GB2048999A - Improvements relating to the construction of in situ piles - Google Patents

Abstract

A method of constructing a pile in situ comprising: forming a hole in the subsoil by allowing an elongate self- propelled soil displacing mole 19 to descend through the subsoil; withdrawing the mole; filling the hole with concrete; allowing such mole to descend within the filled hole before the concrete sets to produce a pile footing 23 and to displace concrete into the hole wall; withdrawing such mole; and refilling the hole with concrete and allowing the concrete to set to form an in situ pile with an integral footing. <IMAGE>

Description

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

In building and civil engineering work it is known to support structures on piles. The piles may be pre-formed and driven into the subsoil: alternatively a hole may be bored and a reinforced concrete pile formed in situ therein. The choice is wide with new building work on accessible sites.

However the need often arises for remedial support work beneath existing structures that have subsided. Such remedial work can present severe problems of access and in consequence underpinning work in general is difficult and expensive.

For example many recently built houses are constructed in the form of an external cavity wall built on a concrete strip footing. The enclosed ground area is then built up and levelled with compacted hard core and a concrete slab is cast thereon. Either the strip footings of the slab or both may in due course subside for various reasons, such as drying out of clay or peat subsoils, wash-outs due to water flow, the presence of mine workings, or due to the house being built on a sloping or slack-filled quarry site.

A problem in some areas is subsidence of the floor slab due to an insufficient depth of insufficiently compacted hard core resting on a peaty subsoil that shrinks in a period of dry weather. A first solution to this problem would be to bore holes through the slab and inject grout under pressure to attempt to fill the pore spaces and to simultaneously restore the slab to its proper level. This first solution is often insufficient in that it gives only short term relief or may prove barely workable for various reasons. In that event the current recommendation by the National House Building Council is to gut the interior of the house, including the interior walls and staircase and usually the mains services, break and dig up the slab, add and consolidate hard core and cast a new slab.This solution is difficult, timeconsuming, and the total expense approaches that of building a new house.

An object of the present invention is to provide a method of constructing piles in situ that is capable of being practised in situations where access, particulariy headroom, is restricted, for example within a room of a modern house with little more than two metres headroom.

According to the present invention there is provided a method of constructing a pile in situ comprising: forming a hole in the subsoil by allowing an elongate self-propelled soil displacing mole to descend through the subsoil; withdrawing the mole; filling the hole with concrete; allowing such mole to descend within the filled hole before the concrete sets to produce a pile footing and to displace concrete into the hole wall; withdrawing such mole; and refilling the hole with concrete and allowing the concrete to set to form an in situ pile with an integral footing.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a section through a portion of a house foundation showing a mole driving a hole as a step of a method of forming an in situ pile according to the invention Figure 2 shows a later step in which the mole causes the first poured concrete to form a footing for the pile; Figure 3 shows a completed reinforced pile supporting a pre-existing concrete slab; Figure 4 shows a step in an alternative embodiment in which the hole is lined as the mole descends; and Figures 5 and 6 are schematic views of two embodiments for supporting a pre-existing wall.

Referring firstly to Figure 1, there is shown a house wall 10 supported on a footing 11 and a ground floor concrete slab 12 supported on a compacted hard core bed 13 on the subsoil 14.

Assume the slab 1 2 has subsided and thus required restoring to its correct position and supporting in that correct position. A normal first step is to drill holes 1 5 through the slab and inject grout under pressure through the holes. The grout 1 6 is forced through any interstices of the hard core and may penetrate voids beneath the hard core caused by subsoil shrinkage, for example due to a drought. The hydraulic pressure exerted by the grout 16 will normally serve to lift the subsided portion of the slab to its correct position.

However, grouting alone is often insufficient to solve the subsidence problem in a reliable and long-term manner. In accordance with the invention piles are formed in situ beneath the concrete slab. The number of piles required and their spacing is calculated in accordance with normal structural engineering practice. The diameter of the piles would normally be in the range 7 to 12 centimetres, e.g. 8 centimetres.

The first step is to form an access hole 1 7 through the slab, e.g. by a pneumatic drill. A vertical hole 1 8 is then formed in the subsoil by allowing an elongate (e.g. about 1.4 metres) selfpropelled soil displacing mole 1 9 to descend through the subsoil. A suitable mole is described in British Patent No. 1,392,868 and is commercially available from the Patentee. That mole is pneumatically actuated, the air line 20 following the mole down the hole. When the pile is being formed from a room in a house the line 20 may extend through a window to a suitable compressor outside.In each cycle the air pressure drives a piston forward which in turn drives a penetrating chisel 21 forward into the subsoil, forward motion of the chisel acting a short time later, through resilient means, also to drive the mole housing forward.

At a suitable depth, which may be in the range 4 to 8 metres, the pneumatic action of the mole is reversed by a pull on a control cable 22, and the mole withdraws itself from the hole. Alternatively the mole may pay out a steel cable and be withdrawn by tension exerted on the cable by a suitable extractor rig. Reverse pneumatic action may be used together with such an extraction rig.

A pile generally supports a load by two basic mechanisms, direct end load and skin friction over its whole length. End load capability is enhanced if the foot of the pile rests on a rock stratum or the like, but this objective must be balanced against the undesirability of increasing the pile length to a point where the pile has insufficient resistance to bending. Skin friction load bearing increases as the pile length increases and also may depend on the nature of the subsoil and the extent to which it is compacted. Formation of the hole by soil displacement, as opposed to soil removal, enhances the skin friction effect. For these reasons the actual depth achieved by the mole and its rate of progress are both monitored, e.g. by monitoring pay-out of the air line or cable, in practice of this invention.The rate of progress gives information relating both to the hardness of the stratum currently being penetrated by chisei 21 and the skin friction on the mole. Charts can be prepared giving the mole operator predetermined combinations of values of depth and rate indicative of optimal pile effectiveness in the particular subsoil.

After withdrawal of the mole the hole is filled with concrete. The mole is then re-introduced, before the concrete sets, and allowed again to work its way down the hole as shown in Figure 2.

This action displaces the concrete to produce a bulbous pile footing 23 and also displaces concrete 24 into the walls of the hole which in turn further laterally consoiidate the subsoil around the hole to enhance the skin friction effect This step also produces a slightly iregular pile outline 25 due to the natural inhomogeneity of the subsoil further to enhance the skin friction effect.

The mole is again withdrawn. Preferably, an upwardly outwardly flared portion 26 is formed at the upper end of the hole, or this could be formed before the first filling with concrete.

Reinforcement rods 27 are then preferably inserted in the hole and inverted V-shaped reinforcement members 28 are inserted into the flared portion 26. The members 28 can be forced through hole 1 7 and expand resiliently beneath the slab as shown in Figure 3. The hole is refilled with concrete and the concrete allowed to set, thereby forming an in situ reinforced pile with an integral footing and an integral flared shoulder support for the slab 1 2.

In certain subsoils, e.g. clay, the walls of the hole may tend to collapse inwardly behind the mole on its first descent. In such cases the modified method illustrated in Figure 4 may be used. The mole carries a tapered hole expander member 29 preferably at its tail end The arrangement is such that the mole carries the expander forward during formation of the hole, but the mole can be withdrawn backwardly ieaving the expander in the hole. This may be accomplished by mutually abutting flanges shown schematically at 30 or by a suitable form of connection detachable by operation of a remote control drag cable. The expander 29 has an internally threaded rear section 31 for connection to a tubular member 32.

The internal diameter of the expander and of the tubular member 32 is larger than the external diameter of the body of the mole. The rear end of the tubular member 32 is also internally threaded to permit successive similar tubular members to be joined end to end as the moled hole deepens. A plurality of the tubular members are threaded onto the pneumatic hose and control cable of the mole before work commences. The successive tubular members are connected as the end of the preceding tubular member descends to adjacent the top surface of the slab, thereby permitting practice of the method of this invention in a site with limited headroom.

When the hole is of sufficient depth the mole is withdrawn up within the bore of the exapnder and the tubular members and the lined hole is filled with concrete. The mole is again introduced and allowed to descend to produce the bulbous footing 23, as before, and also to displace concrete up outside the tubular member into the subsoil wall of the hole. The mole is again withdrawn and the hole refilled with concrete. The tubular member and expander are extracted by means of an extraction rig while continually refilling the hole with concrete. The upper flared portion can be formed before final topping-up of the hole.

Experimental work has shown that the invention as described above can be practised in situations of very restricted access, in particular within a room of a house of little more than two metres headroom and within 5 centimetres of a wall of the room. The piling is believed to be an effective solution to a slab subsidence problem and is substantially less expensive than a slab remaking exercise.

A further example of a use of the invention may be in forming piles beneath the subsided and reraised sleeper bed of an underground railway in a drift mine. Regular passage of laden coal trucks in a mine can cause subsidence problems that the prior art has found very difficult and expensive to solve in view of the restricted headroom, sometimes only about 1.6 metres, in mine tunnels.

The present invention permits the piling of any desired length to be formed beneath existing structures in such situations.

The invention also finds application in supporting pre-existing walls. For example, referring now to Figure 5, first and second in situ reinforced piles 33 and 34 are each formed as described above in relation to Figures 1 to 3 or Figures 1 to 4. The first pile 33 is formed as close to the house wall 35 as possible subject to avoiding the side of the footing 36 which is assumed to be failing due to subsidence. The second pile 34 is spaced further away from the wall. A rigid, reinforced concrete cross-member 37 is then formed, with its reinforcing members tied to the upper ends of the reinforcement of piles 33 and 34, one end of the cross-member 37 extending into an opening 38 formed in the wall 35. The wall 35 is thereby given support independently of the footing 36, which obviates any necessity to excavate down to the existing footings in an attempt to underpin them directly.

The cross-member 37 exerts an upward supporting force on the wall by exerting an upward tensile force on the pile 34 and a downward compressive force on the pile 33. The pile 34 opposes the upward tensile force by the skin friction effect along its length and particularly around its bulbous foot as described above.

An alternative embodiment to Figure 5 is shown in Figure 6. In Figure 6 two reinforced piles 39, 40 are each formed as described in relation to Figures 1 to 3 or Figures 1 to 4, pile 39 being outside the wall 35 and pile 40 being inside. Pile 40 may be formed through a floor slab, as described above, or by removing a suspended floor depending on the ground floor structure of the building at issue. The rigid reinforced concrete cross-member 37 is then formed, as in Figure 5, with its reinforcing members tied to the reinforcement of piles 39 and 40. The crossmember 37 extends through the opening 38 and exerts an upward supporting force on the wall 35 by exerting a downward compressive force on both piles 39 and 40. At corners of the house two.

piles may be constructed as described above in relation to Figures 1 to 3 or Figures 1 to 4 adjacent the two walls forming the corner, and a cross-member is then formed therebetween through a diagonal opening across the house corner. In this case again the two piles are both used in compression to bear the load.

Claims (12)

1. A method of constructing a pile in situ comprising: forming a hole in the subsoil by allowing an elongate self-propelled soil displacing mole to descend through the subsoil; withdrawing the mole; filling the hole with concrete; allowing such a mole to descend within the filled hole before the concrete sets to produce a pile footing and to displace concrete into the hole wall; withdrawing such mole; and refilling the hole with concrete and allowing the concrete to set to form an in situ pile with an integral footing.

2. A method of constructing a pile in situ comprising: forming a hole in the subsoil by allowing an elongate self-propelled soil displacing mole to descend through the subsoil, said mole carrying forward a hole expander member; attaching successive tubular members in turn to said expander member and to the previous tubular member as the mole progressively descends through the subsoil, said expander member and said tubular members having an internal diameter larger than the external diameter of the mole, whereby to line the hole; withdrawing the mole up through the bore of the expander member and tubular member(s); filling the lined hole with concrete; allowing such mole to descend through the filled, lined hole before the concrete sets to produce a pile footing and to displace concrete from the bottom of the hole up outside the tubular member into the actual hole wall;; withdrawing such mole; and withdrawing the or each tubular member and the expander member while refilling the hole with concrete and allowing the concrete to set to form an in situ pile with an integral footing.

3. A method as claimed in claim 1 or claim 2 including inserting an elongate reinforcement member within the hole subsequent to last withdrawing such mole and before said step of refilling the hole with concrete.

4. A method as claimed in any one of claims 1 to 3 including forming an upwardly outwardly flared portion at the upper end of the hole prior to said step of refilling with concrete, whereby the finished pile has an integral flared load-bearing upper end.

5. A method as claimed in claim 4 including inserting an inverted V-shaped reinforcement member within the flared hole portion subsequent to last withdrawing such mole and before said step of refilling the hole with concrete.

6. A method as claimed in any one of claims 1 to 5 including monitoring both the actual depth attained by said mole and its rate of downward progress in said hole forming step, and withdrawing the mole when the actual depth and the rate of progress attain a predetermined combination of values indicative of optimal pile effectiveness in that subsoil.

7. A method of supporting a pre-existing concrete slab comprising forming a number of spaced holes through the slab and, working through said slab holes, constructing a corresponding number of in situ piles each by a method as claims in any one of claims 1 to 6.

8. A method of supporting a pre-existing wall comprising constructing first and second in situ piles, each by a method as claimed in any one of claims 3 to 6, the second pile being spaced further away from the wall than the first, and tying a rigid cross-member to the reinforcement of both said piles at the upper ends thereof, said cross-member extending into an opening formed in said wall, whereby said cross-member exerts an upward supporting force on said wall by exerting an upward tensile force on said second pile and a downward compressive force on said first pile.

9. A method as claimed in claim 1 and substantially as described herein with reference to the accompanying drawings.

10. An in situ pile when constructed in accordance with a method as claimed in any one of claims 1 to 6 and 9.

11. A concrete clab when supported by a method as claimed in claim 7.

12. A wall when supported by a method as claimed in claim 8.