GB2250762A

GB2250762A - Soil treatment

Info

Publication number: GB2250762A
Application number: GB9026874A
Authority: GB
Inventors: Graham Allan Feakins; David Kenneth Young
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-12-11
Filing date: 1990-12-11
Publication date: 1992-06-17
Also published as: WO1992010615A1; AU9060291A; GB9026874D0

Abstract

The invention provides a device for soil treatment, the device comprising a composite of a filter material (4) and a stiff soil reinforcing element (1) combined in such a way as to provide at least one drainage channel (2, 3) within the device. According to another aspect of the present invention there is provided a method of reinforcing and draining soil including incorporating in the soil the invention device.

Description

SOIL TREATMENT This invention relates to treatment of soil to improve its behaviour as a structural material.

The principles of reinforcement of soil have been established in theory and practice over a number of years. Two principal modes of reinforcement action are recognised. In the case of reinforced earth a frictional interaction between the soil particles and reinforcing elements is invoked; in anchored earth geometrical features of the reinforcing element are considered to transfer load between the soil matrix and the reinforcing element at discrete locations. In reality it is likely that all reinforcing elements act to some extent in both modes. Reinforcing elements, whether for reinforced or anchored earth, range from fine polymeric fibres included within a soil matrix at random orientations to large structural sections of steel or reinforced concrete.

The principle of reinforcement of soil has been applied to a wide variety of structural forms. The first form to be identified and widely developed commercially was the reinforced soil wall. It consists of a block of soil incorporating reinforcing elements and provided with a facing to prevent the loss of soil particles between layers of reinforcement. Further unreinforced material may be retained behind and, subject to limitations of the angle of repose of the material, above the reinforced soil block. The reinforced block may also support surcharges such as carriageways, bridge decks, etc.

Similar reinforcing techniques have since been applied to the retention of soils in steep slopes, with the slopes in some cases approaching the vertical. Zones of reinforced soil have also been provided to support embankments, pavements or other structures over founding materials which could not be relied upon to provide adequate support unaided. Differing methods of analysis have been used in the design of these structures.

Prior to discussing the nature of soils amenable to reinforcement, it is first prudent to describe the nature of a soil mass as idealised for analytical purposes.

Engineering soils are considered as an assembly of solid particles and void spaces between those particles. These void spaces may be filled with water, air or both. When considering the strength of soils it is usual to consider only one continuous pore fluid, either air or water, but not both.

In the present state of the art there are two major technical restrictions on the nature of soils that are amenable to reinforcement. In the first case the environment within the soil mass may inhibit the longevity of the reinforcement, either by virtue of moisture or by the presence of deleterious chemical or microbiological conditions within the soil mass. This problem has largely been resolved by the use of suitable polymeric materials, although there remain certain conditions under which these materials are unsuitable.

In the other case the soil may not be free-draining, as is typically the case with soils with a significant fraction of fine particles. These are not necessarily clay soils; soils with significant proportions of silt or even fine sand sized particles may be poorly drained. In all soils the existence of water pressure reduces the strength of the soil mass. Such water pressure may be a static pressure, due to the existence of a water table at a higher level in or above the soil, or an excess pressure, due to seepage or other processes. In poorly-draining soils the processes of construction may lead to the development of excess pore pressures within the soil mass. Depending on the nature and initial state of the soil, and on the construction process, these excess pressures may be either positive or negative.

Negative pressures, or suctions, enhance the available strength of the soil mass in the short term, but may lead to unsatisfactory performance in the long term; positive pressures reduce the available strength of the soil mass in the short term. No soil is absolutely impermeable, and these excess pore water pressures will eventually dissipate; however, the time required for dissipation may be very long, perhaps some tens or even hundreds of years. Even where the dissipation times are relatively short, they may be excessive compared to the timescale of engineering construction.

The development of pore water pressures during construction can . lead to unacceptable loss of soil strength in certain types of soil and thereby prevent their use in engineering construction.

The foregoing discussion relates to the use of soil as fill material in the course of engineering construction.

It is, however, common to encounter fine-grained and poorly-draining soils as natural deposits. Similar problems occur here with the generation of excess pore pressures, and a number of engineering solutions have been developed.

One of the commonest situations which gives rise to the generation of excess pore pressures is the construction of embankments, typically for highway construction, across poorly-draining soils. Similar situations may occur, although less commonly, in other forms of construction. Two solutions are adopted, either singly or in combination.

One solution is the application of controlled amounts of loading in excess of the intended final embankment load to force the water out of the soil. There are risks associated with this approach as the amount of excess load must be carefully controlled to avoid gross instabilitY and the collapse of the founding material on which the embankment or structure is being built. This approach is commonly only used when the stiffness of the founding material, rather than its strength, is the controlling factor.

The other widely used approach is the installation of drains. The early applications of this approach involved the drilling of comparatively small diameter boreholes which were then filled with free-draining materials such as sand. These vertical sand drains then provided an easier drainage path for the dissipation of excess pore pressures while the surcharge - embankment or whatever was constructed above them. There are a number of technical and practical difficulties with this technique, and it has now been largely superseded by the wick drain.

In its early form the wick drain consisted of a strip or tube of corrugated cardboard which allowed passage of water through the cardboard skin and then flow of the water within the corrugations. Cardboard wick drains have now been superseded by geotextile wick drains.

These consist of a sleeve of a geotextile filter cloth surrounding a hydraulically transmissive core. The core is typically formed from a polymeric material, and is designed to provide a free space for the passage of water. It is not designed to reinforce the ground.

It is an object of the present invention to allow the use of reinforcement in poorly-draining soils which might otherwise develop sufficiently high excess pore water pressures as to render their use either uneconomic or impossible.

According to one aspect of the present invention, there is provided a device for soil treatment, the device comprising a composite of a filter material and a soil reinforcing element combined in such a way as to provide at least one drainage channel within the device.

According to another aspect of the present invention, there is provided a method of reinforcing and draining soil including incorporating in the soil a device comprising a composite of a filter material and a soil reinforcing element combined in such a way as to provide at least one drainage channel within the device.

The or each drainage channel can be provided between the filter material and the reinforcing element for the transmission of fluid in a direction defined by the direction of the extent of the reinforcement. In the case of a grid or other sheet reinforcement the drainage channel may be in the plane of the grid or other sheet.

Instead of or as well, the or at least one drainage channel can be provided within the reinforcing element for the transmission of fluid in the direction of the extent of the reinforcement.

The filter material can be a durable geosynthetic material with pore sizes appropriate to prevent migration of particles of the soil (in which the composite is placed in use) into the or each drainage channel.

The filter material can be bonded to the exterior of the reinforcing element, either continuously or at discrete locations, in such a way as to provide mechanical continuity but to prevent migration of particles of soil into the or each drainage channel. The filter material may be applied over all or part of the reinforcing element.

The filter material can be such as to be no less extensible than the reinforcing element.

The cross-section of the or each drainage channel may be of any shape, including but not limited to triangular, cuspidate, rectangular including square, semi-elliptical including semicircular and elliptical including circular. There may be any number of drainage channels, and one or more secondary transverse drainage channels may be provided to aid the passage of fluid to the primary drainage channel(s).

The reinforcing element may be any form of manufactured reinforcement including strips or straps with or without surface texture or ribbing, sheets whether woven or integral, grids whether woven or integral at grid nodes, planks, bars, beams, rods or tubes.

The reinforcing element may be of any metal including steel, whether galvanised or not.

The reinforcing element may be of polymeric reinforcement wherein a polymer or combination of polymers is formed into a structural element. The reinforcing element may be a polymeric reinforcement as where the structural element is embedded in, coated with or otherwise encapsulated within a protective sheath.

The reinforcing element may be a structural composite wherein a material of higher stiffness or strength is embedded in a matrix of lesser stiffness or strength.

Where a drainage channel or channels is/are provided within the reinforcing element, passage of fluid into the or each drainage channel will be allowed by the provision of apertures or permeable zones in the reinforcing element or its sheath.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagrammatic perspective view of one form of device for soil treatment, Figure 2 is a view similar to Figure 1 but showing an alternative embodiment, and Figures 3, 4 and 5 are diagrammatic end views showing three further possible arrangements.

Referring to the drawings, a device for soil treatment is illustrated which comprises, as a composite member, filter material and a soil reinforcing element combined in such a way as to provide at least one drainage channel within the device.

The reinforcing element illustrated is in the form of an elongate metal bar or strip 1 with external drainage channels 2 and internal drainage channels 3.

A layer of filter material 4 is laid over and bonded to both sides of the reinforcing strip 1, thereby covering the drainage channels 2, the filter material 4 acting inter alia to inhibit penetration of soil particles into the drainage channels 2. Drainage channels 2 can be provided on one or both sides of the reinforcing strip 1. The drainage channels 3 can be provided as well as or instead of the drainage channels 2.

A plurality of transverse grid members 5 is provided (only one grid member 5 being shown in each Figure). The transverse grid members can be bonded in any way to the reinforcing strip 1 and filter material 4. In practice, the transverse grid members 5 will be bonded to a plurality of spaced-apart composite members of the type illustrated in Figure 1 or Figure 2.

The embodiment of Figure 2 differs from that of Figure 1 in that the filter material 4 is laid over each transverse grid member 5 so that each grid member 5 is sandwiched between the filter material 4 and reinforcing strip(s) 1. In the Figure 1 embodiment, the filter material 4 is sandwiched between the longitudinal and transverse reinforcing elements. The bonding may be achieved by gluing or other means.

Figure 3 shows another possible form which the device can take and here are illustrated parallel, spaced-apart rod or rope reinforcing members 1A individually embedded in a sheath or matrix 1B defining tubes through which the reinforcing members 1A extend. Some of the tubes do not contain reinforcing members 1A and these empty tubes thereby form internal drainage channels 3. The sheath or matrix 1B can itself comprise a filter material and/or can be covered by layers of filter material 4, in which case the sheath or matrix 1B is provided with apertures 1C to allow drainage into the internal drainage channels 3 comprised by the empty tubes.

In the case of Figure 4, there is a central reinforcing element 1 which is provided with external ribbing 1D, the valleys in the ribbing defining with sheets of filter material 4 drainage channels 2 between the reinforcing element 1 and the filter material 4.

Figure 5 shows a composite reinforcing element 1 incorporating strengthening members 1E such as pre-stressed rods. The reinforcing element 1 shown in Figure 5 is provided with external drainage channels 2 which are covered with layers of filter material 4.

It will be appreciated that the present composite material will allow for the drainage and reinforcement of slopes of poorly-draining soil at steeper inclinations than those which would be stable without reinforcement.

The present composite material for the drainage and reinforcement of poorly-draining soils can be subject to surcharges by placement of additional fill or by other sustained loadings.

The composite material can be used for the drainage and reinforcement of poorly-draining soils in vertically-faced or substantially vertically-faced reinforced soil walls with segmental rigid facings or panels. Such facing segments or panels may or may not interlock, and may or may not have apertures within or between them. The composite material for reinforced soil walls may be used with full-height rigid facings.

Also, the composite material may be used for reinforcing soil walls with flexible facings or other means of soil retention, including both retention by vegetation and by manufactured means.

It will be appreciated that the various features described with reference to the present invention can be incorporated in any of the embodiments illustrated as desired.

Claims

CLAIMS:

1. A device for soil treatment, the device comprising a composite of a filter material and a soil reinforcing element combined in such a way as to provide at least one drainage channel within the device.

2. A method of reinforcing and draining soil including incorporating in the soil a device comprising a composite of a filter material and a soil reinforcing element combined in such a way as to provide at least one drainage channel within the device.

3. A device or method according to claim 1 or 2, wherein the or each drainage channel is provided between the filter material and the reinforcing element for the transmission of fluid in a direction defined by the direction of extent of the reinforcement.

4. A device or method according to claim 1, 2 or 3, wherein the or at least one drainage channel is provided within the reinforcing element for the transmission of fluid in the direction of the extent of the reinforcement.

5. A device or method according to any one of the preceding claims, wherein the filter material is a durable geosynthetic material with pore sizes appropriate to prevent migration of particles of the soil, in which the composite is placed in use, into the or each drainage channel.

6. A device or method according to any one of the preceding claims, wherein the filter material is bonded to the exterior of the reinforcing element.

7. A device or method according to any one of the preceding claims, wherein the filter material is applied over all of the reinforcing element.

8. A device or method according to any one of the preceding claims, wherein the filter material is such as to be no less extensible than the reinforcing element.

9. A device or method according to any one of the preceding claims, wherein the or each drainage channel is formed so as to have a cross section which is triangular, cuspidate, rectangular, semi-eliptical or eliptical, or any combination thereof.

10. A device or method according to any one of the preceding claims and comprising providing one or more secondary drainage channels transverse to the or each firstmentioned drainage channel.

11. A device or method according to any one of the preceding claims, wherein the reinforcing element is made in the form of a strip or strap.

12. A device or method according to any one of claims 1 to 10, wherein the device is made in the form of a sheet.

13. A device or method according to any one of the preceding claims, wherein the reinforcing element is made of metal.

14. A device or method according to any one of claims 1 to 12, wherein the reinforcing element is of polymeric reinforcement.

15. A device or method according to claim 14, wherein the reinforcing element is embedded in, coated with or otherwise encapsulated within a protection sheath.

16. A device or method according to any one of the preceding claims, wherein the reinforcing element is formed as a structural composite wherein a material of higher stiffness or strength is embedded in a matrix of less stiffness or strength.

17. A device or method according to any one of the preceding claims, wherein passage of fluid into the or each drainage channel is facilitated by the provision of apertures or permeable zones in the reinforcing element or its sheath.

18. A device or method according to any one of the preceding claims, wherein a plurality of said reinforcing members is provided, the reinforcing members being spaced-apart and joined by other members.

19. A device or method according to claim 18, wherein said other members are grid members which, together with the firstmentioned reinforcing members, define a grid.

20. A device or method according to claim 19, wherein the filter material is sandwiched between the reinforcing members and the grid members.

21. A device or method according to claim 19, wherein the filter material is laid over the top of the grid members so that the grid members are sandwiched between the filter material and the reinforcing members.

22. A device or method according to claim 19, 20 or 21, wherein the grid members are bonded to the reinforcing members and/or to the filter material.

23. A device for soil treatment, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

24. A method of reinforcing and draining soil, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.