WO1992010615A1

WO1992010615A1 - Soil treatment

Info

Publication number: WO1992010615A1
Application number: PCT/EP1991/002392
Authority: WO
Inventors: David Kenneth Young
Original assignee: Exxon Chemical Patents Inc
Current assignee: ExxonMobil Chemical Patents Inc
Priority date: 1990-12-11
Filing date: 1991-12-10
Publication date: 1992-06-25
Anticipated expiration: 1993-06-11
Also published as: GB2250762A; 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

Technical Field

This invention relates to treatment of soil to improve its behaviour as a structural material for construction purposes.

Background Art

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.

Prior to discussing the nature of soils amenable to reinforcement, it is first prudent to describe the nature of a soil mass as idealized 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. A major technical restriction on the nature of soils that are amenable to reinforcement is that 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. 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 widely used approach for remedy 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. Currently these typically 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. For instance U.K. patents GB 2023496 A and GB 1479772 address materials and geometries for wick drains whereby features are incorporated to reduce the resistance of the product to soil movement in the direction of drainage channels.

UK patent GB 1 599 892 addresses stabilized soil structures provided by utilizing a plurality of flexible lamellae each constituted by two filter layers between which there is a porous spacer layer. The spacer layer is said to provide both drainage capacity and the bulk of tensile strength necessary to support the soil above the lamella and to accept the shear load from the soil. The spacer layer accordingly is porous to permit dissipation of excess pore pressures from the center without reducing stability of the structure at its edges.

Invention Disclosure

The present invention provides means to allow the use of reinforcement in poorly-draining soils which might otherwise develop sufficiently high excess pore water pressures such 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 stiff 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.

Description of the Drawings

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.

Best Mode and Examples of the Invention

The filter material of the invention 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. Such material is typically comprised of engineering thermoplastic fibers, for example those of polyethylene, polypropylene, polyethylene or polypropylene copoly ers, polyethylene terepthalate, polyamides, polyaramides, and polystyrene, or mixtures thereof. These fibers are prepared by methods known in the art.

This 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. Bonding by such methods as heat, adhesive or ultrasonic means known generally can be utilized. 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. Typical non-limiting examples of materials having such properties are the known nonwoven geotextiles from such fibers as polyester, polypropylene, high density polyethylene, polyethylene or polypropylene copolymers and the like, particularly blends with polyethylene.

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.

This reinforcing element is stiff in the sense that it is of limited extensibility, typically less than 10%, in the direction of the reinforcement as in a similar manner to a rope or other fibre element. The stiffness of the reinforcing element in other directions and in bending can be low or not. Further, by use of the stiff reinforcing element of this invention, the material will not be compressible to any significant extent, thus protecting the drainage channels.

By virtue of the design for this invention, the material of the reinforcing element typically will not be porous to any significant extent, although it may be so. Significant extent means porosity that degrades the reinforcing stiffness or compressibility more than about 10% of that of equivalent non-porous material.

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. Typical polymers for such use can be the same as described for the durable geosynthetic material addressed above.

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. A commercially available example is high strength polyester fiber encased in low density polyethylene available as PARALINK^® from EXXON CHEMICAL GEOPOLYMERS LTD. (U.K.)

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.

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.

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 IB 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 IB can itself comprise a filter material and/or can be covered by layers of filter material 4, in which case the sheath or matrix IB 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 ID, 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 IE 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.

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 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.

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.

Industrial Applications

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 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.

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.

What is claimed is :

Claims

CLAIMS :

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

2. The device according to claim 1, 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.

3. The device according to claim 1 or 2, 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.

4. The device according to claim 1, 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.

5. The device according to claim 4, wherein the filter material is bonded to the exterior of the reinforcing element.

6. The device according to claim 1, wherein the filter material is applied over all of the reinforcing element.

7. The device according to claim 1, wherein the filter material is such as to be no less extensible than the reinforcing element.

8. The device according to claim 1, wherein the or each drainage channel is formed so as to have a cross section which is triangular, cuspidate, rectangular, semi-elliptical or elliptical, or any combination thereof.

9. The device according to claim 1 and comprising providing one or more secondary drainage channels transverse to the or each firstmentioned drainage channel.

10. The device according to claim 1, wherein the reinforcing element is made in the form of a strip or strap.

11. The device according to claim 1 or 4, wherein the device is made in the form of a sheet.

12. The device according to claims 1, 2 or 3, wherein the reinforcing element is made of metal.

13. The device according to claims 1, 2 or 3, wherein the reinforcing element is of polymeric reinforcement.

14. The device according to claim 13, wherein the reinforcing element is embedded in, coated with or otherwise encapsulated within a protection sheath.

15. The device according to claim 13, 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.

16. The device according to claim 1, 2 or 3, 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.

17. The device according to claims 1, 2 or 3, wherein a plurality of said reinforcing members is provided, the reinforcing members being spaced-apart and joined by other members.

18. The device according to claim 17, wherein said other members are grid members which, together with the firstmentioned reinforcing members, define a grid.

19. The device according to claim 18, wherein the filter material is sandwiched between the reinforcing members and the grid members.

20. The device according to claim 18, 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.

21. The device according to claim 18, 19 or 20, wherein the grid members are bonded to the reinforcing members and/or to the filter material.

22. A use of the device according to claim 1 as a means of reinforcing and draining poorly-draining soils.

23. The use according to claim 22 with segmental rigid facings or panels in vertically faced or substantially vertically faced reinforced soil walls.

24. The use according to claim 22 with additional means of soil retention.