GB2040655A

GB2040655A - Liquid channelling assembly

Info

Publication number: GB2040655A
Application number: GB8002540A
Authority: GB
Original assignee: John Laing Services Ltd
Current assignee: John Laing Services Ltd
Priority date: 1979-01-30
Filing date: 1980-01-25
Publication date: 1980-09-03
Also published as: GB2040655B

Abstract

An assembly for channelling drainage liquid includes a filter material, as disclosed in U.K. Patent Specification No. 1,536,551, arranged in combination with helical core members 3 to form a channeling member which guides liquid passed through the filter material 5 to a duct or pipe 1, the helical core members 5 being able to flex when bent and retain the core aperture to improve the hoop stress characteristics of the channelling member. <IMAGE>

Description

SPECIFICATION Liquid channelling arrangement This invention relates to a liquid channelling arrangement which has particular, though not exclusive, application to the draining of water in soil.

The most common method of draining soil involves the use of lengths of porous pipe which are laid in the bottom of a trench and coupled loosely together. The trench is then filled with aggregate material which acts as a filter.

This type of drain tends to block because small soil particles are able to migrate into the aggregate material and block both the pipe and the path through the aggregate material. In some cases it has been found that the effective life of such a drain is only ten years.

Drainage arrangements have previously been proposed which use vertical-fin core structures encapsulated by nonwoven plastics or other materials having a random size mesh. Drains made of such materials tend to block-up due to the migration of fine soil particles within the matrix of such random structures.

The present invention provides an arrangement in which the filter of random nature provided in previous arrangements by the aggregate material or by non-woven or other materials is replaced by a filter having a characteristic which is designed specifically to match the material surrounding it. It will be understood that, if all of the material surrounding the pipe could be maintained in a static state, while fluid was permitted to flow through it to the pipe, the material surrounding the pipe and acting as a filter would remain free from blocking. Alternatively, it is possible for the rate of flow of liquid through the surrounding aggregate material and through the pipe to be at such a speed that fine material, that might otherwise block both the pores of the aggregate material and the pipe, is carried through them both by the liquid.

The present invention provides an arrangement for use in channelling liquid which includes a longitudinally extending duct and a channel member which, when in use, conveys liquid to the duct, and in which the channel member includes a tubular core member made of a flexible material and capable of conducting liquid to the duct and a filter arranged to filter liquid passing to the core, the core member incorporating a helical gap extending along its length and the filter having a coefficient Mo/Ds0 as herein defined related to a bridging coefficient Cb as herein defined of the surrounding material as determined by the co-ordinates of a point which lies in a region on a graph between the ordinate axis, when the ordinate represents values of the bridging coefficient Cb, and a line defined by the following respective co-ordinate points of the bridging coefficient 0b and the coefficient M0/D0, 1.00, 2.10; 1.25, 2.60; 1.75, 3.20; 2.50, 4.15; 3.75,5.00; 7.00,6.66; 12.50,8.1; 25,50, 10.05 and where Cb = D90/D40r where D40, D50 and Dso are the minimum dimensions of apertures in given units through which particles which constitute respectively 40%, 50%, and 90% by dry weight of the material in which the filter material is to be used can pass and Mo is the minimum dimension of the openings in the filter material in the same given units.

Afilter having the above characteristics has been described in the specification of our U.K. Patent No.

1,536,551.

A feature of the preferred embodiment of the present invention is the use, with this filter material, of a tubular core member having a helical gap along its length, the core and the filter material being so attached together that the filter material is stressed around the cores whereby the channel member is given a considerable degree of stiffness and strength.

A further feature of the preferred embodiments is the use of a helical core which enables the channel member to be bent around a pipe or duct with a minimum of distortion of the internal cross-section of the core, the helical gaps becoming wider or narrower where the member is bent.

Yet another feature of the preferred embodiments is the ease with which the characteristics of the structure can be varied by varying the dimensions of the helical tubular core used.

In order to enable the invention to be better understood embodiments will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1{a) is a perspective view of a duct and channel member; Figure 1(b) is a partly cut-away perspective view of a part indicated at A of the arrangement shown in Figure 1(a); Figure 2(a) is a transverse section through the duct and a part of the channel member of Figure 1(a); Figure 2(b) is a transverse section through a duct and a part of a channel member terminated in a different mannerfrom that of Figure 1(a); Figure 3 is a graph illustrating co-ordinate points relating the bridging coefficient to Mo/Ds0 coefficient;; Figure 4 is a partly cut-away side view of a channel member; Figure 5 is a side view of a tubular core member; Figure 6is a space diagram plotting flow rate in litres per second per metre run against pressure in pounds per square inch (1 p/s.i = 6.89 kN/m2) and Figures 7(a) and (b) are cross-sectional and side views respectively of an embodiment incorporating a permeable spacer between tubular core members.

Referring to Figures 1(a), (b) and 2(a), there is shown a pipe 1 of a plastics material, which is either wholly water-permeable or water-permeable over only a sector which is positioned so that it can extend upwardly when in use; the pipe acting as a duct. Around the pipe 1 there is wrapped a channel member which is constituted by a plurality of tubular core members 3 arranged between two sheets of mesh filter material 5.

Each of the tubular core members 3 has a helical gap 4 extending along its length so that water passing through the sheets of filter material 5 is able to enter the tubular core members 3 and to pass down the channel member and through the water-permeable part of the pipe 1 into the pipe 1. The two sheets of mesh material 5 are stitched together in the regions between the core members 3 either over a plurality of spaced-apart regions, or continuously along the length of the channel member, as indicated at 6. It will be appreciated that the two sheets of filter material can be attached together by other means. For example by means of staples or rivets, or that the stitching could be arranged in other patterns.It will also be appreciated that a plurality of lengths of channel member can be arranged about a single length of duct and that the lengths of channel member can be spaced apart or arranged to be abutting along the length of the pipe or duct 1. It will be seen tht the ends of the core members 3 of the channel members are closed by means of tape 9 or by other suitable closure means.

In Figure 2(b), there is illustrated an alternative method of terminating the channel member. Instead of closing the ends of the core members 3 by means of a tape 9, the channel member is wrapped around the pipe 1 in such a way that it overlaps. The overlapping portion is held, for example, by means of staples, as indicated at 11.

Successive lengths of pipe 1 and of the channel member can be joined together easily end to end, for example by arranging that a length of the channel member overlaps the joint between two lengths of duct or pipe 1.

In use, a narrow trench is generally first excavated and the completed assembly is lowered in the trench to a depth at which the soil is free draining. It is possible, since the structure is very flexible, to lay the water channelling structure by means of mole draining techniques. Where deeper installations are required, deeper and wider trenches can be excavated and, after the water channelling structure has been introduced, the excavated material can be returned to the trench.

The tubular core members 3 can easily be made from lengths of thermoplastics strip material which is wound around a core and set with a suitable helical gap 4 along its length. Alternatively it can be extruded in such a way that it forms a tube with a helical gap along its length. Afurther means of manufacture of the helical core is to cut a slot around a tube along its length.

In one embodiment, the tubular core members 3 are formed from a member which has perforations or other apertures along its length. A tubular member with such perforations or other apertures can conveniently be formed of a plastics lattice or mesh-type material, which can be manufactured as a tube, and the gap can easily be formed by slitting the mesh tube helically. Preferably, the gap, whether it be a slit or a wider opening, together with the perforations or other apertures occupy between 10% and 75% of the total area of the outer surface of each tubular core member 3.

It will be appreciated that the tube with a helical gap along its length can be made in any other suitable way. The sheets of filter material 5 can be woven as a cloth from filaments or tapes or from combinations thereof of plastics or other suitable material; made from permeable sheet material, for example perforated plastics sheet material, having the required characteristics; or it can be made by the use of techniques employing melded filaments and plastics material, or in any other way.

It will be understood that, in operation, the liquid, which will commonly be water, to be drained from the surrounding soil, is able to filter through the sheet material 5 and to pass down the core members 3 into the pipe or duct 1. The pipe or duct 1 is laid with a gradient so that the liquid is carried away.

It is important that the core members 3 provide sufficient liquid carrying capacity to ensure that the liquid carries away any fine material capable of passing through the filter and that it prevents any such material from being left in the pipe or duct 1. It has been found that a particular filter material 5 in accordance with the invention which allows a liquid flow of at least 400 litres per second over 1 square meter of the surface area of the filter material under a head of 100 millimetres is suitable for the purposes of the invention.

The porosity of the filter material 5 used is determined upon the basis of the particle size distribution of the surrounding soil material to be drained. The openings in the filter material initially permit the finer particles of the adjacent soil material to pass into the pipe 1 and to be flushed through the system. Under these conditions, there will remain next to the filter material 5 a thin layer of soil which is of a much higher permeability than the mass of the surrounding soil, which remains intact. It will be understood that, in order to enable this initial flushing through of the fine material to take place, the ends of the pipe or duct 1 are left open so that water can be flushed through.

In the case of a particular saturated clean sand and gravel mixture, with the water table at ground level, it has been found that the flow of water per square metre into the filter material is approximately equal to 1 litre per second. That is, the required permeability (k) of the filter material 5 is 10-3 metres per second. For most applications, this will be sufficient and for sands with a permeability (k) of 10-4 metres per second, this value will be reduced to 0.1 litres per second per square metre of filter material. It will thus be seen that a range of filter materials which is capable of passing at least 400 litres per second per square metre of liquid under a head of only 100 millimetres would be able to cope with any of the rates of flow to be expected from normal soils.

The soil structures can conveniently be characterised by a coefficient Cb, known as the bridging coefficient, which is a ratio between dimensions of soil particles below which the soil surrounding a liquid channelling system is constituted in respective predetermined percentages by dry weight. The significant percentages are 90% and 40%. Thus if a diameter D of 90% ofthe material in the soil is less than 12.5 millimetres and the diameter D of 40% of the particles in the soil is less than 2.5 millimetres, the ratio Dgo/D40 which is the bridging coefficient Cb, will be 12.5/2.5 = 5.0.In order to relate the porosity of the filter material 5 to the soil particle size, a further coefficient Mo/DSO is used determined by a ratio of mesh opening dimension M,, which is easily determined where the filter material is a woven mesh material, to the dimension D50 of particle size below which 50% (by dry weight) of the particles in the surrounding soil fall.

The mesh opening Mo is the minimum dimension of each opening in the filter material 5 in the same units as those in which the soil particles were measured. A table of Cb against MUD50 set out below gives the co-ordinate points in the terms of these coefficients which define a curve which provides the empirically derived maximum limits of mesh opening for soils of particular bridging coefficients in accordance with the present invention.

TABLE 1 Cb Mo/D50 1.00 2.10 1.25 2.60 1.75 3.20 2.50 4.15 3.75 5.00 7.00 6.66 12.50 8.10 25.50 10.05 Referring to Figure 3, which shows at 8 a curve of bridging coefficients Cb against Mo/D50 in accordance with the figures in the Table 1, there is defined an area, between the curve 8 and the ordinate on which the bridging coefficient values Cb are set out, in which the co-ordinate points relating the bridging coefficient Cb to the ratio Mo/D50 define suitable values of mesh opening.

Thus, if the bridging coefficient Cb were 23 and the D50 value were 0.5 units, the mesh opening could be up to 5 units, since the maximum value of Mo to D50 ratio is 10. If the bridging coefficient were 5, then, it can be seen from the curve that the maximum value of the Mo to D50 coefficient is 5.7 and the mesh opening could be up to 2.85 units for a D50 size of 0.5 units.

From the figures previously given, it will be seen that the flow of liquid is limited more by the porosity of the surrounding soil than by the porosity of the filter material, since the flow per square metre through the soil is commonly between 0.1 litres per second per square metre and 1 litre per second per square metre of surface area of the interface between the soil and the filter material, while the permeability of the filter material is at least 400 litres per second per square metre.

Referring now to Figure 4, a part of a channel member is shown with a part of one of the sheets of filter material 5 cut-away to disclose a plurality of tubular core members 5 having helical gaps 4 along their length.

The core members 3 are in fact made in the particular embodiment shown from tubes of polypropylene or other materials with similar mechanical and chemical properties which are formed helically to provide the gaps along their length. The filter material 5 is a woven plastics material and the sheets of the material 5 on opposite sides of the core members 3 are stitched together, as indicated at 6. It will be noted that the dimension of the space between the core members 3 is indicated byx.

In preferred embodiments, the dimension xis between 2mm and 8mm. Arrangements having a core spacing up to 15mm have been usefully employed in tests.

Referring to FigureS, in which one of the core members 3 is illustrated, the width of the helical gap in the tubular core member is indicated by the letter a. The width of the strip of polypropylene forming the core member 3 is indicated by b. The internal diameter of the tubular core member 3 is indicated by c and the wall thickness of the tubular core member 3 is indicated by the letter d. Tubular members having dimensions in the ranges set out in Table 2 below have been found suitable to accommodate the water flow requirements for channelling arrangements in accordance with the present invention.

TABLE 2 Dimensions Range in mm.

a 2to 15 b 1 to 10 c 5 to 20 d 0.5 to 2.0 The tubular core member 3 fulfil a number of functions. They support the filter material 5 and provide a path along which water can flow to the pipe or duct 1. The width b determines the angle of the helical gap in the tubular core member 3. A narrow dimension of a produces a gap 4 of small pitch and therefore the compression strength of the core member 3 is greater as the hoop stresses at right angles to the longitudinal axis of the structure are only slightly different from those experienced on the longitudinal axis of the structure. On the other hand, the dimension of the helical gap is critical from the point of view of the capacity of water accepted into the bore of the tubular member c.The wall thickness of d of the tubular member 3 partially controls the compressive strength of the tubular member. Thus by using relatively thin material to form the helical core adequate strength is achieved to prevent crushing in situ.

By attaching the two sheets of filter material 5 together, for example by stitching or stapling, the sheets of filter material provide a further support to each of the tubular members 3 in the channel member.

As an alternative the helical tubes may be assembled independently of the filter fabric between two layers of wide mesh fabric having filaments with small extension characteristics, that is they can be stretched by only a small amount, such as glass fibre, over which the filter fabrics are laid and attached.

It will be appreciated that the dimension x shown in Figure 4 of the spacing between the tubular core members 3 has a direct effect upon the flow capacity per square metre of the channel member.

The flow capacities of the tubular core members 3 must also be capable of accommodating the water which runs-off from the surface of the soil in flood conditions, under which it is required that there be a free flow of a high volume of water through the channel member to the pipe or duct 1. This free flow capacity relative to soil pressure is shown in Figure 6, and it is a particular feature of the present invention that its flow capacities fall within the area bounded by the upper and lower curves 10 and 11 of Figure 6.

It is a feature of a preferred embodiment of the present invention that this tubular core member 3 maintains its liquid carrying capacity even when bent around the circumference of the pipe or duct 1. This characteristic is not exhibited by cores from cylindrical wire mesh or extruded plastics mesh structures which crush or kink when bent. However, a core made from a cylindrical wire mesh or other, for example extruded plastics, mesh structure is suitable, if, in accordance with the present invention, it is provided with a helical gap, for example in the form of a slit, along its length, the gap together with the meth or other apertures occupying between 10% and 75% of the surface of the wall of the tubular core member.

In Figures 7(a) and 7(b) there is shown a water-permeable spacer 7 positioned between the tubular members 3 and the two sheets of filter material 5. The spacers 7 and the sheets of filter material 5 are attached to one another by any suitable means, for example stitiching, stapling or welding. The spacers 7 which allow water to pass to the adjoining tubular members 3 have a thickness, in one embodiment, of 2 millimetres. The purpose of the spacers 7 is to ensure that a maximum amount of water is conveyed to the tubular members 3 when the assembly is placed against a sub-surface structure, such as bridge abutment or retaining wall.

On installation, the entire assembly of the core member 3 and filter material 5 is wrapped around a perforated or slotted pipe or duct 1 and fixed in position by means of push-through fasteners of plastics or other material or staples. The edge of the channel member is closed with a tape as shown at 9 in Figure 1, or by means of a deposition of a filling of plastics material, such as urethane foam or by other closure means.

In a further embodiment the water-permeable spacers 7 are omitted and the tubular members 3 are positioned between the two layers of filter material 5 at horizontal intervals of between 4mm to 8mm. As previously mentioned, the tubular members 3 are retained in position by stitching, welding, stapling, rivetting or other mechanical or chemical means, such as glueing. It will be appreciated that the embodiments described having layers of filter material 5 on each side of the channel member are suitable for drainging water from either side.

In a further embodiment, in which there is no spacer 7, a sheet which is impermeable to water, for example, polythene film or a urethane coated woven fabric is incorporated on one side of the channel member instead of one of the sheets of filter material 5, in order to form a barrier to sub-surface liquids. As previously mentioned, in a further embodiment the tubular cores 3 are assembled independently of the sheets of filter material 5 and the sheets of filter material 5 are laid over the tubular cores in a subsequent operation. In this particular embodiment, the tubular cores 3 are located between sheets of filter material 5 made of a glass fibre mesh having comparatively large-size apertures (5mm to 1 0mum), and the spaces between the tubular members 3 are stitched in the way described with reference to Figures 1 to 6.This arrangement is then assembled between two layers of filter material 5, having the characteristics previously described with reference to Figure 3, and the layers 5 are attached to the assembly of glass fibre mesh and tubular core members by means of stitching. Alternatively one of the layers of filter material 5 can be replaced by an inmpermeable sheet In yet another embodiment the tubular core members 3 are held in their relative positions by means of random glass fibre rovings and adhesive and this structure is then arranged between two layers of filter material 5 or one layer of filter material 5 and a layer of impermeable sheeting and the assembly is held together by stitching, stapling, welding or other means of positive attachment.

Although the invention has been described, by way of example, with reference to particular embodiments, it will be appreciated that variations and modifications can be made within the scope of the invention claimed.

For particular applications, one of the two filter layers 5 could be replaced by an impermeable layer, for example a sheet of plastics material. In one method of assembling the channelling member, the tubular cores, which may or may not have a circular cross-section, are first positioned between sheets of glass-fibre mesh material, which are subsequently overlaid with the layers of filter material. It is also possible to retain the tubular cores in their correct orientation by other means than the stitching 6. For example, random glass fibre rovings impregnated by an adhesive can be positioned between the cores 3 before the filter layers 5 are positioned about the cores 3.

Claims

1. An arrangement for use in channelling liquid which includes a longitudinally extending duct and a channel member which, when in use, conveys liquid to the duct, and in which the channel member includes a tubular core member made of a flexible material and capable of conducting liquid to the duct and a filter arranged to filter liquid passing to the core, the core member incorporating a helical gap extending along its length and the filter having a coefficient Mo/D50 as herein defined related to a bridging coefficient Cb as herein defined of the surrounding material as determined by the co-ordinates of a point which lies in a region on a graph between the ordinate axis, when the ordinate represents values of the bridging coefficient Cb, and a line defined by the following respective co-ordinate points of the bridging coefficient Cb and the coefficient M0/D50, 1.00, 2.10; 1.25, 2.601.75,3.20; 2.50,4.15; 3.75, 5.00; 7.00, 6.66; 12.50,8.1; 25.50, 10.05 and where Cb = Dgo/D40 where D40, D50 and D90 are the minimum dimensions of apertures in given units through which particles which constitute respectively 40%, 50% and 90% by dry weight of the material in which the filter material is to be used can pass and Mo is the minimum dimension of the openings in the filter material in the same given units.

2. A liquid channelling arrangement as claimed in claim 1 including a duct which is water permeable throughout its length, a portion of the channel member being wrapped around the duct, the ends of the core members of the channel member being closed by a closure means.

3. A liquid channelling arrangement as claimed in claim 1 including a duct which is water permeable throughout its length, a portion of the channel member being wrapped around the duct in such a way that the ends of the core members of the channel member are overlapped on the duct by the channel member.

4. An arrangement as claimed in any one of the preceding claims including a plurality of tubular core members arranged between two sheets of the filter material.

5. An arrangement as claimed in claim 3 including means attaching together the two sheets of filter material between the tubular core members.

6. An arrangement as claimed in any one of the preceding claims including a tubular core member formed from a tube of plastics mesh-type material and having a helical gap formed therein, the gap being formed by slitting the mesh tube helically, the gap together with the mesh apertures occupying between 10% and 75% of the total area of the outer surface of the tubular core member.

7. An arrangement as claimed in any one of the preceding claims including a water-permeable spacer arranged between the tubular core members in the channel member.

8. An arrangement for use in channelling liquid as claimed in claim 1, substantially as described herein with reference to Figure 3 and either Figures 1(a), (b) and 2(a) or Figure 2(b) or Figure 4 or Figure 5 or Figures 7(a) and 7(b) of the accompanying drawings.