GB2094870A - Construction of in situ piles - Google Patents

Abstract

Tubular means comprises a tip member (11) having a closed leading end (13) and a number of trailing tubes (12) connected in longitudinal alignment. The tubular means is driven down by the action of both a self-propelled mole (16) received within the tubular means and acting on the tip member (11) and by a vibratory hammer (19) acting on the upper end of the upper-most tube (12). The aim is for the mole to provide the force for soil displacement and the hammer to provide the force for overcoming the increasing skin friction as the hole deepens. Concrete may be poured to form a pile in situ in the hole during or after withdrawal of the tubular means, or the tubular means may remain in situ to enable formation of a cased pile. <IMAGE>

Description

SPECIFICATION Construction of in situ piles This invention relates to the construction of in situ piles.

My published U.K. Patent Application No. 2 048 999 A discloses the construction of in situ piles wherein a pile hole is formed by allowing an elongate self-propelled soil displacing mole to descend through the subsoil. That case also disclosed the connection of a train of tubular members to an expander carried down by the mole, the mole being subsequently withdrawn up through the bore of the tubular members. The pile is formed and the tubular members extracted. My Patent Applications Nos. 7925405 and 8023589 further disclose a method wherein the mole draws down a tubular member, e.g. a UPVC pipe by means of a draw cable, and the pile is formed in the tubular member to provide a cased pile.

Working of the above inventions has enabled practical experience to be obtained and certain problems have been identified. Thus, in certain subsoil conditions, e.g. clay, it can be very difficult and time-consuming to extract the expensive mole, if used alone, or to extract the metal tubular members and expander where they are used. The mole is primarily intended for forward movement biting through hard ground material and is found to be adversely affected if used in the reverse mode for a prolonged period while attempting to work itself backwards up a closing hole. The use of an expander and tubular members is only effective in a limited range of soil conditions.As the hole deepens, the increasing frictional drag of the tubular members on the hole wall can slow progress down increasingly severely, to the point where it is practically impossible to form the hole any deeper or as deep as required and the mole is again adversely affected by not being able to move down to bite on fresh subsoil. The use of a draw cable and a UPVC tube suffers from the same problems.

An object of the present invention is to provide a new method of constructing in situ piles wherein the above disadvantages are mitigated.

According to the present invention there is provided a method of constructing a pile in situ comprising: forming a pile hole by driving tubular means down through the subsoil, said tubular means comprising a tip member having a substantially closed leading end and at least one trailing tube member connected in longitudinal alignment with the tip member, said tubular means being driven by both first driving means acting on the tip member and by second driving means acting on the upper end of a said tube member, and forming a pile in situ in said hole.

The first drive means may be an elongate pneumatically actuated self-propelled mole received within and descending with the tubular means. The second drive means may be a vibratory hammer acting directly or via a dolly on the upper end of a said tube member. The aim is for the first drive means to provide all or most of the necessary force for soil displacement to the diameter of the tubular means, and for the second drive means to provide the necessary force to overcome the increasing skin friction between the trailing tubes and the subsoil as the hole deepens.

The two drive means are thus preferably individually controllable. The connection between the tip member and the first trailing tube member is preferably a sliding connection to permit mutually independent longitudinal driven movement over a limited range while preserving the longitudinal alignment and also allowing surplus driving energy from the second drive means to be transferred to the tip member for soil displacement.

The driven tubular means may remain in situ and be filled with concrete to provide a cased in situ pile. That course may be preferred for poor subsoil conditions, e.g. sand or clay, or where headroom is limited so that the tubular means coprises a plurality of successive said tube members. The successive tube members are conveniently provided with simple elongate welded collars at their lower ends to maintain their mutual longitudinal alignment.

Alternatively, the whole tubular means, or at least the tube members thereof, may be withdrawin for re-use. That course may be preferred on the grounds of cost-saving and may be economic for new work in the open and where subsoil conditions are suitable. The concrete to form the in situ pile may be charged into the pile hole after or during withdrawal of the tubular means. The tube members are suitably threadedly connected to one another to permit their withdrawal, e.g. by reversing the action of a said vibratory hammer.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a vertical section through apparatus forming a pile hole in practice of a method according to the invention; and Figure 2 is a partial sectional view of one way of filling the pile hole with concrete.

Figure 1 shows a pile hole being formed in subsoil 10. Tubular means comprises a tip member 11 and at least one trailing tube member 1 2. The tip member is a short steel tube with a closed leading end 1 3. The end is preferably closed by crimping in a simple hydraulic press so that the end appears cross-shaped in end view. The tube member 12 is also of steel and is connected in longitudinal alignment with the tip member. The connection is suitably a sliding spigot and socket connection to permit mutual independent longitudinal driven movement while preserving the longitudinal alignment. The tip member and tube member are of the same diameter, e.g. 10 centimetres, the socket being formed by an external steel collar 14 of substantial axial length and welded over half its length to the trailing end of the tip member.The collar is internally grooved and the connection is packed with waterproofing compound to prevent ingress of ground water.

A single tube member of, say 5 to 6 metres in length may be used, but preferably a train of tubes of, e.g. 1.5 metres in length, is used. Two methods will be described, in which the tube members are respectively recovered and left in situ. For recovery, successive tube members are provided with male and female threaded portions at opposite ends. When the tubes are to be left in situ a simple spigot and socket connection may be used. The lowermost tube has no collar, serving as a spigot at both ends, subsequent tubes have a welded-on collar at their lower ends to serve as sockets, the connection being packed with waterproofing compound. The collars are at the lower ends to avoid damage by hammering as will become clear.

In the first method to be described, a complete tubular means is assembled in the longitudinal position on the ground, i.e. a tip member 11 and a train of threadedly connected tube members 1 2. A quantity of sand, rag, paper or the like matrix material 1 5 is packed into the end of the tip member to take shock loading, and a elongate pneumatically actuated self-propelled mole 1 6 is received within the tubular means near the tip end with its trailing air supply tube 1 7 and draw wire 18 threaded down the tubular means. Such a mole is described in my published U.K. Patent Application No. 2 048 999 A and in more detail in U.K. Patent No. 1,392,868.To avoid the heavy mole knocking the tip member off at this stage, a temporary retaining pin may be provided extending radially inwards through the collar 14 to hold the tip member onto the first tube member.

The assembly is then raised to the vertical position and a vibratory hammer 19 located in and on the top end of the uppermost tube member 1 2.

The hammer has a bore or channel 20 for free passage of the air tubes 17 and draw wire 18 of the mole 1 6. The hammer is suspended by a cable 21 from a crane or scaffold tower or the like (not shown). The hammer may be pneumatic, hydraulic, diesel-driven or powered by any other convenient power source.

The mole 1 6 is operated, as the first drive means, to commence driving the tip member down through the subsoil, and shortly thereafter the hammer 1 9 is activated to commence driving the tube members. Initially the hammer may be operated at relatively lower power, and then with increasing power as the hole deepens and skin friction increases. The aim is for the mole to provide most of the necessary force for soil displacement to deepen the hole, with none of its efforts being dissipated in trying to overcome the skin friction drag on the tube members.The hammer 20 provides the necessary driving force to overcome the increasing skin friction and any surplus driving energy, particularly in the initial stages, is transferred to the top member by its abutment with the lowermost tube member to assist the efforts of the mole in soil displacement. The hammer power is controlled, by experience of the operator, to ensure that the tubes at least keep pace with, and preferably assist, downward motion of the tip member so as to avoid disconnection of the tip from the tube.

The vibration frequencies of the mole chisel and of the hammer preferably differ substantially so as to avoid any undesirable beat effects between the two vibratory driving motions.

When the pile hole has attained the desired depth, the hammer is removed and the mole withdrawn, e.g. by attaching its draw wire 1 8 to the lifting cable 21. The lined hole is then filled with concrete. For larger diameter holes, e.g. 1 5 centimetres or more, the concrete may simply be poured in via a tundish. The mole may then be lowered in and actuated to compact the concrete.

The tube members 12 are withdrawn from the hole by pinning the vibratory hammer to the top of the uppermost tube and actuating it in reverse to hammer the tube members upwardly out of the hole. The lifting cable 21 takes the weight of the hammer and the growing weight of the tubes as they are withdrawn from the frictional grip of the subsoil. Reinforcement is then inserted in the hole and the concrete may be further compacted by vibration as desired.

For narrower diameter holes it may be difficult to ensure that the poured concrete fills the hole without trapping air pockets. It may be preferred to adopt the following procedure illustrated in Figure 2. A delivery pipe 30 from a ready-mixed concrete supply pump (not shown) is lowered down to near the bottom of the tubular means. An inflatable grout bag 31 is carried by collars 32 near the lower end of the pipe 30, and an air line 33 for inflating the bag 31 is paid out down the tube. Having first pumped residual air out through pipe 30 the bag 31 is infiated hard to seal the annular space between the pipe 30 and the lowermost tubular member 12.

The vibratory hammer is again pinned to the top of the uppermost tube and actuated in reverse to start withdrawing the tube members including the sealed-in grout bag from the hole. The concrete pump is simultaneously actuated to supply concrete under maintained pressure to the progressively increasing unlined hole length beneath the grout bag. The tip member remains in situ at the bottom of the hole. By thus maintaining pressure below the grout bag the unlined hole may be filled with concrete even in poor soil conditions in which the hole would tend to collapse were the tubes to be withdrawn prior to filling the hole with concrete. The problem of filling a narrow diameter hole with concrete is thus also solved. When the hole is filled and the tubes withdrawn reinforcement may be inserted and the concrete may be compacted by vibration as desired to complete the in situ pile.

As mentioned above there are situations in which it may be preferred to form cased piles. In those situations the tubes are preferably connected in longitudinal alignment by simple spigot and socket connections as described above rather than by the more expensive threaded connections, and the cased hole is filled with concrete either via a tundish from the top or via a concrete pump delivery pipe from the bottom. The steel tubes may be of lighter gauge than if they were to be hammered down by only a single drive means acting solely at the top or solely at the bottom.

In an alternative method according to the invention the tube members are not assembled together in a train prior to driving the hole. For example it may be desired to form pile holes within a building or other area having severely restricted headroom. In that case only the tip member and the first tube are assembled, the mole inserted therein, and raised to the vertical position for driving. The succeeding tubes are threaded on the air tube 17 and draw cable 18 of the mole, but are not connected. It may not be convenient or possible to use a full size vibratory hammer: instead a dolly is seated in the top end of the first tube, the dolly being split about a central bore to permit free passage of the air tube 1 7 and draw cable 18.The dolly has a robust yoke or stirrup depending therefrom by about half the length of the tube, e.g. 75 centimetres for a 1.5 metre long tube.

The tubular means is then driven down both by the mole as the first drive means and by a pneumatic jack hammer with its vibratory tip engaged in the sitrrup of the dolly, the jack hammer constituting the second drive means.

Hammering steel tube by means of a jack hammer is described in my Patent Application No. 8040772. When the stirrup has descended to ground level the stirrup is removed and the tip of the jack hammer moved up to engage the dolly directly to complete hammering of that length of tube. Driving by both the mole and by the jack hammer ceases while the subsequent tube member 12 is connected, e.g. by spigot and socket as described, to the first tube member, and driving then re-commenced by both drive means.

When complete the pile hole is filled with concrete as described previously to form a cased pile. In this situation of restricted headroom it would not normally be cost effective to seek to withdraw the tube lengths one by one.

In a further embodiment of the invention the tip member is connected to the lowermost tube member by annular shock-absorbing means, e.g.

of elastomeric material, in a manner maintaining the axial alignment and also thereby permitting withdrawal and recovery of the tip member with the tubes where the soil conditions are of sufficient stability to permit such withdrawal prior to filling with concrete.

Claims (12)

1. A method of constructing a pile in situ comprising: forming a pile hole by driving tubular means down through the subsoil, said tubular means comprising a tip member having a substantially closed leading end and at least one trailing tube member connected in longitudinal alignment with the tip member, said tubular means being driven by both first driving means acting on the tip member and by second driving means acting on the upper end of a said tube member, and forming a pile in situ in said hole.

2. A method according to claim 1 wherein the first drive means comprises an elongate pneumatically actuated self-propelled mole received within and descending with the tubular means.

3. A method according to claim 1 or claim 2 wherein the second drive means comprises a vibratory hammer acting on the upper end of a said tube member.

4. A method according to claim 3 wherein said vibratory hammer acts via a dolly on the upper end of a said tube member.

5. A method according to any one of claims 1 to 4 including individually controlling the power applied to the tubular means by each of the first and second drive means.

6. A method according to any one of claims 1 to 5 wherein the first and second drive means have vibration frequencies that differ substantially from one another.

7. A method according to any one of claims 1 to 6 wherein the connection between the tip member and the first trailing tube member is a longitudinal sliding connection.

8. A method according to any one of claims 1 to 7 wherein the driven tubular means remains in situ and is filled with concrete to provide a cased in situ pile.

9. A method according to claim 8 wherein a plurality of successive said tube members are provided and are maintained in mutual longitudinal alignment by means of collars at their lower ends.

10. A method according to any one of claims 1 to 7 wherein the driven tubular means is withdrawn and the pile hole is charged with concrete after said withdrawal to form said in situ pile.

11. A method according to any one of claims 1 to 7 wherein the driven tubular means is withdrawn and the pile hole is charged with concrete during said withdrawal to form said in situ pile.

12. A method according to claim 11 wherein said pile hole is charged with concrete through a concrete supply pipe extending down within the tubular means and sealed therewith by means of an inflated grout bag.

1 3. A method according to any one of claims 10 to 12 wherein a plurality of successive said tube members are provided and are threadedly connected to one another.

1 4. A method of constructing a pile in situ substantially as described herein with reference to Figure 1, or Figure 2, of the accompanying drawings.