GB2237315A - Land improvement - Google Patents

Abstract

A method of improving land, for example an infill site, comprises surrounding the land with a sub-terranean continuous relatively thin wall (10) which is formed in situ by displacing soil to form a void and filling the void with a gas and water impervious material, forming a plurality of columns (20) filled with a liquid and gas permeable material in the land surrounded by the wall with at least some of said columns located as close to the wall that liquid therein can permeate through the soil to moisten the neighbouring wall, and covering the surrounded land with an impermeable barrier sheet (18, 28). <IMAGE>

Description

Improved Method and Apparatus for Land Improvement The present invention concerns an improved method and apparatus for land improvement, especially but not exclusively for land which has been reclaimed by infilling. Land of this nature is often unsuitable for re-use in view of the decomposition of the infill giving rise to the exhaust of sometimes flammable and/or noxious gases and the leaching of contaminated water from the infill area into the surrounding land.

It has been proposed in the past to improve infilled land by surrounding the area of infill with a gas and liquid impervious barrier, providing throughout the surrounded infilled area a plurality of gas escape passages, connecting said gas escape passages to venting apparatus and, after covering the infilled area with an impermeable barrier, covering over the impermeable barrier with top-soil or other suitable material.

Past 'methods have to a certain extent been satisfactory but suffer from the considerable disadvantage of being expensive. For example, in one prior arrangement the surrounding impervious wall has been provided by driving interconnected steel sheet piles. In another method the impervious wall has been provided by excavating a trench to the required depth and subsequently filling the trench with, for example, concrete.

Not only are both these methods expensive but also the latter method is extremely difficult to achieve in view of the difficulty in excavating in such situations.

One past method of providing an escape route for gases has been to form within the in filled area columns of loose stone through which the gas can permeate. Such columns have normally been of at least 300mm diameter and in view of the expense of forming columns of this nature have often been spaced at intervals of around 25m.

It is an object of the present invention to obviate or mitigate these disadvantages.

According to the present invention there is provided a method of improving land comprising surrounding the land to be improved with a relatively thin continuous wall at least the lower regions of which are formed in situ by displacing ground to form a void, filling the void with a gas and water impervious material, forming a plurality of columns within the area of land surrounded by the wall said columns being filled with a material which is permeable to liquid and gas with at least some of said columns being formed sufficiently close to said wall to enable moisture from the said columns to moisten the surrounding ground up to the wall and covering the land surrounded by the wall with an impermeable barrier sheet.

Preferably the surrounding wall is formed by filling the void with bentonite. Alternatively grout or concrete may be employed. Preferably the thickness of the wall is in the range 700 to 1500mm. Preferably- the upper region of the wall is formed by a trenching tool and is of a thickness greater than the lower region formed by soil displacement.

The lower regions of the wall may be formed by driving a hollow pile member into the ground to form a void, removing said member and simultaneously filling the void through the open bottom of the pile member with a cementitious mixture.

Preferably the stone filled columns have a diameter of around 120 and 250mm, are located at no more than 3m centres and those adjacent the wall are spaced as close as 0.75m from the wall.

The stone columns can be formed by driving a hollow pile casing into the ground, the casing being driven through a hopper arranged at ground level and containing a supply of stone for filling the column, the casing having lateral passages therethrough to permit stone to fill the casing as it passes through the hopper and thereafter opening the base of the casing and removing it from the ground, the stone therein remaining in the hole formed by the casing to form the column.

Any other suitable method may be employed.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying diagram which shows a cross-section through the outer regions of an area of land to be improved.

The method involves forming a gas and liquid impermeable wall 10 around an infilled area of land 12 which is surrounded by what can be referred to as "good" or original ground 14.

Normally the method is employed to surround areas of land which have been reclaimed by infilling, for example gravel pits which have been infilled with domestic and industrial waste. Such ground often extends to below the water table 16 such that the infilled area 12, at least at its bottom, is water covered. As a result of this and of the nature of the infill material, flammable and noxious gases are often given off and contaminated water seeps into surrounding good ground.

If these problems can be overcome the reclaimed land can be used for profitable purposes.

The method of the present invention comprises utilising a normal trenching tool to cut a trench of 160mm width to a depth of up to 1m in the ground around the infilled ground 12.

This forms the top of the wall 10 and has not been shown in detail in the drawing. Thereafter the trench is deepened to a suitable depth by a soil displacement method, that is a method which does not involve removing earth but simply displaces it to the side, and as or immediately after the soil has been displaced, filling the void created by a cementitious settable material, for example bentonite, grout or concrete. Any suitable soil displacement means can be employed and the means disclosed in our pending U.K. Patent Application 2222191A are particularly suitable. In this method a hollow pile member is driven into the ground to create the void and as the member is being removed the void thus created is filled through the now open bottom of the pile with the cementitious mixture.As described in our earlier Application, the surrounding wall is provided to a level lower than the lowest level of the infill material and the base of the area containing infill material may also be treated to prevent transmission of contaminated liquid downwardly therethrough. In the present method the thickness of the wall extending downwards from the upper trench is around 100mm.

After the continuous wall 10 has been constructed around the infilled land 12 gases and leachate from the infill is laterally contained and it will be appreciated that containment can be completed by covering the infilled land 12 with a gas impervious barrier sheet 18.

To remove gases generated within the infill material 12 the present invention provides chimneys or stone filled columns 20. These columns are also preferably formed by a soil displacement method whereby the infill material is pushed aside by a penetrating member which, when removed, is replaced by a column of loose stones, for example pea gravel, providing for gas and liquid movement therethrough. The diameter of the column is preferably of the order of 175mm and the columns are located at substantially 2m centres. The columns or chimneys can be formed by any suitable technique but it has been found that the technique described in our co-pending application PCT/GB90/00880 is particularly applicable. In this method a hollow pile having lateral passages therein is driven through a hopper containing the stone material, the material from the hopper passing into the pile as it is driven so that when the pile is removed and its tip opened the material is left in the ground to form the column or chimney 20.

It has been realised that the retaining wall 10, especially if it is formed from bentonite, must be moist to remain efficient 'and the invention makes use of the fact that the stone columns or chimneys 20 penetrate below the water table 16. Thus moisture can permeate up through the column and if the columns nearest to the wall 10 are located as close to the wall that moisture can seep through the material to the wall, then assurance can be given that the wall will remain moist even if upper ground conditions are dry. As a result, the present invention provides that the columns or chimneys 20 closest to the wall are spaced at around 0.75m therefrom.

The present invention provides that a porous hoggin 22 is provided at the top of the wall 10 and chimneys 20 adjacent the wall, the hoggin being covered by the impervious sheet 18.

Gas can be removed from the infill area by providing transversely extending pipes 24 which extend from column to column and have gas inlet means at the top of each column. Vent pipes 26, which may have flare stacks to burn off flammable gases, extend from the pipe at intervals through the porous sheet 18, the sheet being gas tight around the vents 26. It will be realised that the impermeable sheet 18 must also surround the edges of the porous hoggin 22 between the upper sheet 18 and the wall 10, as shown by 28 in the drawing.

Various modifications can be made without departing from the scope of the invention. For example, as described above, the method of forming the wall 10 and columns 20 can take any convenient form provided that they mainly utilise soil displacement rather than soil excavation techniques. In this respect the upper trenching arrangement need not be utilised.

If necessary, reinforcement can be provided in the wall 10 and gas vent pipes can be provided in the columns or chimneys 20.

In a further modification the spacing of the chimneys may be altered but the critical dimensions referred to above should be maintained.

Claims (10)

Claims

1. A method of improving land comprising surrounding the land to be improved with a relatively thin continuous wall at least the lower regions of which are formed in situ by displacing ground to form a void, filling the void with a gas and water impervious material, forming a plurality of columns within the area of land surrounded by the wall said columns being filled with a material which is permeable to liquid and gas with at least some of said columns being formed sufficiently close to said wall to enable moisture from the said columns to moisten the surrounding ground up to the wall and covering the land surrounded by the wall with an impermeable barrier sheet.

2. A method as claimed in claim 1, in which the surrounding wall is formed by filling the void with bentonite.

3. A method as claimed in claim 1, in which the surrounding wall is formed by filling the void with grout or concrete.

4. A method as claimed in any of claims 1 to 3, in which the thickness of the wall is in the range 700 to 1500mm.

5. A method as claimed in any of claims 1 to 4, in which the upper region of the wall is formed by a trenching tool and is of a thickness greater than the lower region formed by soil displacement.

6. A method as claimed in any one of the preceding claims, in which the stone filled columns have a diameter of around 120 and 250mm, are located at no more than 3m centres and those adjacent the wall are spaced as close as 0.75m from the wall.

7. A method as claimed in any one of the preceding claims, in which the lower regions of the wall are formed by driving a hollow pile member into the ground to form a void, removing said member and simultaneously filling the void through the open bottom of the pile member with a cementitious mixture.

8. A method as claimed in any one of the preceding claims, in which the columns are formed by driving a hollow pile casing into the ground, the casing being driven through a hopper arranged at ground level and containing a supply of stone forXfilling the column, the casing having lateral passages therethrough to permit stone to fill the casing as it passes through the hopper and thereafter opening the base of the casing and removing it from the ground, the stone therein remaining in the hole formed by the casing to form the column.

9. A method of improving land, substantially as hereinbefore described with reference to the accompanying drawing.

10. Any novel subject matter or combination including novel subject matter disclosed in the foregoing specification or claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.