced around the circumference of the member 2, and each extends downwardly from the lower edge of the tube 1 into a corresponding bore 6 a sufficient distance to form a sealing fit when the cutting member is welded in position on the tube.

Each duct (such as 8 and 9) is provided at intervals along its length with orifices 11 facing generally inwardly, so that on assembly and operation liquid supplied to the ducts emerges inwardly and upwardly from the groove 5 and also inwardly from the orifices 11.

In operation, the pile is given repeated blows on its upper i.e. its anvil end by, for example, a hydraulic hammer or a heavy weight dropped upon it, to drive it downwardly. In the absence of applied pulses of fluid, the repeated downward driving will cause impacting of sand beneath the cutting edge 4. The sand cannot normally be displaced outwardly due to the already high degree of compression of the sand outside the pile, and further compaction of sand beneath the cutting edge and arching may well take place within the pile. In these circumstances the pile cannot be driven into compacted sand a sufficient distance to provide the necessary degree of support for a structure such as a drilling rig.

In operation of this embodiment of the invention water is used as the fluid to be introduced into the pile. It is forced down the three ducts, including ducts 8 and 9, at high pressure and emerges as previously described from the groove 5 and orifices 11, the effect of which is to ease compression at the cutting edge and reduce arching of sand within the tube so that as the pile is driven further into the sand, material can move upwardly within the tube allowing sand below the cutting edge to be displaced inwardly on advance of the pile.

Preferably the high pressure water is fed to the ducts in pulses harmonised with the driving blows applied to the upper end of the pile. Synchronisation can be obtained by the use of a valve for the high pressure water circuit operating according to the position of the heavy weight or hydraulic piston driving the pile. An additional effect of pulses of the same frequency as the blows but occurring between each blow appears to be to gener ate a hydrostatic pressure tending to lift the pile and the particulate material within it.

This lifting effect is small compared with the downward penetration of the pile with the following blow, so that the overall penetration of the pile with each blow is considerably greater than in the absence of a fluid pulse owing to the more fluid consistency of the sand beneath the cutting edge.

In cases where. as may frequently be the case, a pile has to be driven into the seabed for a distance of some hundreds of feet. it is necessary for the tube 1 to be constructed from a number of sections, each section being welded to adjacent sections whilst still above the water surface. The three ducts.

including ducts 8 and 9, extend the whole length of the pile, being pre-assembled on the inside of each length of the tube 1 before welding, after which connecting members.

such as hydraulic hoses. can be applied in conventional fashion to interconnect adjacent lengths of each duct. Such connecting members do not of course take any stress of the impacts driving the pile.

It can be seen readily that three is not necessarily the optimum number of ducts for any application. For example. one or two may suffice. The groove 5 may be arranged to face horizontally or even downwardly, and indeed more than one such groove may be provided.

WHAT I CLAIM IS: 1. A method of driving into a particulate material a hollow pile having an anvil end which is adapted to receive impulsive blows and an open cutting end, the method comprising the steps of a. imparting a succession of implusive blows to the anvil end of the pile to drive the cutting end of the pile into the material.

b. generating a supply of fluid, and c. introducing said supply to a volume within the hollow pile and adjacent the cutting end thereof; wherein the fluid is supplied as a series of pulses and at a rate which is sufficiently small that no more than a small proportion of the particulate material located within the hollow pile is conveyed by the fluid to the anvil end of the pile while driving of the pile is taking place.

2. A method according to claim 1, wherein the frequency of the pulses is controlled by a valve which is responsive to the position of the driving means imparting the impulsive blows to the anvil end of the pile.

3. A method according to claim I or claim 2. wherein the frequency of the pulses is equal to that with which impulsive blows are imparted to the anvil end of the pile.

4. A method according to any of claims 1 to 3. wherein the pulses are introduced to the volume within the pile at timcs between those at which impulsive blows are imparted to the anvil end of the pile.

5. A method according to any one of the preceding claims wherein the fluid is introduced through an annular groove located at the cutting end of the pile.

6. A method according to any one of the preceding claims. including the step of introducing a fluid to the volume within the pile at at least one location intermediate the annular groove and the anvil end of the hollow pile.

7. A method of pile driving substantially as hereinbefore described with reference to the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. ate a hydrostatic pressure tending to lift the pile and the particulate material within it. This lifting effect is small compared with the downward penetration of the pile with the following blow, so that the overall penetration of the pile with each blow is considerably greater than in the absence of a fluid pulse owing to the more fluid consistency of the sand beneath the cutting edge. In cases where. as may frequently be the case, a pile has to be driven into the seabed for a distance of some hundreds of feet. it is necessary for the tube 1 to be constructed from a number of sections, each section being welded to adjacent sections whilst still above the water surface. The three ducts. including ducts 8 and 9, extend the whole length of the pile, being pre-assembled on the inside of each length of the tube 1 before welding, after which connecting members. such as hydraulic hoses. can be applied in conventional fashion to interconnect adjacent lengths of each duct. Such connecting members do not of course take any stress of the impacts driving the pile. It can be seen readily that three is not necessarily the optimum number of ducts for any application. For example. one or two may suffice. The groove 5 may be arranged to face horizontally or even downwardly, and indeed more than one such groove may be provided. WHAT I CLAIM IS:

1. A method of driving into a particulate material a hollow pile having an anvil end which is adapted to receive impulsive blows and an open cutting end, the method comprising the steps of a. imparting a succession of implusive blows to the anvil end of the pile to drive the cutting end of the pile into the material.

b. generating a supply of fluid, and c. introducing said supply to a volume within the hollow pile and adjacent the cutting end thereof; wherein the fluid is supplied as a series of pulses and at a rate which is sufficiently small that no more than a small proportion of the particulate material located within the hollow pile is conveyed by the fluid to the anvil end of the pile while driving of the pile is taking place.

2. A method according to claim 1, wherein the frequency of the pulses is controlled by a valve which is responsive to the position of the driving means imparting the impulsive blows to the anvil end of the pile.

3. A method according to claim I or claim 2. wherein the frequency of the pulses is equal to that with which impulsive blows are imparted to the anvil end of the pile.

4. A method according to any of claims 1 to 3. wherein the pulses are introduced to the volume within the pile at timcs between those at which impulsive blows are imparted to the anvil end of the pile.

5. A method according to any one of the preceding claims wherein the fluid is introduced through an annular groove located at the cutting end of the pile.

6. A method according to any one of the preceding claims. including the step of introducing a fluid to the volume within the pile at at least one location intermediate the annular groove and the anvil end of the hollow pile.

7. A method of pile driving substantially as hereinbefore described with reference to the drawings accompanying the provisional specification.