GB2088450A - Grouting piles - Google Patents

Abstract

This invention relates to piling where the lower end of the pile is to be grouted to the bedrock. A hole is drilled downwardly from ground level through the over-burden into the bedrock 12. The bore in the bedrock may however be a very long way below ground level but despite this the grout bond should be of high quality. The lower end of the pile is provided with a radially inflatable closure 38 which is spaced from its lower end by an amount substantially equal to the length over which it is to be bonded to the bedrock. When the hole has been drilled and the pile lowered into it, that closure is then inflated to seal off the region bored in the bedrock and liquid grouting material is pumped into that region. Before this is done, however, it may be desirable to flush the region with water until the water leaving that region is sufficiently clean. In addition the grout may be supplied under high pressure to penetrate and seal any fissures in the bedrock. <IMAGE>

Description

SPECIFICATION Provision of piles for buildings This invention relates to the provision of piles for buildings.

In places such as Hong Kong where many buildings stand on reclaimed land, it is essential to provide a foundation which includes piles which extend down through the fill to the bedrock beneath it. In this way, a secure foundation can be provided.

Where the pile used is a steel H beam, one possible approach has been to rest the lower end of the beam on the surface of the bedrock. The disadvantage of this however is that the load applied to the beam is applied to the rock over the relatively small surface area of the end of the beam. As a result, the actual load which this limited surface area of the rock can take is often little more than about a third of the theoretical bearing strength of the beam.

It has therefore been proposed to drill a hole into the bedrock itself, to insert the lower end of the beam into that hole, and then to grout that lower end to the rock so that the load is spread through the grout over a large surface area of rock.

Such a beam could then be subjected to a load somewhat closer to its theoretical bearing strength and so the number of piles could be reduced.

However, the bedrock can be a very long way below ground level and so its is not easy to ensure that the grouting between the lower end of the pile and the rock is of high quality. To keep the drilled surface of the rock relatively clean so that a good grout bond can be obtained, a temporary outer casing can be provided for the hole drilled through the fill but the use of this casing makes the piling operation expensive.

It is therefore an object of the invention to provide an improved and simplified manner for socketing the lower end of a pile into bedrock or other stratum capable of bearing loads.

According to the invention there is provided a method of bonding the lower end of a pile into bedrock or other load bearing stratum (as hereinafter defined) in which a hole is bored into the bedrock or other stratum, a pile, provided with a radially inflatable closure spaced from its lower end by an amount substantially equal to the length over which it is to be bonded to the bedrock or stratum, is lowered into the hole, the closure is inflated to seal a region between the bored hole and the lower end of the pile, liquid grouting material is pumped into that sealed region under pressure, and thereafter allowed to set to bond the lower end of the pile to the bedrock or other stratum.

In this way, the closure enables one to apply pressure to grout so that this penetrates into loose material and bonds this to give a good frictional bond between the pile and the stratum.

The load bearing stratum need not be bedrock itself provided it is sufficiently strong to support the pile and it can withstand the force of the inflatable closure and give a good seal with it. For example, a suitable load bearing stratum might be decomposed granite once it has been well bonded by the application of the pressurised grout which will penetrate into any crevices or the like and seal up a region at the lower end of the pile.

It is also necessary when the hole is drilled into bedrock to ensure that the surface of the rock is clean so that a good bond can be achieved despite the fact that that hole may be a long way below ground.

Therefore, according to another aspect of the invention there is provided a method of providing a building pile in which a hole is drilled down to and into the underlying bedrock for a distance sufficient to give satisfactory socketing of a pile, such as a steel H beam, into the rock, the drill is withdrawn and the pile, provided with a radially inflatable closure spaced from its lower end by an amount substantially equal to the length of socketing is lowered into the drilled hole whilst flushing water under pressure is supplied adjacent the lower end of the pile until the pile enters the hole bored in the bedrock, inflating the closure so as to seal the drilled hole in the bedrock from the soil above it and continuing to supply flushing water adjacent the lower end through the closure whilst allowing the flushing water to escape through an outlet from the closure, continuing that flushing until the drilled hole within the bedrock is sufficiently clean, pumping liquid grouting material into the space closed by the temporary closure between the lower end of the pile and the bore in the bedrock so as to displace the flushing water, and thereafter allowing the grouting material to set so as to socket the lower end of the pile into the rock.

By operating in this fashion the hole bored in the bedrock can be cleansed of soil and debris from fill, by means of the flushing water and so a good bond can thereafter be achieved between the bedrock and the lower end of the pile despite the fact that this may be along way below ground level.

The liquid grouting material is supplied under high pressure until all the flushing water has been displaced (and this can be achieved by observing the outlet from beneath the closure and continuing to supply grout until clean fresh grout is expelled through the outlet) and then preferably the outlet is closed and grout continues to be supplied under high pressure so as to cause the grout to enter and fill any cracks or cavities in the rock. In this way, the socketing of the pile will not only give a good bond between the lower end of the pile and the rock but can also be used to seal up fissures in the rock and so bond the rock in that region.

The inflatable seal preferably comprises a circular metal backing plate welded to the steel beam at the upper intended level of the socketing and beneath this plate is provided an inflatable elastomeric toroidal member. As the beam is lowered into position this toroidal member can be completely deflated and so does not obstruct the entry of the pile. To assist in protecting the toroidal member, it may be contained within a protective member such as within the annular depression is a member similar to a wheel rim, but once the pile is in position the toroid is inflated under high pressure causing it to expand outwardly into tight contact with the wall of the bore in the bedrock.

The invention also extends to a steel beam provided with an inflatable seal as defined above.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figures 1 to 3 are diagrams showing steps in the boring of a hole in which an H beam steel pile is to be provided; Figure 4 is an enlarged perspective view of the H beam pile prior to its lowering into a bored hole; and Figure 5 is a diagram showing the beam in place as a pile.

A drill is used to bore a hole down to arid into underlying bedrock 1 2. In order to reach the bedrock the drill 10 has first to pass through fill 14 which may be of very erratic composition and may include for example boulders 1 6. Beneath the fill there is frequently a stratum of marine mud 18 and beneath that and above the surface of the bedrock a layer 20 of decomposed granite.

As shown in Figures 1 and 2, the drill will penetrate relatively freely through the layers 14, 18 and 20 and this is used to bore a hole 22 down into the bedrock 12. The length by which the drill penetrates the bedrock is determined by the required length of a socket to be made between a pile and the bedrock so as to absorb the load imparted by the pile with substantially no load applied by the lower end face of the pile to the bedrock. This length is determined by conventional calculations by a geotechnical engineer and the determination of this length forms no part of the present invention.

The hole 28 which is drilled as shown in Figures 1 and 2 is not provided with any form of casing.

Therefore, when the drill 10 is removed as shown in Figure 3, the hole will probably collapse and soil and rock particles will tend to fill the bore 22.

Next, an H beam steel pile 30 shown in Figure 4 is lowered into the hole 28 or at least is lowered along the axis of what was originally the hole 28.

The H beam steel pile 30 comprises a conventional H beam, e.g. one meeting British Standard BS 4360 Grate 50B. If required, additional fins or ribs 31 may be welded so as to increase the cross-section of metal and so increase the load bearing ability of the pile.

Welded to the beam is a circular plate 32. This plate is joined at a spacing from the lower end 34 which is slightly less than the length of the bore 22 and slightly more than the intended length of the socketing, e.g. about 0.5m more. Beneath the circular plate 32 is fixed an annular rim 36 of a shape comparable to a car wheel rim. The diameter of the rim 36 is slightly smaller than the diameter of the plate 32. Within the annular recess of the rim 36 is positioned an inflatable toroidal bladder 38 of elastomeric material such as reinforced rubber. In the deflated condition of this bladder 38 shown in Figure 4, its maximum diameter is less than the diameter of the rim 36 which therefore affords the bladder protection as the pile 30 is lowered into the hole 28.The bladder can be connected to a source of pressurised air or other liquid fluid through a tube 40 extending up through the plate 32 to the top of the H beam.

Extending down within the H beam is a pipe 44.

This passes through the plate 32 and rim 36 to a branch 46 adjacent the lower end of the pile 30.

There the pipe 44 branches into two or more branches 48 which terminate a short distance above the lower end of the pile, e.g. about 50 mm short of the end of the pile, the branches 48 terminating one on one side of the beam and the other on the other side of the H beam. It may be necessary to provide sufficient branches 48 to give an annular ring of water jets from the branches 48.

Also extending down within the beam are a pair of outlet tubes 50. These outlet tubes 50 pass through the plate 32 and rim 36 and terminate adjacent the underside of the rim 36, one on either side of the H beam.

As shown in Figure 5, the pipe 44 is connected via a stop valve 54 to a source of pressurised water 56, e.g. water at a pressure at least 100 bar.

As the pile 30 is lowered into the hole 28 water is supplied from the source 56 to the pipe 44 and leaves the branches 48 in the form of two high pressure jets of water. These jets have the effect of clearing a path for the pile 30 as it is lowered since they remove any loose soft material and of course holes have already been drilled through hard objects such as boulders 1 6 in the line of a hole 28 and the hole 22 has been drilled into the bedrock 12. The pile 30 can therefore be lowered relatively easily into the hole 28.

The pile 30 is lowered until it reaches a position shown in Figure 5 where the lower end of the pile is just above the lower end of the drilled hole 22. To assist the lowering of the pile it may be necessary in some circumstances to attach a vibrating hammer to the top of the pile. This position can be determined precisely by measuring the depth of boring and the length of the pile and once the pile is in this position, as best shown in Figure 5, the plate 32 has entered the top of the hole 22 bored in the bedrock. This has been possible because the high pressure jets of water from the branches 48 have flushed out any loose debris which has entered the hole 22 and this debris together with the waste water passes upwardly and outwardly through the tubes 50.

Next the inflatable bladder 38 is inflated through the tube 40. The inflation pressure is relatively high and so the bladder is brought into close tight sealing contact with the wall of the bore 22 so providing an entirely enclosed region of the bore 22 beneath the bladder 38 and plate 32.

The supply of flushing water to this enclosed region is continued until the water leaving the outlet tubes 50 is seen to be clean. Then the supply of water is stopped. The pipe 44 is next connected to a source of liquid cement grout. This can be conventional grout such as that meeting British standard BS 12. The liquid grout is pumped under pressure, e.g. 20 kpa, down through the pipe 44 to the enclosed region of the bore 22 to displace the water within that region out through the outlet tubes 50. Pumping of the grout continues until all of the water is displaced and one can observe clean grout being displaced through the outlets 50. Then a stop valve 60 on the outlet of the tubes 50 is closed. The supply of grout under high pressure is continued until one observes no further flow of grout through the pipe 44.This continued application of the grout under pressure forces the grout into any cracks or crevices in the bedrock surrounding the hole 22 so sealing those cracks and crevices.

Once no further flow of grout occurs through the pipe 44, a stop valve 54 at the end of the pipe is closed. The cement grout is then left to set and secure the lower end of the beam 30 to the bedrock 12.

After the grout is set the tubes 40, 44 and 50 can be severed below ground level and in conventional manner gaps between the pile 30 and the drilled hole 28 filled with vibrated granular material mixed with weak grout so as to give some lateral support to the pile.

As will be seen, this method of piling according to the invention has the advantage that during drilling a casing for the hole 28 is not necessary yet it is possible to clean the region where the grout is to form a strong bond between the bedrock and pile very thoroughly indeed so that loose soil particles and the like do not interfere with the obtaining of a strong bond.

Claims (14)

1. A method of bonding the lower end of a pile into bedrock or other load bearing stratum (as hereinafter defined), in which a hole is bored into the bedrock or other stratum, a pile, provided with a radially inflatable closure spaced from its lower end by an amount substantially equal to the length over which it is to be bonded to the bedrock or stratum, is lowered into the hole, the closure is inflated to seal a region between the bored hole and the lower end of the pile to define the length over which it is to be bonded to the bedrock or stratum, liquid grouting material is pumped into that sealed region under pressure, and thereafter allowed to set to bond the lower end of the pile to the bedrock or other stratum.

2. A method of providing a building pile in which a hole is drilled down to and into the underlying bedrock or other load bearing stratum (as hereinafter defined) for a distance sufficient to give satisfactory socketing of a pile into the rock, the drill is withdrawn and the pile, provided with a radially inflatable closure spaced from its lower end by an amount substantially equal to the length of socketing is lowered into the drilled hole whilst flushing water under pressure is supplied adjacent the lower end of the pile until the pile enters the hole bored in the bedrock, inflating the closure so as to seal the drilled hole in the bedrock from the soil above it and continuing to supply flushing water adjacent the lower end through the closure whilst allowing the flushing water to escape through an outlet from the closure, continuing that flushing until the drilled hole within the bedrock is sufficiently clean, pumping liquid grouting material into the space closed by the temporary closure between the lower end of the pile and the bore in the bedrock so as to displace the flushing water, and thereafter allowing the grouting material to set so as to socket the lower end of the pile into the rock.

3. A method as claimed in Claim 2 in which the liquid grouting material is supplied under high pressure until all the flushing water has been displaced.

4. A method as claimed in Claim 2 or Claim 3 in which, once all the flushing water has been displaced, the outlet is closed and grout is continued to be supplied under high pressure so as to cause the grout to enter and fill any cracks or cavities in the rock.

5. A method as claimed in any preceding claim in which the pile is a steel H beam.

6. A method as claimed in any preceding claim in which the inflatable seal comprises a circular metal backing plate joined to the beam at the upper intended level of the socketing and beneath this plate is provided an inflatable elastomeric toroidal member.

7. A method as claimed in Claim 6 in which, to assist in protecting the toroidal member, it is contained within the annular depression of a protective member similar to a wheel rim, but once the pile is in position the toroid is inflated under high pressure causing it to expand outwardly into tight contact with the wall of the bore in the bedrock.

8. A method of bonding the lower end of a pile into bedrock or other load bearing stratum, substantially as herein described with reference to the accompanying drawings.

9. A method of providing a building pile substantially as herein described with reference to the accompanying drawings.

10. A pile which has been formed by a method as claimed in any preceding claim.

11. A steel beam to be used as a building pile which has at its lower end to be bonded to the bedrock or other load bearing stratum a radially inflatable closure spaced from that lower end by an amount substantially equal to the length over which it is to be bonded over the bedrock or stratum, conduit means extending along the beam for supplying a pressurised fluid to inflate the closure when required, and additional conduit means for supplying liquid grout extending along the length of the beam to the region at the lower end of the beam beyond the radially inflatable closure.

12. A beam as claimed in Claim 11 which is a steel H beam.

13. A beam as claimed in Claim 11 or Claim 12 in which further conduit means are provided along the length of the beam and through the closure, for supplying flushing water to the lower end of the beam, and outlet means for the escape of flushing water from beneath the closure.

14. A steel beam to be used as a building pile substantially as herein described with reference to the accompanying drawings.