GB2266738A - Ground treatment - Google Patents

Abstract

A method of preventing subsidence in buildings comprises reducing dissipation of moisture from the clay layer in which the footings of the building are founded. The method employed may be used to replace dissipated moisture and may result in some rehydration of a dehydrated foundation clay layer thus correcting subsidence which might already have occurred. The method primarily comprises locating in or above said clay layer water irrigation means within an area extending from a side surface of a wall supported by footings founded in the clay layer wherein said water irrigation means is connected to a water supply. A membrane may be placed over the clay layer to aid the retention of moisture in the clay layer.

Description

THE ARREST, CORRECTION AND PREVENTION OF SUBSIDENCE IN BUILDINGS The present invention relates to the arrest, correction and prevention of subsidence in buildings and, in particular, methods and apparatii for arresting correcting and preventing subsidence in buildings whose foundations are founded in clay.

Subsidence in a building can be the result of movement in its foundations. This movement is rarely uniform and results in damage to the structure of the building. In extreme cases, subsidence may be so great as to threaten the structural integrity of the building.

Subsidence is a particular problem where buildings have their foundations founded in a clay layer. Normally, the clay layer is covered by topsoil layers which prevent dehydration of said clay layer and the moisture content of the clay varies little about an average level. However, dehydration of portions of the clay layer leads to contraction of said portions and to cracks and fissures forming therein. Where the foundations of a building are, in part, founded in those portions of a clay layer which suffer dehydration, the result can be movement of the foundations of the building resulting in subsidence of the building structure. This becomes noticeable by the resultant cracks which form in the structure of the building.

The conventional method of preventing further subsidence in a building is by underpinning its foundations. This involves subsequent excavations down to and below the level of the foundations so that additional footings can be built which extend further into the clay layer and which support the original footings of the building. However, this is a preventative measure which merely arrests the problem of further subsidence in the building structure. It is both labour intensive and expensive.

It is an object of the present invention to obviate and mitigate the aforementioned problems by providing methods and apparatii to prevent, reduce and arrest dehydration of foundation clay layers and to assist rehydration of dehydrated layers and maintain hydration levels once hydrated.

According to a first aspect of the present invention there is provided a method of reducing moisture dissipation from (dehydration of) a foundation clay layer comprising placing an impermeable membrane over said clay layer to extend from a point adjacent a side surface of a wall supported by footings founded in the clay layer to a point remote from said side surface.

The method may include the step of removing any top-soil covering the clay layer prior to placing the impermeable membrane over said clay layer.

Preferably, removed top-soil is replaced after the membrane has been placed over the clay layer.

Alternatively, the membrane may be placed over a layer of fill material covering the clay layer.

Preferably, removed top soil is replaced to cover the membrane after it has been laid over the fill material layer.

The method may also include the step of forming a ditch in the fill material layer at a distance from the side surface of the wall with the membrane laid over the fill material layer such that an edge of the membrane furthest from the surface of the side wall depends downwards into the ditch.

The ditch may extend downwards to at least the surface of the clay layer.

The membrane may be laid such that an edge nearest the surface of the side wall contacts with said side wall and a portion of said membrane adjacent said edge may be laid to lie coplanar and in contact with said side surface.

According to a second aspect of the invention there is provided a method according to any of the next eight proceeding paragraphs wherein the membrane is formed from a semi-permeable sheet form material, the material being such that it allows moisture to pass through it in one direction but prevents moisture flow in an opposing direction, the membrane being located over the clay layer such that it acts to reduce moisture dissipation from said layer.

According to a third aspect of the invention there is provided a device for use with the impermeable membrane utilised in the method according to the first aspect of the invention, the device comprises a housing having an interior chamber with which an inlet and an outlet communicate, the outlet being closable by a one way valve means, a spike-like piercing means located adjacent the outlet and depending outwardly from the housing beyond fixing means arranged to enable said device to be fixed to a surface of the membrane such that the outlet is spaced above said surface and the piercing means pierces the membrane forming an aperture therein.

The one-way valve means may comprise a flap pivotally mounted adjacent an edge of the outlet and biased to depend into the chamber but prevented by means of a stop located adjacent the outlet such that the flap closes said outlet.

The fixing means may be capable of sealably fixing said device to the surface of the membrane.

According to a fourth aspect of the invention there is provided an alternative device for use with the impermeable membrane utilised in the method according to the first aspect of the invention wherein said membrane is formed from a number of strips of impermeable sheet material placed sideby-side with corresponding edges adjacent or overlapping.

The device according to this aspect of the invention is provided as an adjoining strip for said adjacent strips of sheet material.

The device comprises an elongate member having a first surface for fixing said device to an edge portion of a first strip of sheet material, a second surface spaced apart from said first surface for fixing the device to an edge portion of a second adjacent strip of sheet material, a one-way valve means located within a space between said surfaces, an inlet communicating with the space on an inlet side of the valve means and an outlet communicating with the space on an outlet side of the valve means.

The member may include more than one valve means with corresponding inlets and outlets.

According to a fifth aspect of the invention there is provided a further method of reducing moisture dissipation from a foundation clay layer by locating in the clay layer water irrigation means within an area extending from a side surface of a wall supported by footings founded in the clay layer wherein said water irrigation means are connected to a water supply.

Alternatively, the method may involve locating the water irrigation means in a fill material covering the clay layer.

The method may include monitoring the supply of water to the water irrigation means by water metering means.

This method may be utilised separately or in combination with the foregoing methods and devices.

Where this method is utilised in combination with any of the foregoing methods, the water irrigation means may be located directly below the impermeable or semi-permeable membrane.

The water irrigation means used in this method may comprise an array of pipes each connected to a water supply, some of said pipes being perforated along their lengths to enable water to seep outwardly therefrom.

The irrigation means may comprise French drains.

The water supply may be mains water or any other suitable water supply such as waste water or rain run-off water.

According to a sixth aspect of the invention there is provided a method for rehydrating a foundation clay layer by inserting a first end of a water injection means into the clay layer, a second end of said means being located above the clay layer and connected to a water supply wherein water is fed from the water supply to the water injection means for injection into the clay layer.

The water injection means may comprise a pipe. Preferably, the pipe is perforated along at least a portion of its length adjacent said first end.

The water injection means may include a plurality of secondary water injection means located between said first and second ends and extending generally radially outwardly therefrom.

The water supply may be located in a ditch formed in a top soil layer or fill material layer covering the clay layer.

The water supply may be a ring main to which the first end of said water injection means is connected.

The method may include inserting a plurality of water injection means into a foundation clay layer in an area adjacent a side surface of a wall supported by footings founded in said clay layer.

According to a seventh aspect of the invention, there is provided a tool for use in inserting water injection means in the form of pipes into a foundation clay layer, the tool comprises an elongate rod member, a collar which is movably mounted on the rod and is fixedly locatable on the rod at axial positions intermediate first and second ends thereof, and a spike-like member which has a first end locatable in an axially extending bore in the first end of the rod such that the first end of the rod abuts a shoulder of said spikelike member, and a second end of the spike-like member which is oppositely directed to the first is generally spike-shaped.

According to an eighth aspect of the invention, there is provided a method for rehydrating a foundation clay layer by forming in an area adjacent a side surface of a wall supported by footings founded in the clay layer at least one bore-hole extending through layers covering said clay layer, and supplying water to said bore-hole for diffusion into said layers.

Preferably, the bore-hole depends down into the clay layer.

Preferably also, the bore hole is filled with an inert drainage material, for example sand.

Preferably further, the inert drainage material is compacted within the bore hole.

This method may also involve forming secondary holes radiating generally radially outwardly from the bore-hole at predetermined axial positions along the length thereof.

The secondary holes may be formed by a device substantially similar to the device according to the seventh aspect of the invention.

Alternatively, the method may include forming secondary holes in the form of fissures by using water under high pressure within the bore of the bore-hole to rupture and expand natural fissures in the foundation clay, for example.

Preferably, however, the surface area through which water can diffuse from the bore-hole into the various layers including the clay layer may be increased by forming in at least the clay layer planar-type cuts extending radially outwardly from the bore-hole.

Preferably, the bore-hole and planar-type cuts are filled with an inert drainage material which may also be compacted.

According to a ninth aspect of the present invention, there is provided a tool for insertion into a bore-hole for forming outwardly extending planartype cuts in the medium in which the bore-hole is formed, said tool comprising an elongate rod having pivotally mounted to a first end thereof a number of blades, each of which , on insertion of the tool into the bore hole, may lie generally parallel and adjacent to a lower portion of the rod and which, in use, can be made to pivot outwardly about said first end thereby extending into the medium causing planar-type cuts therein which extend radially outwardly from the bore-hole.

When the tool is inserted in a bore-hole, pivotal movement of said blades about said first end may be effected by the action of withdrawing of the tool from the bore-hole.

The tool may be withdrawn from the bore hole using a rig such as a drilling rig for example.

Initial pivotal movement of the blades about said first end may be effected by means of a second tool cooperable with said cutting tool, said second tool comprising a sleeve adapted to slide along the length of the rod and having at a primary end thereof a wedge means for engaging with the blades of the cutting tool whereupon movement of the sleeve down the rod causes the wedge means to engage the blades of the cutting tool and further movement of the wedge means drives said blades outwardly away from the lower portion of the rod to depend into the medium in which the bore hole is formed.

The foregoing and further features of the present invention will be more readily understood from the following description of preferred methods and apparatii which are given by way of example only and described with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic illustration of a method according to first and second aspects of the invention; Figure 2 is a perspective view of a device according to a third aspect of the invention; Figure 3 is a cross-sectional view of the device of figure 4; Figure 4 is a perspective view of a device (in use) according to a fourth aspect of the present invention; Figure 5 is a cross-sectional view of the device of figure 4; Figure 6 is a diagrammatic illustration of a further method and apparatus according to the invention; Figure 7 is a further diagrammatic illustration of the method illustrated in figure 6;; Figure 8 is a diagrammatic illustration of a method and apparatus for a still further aspect of the present invention; Figure 9 is a schematic diagram of a water supply for the apparatus of figure 8; Figure 10 is an enlarged view of a portion of figure 8 showing additional apparatus for use in a still further method according to the invention; Figure 11 is a schematic diagram of a water supply for the apparatus and method illustrated by figure 10; Figure 12 is a cross-sectional view of a tool for use in inserting the water injection means shown in figure 8; Figure 13 is a side view of a tool for inserting secondary water dissipation means; and Figure 14 illustrates an anti-extraction device for the water injection means shown in figure 8.

Figures 15a-15e are diagrammatic illustrations of a still further method and apparatus according to the present invention; Figure 16 is a side sectional view of a bore-hole formed using the apparatus in accordance with the method illustrated in figures 15a-15e; Figures 17a and 17b are side sectional views of water supplies suitable for use in at least the method illustrated by figures 15a-15e.

Figure 18 is a side sectional view of a system utilising the method illustrated by figures 15a-15e.

Referring to the drawings and, in particular, figure 1, the first aspect of the invention provides a method of reducing moisture dissipation from a foundation clay layer 10 by locating over said clay layer 10 an impermeable membrane 12 which acts to contain moisture within said clay layer 10 and prevent dissipation of moisture upwardly therefrom. Figure 1 shows a diagrammatic illustration of this method in use. It can be seen from said figure that an impermeable membrane 12 has been placed over a fill material layer 14 covering the foundation clay layer 10. The membrane 12 is placed with a first edge 12a in contact with a side surface 16 of a wall 18 supported upon footings 20 founded in the clay layer 10. A portion of the edge 12a of the membrane 12 is placed to lie coplanar and in contact with said side surface 16 of the wall 18.An edge 12b of the membrane 12 furthest from the side surface 16 of the wall 18 depends downwards into a ditch 22 formed in the fill material layer 14. The ditch depth may extend down to at least the surface of the clay layer 10 as shown in figure 1. The membrane preferably extends a distance of between 2 and 4 metres from the side surface 16 of the wall 18 outwardly therefrom.

This method of reducing moisture dissipation from a foundation clay layer 10 might be employed at the time of constructing a building and a membrane 12 might be laid to cover said foundation clay layer 10 on both the external and internal sides of the exterior supporting walls of the building under construction. It is envisaged that the method would be particularly useful in areas where subsidence caused by dehydration of the foundation clay layer is a known problem.

An advantage of this method is that it is a low technology method and relatively inexpensive to implement when compared with known methods of preventing subsidence in buildings. It is understood, however, that where subsidence in a building is acute, this method alone would not be sufficient to prevent further subsidence or to correct that subsidence.

A disadvantage of this method is that whilst the membrane acts to reduce upward moisture dissipation from the foundation clay layer it also acts to prevent the natural drainage of rain water from covering topsoil layers dissipating into the clay layer 10 to replace water lost by natural drainage and upward dissipation.

An alternative to the impermeable membrane 12 utilised in the method according to the first aspect of the invention would be a membrane 12 made from a semi-permeable sheet form material where the structure of such material allows moisture to flow through it in one direction but acts to prevent moisture flow in the opposing direction. A membrane 12 formed of this material and laid such that its surface, towards which moisture can flow, faces the foundation clay layer 10 would allow the natural percolation of rain water into the foundation clay layer over which it is located but act to reduce upward water dissipation therefrom.

Alternatively, in order to overcome the problem presented by the impermeable membrane 12 utilised in the method according to the first aspect of the invention, there is provided a device 30 which is adapted to form perforations or apertures through the membrane 12 to allow rain water to percolate down to the foundation clay layer 10 but also prevent upward moisture dissipation via said apertures from said clay layer 10. Figures 2 and 3 illustrate such a device 30 which comprises a housing 32 which is generally rectangular in cross-section. The housing 32 has an interior chamber 34 with which inlets 36 communicate. The housing also has an outlet 38 closable by a one-way valve means 40 formed from a flexible flap 42 pivotally mounted to an edge 38a of said outlet 38 and biased to depend into the chamber 34.The flap 40 is prevented from depending into said chamber by a stop 44 against which the flap rests to seal said outlet 38. A spike-like piercing means 46 is located adjacent said edge 38a of the outlet 38 and depends outwardly from the housing 32 extending beyond fixing means 48 which allow the device 30 to be sealably fixed to a surface of the membrane 12.

In use, the device 30 is sealably adhered to an upper surface of the membrane 12 and held in position by the fixing means 48 which comprise double-sided adhering strips. On locating the device 30 upon an upper surface of the membrane 12 downward pressure causes the piercing means 46 to pierce through the membrane 12 and form an aperture 50 therein.

Thereafter, the device 30 is covered by topsoil. Rain water draining through the topsoil can enter the chamber 34 of the housing 32 via the inlets 36. Rain water gathered in the chamber 34 causes the flap 40 to flex downwards away from the stop 44 and enables the water contained within the chamber 34 to flow through the outlet 38 onto the surface of the membrane 12 into a space 52 defined between the fixing means 48, the membrane 12 and device 30. This rain water can then percolate via the aperture 52 formed by the piercing means 46 into the foundation clay layer over which the membrane 12 has been placed or into the fill material layer covering said clay layer.

Moisture dissipating upwards from the clay layer can progress above the membrane 12 via the aperture 52 formed by the piercing means 46 of the device 30. However, such moisture is contained within the sealed space 52 and is unable to progress beyond the one way valve means which remains closed.. In this way, the device 30 prevents upward moisture dissipation from the clay layer into topsoil layers.

The device 30 may comprise an elongate housing 32 with a plurality of inlets 36 and piercing means 46 and a single elongate outlet 38.

Figures 4 and 5 illustrate an alternative device to which is provided for use with an impermeable membrane 12 utilised in the method according to the first aspect of the invention where said membrane is formed from strips of an impermeable sheet material placed side-by-side with corresponding edges adjacent or overlapping.

The device 60 is provided as an adjoining strip for joining corresponding edges of adjacent strips of impermeable material. The device 60 comprises an elongate member 62 of indeterminate length which is generally H-shaped in cross-section. The elongate member has first flanges 64 which depend outwardly from said member 62 to define a first surface to which an edge of a first sheet 66 of impermeable material can be secured by fixing means 68 located on the outwardly facing surfaces of said flanges 64. Similarly, the member 62 has second flanges 70 which define a second surface to which an edge of an adjacent second sheet 72 of impermeable material can be securedly attached thereto. The first and second surfaces define therebetween a space 80 in which there is located at least one non-return valve means 82.An inlet 84 is located in the upright of the member on an inlet side of the valve means 82 which communicates with said space 80 and an outlet 86 is located in the opposing upright on an outlet side of the valve means 82.

The valve means 82 may simply comprise a flexible flap which is pivoted to one upright of the member 62 on the inlet side and biased to depend away from a cross member 88 spanning between said uprights. The flap may depend outwardly beyond the surface defined by said first flanges 70 such that the flap abuts against the surface of an edge portion of a sheet of impermeable material secured to said first surface to form a seal. In use, the device 60 is placed with a first surface adhered to an edge portion of a first sheet 66 of impermeable material such that the inlet 84 communicates with the space above said first sheet 66 of the membrane 12 and an edge portion of a second sheet 72 is adhered to said second surface such that the outlet 86 communicates with the space below said second sheet 72.

Where a membrane 12 is formed from a series of adjacent sheets adjoined by devices 60 according to this aspect of the invention, rain water draining through top soil layers placed over the membrane 12 can enter the space 80 defined within the device 60 via the inlet 84 and progress via the oneway valve means to exit via the outlet 86 on the underside of the membrane 12 to dissipate into the foundation clay layer. Conversely, moisture dissipating upwardly from the clay layer may enter the space 80 defined within the device 60 via the outlet 86 but cannot progress via the one-way valve means which acts to effectively close the inlet 84.

Figures 6 and 7 illustrate a further method and apparatus to reduce moisture dissipation from a foundation clay layer 10. This method involves the use of water irrigation means 100 to introduce water into the clay layer to replace water lost through upward dissipation. The water irrigation means 100 may comprise a number of pipes 110 connected to a water supply (not shown) wherein some of said pipes 10 are perforated along their length to allow water to seep therefrom. The pipes may be located in the clay layer but it is preferable that they are located in topsoil 15 or fill material layers 14 covering said clay layer 10 since water will more readily seep into the structure of the less dense topsoil 15 or fill material layers 14 than the foundation clay layer 10. As an alternative to perforated pipes 110, the water irrigation means 100 may comprise French drains 120.The water supply may be a mains water supply to which each of the perforated pipes 110 or French drains 120 are connected or alternatively water may be supplied from waste water drains or rain run-off drains. A disadvantage of using waste water is that it may lead to contamination of the soil and clay layers and particulate matter contained in the waste water may block the water irrigation means 100.

This method is rather more complex than the method according to the first aspect of the invention and is primarily intended to be utilised in combination with said method. It is intended that the water irrigation means 100 would be located adjacent the underside of a membrane 12 covering the fill material layer 14, for example.

The foregoing methods are primarily intended to reduce moisture dissipation from a foundation clay layer 10 in order to prevent subsidence from occurring or subsidence from continuing where some subsidence has occurred. However, since clay is extremely impermeable it takes a considerable time to re-establish the moisture content where this has been lost. The foregoing methods would, over a considerable period of time, help re-establish the natural level of moisture content of a foundation clay layer 10 where a significant portion of that moisture content was lost. However, further subsidence could occur whilst this process was continuing and accordingly a more direct method of introducing water into the foundation clay layer is required.

According to a further aspect of the invention there is provided a method for rehydrating a foundation clay layer 10 by injecting water into said clay layer. This method is recommended in circumstances where a rapid recovery from shrinkage of the clay layer is required. However, because this method injects water directly into the foundation clay layer causing rehydration and expansion of dehydrated portions thereof it is necessary that a thorough investigation is conducted of the clay layer to determine Atterburg limits and the moisture profile of the layer in order that the method can be correct utilised without causing over-hydration of the clay.

The method involves inserting a first end 200 of a water injection means 210 into a clay layer 10 and connecting an opposing second end 220 of said means which is located above the clay layer, to a water supply 230 wherein water can be fed from the supply 230 via the water injection means 210 to be injected into the clay layer 10. The water injection means may comprise a pipe 240. A portion of the pipe 240 adjacent the first end 220 may be perforated to enhance injection of water into the foundation clay layer 10.

Alternatively, the pipe may be provided with a number of routelet pipes 250 which extend radially outwardly therefrom into the clay layer 10 in order to increase the number of injection points into the clay layer and the volume through which water is injected into said layer. Because of the impermeable nature of clay it is important that the injection means 210 has as large a surface area of contact with the clay as possible. This may be enhanced by using routelets 250 which are also perforated. Figure 8 illustrates this method.

A ditch 260 is formed in a topsoil layer 15 in which a water supply 230 is located. A water injection means 210 in the form of a pipe 240 connected by its second end 220 to the water supply means 230 protrudes down through a fill material layer 14 covering the clay layer 10 and into said clay layer. At axial locations along the portion of the pipe 240 protruding into the clay layer are located routelets pipes 250 which extend radially outwardly from the water injection pipe 240.

Water may be supplied to the injection means 210 under mains pressure or alternatively the water supply 230 may include a pump which allows water to be injected under higher pressure into the foundation clay layer 10. A problem encountered with using a high pressure water supply is that the moisture take-up of the clay might cause it to expand in such a manner that it tends to force the water injection means 210 out of the clay. In order to overcome this problem the ditch 260 in which the water supply 230 is located may be filled with concrete and may include reinforcing rods 300 (figure 10).

In its simplest form, this method entails supplying water to water injection means 210 in order to inject water into a foundation clay layer 10 in order to rehydrate it at the fastest possible rate. The supply of water may be under gravity, under mains pressure or under pump pressure. Whilst the method in its simplest form will achieve the desired result it may also create a problem of over-hydrating the clay layer with the result that not only will the rehydration of the clay layer correct subsidence which has occurred by returning foundations to their original positions but might cause the foundations to lift unevenly upward from said positions. As a result, this proposed method for preventing, and correcting, subsidence which has already occurred might in fact create a greater problem than that which it is intended to solve.It is therefore important that this method is employed only where careful study and investigation of the factors which might affect moisture dissipation from the foundation clay layer has been undertaken. For example, factors which affect soil dissipation from a foundation clay layer include rainfall levels, amounts of vegetation growing on topsoil layers adjacent to or covering the foundation clay layer, the depth of the foundation clay layer, etc.

Such factors which affect moisture dissipation from a foundation clay layer can be estimated and the amount of moisture dissipation can be calculated such that the water injection means 210 can be utilised to accurately inject over a period of time an amount of water sufficient to rehydrate the clay layer 10 without causing it to over-expand. The method and apparatus of this aspect of the invention might include water metering means which allows controls of the amount of water supplied to the water injection means 210 and a means of measuring the amount of water supplied.

Figure 9 illustrates a water supply 230 for the water injection means 210.

The water supply 230 includes water metering means 400 to measure the flow of water supplied to a water cistern 410 and thus allows control of the amount of water supplied to the cistern 410 which thereafter flows under gravity to the water injection means 210 via an outlet pipe 430. The water supply 230 may also include a water softener 440 to remove minerals from the water and a chemical dosing device 450 for introducing chemicals into the water to be supplied to the water injection means 210. Because the water is injected into the clay through relatively small bore tubing, minerals such as carbonates in the water can produce furring which might block the tubing, particularly in view of the fact that the rate of flow of water through the tubing is small. It is therefore important that water is softened before use and this might be achieved by use of a simple ion exchanger such as a salt reservoir.

The water supply may also include a chemical dosing device which injects into the water chemicals which assist speed of water dissipation into the clay matrix.

In circumstances where it is considered necessary to inject water into the clay layer 10 at pressures greater than that produced by a head of water in a cistern or mains pressure, the water supply may also include a pump 460 (figure 11). This might be a single piston solenoid operated pump which is set to maintain a predetermined pressure upon the water supplied to the injection means 210. The water supply may also include an adjustable pressure sensitive switch 470 in order to vary the pressure at which water is supplied to the injection means 210 such that on subsequent monitoring of the rehydration process the pressure at which water is injected into the clay can be varied accordingly. The supply may also include metering means 480 and a non-return valve 490.

It is envisaged that this method will be used where it is important that a clay layer 10 be rapidly rehydrated to prevent further subsidence of a building structure and to correct subsidence which has already occurred. The advantage of this method over conventional alternatives is that once installed the rate of water injection into the foundation layer 10 can be monitored and controlled to take account of variations such as change in average rainfall levels which have an affect on the degree to which the clay layer requires further water injection. This method is not regarded as a short term solution to the problem of subsidence but a long term method of maintaining the foundations of a building stable after subsidence has been arrested.

The water injection means 210 preferably comprises a pipe 240 made of a plastics material. However, a problem encountered with using such pipes 240 is that, due to the dense nature of clay, injecting the pipes 240 into the clay layer 10 can result in damage to the pipes 240 or even blocking the ends so that water supplied to the pipes 240 is not able to easily dissipate into the clay layer 10. Accordingly, a tool 500 (figure 12) is provided to assist in the insertion of the water injection pipes 240. The tool 500 comprises an elongate rod 510 having at a first end 510a an axial extending bore 520 which is adapted to receive a first end 53a of a spike-like member 530. The rod 510 abuts against a shoulder 540 of the spike-like member 530 which shoulder 540 extends radially outwardly beyond the diameter of the rod 510.An oppositely directed end 530b of the spike-like member 530 is generally conical in shape.

The rod 510 also has a collar 560 which is screw threaded engagable with a screw thread formed on a portion of the rod adjacent a second end 510b thereof. The rod 510 is adapted to receive, in use, a water injection pipe 240 which slides over the outer surface of the rod and locates between the collar 560 and the spike-like member 530 which engages with the axial bore 520 of the rod 510.

In use, this tool 500 is used to drive water injection pipes 240 downward through topsoil layers 15 into the clay layer 10. The spike-like member 530 is made of steel or other suitably rigid material and under downward pressure from the rod 510 abutting against the shoulder 540 thereof can be forced through topsoil layers down into a foundation clay layer 10. The width of the shoulder 540 is such that it extends at least to the diameter of a water injection pipe 240 located on the rod 510 therefore protecting the pipe 240 from damage as the rod is forced downward towards the clay layer 10. Once the rod 510 has been forced down into the clay layer 10 to a predetermined depth the screw threadable collar is released from contact with an upper end of the water injection pipe 210 by a small axial distance.The rod 510 is then forced down again by an equivalent distance in order to drive the spike-like member 530 away by said axial distance from the end of the water injection pipe 240 which has now been inserted into the clay layer 10. At this stage the rod 510 can be withdrawn leaving the water injection pipe 240 extending with a first end 200 located in the foundation clay layer 10 at a predetermined depth and a second end 220 located above said layer which can later be connected to a water supply 230. The spike-like means 530 remains in the clay layer at an axial distance spaced from the first end 200 of the water injection pipe 240.

The first end 530a of the spike-like member 530 which, in use, is accommodated in the axially extending bore 520 of the rod 510 has inclined surfaces 580 directed away from a central axis towards outer sides.

Accordingly, water flowing down the water injection pipe 240 would be deflected by the inclined surfaces 580 to flow between an annular gap which exists between the first end 200 of the pipe and the spike-like member 530.

More importantly, the purpose of the inclined surfaces 580 is to deflect thin wire members 600 carrying routelet pipes 250 (figure 13) which are fed down the bore of the pipe 240 such that the wires 600 contact with the inclined surfaces 580 of the spike-like member 530 and are deflected away therefrom to exit the first end of the pipe 240 via the annular gap. The wires are thereafter forced through the clay layer 10 such that they extend generally radially outwardly from the first end of the pipe. Withdrawal of the wires leaves the narrow routelet pipes 250 remaining in the clay. Water supplied to the water injection pipe will flow through the bore of said pipe but also through the routelet pipes which extend outwardly therefrom into the clay thereby increasing the effectiveness of the water injection into the clay layer to be rehydrated.

A further tool which is adapted to locate within the bore of the rod is provided (not shown) which can be passed down the bore of the water injection pipe 240 to a predetermined axial position therein and upon further pressure from the rod can form side apertures in the wall of the pipe 240 through which a further cluster of routelet pipes may be inserted to extend radially outwardly from the pipe at said axial position.

Where the water supply 230 includes means for pumping water at higher than mains pressure to the water injection pipes 240 it is necessary to provide said pipes with means to prevent their extraction under pressure from the rehydrated clay layer 10. Figure 14 shows an anti-extraction device which comprises a generally tubular member 700 which is fixed to a water injection pipe 240 to prevent extraction of the injection pipe 240 when inserted in the clay layer 10.

The member 700 is solvent welded to the pipe 240 such that opposing side blade members 700a,b thereof face rearwardly when the pipe 240 is inserted into a clay layer 10. As the pipe 240 with member 700 affixed is inserted into the clay layer 10, the blades are pressed against the pipe 240 by soil pressure. In the event that pressure upon the pipe 240 forces it to move in a reverse direction to its direction of insertion, the blades 700a,b of the member 700 flex open to embed into the soil/clay layer and anchor the pipe 240 in position.

According to a still further embodiment of the invention there is provided another method of directly introducing water into a foundation clay layer 10. This method may be utilised separately, or in cooperation with any of the aforementioned methods.

This method involves forming in an area adjacent a side surface of a wall (not shown) whose footings are founded in a foundation clay layer 10 boreholes 800 which preferably extend at least through any layers covering the clay layer 10 and preferably into the clay layer 10 itself. Water introduced into the bore-holes 800 will diffuse into said layers (including the clay layer 10) thereby replacing water lost from the clay layer 10 by dehydration and drainage.

In order to support the bore-holes 800 and to assist in the diffusion of water from the holes 800 into the foundation clay layer 10, the bore-holes 800 may be filled with an inert drainage material 810 such as sand, for example.

The inert drainage material may also be compacted within the bore-holes.

The surfaces of the bore-holes 800 form interfaces with the clay layer 10 through which water diffusion takes place. In order to increase the surface available for water diffusion, secondary holes (not shown) may be formed at predetermined axial positions down the bores of the holes 800. These secondary holes may be formed by a tool substantially similar, or identical to, that described with reference to figures 10 and 11. Alternatively, secondary holes in the form of fissures (not shown) may be formed by the use of high pressure water within the bores 800 to rupture and expand natural fissures in the clay layer 10. These may then also be filled with the inert drainage material 810 which will assist in the diffusion of water into the clay layer 10.

The preferred method of increasing the surface area for water diffusion is as illustrated in figures 15a-e of the drawings. This method consists of making planar-type cuts 820 in the clay layer 10 which extend radially outwardly from the bore-holes 800.

The purpose of achieving maximum surface area of water in contact with clay is two-fold. Firstly, to facilitate natural diffusion of water into the clay layer matrix via the surface interfaces formed and secondly to maximise the possibility of intersecting natural fissures which will themselves greatly enhance the surface area of contact for water diffusion into the clay layer 10.

Figure 15a shows a side sectional view of a cutting tool 830 provided to form such cuts 820. The tool 830 is shown inserted in a bore-hole 800. The tool 830 comprises a rod 840 which is shown with a first end 840a in contact with the base of the bore-hole 800. The tool 830 has a number of blades 850 pivotally mounted to said first end 840a which, when the tool 830 is inserted in the bore-hole 800, are each held substantially parallel and adjacent to a lower portion of the rod 840. In use, the blades 850 are made to penetrate into the clay layer 10 forming planar-type cuts 820 in said layer 10 which extend radially outwardly from the bore-hole 800. Figure 15b shows with the blades 850 having been caused to partially depend outwardly into the clay layer 10.

The blades 850 can be made to penetrate into the clay layer 10 and to slice therethrough as the tool 830 is withdrawn from the bore-hole 800.

Experience has shown, however, that withdrawal of the tool 830 after insertion does not always cause the blades 850 to penetrate into the clay layer 10. Accordingly, a second tool 860 is provided in association with the cutting tool 830. This tool 860 comprises a sleeve 870 adapted to slidably locate over the rod 840, said sleeve 870 having at a lower end wedge means 880 adapted to engage with the blades 850 to cause them to move away from their position lying adjacent the lower portion of the rod 840 and to depend partially into the clay layer 10 (figure 15b). The force required to cause the blades 850 to move partially into the clay layer 10 is considerable but further penetration of said blades 850 into said layer 10 to form cuts 820 therein is then effected by withdrawal of the tool 830 from the bore-hole 800.An upper end 840b of the rod 840 is attached to a rig 900 which provides the pulling force necessary to remove the tool 830 from the bore-hole 800.

Figure 15c is an above sectional view of the cutting tool 830 along line xx of figure 15b.

Figure 15d shows the cutting tool 830 with the blades 850 thereof depending downwardly into the clay layer 10 to occupy a position generally below the rod 840. It can be seen that the second tool 860 is no longer in contact with the blades 850. This tool 860 can be withdrawn simultaneously with the cutting tool 830, or alternatively, can be firstly removed after having been used to cause the blades 850 to depend partially into the clay layer 10.

Figure 15e shows the cutting tool 830 with the blades 850 in a collapsed state depending downwardly below said first end 840a of the rod 840 ready for removal from the bore-hole 800. The cuts 820 made in the clay layer 10 are as indicated by the broken outline and it can be seen that the cuts 820 formed by opposing blades 850 form a generally kite-shaped planar-type cut 820.

Whilst the cutting tool 830 is shown as having two pairs of opposed blades 850, it will be understood that the tool 830 may have any number of blades 850 suitable for a particular clay-type layer 10. It will also be understood that the tool 830 need not be inserted into a bore-hole 800 with its first end 840a in contact with the base of the hole 800 but may be utilised to effect cuts 820 in the clay layer 10 at positions spaced above the base of the bore-hole 800.

Figure 16 is a sectional view of a bore-hole 800 formed in accordance with the aforesaid method having two generally kite-shaped orthogonally intersecting planar-type cuts 820 whose line of intersection lies generally coincident with the longitudinal axis of the bore-hole 800. The bore-hole 800 and planar-type cuts 820 have been filled with an inert drainage material 810 such as sand to assist with the diffusion of water from said hole 800 and cuts 820 into the clay layer 10. The inert drainage material 810 may be compacted to support the hole and assist water diffusion.

In a system utilising the above method, water may be supplied to the bore-holes 800 by any suitable means including those described in connection with other aspects of the present invention. However, the most effective means of controlling the rate of uptake of moisture by the clay layer 10 is by using a very low hydrostatic head of pressure allowing the natural clay suction to take up the moisture it requires. Once the moisture content of the clay layer 10 reaches its equilibrium level the suction reduces to virtually zero and flow ceases.

The equilibrium moisture content of the clay layer 10 is affected by the level of horizontal stress in the layer 10. It is important to ensure that the horizontal stress is controlled by the process. As a guide, the existing horizontal stress in the layer 10 should be taken as the minimum acceptable and the effect of forming bore-holes or other holes in the clay layer 10 should be so as not to allow any reduction below that figure.

Horizontal stress in the layer 10 may be increased by adjusting the size of the bore-holes 800 in relation to the size of a pressure inserted probe or by using probes which are capable of exerting a lateral pressure once installed in the bore-holes 800.

Compaction of sand in a sand-filled bore-hole 800 increases the horizontal stress in the clay layer 10 and increases the ability of the layer 10 to absorb water.

The supply of water in the vicinity of the footings of a building should be such that the hydrostatic head is approximately at the level of the footings or slightly lower. This may be achieved by placing in a trench at footings level a water manifold and setting a pressure breaking device in the ground at the level of the manifold. Figures 17a and b show two suitable water supplies 950 which differ only in the depth at which the manifold is placed. Each such supply 950 comprises a trench 960 which depends below the level of the ground to the approximate depth of the footings (not shown), the base 960a of said trench forming a water trap from which water may exit via an outlet pipe 970 which connects to a pipe (not shown) located in a bore-hole 800.The water trap may be supplied by mains supply water which is regulated by a ballcock which maintains the level of the water in the trench at a desired level.

The trench also has at least a cover 980 to allow some airflow thereinto.

The flow of water from this water supply 950 to the bore-holes 800 is maintained at low pressure and even where the foundation clay layer 10 is located at a deep level such a water supply 950 still allows water to be supplied to bore-holes 800 with a low head of pressure. The level of water in the trap may be selected so as to equal the natural water table level for the clay layer 10 being rehydrated such that where the water table level tends to fall below its natural level, supply from the trap can make up the difference thereby preventing dehydration of the foundation clay layer 10.

Figure 18 illustrates a system of maintaining hydration levels of a foundation clay layer 10 using the method as aforementioned. The system comprises a number of adjacent bore-holes 800 which each extend into the foundation clay layer 10. A first trench 960 is located to one side of the boreholes 800. This trench forms a water supply 950 in the manner described with respect to figures 17a and b. An outlet pipe 970 from the supply 950 rises therefrom before depending downwardly into a first bore-hole 800. Thereby water can be diffused via the draining material 810 compacted in the boreholes 800 into the foundation clay layer 10 under syphonic action.However, the system is more complex insofar that a further pipe 990 located in the first bore-hole 800 carries water onward to a further bore-hole 800 in order to allow further water diffusion into the clay layer 10 via this bore hole 800. A final pipe 995 located in the second bore-hole 800 exits to a second trench 965 located on the other side of the bore-holes 800 and depends downwardly thereinto. Excess water passes through this pipe and can be pumped away by a float operated pump means located in the second trench 965. In this way, the system allows for water to be supplied to the bore-holes 800 when the natural water table reduces below that of the level set by the water supply manifold of the first trench 960 and for water to be extracted when the water table rises above that determined by the water pump manifold in the second trench 965.Accordingly, this system allows accurate continuous control of the hydration of the clay layer 10.

Upper portions of the bore-holes 800 may be filled with concrete in order to seal the various pipes in the bore-holes 800. Alternatively, the pipes might be sealed by use of a bentonite plugs.

In addition to the methods already described, in instances where damage to a building's foundations is being caused by dehydration by the roots of a tree, for example, a line of soil moisture probes may be located so as to intersect the offending roots at a position midway between the tree and the footings of the building. The moisture curtain produced by this will provide the water required to encourage growth of water seeking rootlets. In encouraging the uptake of moisture midway along the tree's root system, the tips of the roots nearest to the foundations will be required to supply less moisture and as a result the effect of dessication adjacent the building footings will be diminished.

For the long term provision of moisture to this mid-point the root system the various membrane arrangements described with reference to figures 1 to 7 could be used. The membrane might be set so as to have a fall away from the side surface of a wall supported by footings founded in a clay layer towards the root system mid point. At a mid-point a trench might be provided into which an edge portion of the membrane might depend and within which a water supply in any of the manners hereinbefore described might be located.

Precipitation falling on the soil layer or other layers covering the membrane would drain towards the root system mid-point. The membrane would prevent precipitation reaching those root system tips adjacent to the footings which might affect the moisture content of the foundation clay layer. This combined with irrigation of the mid-point of the root system would greatly reduce the activity of these root tips.

Any of the aforesaid methods, particularly those for injecting or introducing water into the foundation clay layer 10 might be used to deliberately raise the water table level in the layer 10 above a desired level for a period of time sufficient to produce anaerobic conditions to naturally destroy portions of a tree root system below the raised level which may be affecting the moisture content of the layer 10 adjacent the footings of a building. This would thereby negate the need to employ chemical treatments or to physically excavate said roots.

The methods and apparatii described in the present application are provided for the arrest, correction and prevention of subsidence in buildings.

The methods and apparatii are provided as alternatives to the conventional method of underpinning which is both time consuming and expensive and involves excavating under the footings of buildings. The methods and apparatii of the present invention may be utilised to correct subsidence when it has occurred and arrest and prevent further subsidence but, equally importantly, may be utilised to prevent subsidence from occurring where it is deemed there is a possibility that subsidence might occur. This is particularly applicable in regions where the characteristics of a foundation clay layer are well known and considered stable but where extended periods of below average rain fall leads to unexpected dehydration of portions of the clay layer.

Correction of subsidence by underpinning is normally only undertaken where evidence of subsidence becomes obvious ie. cracks appearing in the structure of the buildings. However, some of the methods and apparatii of the present invention can be utilised even where no such evidence is available but where the likelihood of subsidence is considered a realistic risk. The methods and apparatii of the present invention vary in their complexity and cost but cost of implementation remains favourable when compared to the cost of the conventional method of underpinning.

The methods and apparatus as hereinbefore described might be utilised during construction of new buildings in order to provide the buildings with means to prevent subsidence from occurring and ensure stability of said foundations for the life-span of the building. Careful monitoring and control of the apparatus employed will enable the moisture content of the foundation clay layer to be maintained within a close tolerance to a preferred level.

Monitoring and control of the apparatus may be achieved electronically by means of electronic monitoring means which provide signals indicative of moisture content of the clay layer to a control apparatus which in turn actuates control devices to supply water to water irrigation or injection means as necessary. The monitoring means may detect when the moisture content falls below a threshold valve before actuating the control devices.

Such a system installed to prevent subsidence at the time of construction of a building would add negligibly to the overall cost of construction but would negate the need for possible expensive corrective action at a later date.

The foregoing description of the various aspects of the invention refers in the main to foundation clay layers. It will clearly be understood by those skilled in the art that the abovementioned methods and apparatii could be employed in connection with any type of foundation layer susceptible to shrinkage caused by loss of some of its moisture content. Accordingly, it will be clearly understood that the foregoing description in no way limits the scope and application of this invention to foundation clay layers only.

Claims (29)

1. A method of reducing moisture dissipation from or assisting rehydration of a foundation clay layer by locating in or above said clay layer water irrigation means within an area extending from a side surface of a wall supported by footings founded in the clay layer wherein said water irrigation means is connected to a water supply.

2. A method as claimed in claim 1, wherein the water irrigation means comprises an array of pipes each connected to the water supply, some of said pipes being perforated along their lengths to enable water to seep outwardly therefrom.

3. A method as claimed in claim 1, wherein the irrigation means comprise French drains.

4. A method as claimed in claim 1, wherein the water irrigation means comprise means for injecting water into the clay layer, said means having a first end inserted into the clay layer and a second end of said means being located above the clay layer and connected to the water supply wherein water is fed from the water supply to the water injection means for injection into the clay layer.

5. A method as claimed in claim 4, wherein the water injection means comprises a pipe.

6. A method as claimed in claim 5, wherein the pipe is perforated along at least a portion of its length adjacent the first end thereof.

7. A method as claimed in claims 4 to 6, wherein the water injection means includes a plurality of secondary water injection means located between said first and second ends and which extend generally radially outwardly therefrom.

8. A tool for use in the method of claims 4 to 7 for inserting the first end of a water injection means into the clay layer said tool comprising an elongate rod member, a collar which is movably mounted on the rod and is fixedly locatable on the rod at axial positions intermediate first and second ends thereof, and a spike-like member which has a first end locatable in an axially extending bore in the first end of the rod such that the first end of the rod abuts a shoulder of said spike-like member, and a second end of the spike-like member which is oppositely directed to the first is generally spike-shaped.

9. A method as claimed in claim 1, wherein the water irrigation means comprises a bore-hole extending through layers covering said clay layer, and water supplied from the water supply diffuses from the borehole into said layers.

10. A method as claimed in claim 9, wherein the bore-hole depends down into the clay layer.

11. A method as claimed in claims 9 or 10, wherein the bore-hole is filled with an inert drainage material.

12. A method as claimed in claim 11, wherein the inert drainage material is compacted within the borehole.

13. A method as claimed in claims 9 to 12, wherein the water irrigation means includes secondary holes radiating generally outwardly from the bore-hole at predetermined axial positions along the length thereof.

14. A method as claimed in claim 13, wherein the secondary holes comprise fissures formed by using water under high pressure within the bore of the bore-hole to rupture and expand natural fissures in the foundation clay layer.

15. A method as claimed in claim 13, wherein the secondary holes comprise planar-type cuts formed in the clay layer which extend radially outwardly from the bore-hole.

16. A method as claimed in claims 13 to 15, wherein the secondary holes are filled with an inert drainage material.

17. A tool for use in the method according to claim 15 for forming said outwardly extending planar type cuts, said tool comprising an elongate rod having pivotally mounted to a first end thereof a number of blades, each of which, on insertion of the tool into the bore-hole, lie generally parallel and adjacent to a lower portion of the rod and which, in use, are caused to pivot outwardly about said first end by means of a second tool cooperable with said cutting tool, said second tool comprising a sleeve adapted to slide along the length of the rod and having at a primary end thereof a wedge means for engaging with the blades of the cutting tool whereupon movement of the sleeve down the rod causes the wedge means to engage the blades of the cutting tool and further movement of the wedge means drives said blades outwardly away from the lower portion of the rod to depend into the medium in which the bore-hole is formed thus forming the planar-type cuts.

18. A method as claimed in any preceding claim, wherein the water supply may be mains water or any other suitable water supply such as waste water or rain run-off water.

19. A method as claimed in claim 18, wherein the supply of water to the water irrigation means is monitored by water metering means.

20. A method as claimed in claims 18 or 19, wherein the water supply may be located in a ditch formed in a top soil layer or fill material layer covering the clay layer.

21. A method as claimed in claims 4 to 7, wherein the water supply connected to the water injection means includes a pump for supplying water to said means under pressure greater than mains pressure.

22. A method as claimed in claims 9 to 16, wherein the water supply comprises a hydrostatic water supply located at or below the level of the footings founded in the clay layer.

23. A method of reducing moisture dissipation from a foundation clay layer caused by the root system of a tree comprising locating in or above the clay layer at a position between a wall whose footings are founded in the clay layer and a trunk of the tree water irrigation means to thereby produce a moisture curtain in at least the clay layer at said position whereby the root system draws moisture from the moisture curtain thus reducing moisture uptake by a part of the root system extending into the clay layer adjacent the footings.

24. A method as claimed in any preceding claim, wherein a membrane is placed over said clay layer to extend from a point adjacent the side surface of the wall supported by footings founded in the clay layer to a point remote from said side surface.

25. A method as claimed in claim 24, wherein the method includes the step of forming a ditch in a layer covering the clay layer at a distance from the side surface of the wall with the membrane laid over the said layer such that an edge of the membrane furthest from the surface of the side wall depends downwards into the ditch.

26. A method as claimed in claim 25, wherein the ditch may extend downwards to at least the surface of the clay layer.

27. A method as claimed in claims 25 or 26, wherein the membrane is laid such that an edge nearest the surface of the side wall contacts with said side wall and a portion of said membrane adjacent said edge is laid to lie coplanar and in contact with said side surface.

28. A method as claimed in claims 24 to 27, wherein the membrane is formed from a semi-permeable sheet form material, the material being such that it allows moisture to pass through it in one direction but prevents moisture flow in an opposing direction, the membrane being located over the clay layer such that it acts to reduce moisture dissipation from said layer.

29. A method as claimed in claims 24 to 28, wherein the membrane slopes downwardly away from the wall.