EP1473413A1

EP1473413A1 - Pile cap removal

Info

Publication number: EP1473413A1
Application number: EP04251943A
Authority: EP
Inventors: Claire Laing O'Rourke Plc Watson
Original assignee: Laing Orourke PLC
Current assignee: Laing Orourke PLC
Priority date: 2003-05-02
Filing date: 2004-03-31
Publication date: 2004-11-03
Also published as: GB2401140A; GB2401140B

Abstract

A pile cage is fitted with a frame (30) held to rebar (4) of the cage at a cut-off level (20). When a pile is cast using such a cage, a column (54) of concrete is formed between the main part of the pile (56) and a pile head (56) passing through a central aperture (40) of the frame. This column (54) can then be snapped by a mechanical grab without the need for use of hand tools in breaking or trimming the pile. The frame (30) provides a plurality of slots (42) to enable it to be passed over irregularly spaced rebars (4). Prior to pouring the concrete the remainder of the slots (42) are filled with builder's filler foam, preventing the ingress of concrete. The frame (30) may be reused.

Description

Background of the Invention

The present invention relates to the breaking of concrete piles in order to remove the surplus concrete cap or head. More specifically, the present invention relates to a pile cage and a novel composite frame that facilitates removal of the pile head.
Reinforced concrete piles are widely used in civil engineering for retaining walls and foundations for structures. In accordance with the Institution of Civil Engineers "Specification for Piling and Embedding Retaining Walls" published on 17 April 1996 under ISBN 0-7277-2566-1, it is taught to cast a pile to a level above a specified cut-off so that there remains a pile head or cap that must be removed. This is to ensure that:
(a) there is a sound concrete connection with the pile when it is incorporated into the remainder of the structure; and
(b) the concrete comprising the top portion of the pile is of good quality and is not, as would otherwise be the case, contaminated with soil or poorly compacted.
It is standard practice to cast the concrete of the pile so that it covers the entirety of the pile cage or pile reinforcement.
Traditionally, the breaking of concrete piles is carried out by manual labour and is a slow, arduous and expensive process which produces a considerable amount of loose debris for disposal. Even if mechanical means are used, close supervision is necessary as the cut-off level is approached, in order to prevent damage to the pile below the cut-off level.
The use of hand-held pneumatic tools, as employed in manual breaking processes, is associated with industrial injuries including hand-arm vibration syndrome ("HAV") including vibration white finger.
Other health and safety issues arising from manual methods include noise and the risk of injury from displaced concrete fragments.
One method of breaking a reinforced concrete pile, which is in current use, is described in WO9736058 (Merritt & Elliott). In this method the reinforcement in the pile head to be removed is treated so that it is debonded from the concrete of the pile, a hole is formed in the pile at the desired cut-off level, and a hydraulic tool is applied in the hole to split the pile in a substantially transverse plane. Use of this method reduces the risk of industrial injury but does not eliminate it.
The concept of debonding is also described in JP58-11218 (Yahagi Kensetsu KKK) and WO8102757 (Asakura). Asakura also attempts to eliminate the need to use mechanical tools by placing a substantially flat circular metal pipe into the pile cage at the cut-off level. After casting is complete and the concrete has hardened, fluid is supplied with increasing pressure to this pipe to break off the pile cap. The pile cap may then be lifted and removed. Crack propagation using the Asakura method is unpredictable. The requirement for a high pressure fluid supply is impractical on most construction sites.
Chemical methods of splitting the pile head at cut-off level are also used. Such methods include the RECEPIEUX ® technique, which uses expanding grout in conjunction with debonding of the reinforcement to break the piles down. It is also possible to leach out the cementious material, above the cut-off level from the wet concrete of a cast-in-situ pile as part of the construction process. While these chemical techniques avoid the necessity for the use of vibrating hand tools, there are safety and environmental implications in keeping and using chemicals (which may be volatile) on site. There is also an explosion risk if the process is not operated correctly.
Pile cap removal has recently been developed further using a pile cage in which pieces of polystyrene are fitted to the pile reinforcement projecting above the pile cut-off level as shown in Figure 1 of the accompanying drawings. Two large pieces of high density polystyrene supplied by Cordek Limited are fixed to the rebar. The rebar above the polystyrene is protected by debonding material as with the Merritt & Elliott method. The polystyrene elements are positioned so that their bases are at the cut-off level. A tremmie pipe for pouring the concrete passes through the centre of the cage and polystyrene pieces. When the concrete is set the pile cap is connected to the remainder of the pile by a restricted and unreinforced column of concrete defined by the diameter of the void within the polystyrene pieces through which the tremmie pipe passes. The surface of the concrete below the polystyrene is debonded from the pile cage. The pile cap can therefore be grasped by a grab and rocked about a horizontal axis parallel to and above the gap between the two polystyrene pieces. The compressibility of the polystyrene allows this movement to occur, thus snapping the unreinforced column above the cut-off level. This cap can then be lifted off. The use of this experimental pile cage was described, and illustrated with photographs in a presentation given to the Health and Safety Executive on 27 March 2003, at T5, B2 Offices of Substructures Team on the Terminal 5 site at Heathrow Airport, London, UK by Bachy Solentanche Ltd.
The polystyrene pieces have to be removed and cannot be re-used. There is a tendency to flaking of the polystyrene on its removal. Disposal is a problem and loose, flying pieces of polystyrene on site can represent a safety hazard, especially on airport construction sites. Mechanically operated hand tools are still needed to trim the column of concrete to the cut-off level. The polystyrene pieces need to be relatively deep to prevent them floating up as the concrete is poured. This creates a significant amount of concrete to be trimmed. The polystyrene pieces need to be custom manufactured for each pile cage as the rebar is typically irregularly spaced. This makes their production expensive. They are also bulky to store and may need to be weighted down and protected if stored outside. If pile cages are fabricated off site, the polystyrene pieces are liable to be damaged in transit.

Technical Problem

Technical problems associated with the prior art methods include the need to ensure that the break is level in order to produce a horizontal surface at cut off level. Where the break is inclined to the desired cut-off level remedial work is necessary. It is difficult to control the propagation of the crack created by the existing methods. Accordingly, pile caps are frequently removed at a level above the cut-off level then trimmed down using hand tools to the required level.
The present invention accordingly addresses the technical problems of achieving a clean break at the desired cut-off level without the use of pneumatically operated hand tools.

Solution of the Invention

The present invention accordingly provides a pile cage comprising a reinforcement structure having a first part adapted to remain in a pile, the first part terminating at a predefined level, and a second part that is protected in order to debond it from concrete cast around it; and a frame having a substantially flat base aligned at the cut-off level, the frame having an outer periphery substantially corresponding to the intended outer periphery of the pile to be cast around the cage, a central aperture, and a wall defining a plurality of slots passing through it in order to accommodate the second part of the reinforcement, the frame wall having a compressible core sandwiched between rigid layers to which concrete will not bond.
By using a composite frame, the necessary compressibility to allow an unreinforced column of concrete formed in the central aperture to be snapped by rocking can be achieved, while ensuring that the resultant nib of concrete above the cut-off level is small and can be arranged to be within the design tolerance so that no manual trimming is necessary. This eliminates the health and safety risks associated with such tools and provides a safer working environment.
When a pile is cast using such a pile cage, the slots surrounding the second part of the reinforcement are filled with a material such as spray-applied, polyurethane based, expanding filler foam ("builder's foam") to prevent ingress of concrete. Accordingly, the only concrete connection between the structural pile and pile head is that defined by means of the central aperture of the frame. When such a pile head is grasped and rocked by a hydraulic grab, this small column will snap at or above the cut-off level within the height of the frame, and the pile head can be lifted and removed leaving the second part of the reinforcement exposed.
Such a frame can be reused several times. It can be removed from the reinforcement after the removal of the pile cap and the filler foam from the slots. The frame does not break-up or flake on removal, eliminating the health and safety risks and environmental implications of polystyrene pieces of the prior art. The use of slots rather than respective holes for each rebar of the reinforcement, eliminates the need for custom design where the rebar is not equally spaced.
The present invention also provides a method of breaking concrete piles reinforced with a pile cage as previously described.
Other features of the invention are set out in the appended claims.

Brief Description of the Drawings

In order that the invention may be well understood an embodiment thereof will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which:

Figure 1: shows a diagram of an experimental pile cage, which is part of the prior art, but on which similar parts are identified by like reference numerals;
Figure2: shows a diagrammatic cross section through a pile cage of the present invention;
Figure 3: shows a horizontal section through the pile cage along the line 3-3 of Figure 2;
Figure 4: shows a cross section of the frame along the line 4-4 of Figure 3;
Figure 5: shows a diagrammatic cross section through a pile cast around the pile cage of Figure 2;
Figure 6: shows a grab being used to remove a pile head; and
Figure 7: shows a vertical section through a pile after removal of the pile head.

Detailed Description of a Preferred Embodiment

A pile cage comprise a reinforcement 2 for a pile. The reinforcement 2 is typically fabricated from an assembly of rebars 4 held together in the desired configuration by means of a coil or links (not shown). Steel mesh may also be used to create the reinforcement. A lifting band 6 is provided. The reinforcement 2 has a first part 8 which remains in situ in a cast pile and a second part 10 which will be in a pile head is to be incorporated into reinforced concrete slabs or caps. The first and second parts 8, 10 are separated by the plane of a cut-off level indicated at 20 in Figure 2. The lifting frame 6 is positioned below the cut-off level 20.
The rebar 4 in the second part 10 of the reinforcement 2 is protected by means of a debonding foam tubing 22. The type of foam tubing which is slit axially like pipe insulation is preferred as this can be easily assembled to the rebar 4 in order to form a protective layer. Other debonding products or techniques may be employed. The debonding 22 terminates at the cut-off level 20.
The pile cage is also fitted with an annular frame 30 that has a base 32 which rests exactly at the cut-off level 20. The outer periphery 34 of the frame 30 is substantially the same as the intended outer periphery of the pile less a small tolerance. Although a circular cross section has been shown for the frame 30, it will be appreciated that rectangular or other cross sections as required by the particular application could be provided. The frame 30 comprises an annular wall 36 defining a central aperture 40. Slots 42 extend through a central region of the frame wall 36 in order that the frame 30 can be passed over the debonded second reinforcement part 10. Each of the slots 42 is arcuate and runs around the middle of the wall and extends through the full height of the wall. The width of the slots 42 is sufficient to accommodate the rebar 4 and debonding 22 plus a small tolerance. The remainder of the space in the slots is to be filled with an expanding polyurethane builder's foam or similar packing once the frame 30 is positioned on the reinforcement 2. Four slots are shown in the illustrated embodiment but three or two are possible.
The frame 30 is made in a sandwich construction. A base layer 44 which sits at the cut-off level is a 6mm neoprene layer or similar which will not bond to concrete. This facilitates the creation of a smooth surface at the cut-off level 20 when the pile head is removed and protects the frame 30 from damage in storage and during installation. A central layer or core 46 of the frame 30 is made of 25mm high density closed cell polyethylene. Other compressible materials may be employed. The compressibility and depth of this core 46 are chosen to enable the pile head to be rocked for removal. The height should be as low as possible to reduce trimming. The compressible central core 46 is surrounded above and below by two rigid layers 48,50, preferably formed of 10mm thick plywood. The upper surface of the frame 30 is protected by another neoprene layer 52.
The cross section of each of the layers 44-52 of the wall 36 is identical and as shown in Figure 4.
The aperture 40 is as small as possible in order that only a small, easily cracked column of concrete will be created between the pile head and body. Nevertheless the aperture 40 must be large enough to allow a tremmie pipe to pass through and to allow displaced concrete to escape from the bore.
The frame construction described is particularly convenient for assembly on site. The frame is held in place on the reinforcement 2 at the cut-off level 20 by the foam which bonds the layer 22 to the walls of the slots 42.

Method of Use

The method of use of such a pile cage will now be described.
The reinforcement 2 is fabricated and the second part 10 of the reinforcement has debonding tubing foam 22 carefully fitted to the rebar 4 above the cut-off level 20.
A frame 30 of the correct diameter of the required pile is fitted over the debonded rebar 4 and secured in place with builder's foam. For large piles the size of the cage may result in it being too heavy for the frame to be assembled to it on the ground. In this case hanging chains and a crane are used to position the cage in the open bore and to place the frame 30 over the reinforcement. The chains will pass through slots 42 or other voids (not shown) and be secured to the lifting frame 6. The base 32 of the frame 30 is positioned exactly at the required cut-off level 20. Filler foam is then used to stuff the slots 42 around the rebar 4 in order to prevent ingress of concrete. Voids where lifting chains 66 pass through the frame 30 to the pile cage are also filled with expanded foam to prevent ingress of concrete there.
The pile is bored to the desired depth and the cage installed and the fully assembled cage including the frame 30 installed.
The pile is then concreted resulting in a pile as illustrated in Figure 5. This pile has a small column 54 of unreinforced concrete formed between the structural element of the pile 56 and a pile head 58. This column 54 is created in the central aperture 40 of the frame 30. Once the concrete has gained strength, the ground around the piles is excavated to locate the cut-off level 20. A hydraulic grab 60 on an excavator 62 is then used to grip the pile head and rock it slightly to create a fracture at the cut-off level 20 within the aperture 40. The grab is then used to lift the fractured pile head and load it directly into a truck. The pile head can then be taken to a concrete crushing plant to be recycled. Other types of mechanical or hydraulic lifting equipment may be used for this purpose.
As shown in Figure 7, when the pile head is removed, a small nib of concrete 70 will typically project from the surface of the pile at the cut-off level 20. The fracture will always be created by the force of the grab in the weakened smaller diameter section of concrete that fills the central aperture 40 of the frame 30. Even though the fracture may not be at the base of this smaller diameter portion and therefore at the cut-off level 20, this is usually of a depth small enough to require no trimming. The surrounding concrete, which abutted the base 32 of the frame 30 will be at the set cut-off point and also be level and have a texture influenced by the abutting surface of the frame.
The frame 30 may be reused a number of times in order to increase efficiency. Ten will be a typical number of reuses possible with the materials described.
The grab 60 is preferably specially adapted so that it can hold the pile head firmly from above and rock it in the same manner as proposed by Bachy Solentanche Ltd and Laing O'Rourke plc.
The pile cage as described can readily be adapted for use with other pile casting methods in which the concrete is poured first and the reinforcement inserted second.

Claims

A pile cage (2) comprising a reinforcement structure having a first part (8) adapted to remain in a pile, the first part terminating at a predefined level (20), and a second part (10) that is protected in order to debond it from concrete cast around it; and a frame (30) having a substantially flat base (32) aligned at the cut-off level (20), the frame having an outer periphery (34) substantially corresponding to the intended outer periphery of the pile to be cast around the cage (2), a central aperture (40), and a wall defining a plurality of slots (42) passing through it in order to accommodate the second part (10) of the reinforcement, the frame wall having a compressible core (46) sandwiched between rigid layers (48, 50) to which concrete will not bond.
A pile cage as claimed in claim 1, wherein the reinforcement is steel bars (4) or mesh.
A frame for use in constructing a pile cage as claimed in claim 1 or 2.
A method of breaking reinforced concrete piles comprising the steps of preparing a pile cage as claimed in any one of the preceding claims; casting the pile; and removing the pile head (58) by grabbing the pile head above the cut-off level (20) with a mechanical or hydraulic grab (60) in order to fracture a column of concrete (54) that passes through the central aperture of the collar (40); and lifting and removing the pile head (58).