GB2145370A

GB2145370A - Textile material

Info

Publication number: GB2145370A
Application number: GB08420526A
Authority: GB
Inventors: Henry Melville Green
Original assignee: NAT RES DEV; National Research Development Corp UK
Current assignee: NAT RES DEV; National Research Development Corp UK
Priority date: 1983-08-23
Filing date: 1984-08-13
Publication date: 1985-03-27
Also published as: GB8322644D0; DE3430842A1; GB8420526D0; AU3191984A; ZA846533B

Abstract

A textile material (10) comprises a base web (11) having united therewith an overlying fibrous layer (12) which can be filled with particulate material such as sand. The base web (11) has on its underside an enhanced friction layer which can be created by needling tufts (13) of said fibrous layer through the web (11). A composite material consisting of said textile material (10) and a filling of sand is useful as a component of a road structure overlying a substrate (14) and supporting an overlying layer (15) of fragmentary material such as stone which in turn supports a road surface layer 16. <IMAGE>

Description

SPECIFICATION Textile material This invention relates to a textile material which is particularly useful in reinforcing a layer of particulate material such as sand in a civil engineering structure such as a road base.

Sand layers are often used as part of a road base construction. Such layers of varying depths are placed directly on top of the soil or subgrade on which the road is to be constructed. The sand is compacted to the required thickness and density, and graded stone fill is then tipped, compacted and levelled to provide a granular stone layer whose function is to spread loads applied to the bearing surface layer of the road.

The purpose of the sand layer is to provide free drainage of both surface water and base water (water from the subgrade) at the interface between soil and stone since the presence of such water can lead to breakdown of the road function. This is particularly so if the base (pore) water pressure is allowed to build up since such condition may lead to the migration of soil or clay particles (fines) through to the stone layers. This can lead to loss of stone binding and cohesion. The present operation of sand spreading 25 and compacting to controlled depths is difficult. Such layers, however carefully placed, are disturbed by the dumping of stone fill at the next stage of construction. Stones can be 'punched' through the sand layer to contact or even become embedded in the soil surface.

Large volumes of sand may be used in an attempt to stop such punching.

It should be understood that once stone has penetrated the sand layer there is a virtual short-circuit of the filtration function of the sand.

The efficiency of a sand layer as described above depends on the maintenance of the thickness of the layer sufficient to allow free passage of water in any direction and to provide an effective separation function, keeping the stone fill from direct contact with the soil. The maintenence of the sand layer thickness ensures that all point loads are dissipated first by the stone and then by the layer of sand. This results in more even stress distribution at the subgrade level.

Accordingly the invention provides a textile material suitable for use as reinforcement for a layer of particulate material in a civil engineering structure, comprising a porous dimensionally stable base web, a fibrous layer secured to the web and overlying one face thereof, and an enhanced friction layer on the face of the base web remote from the fibrous layer.

The base web can be an open mesh fabric, and the fibrous layer may be secured to the base web by needling.

The enhanced friction layer can be constituted by a pile or by tufts. Alternatively the enhanced friction layer can be provided by a pile or tufted layer secured to the base layer. The pile or tufts can be generated by causing portions of the fibrous layer to project through the base web. This can be effected by securement of fibrous layer to the base web and the generation of said pile or tufts can be effected by the same needling operation. The fibrous layer can be a mat or polypropylene staple monofilaments, having a weight of from 750 to 1750 gms/m2.

THe invention also provides a composite material comprising a textile material as aforesaid the fibrous layer is filled with particulate material. The particulate material can be sand or ash.

The base web should be sufficiently closely formed to prevent the passage of particulate material through it, but sufficiently porous to allow the free passage of water.

The invention further provides a civil engineering structure including a substrate, a composite material 25 as aforesaid overlying the substrate, a layer of fragmentary material overlying the composite material, and a surface forming layer overlying the layer of fragmentary material.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a cross-sectional view illustrating a preferred textile material of the invention; and Figure 2 shows the material of Fig. 1 in use in a civil engineering structure.

A preferred textile material 10 of the invention comprises a base web 11 which is a fabric woven from polypropylene tape and having 12 x 10 tapes per 25 mm to give a fabric weight of 120 gms/m2, with warp and weft tapes of 1000 and 1200 denier respectively. The tapes can be flat or profiled having respective strengths of 18KN/m and 16KN/m.

A fibrous layer 12 of the material 10 is constituted by a mat of polypropylene staple monofilaments in the range 150 to 400 denier. The mat may be from 10 to 25 mm thick and a preferred weight of the mat is 1250 gms/m2, The fibrous layer is secured to the base web by needling using a double row of needles at 15 mm pitch. Groups of the fibres of the fibrous layer 12 are punched through the base web 11 firstly to secure the fibrous layer 12 to the base web 11 and secondly and simultaneously to form an array of tufts 13 on the underside of the base web 11. After needling, the thickness of the fibrous layer 12 may be reduced to 10-15 mm, preferably 10mm.

The lengths of the tufts can be from 3 to 5 mm.

The tufts 13 constitute an enhanced friction layer on the underside of the material, the significance of which will be later described.

The textile material of the invention is particularly suitable for use as reinforcement for a layer of particulate material such as sand in the construction of civil engineering structures such as road bases. In constructing such a structure, the textile material of the invention is laid on top of a cleared subgrade such as soil and then filled with particulate material, such as sand, by blading and/or brushing and/or washing the particulate material into the textile material to fill the insterstices of the fibrous layer as completely as possible. The volume of sand in the fibrous layer may amount to 60-90% of the volume of the layer. The grain size of the sand can be DGO=0.175 m/D10 = 0.110 mm with Cu (coefficient of uniformity) = 1.6. The weight of sand in the fibrous layer may amount to six times or more the weight of the textile material per unit area.The sand thus trapped in the fibrous layer 12 is virtually incompressible while the short fibrous stubble or tufts 13 underneath grips the subgrade or soil with great tenacity thereby effectively anchoring the composite to provide maximum shear resistance at the soil interface.

The adhesion developed between the enhanced friction layer and the cohesive material (soil) which it attempts to reinforce is most important. If such adhesion is low, anchorage of the textile material in the soil is poor, and transfer of stress from the soil to the fabric is therefore low. Laboratory tests, using shear-box equipment in which a composite fabric comparable to the fabric of the invention is clamped in a soil bed with normal applied stress loadings of from 140KN/m2, to 200KN/m2, shows that at a fabric pull-out rate of 1.22mm/min, this comparable composite textile material of the invention can develop adhesion values of up to 90 per cent of the undrained shear strength of the soil. A textile material having the enhanced friction layer constituted by the fibrous stubble face develops surprising grip on the soil under normal stress loading.This grip discourages movement such as rucking or creep at the interface under dynamic loading conditions. Such movement of the textile between the soil and the stone fill at the road base can result in rupture of the fabric.

In a recent major road trial in the U.K. a number of geotextiles of different constructions were tested in a road bed under fully instrumented and controlled conditions. The road was trafficked, under known loads and speeds, and surface deformation of the road measured. A number of conclusions were made, the first of which was that the rate of deformation of granular pavements (road) is reduced and the pavement life extended by use of goetextile, provided it is not torn, (see CIRIA Report RP 245-The Sandleheath Experiment).

Referring now to Fig. 2, the textile material 10 of the invention is laid on a soil or similar subgrade 14, the enhanced friction layer constituted by tufts 13 ensuring that there is a firm bond between the material 10 and the sub-grade 14. The fibrous layer 12 is now filled with sand by any of the described methods. When this has been done a layer 15 of fragmentary material, such as broken rock, is laid on top of the sand-filled textile material. A surface forming layer 16 can then be laid on top of the layer 15 as in conventional road construction practice.

The composite formed by filling the textile material 10 with sand serves several functions. Firstly it provides a physical barrier between the layer 15 and the subgrade 14, particularly during the construction phase preventing stone from layer 15 from penetrating the subgrade 14 (as can easily happen with an unreinforced sand layer) and thus allowing silt and fines from the soil invading the stone and causing lack of adhesion and stone layer cohesion. This function is maintained in use, as upward migration of silt and fines under the influence of ground water is prevented by the composite. Secondly, the reinforced sand layer facilitates drainage of descending and ascending water lat rally outwards through the sand layer. This reduces the possibility of the structure becoming waterlogged and losing its local bearing capabilities.Thirdly, the reinforced sand layer is rendered much less suscep table to extrusion under high localised loads. Fourthly, the spreading of applied loads to the subgrade is consistently enhanced.

The combination of these properties leads to enhanced roadway life compared with conventional road constructions under comparable conditions.

A road trial was carried out on a section of the main Belfast/Newry motorway in Northern Ireland in which five fabrics of various constructions were used. Three commercial textiles were compared against two three-dimensional composites of the type described in this patent application one filled with sand and the other with fine quarry ash. The trial was completed over a 5-month period with the help of the Department of the Environment (N. Ireland). The road section containing the five fabrics which had been laid under controlled conditions was carefully excavated after the five month service to expose these fabrics at the road base. Visual examination showed the two composite materials to be more effective than the commercially available geotextiles in terms of degree of contamination by the soil 'fines' and the composites resilience to puncture.This road trial was supervised by the Civil Engineering Department of Queen's University, Belfast.

This invention is not limited to the precise details of the foregoing and variations can be made thereto. For example, although the base web is described as a woven fabric, it can be non-woven if desired. The base web must, however, have sufficient strength to support the fibrous layer adequatly and be capable of resisting deformation loads it is likely to encounter in use. It should not have any marked degree of resilience or tendency to change dimensionally due to changes in ambient conditions such as wetness or temperature.

The fibrous layer can be of any convenient structure, provided that it is sufficiently open to allow it to be filled with particulate material without substantial risks of voids being formed. A fibrous layer consisting of pile or a pile fabric may be suitable, but would probably be more expensive than the staple fibre mat described above.

The enhanced friction layer need not be an array of tufts as described and can be constituted by a pile applied to the base web or a pile fabric secured by its backing to the base web. However each of these possibilities would probably be more expensive. As a further possibility the enhanced friction layer could be constituted by a mat similar to but thinner than the layer 12 needled or otherwise secured to the base web on its underside.

The fibrous layer, the base web and the enhanced friction layer (if separate) can be united by means other than needling, for example by ther mal or chemical adhesion. For example the fibrous layer could be constructed by a felting process using fusible or part fusible shape fibres, and the formed layer subsequently thermally adhered to a compatible base web.

The various components of the fabric should be made from material capable of resisting biological and chemical attack in a civil engineering environment. Polypropylene is a material having excellent properties in this respect, but other synthetic materials having suitable properties may be used.

It will be appreciated that the term 'road' includes such structures as conventional roads and motorways, aircraft runways, car parks and playgrounds, railway tracks and any structure intended to provide a load-bearing base over a subgrade.

When relatively thin or two-dimensional textiles have been used for road base reinforcement in the past, problems have arisen during laying thereof in windy conditions. The wind has tended to lift and fold and/or crease the material between laying and stone loading. Folds and/or creases formed in this way subsequently can form areas of stress in the fabric beneath the later applied stone layer. Fabric failure often occurs at such positions. The textile material of the present invention, because of its thicker construction is much less susceptible to wind movement and associated creasing and folding leading to fabric rupture. Further, there is no need for the fabric to be weighted down between laying and the application of the stone layer(s).

Claims

1. A textile material suitable for use as reinforcement for a layer of particulate material in a civil engineering structure, comprising a porous dimensionally stable base web, a fibrous layer secured to the web and overlying one face thereof, and an enhanced friction layer on the face of the base web remote from the fibrous layer.

2. A textile material as claimed in claim 1, wherein the base web is an open mesh fabric.

3. A textile material as claimed in claim 1 or 2, wherein the fibrous layer is secured to the base web by needling.

4. A textile material as claimed in claim 1, 2 or 3 wherein the enhanced friction layer is constituted by a pile or by tufts.

5. A textile material as claimed in claim 4, wherein the enhanced friction layer is provided by a pile or tufted layer secured to the base layer.

6. A textile material as claimed in claim 4, wherein the pile or the tufts is or are generated by causing portions of the fibrous layer to project through the base web.

7. A textile material as claimed in claim 6 wherein said portions of the fibrous layer are caused to project through the base web by needling.

8. A textile material as claimed in claim 7, wherein securement of fibrous layer to the base web and the generation of said pile or tufts is effected by the same needling operation.

9. A textile material as claimed in any preceding claim, wherein the fibrous layer is a mat of polypropylene staple monofilaments, having a weight of from 750 to 1750 gms/m2.

10. A textile material substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. A composite material comprising a textile material as claimed in any preceding claim wherein the fibrous layer is filled with particulate material.

12. A composite material as claimed in claim 11 wherein the particulate material is sand.

13. A composite material as claimed in claim 11 wherein the particulate material is ash.

14. A composite material as claimed in claim 11, 12 or 13 wherein the base web is sufficiently closely formed to prevent the passage of particulate material through it, but sufficiently porous to allow the free passage of water.

15. A composite material substantially as hereinbefore described with reference to the accompanying drawings.

16. A civil engineering structure including a substrate, a composite material as claimed in any of claims 11 to 15 overlying the substrate, a layer of fragmentary material overlying the composite material, and a surface forming layer overlying the layer of fragmentary material.

17. A civil engineering structure substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.