WO2003012484A1

WO2003012484A1 - Detecting buried geosynthetic

Info

Publication number: WO2003012484A1
Application number: PCT/GB2002/003509
Authority: WO
Inventors: Anthony Jay; Peter James Jackson; David Carpenter
Original assignee: Terram Limited; Imc Geophysics Limited
Priority date: 2001-07-31
Filing date: 2002-07-31
Publication date: 2003-02-13
Also published as: GB2378250A; GB0118663D0

Abstract

A geosynthetic structure (1) is buried between a subgrade layer (5) and a ballast layer (6) of a railway track (7) and incorporates metallic stripes (33) with well-defined edges which respond to ground-probing radar to enable the presence of the stripes (33) to be remotely detected from the surface. This enables the radar velocity through the ballast layer (6) to be calculated rather than estimated and thus the depth of burial of the geosynthetic structure (1) may also be accurately calculated rather than estimated from an assumed radar velocity value.

Description

DETECTING BURIED GEOSYNTHETIC

FIELD OF THE INVENTION The invention relates to detecting buried geosynthetic. The invention may, for exam Lple, be used in the railway industry where geosynthetic is laid between the ballast and underlying subgrade of a railway line. Other possible fields of use are in constructing roads, airfields and landfill sites.

DESCRIPTION OF THE PRIOR ART It is common practice to install a sheet of geosynthetic beneath the ground in order, for example, to control the behaviour of the surrounding soil, ballast or other material, including controlling any components contained therein such as fluids, contaminants, clays etc.

A geosynthetic layer may be installed between the ballast and subgrade of a railway line. Alternative applications include laying a geosynthetic layer underneath a road or underneath an aircraft runway. Geosynthetic layers are also used in the construction of landfill sites.

Geosynthetics are frequently installed by contractors working to a contract. A contract may specify the sort of geosynthetic to be used, where the geosynthetic is to be buried, the extent of the geosynthetic and the depth of the geosynthetic. When the civil engineering works under the contract have been completed, the customer may wish to check that the contractor has complied fully with the contract.

It is already known in some respects to interact with a buried geosynthetic. For example, EP-0,418,209 discloses a geosynthetic which has perpendicular sets of wires between which a voltage is applied to detect a fault in an adjacent insulation layer, when buried underground.

FR-2,694,773 discloses a geosynthetic with a metallic conductor layer through which current is passed in order to produce a heating effect.

US-4,404,516 discloses a network of electrically-conductive wires for detecting leaks, which may be incorporated in a geosynthetic. US-5,980,155 discloses a geosynthetic with electrically-conductive filaments woven therein. For example, a stainless steel mesh may be incorporated in the geosynthetic. wO-00/39405 discloses an electrically-conductive geosynthetic for making use of the electrokinetic phenomenon to remove water, particles, ions etc. The geosynthetic may incorporate metal strips, a conducting layer or bands of conductive material in order to produce the electrokinectic effect.

With the above prior art techniques, electrical connections (i.e. electrical wires) have to be provided from the surface down to the buried geosynthetic, specifically down to the electrically-conductive component thereof. Thus with the prior art there is no remote or indirect interaction with the buried geosynthetic.

Ground-probing radar and electromagnetic survey are well known and understood. Ground-probing radar has been used in the railway context in order to detect the soil and ballast interface. In general, existing ground-probing radar technology can be used to estimate approximately the depth of a generally planar surface such as the interface between two strata (e.g. a ballast layer and an underlying subgrade layer). The radar reflects off the interface and the reflection and its timing are detected at the surface. The depth may be calculated based on the velocity of the radar pulse through the ground and the transmission duration. For different types of ground the pulse velocity will vary, and because of this variability the calculation of the depth of burial of the interface can only be an approximate estimate and will not be sufficiently accurate.

Since making the invention and filing our U.K. Patent Application No. 0118663.4, we have become aware of two extra prior art documents, from the Search Report issued by the U.K. Patent Office.

Firstly, WO-91/04503 discloses a tape which incorporates passive markers which re-emit a received electromagnetic signal. The tape is buried above a conduit and a hand-held locating device is used to remotely detect the tape. The distance between the markers on the tape may encode information. Instead of the tape, a slotted ribbon may be used with passive electronic markers which are inserted in an encoded pattern. Secondly, CA-2,204,174 discloses, according to its English abstract, a geosynthetic to which is laminated a metal sheet such as very thin aluminium foil. The depth of burial of the geosynthetic under a backfill layer may be determined with a "reflectometer".

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of remotely detecting a geosynthetic structure which is buried beneath a surface and comprises a geosynthetic layer and AC electrically-conductive material co-located with the geosynthetic layer and provided as a pattern having well-defined edges, the method comprising:- using a geophysical ground-probing technique to look beneath the surface and produce electromagnetic output from the AC electrically-conductive material; and detecting and processing the electromagnetic output in order to determine that the geosynthetic structure is present.

A suitable geophysical ground-probing technique would be ground-probing radar or electromagnetic survey.

Using the method, a customer could check that a contractor has satisfactorily complied with the terms of a contract. For example, if the contract requires that geosynthetic should be installed beneath a particular area of the surface, the geophysical ground-probing technique could be used within that area to determine that the geosynthetic is buried underneath that area. For example, by moving ground- probing radar along a line starting outside the area, passing across the area, and then passing out of the area, the electromagnetic output will first of all show that there is no buried geosynthetic structure. As the path enters the relevant surface area, the output will change to show that there is now geosynthetic structure buried below the ground. As the path exits the surface area, the output will now again show that there is no buried geosynthetic structure. By passing along various different paths crossing the surface area at different angles (such as mutually orthogonal paths), a picture could be built up as to the underground extent of the buried geosynthetic. Thus it would be possible to determine whether or not the contractor has complied with the specification of the contract. In the context of a railway line that is being laid for the first time or relaid, the contract may specify that a particular length of the railway line should have a geosynthetic layer buried therealong. The required width of the geosynthetic layer and its positioning relative to the rails of the railway line may also be specified in the contract. A ground-probing radar which has an antenna which will fit within the width of the railway track could pass along the track and detect where the buried geosynthetic layer begins and where it ends, in order to ensure compliance with the contract. The detected width of the geosynthetic layer and its positioning relative to the rails could also be detected and checked against the contract specification. When using ground-probing radar, the function of the AC electrically- conductive material is to be a target for the electromagnetic radiation emitted by the radar (at a typical frequency range of about 10MHz to 1,000MHz) and to reflect that radiation back to the radar antenna. The preferred frequency range is about 400MHz to 1,000MHz. When using the electromagnetic survey technique, the function of the AC electrically-conductive material is to act as a target which will reradiate electromagnetic radiation induced by a ground conductivity meter, with the frequency range typically being about 10kHz to 1MHz. The preferred frequency range is about lOkHz to lOOkHz.

Because the detection is done remotely, there is no need to go to the difficulty and expense of installing and maintaining the integrity of wires running up from the buried geosynthetic structure to the surface.

In other words, the remote detection is performed entirely at or above the surface by inducing the AC electrically-conductive material to radiate information up to the surface where it may be detected and processed. There is no need for any physical link or contact between the remote detection apparatus and the target geosynthetic structure. Instead, the link up from the buried geosynthetic structure to the detection apparatus at the surface is simply an intangible electromagnetic link. Thus there is no need to excavate down to the geosynthetic structure to connect electrical wires to it. The well-defined edges of the pattern of electrically-conductive material produce distinctive signatures within the detected electromagnetic output that may be used to calculate the velocity through the overburden between the surface and the geosynthetic structure. Because the velocity is known (rather than estimated as in the prior art) the depth of burial of the geosynthetic structure may also be correspondingly accurately calculated, rather than calculated using an estimated velocity value.

Along a railway line, the radar velocity through the overburden (ballast) may vary (e.g. because of different degrees of contamination of the ballast by fines or clay material, or different moisture contents) and the present invention enables the true depth of burial of the geosynthetic layer to be calculated at different positions along the railway line by using the true local radar velocity, rather than having to estimate the burial depths using an estimated common value for the radar velocity. The radar velocity is indicative of ballast condition, and thus by monitoring radar velocity over a period of time (e.g. several weeks, months or years) it is possible to detect gradual changes in ballast condition. For example, downwardly-directed radar apparatus could be carried on a railway vehicle and a length of railway line regularly scanned as the vehicle moves along the line. The calculated velocity through the ballast (and calculated burial depth) could be recorded at many positions along the railway line during a particular scan along the line. By analysing changes between successive scans, the ballast condition along the line may be monitored over time.

A well-defined edge of the pattern of electrically-conductive material produces a discernable signature characteristic in the electromagnetic output, such that the edge can be identified.

The electrically-conductive material may be layer material (e.g. metal foil) laid out within the overall area of the geosynthetic layer as smaller sub-areas (e.g. stripes) which provide the pattern having well-defined edges. Where a metal area stops, there is a (sudden) transition from radar-reflective metal to radar-permeable geosynthetic material and this (sharp) edge produces a distinctive signature in the reflected radar as the radar moves overhead.

The electrically-conductive material (e.g. foil) is preferably thinner than the geosynthetic layer to avoid an excessive increase in weight, cost and rigidity of the overall geosynthetic structure. In quantifying the degree to which the pattern of electrically-conductive material is not coterminous in extent with the geosynthetic layer, the blank or open areas outside the pattern may be 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95% of the area of the geosynthetic layer.

It is beneficial to have a range of say 5 to 80%, 10 to 70%, 20 to 60% or 30 to 50%) in order to provide a good number of preferably straight edges to interact with the ground-probing signal.

A simplest mode of using the present invention is merely to discriminate between areas of the surface beneath which the geosynthetic structure is and is not buried.

In a more sophisticated mode, the electromagnetic output is processed to determine the extent of the geosynthetic structure.

Preferably, the electromagnetic output is processed to determine the depth of burial of the geosynthetic structure.

The pattern of electrically-conductive material may be painted, printed or laminated as a series of stripes on the geosynthetic layer with each stripe having two well-defined longitudinal edges.

Accurately determining the depth of burial of the geosynthetic is useful if the installation contract specifies the depth and the client wishes to check that the contractor has complied with the contract conditions. In the railway context, the depth of the geosynthetic will be the same as the depth of the layer of overlying ballast, and the contract may specify the ballast depth needed to meet the loading and speed requirements of a particular length of track.

The electromagnetic output detected by the ground-probing radar or the ground conductivity meter will vary depending on the depth of burial of the geosynthetic structure. The nature of this variation with depth is well understood. The characteristics of the detected electromagnetic output may therefore be used to calculate the depth of burial of the geosynthetic structure.

For example, we have shown that stripes of electrically-conductive paint on an electrically-insulating geosynthetic layer placed beneath approximately 50cm of railway ballast may be readily detected by using standard ground-probing radar techniques. The depth of burial may be determined by using well-established analysis techniques which make use of the rate of change of the time of arrival of the radar reflection as the horizontal position of the radar antenna is changed. The well-defined or sharp edges of the electrically-conductive material and the fact that radar information is obtained at a number of different positions on the surface enable an accurate calculation of the burial depth to be made, unaffected by variability of the velocity of the radar pulse through the overlying ground material. To make the geosynthetic layer detectable, it is only necessary to add the electro-conductive material to the layer to act as a detectable geophysical marker. No other components need be added to make the remote detection possible.

It is by using the AC electrically-conductive material in combination with the geosynthetic layer that it is possible to look or map beneath the surface to determine or sense the extent of the geosynthetic layer that has been laid.

The geosynthetic structure is buried underground. It will usually separate two strata, such as, in the railway context, an upper stratum of ballast and a lower stratum of subgrade.

The preferred embodiments of the geosynthetic structure are flexible in addition to being planar so as to facilitate installation between the two strata when being buried.

It is the electromagnetic interaction of the detection technique with the AC electrically-conductive material that enables an indication to be produced at the surface of the presence of the geosynthetic structure, and preferably also its extent and its depth of burial. The necessary analysis of the electromagnetic output that is detected may be achieved using a computer.

In our preferred embodiments, using the geophysical ground-probing technique comprises moving geophysical ground-probing apparatus along a path on the surface, and detecting the electromagnetic output comprises detecting the electromagnetic output at a plurality of positions along the path.

For example, moving along the path comprises moving along a strip of the surface with an antenna extending over e.g. transversely of the strip.

In our preferred embodiments, processing the electromagnetic output comprises comparing the electromagnetic output against a plurality of different electromagnetic output signatures corresponding to types of geosynthetic structures having respective patterns of AC electrically-conductive material, and identifying the type of geosynthetic structure that is buried below the surface. This information may, for example, be used to verify that the contractor has installed the correct geosynthetic structure required under the contract.

For example, the electromagnetic output signatures could be coded to indicate the thickness of the geosynthetic layer with which they are associated. Alternatively, the coding could be a date such as the year of manufacture.

The electromagnetic output signatures could also indicate the sort of material of the geosynthetic layer. Combinations of the parameters could also be indicated by the coded output signatures.

Conveniently, the electrically-conductive material may be provided as different patterns on the respective geosynthetic layers. A predetermined non-regular or non- uniform pattern may be used, such as one similar to the stripes of a bar code on a product purchased in a shop. The pattern could be coded with the desired information, such as year of manufacture, the material or sort of the geosynthetic layer and the thickness of the geosynthetic layer. Apart from providing information regarding parameters of the geosynthetic structure, the patterns of electrically-conductive material will still enable the presence, extent, depth etc. of the geosynthetic structure as a whole to be determined.

The coded information will enable the regular monitoring over the years after installation. Thus the condition of the geosynthetic structure may be estimated years into the future after installation, in addition to continuing to check the extent and depth of burial of the geosynthetic structure to ensure that no undesirable changes have occurred since it was originally laid.

It is convenient to print or paint the electrically-conductive material onto the geosynthetic layer by using electrically-conductive paint (such as metallic paint) that could be applied in the desired (coded) pattern. Alternatively, the electrically- conductive material could be laminated onto the geosynthetic layer in the desired pattern.

As a web of the geosynthetic layer is produced it could have the pattern applied to it before the geosynthetic layer is rolled up for delivery to site for installation under the ground. In some embodiments, the geosynthetic structure is buried in proximity to an underground feature and the type of geosynthetic structure identifies the kind of underground feature.

For example, the geosynthetic structure may overlie the underground feature. Often, the underground feature is linear and the geosynthetic structure is also linear. For example the underground feature could be a plastic gas pipe, a plastic water pipe or a fibre-optic cable. In themselves, these may be difficult to detect as to where they are buried, and it might be necessary to perform a series of trial excavations. This is not necessary with the present invention because the geosynthetic structure is remotely detectable and thus identifies the presence and positioning of the adjacent buried underground feature.

By coding the output signature of the electrically-conductive material, the electromagnetic output information detected at the surface will indicate the nature of the underground feature. A regular or uniform pattern of the electrically-conductive material could be used. For example, stripes spaced apart every 1 metre along the length of the geosynthetic layer could indicate a gas pipe. Stripes spaced apart every 0.5 metre could indicate a water pipe, and so on. Alternatively or additionally, coding in the form of electrically-conductive bar codes as discussed above could be used as the electrically-conductive material in order to provide more information. In many embodiments, the underground feature is part of a transmission system of a buried service, such as of a public utility.

The AC electrically-conductive material might have its own structural integrity as a layer, which could be manufactured separate from the geosynthetic layer, and the two layers would be laid on top of one another when being buried, without actually being permanently connected together. However, for ease of manufacture and for greater certainty, we prefer that the AC electrically-conductive material should be integral with the geosynthetic layer, such as by means of spraying, printing or laminating the electrically-conductive material as a pattern onto the geosynthetic layer.

According to a second aspect of the present invention, there is provided a method of installing a geosynthetic structure which comprises a geosynthetic layer and AC electrically-conductive material co-located with the geosynthetic layer and provided as a pattern having well-defined edges, the method comprising:- burying the geosynthetic structure beneath a surface without providing an electrical connection from the AC electrically-conductive material to the surface.

Preferably, the geosynthetic structure is selected from a plurality of such geosynthetic structures each having a respective pattern of AC electrically-conductive material.

For example, each pattern may be non-regular in order to assist with the coding of information.

Preferably, each pattern comprises a series of stripes arranged side by side and having different widths and/or spacings. These stripes may resemble the bar code found on products in a shop.

For ease of manufacture, and for ease of transportation to the site, and then installation at the site, the geosynthetic structure is preferably a strip. In order to ensure the necessary extent of underground coverage provided by the geosynthetic, a plurality of strips (e.g. lengths cut off from a roll delivered to the site) may be buried end to end and/or side by side in order to cover the necessary extent underground. It will be usual for the edges (the longitudinal edges or end edges) to overlap. This will mean that, when sweeping the radar or electromagnetic survey over the finished installation, the buried geotextile will appear to be continuous based on the electromagnetic output received at the surface. There is no need to electrically connect together neighbouring sheet-like portions of the overall geosynthetic structure.

The geosynthetic structure can tolerate some localized damage (e.g. punctures) to the electrically-conductive material whilst still being able to produce a satisfactory electromagnetic output when interrogated by the surface apparatus. Preferably, there is an underground feature in situ, the plurality of geosynthetic structures have patterns coded to identify different kinds of underground features, the particular geosynthetic structure for burial is selected according to the particular kind of the underground feature in situ, and the selected geosynthetic structure is buried overlying the underground feature. Often, the underground feature is part of a transmission system for a buried service and the geosynthetic structure is buried extending therealong. According to a third aspect of the present invention, there is provided a geosynthetic structure remotely detectable when buried by ground-probing radar or electromagnetic survey, the geosynthetic structure comprising :- a geosynthetic layer; and AC electrically-conductive material co-located with the geosynthetic layer and provided as a pattern having well-defined edges so as not to be electrically continuous across the width and/or across the length of the geosynthetic layer.

For the preferred method of manufacturing, the geosynthetic structure is formed as a strip and the AC electrically-conductive material is integral with the geosynthetic layer.

Preferably, the pattern of electrically-conductive material defines a plurality of planar regions having marginal edge regions such that some of the edge regions extend generally longitudinally of the geosynthetic structure and some extend generally transversely of the longitudinal direction. By having a large number of well-defined edges, it is easier to determine the depth of burial. If it is known that the sweep direction of the radar or electromagnetic survey is only ever going to be in one direction, such as in the railway context where the sweep or scan will be along the railway line, then the edges could simply be provided transverse to that known sweep direction. Thus, the edges of the regions of electrically-conductive material could be provided transverse to the strip of the geosynthetic structure.

The areas or regions of electrically-conductive material forming the pattern may be entirely separate (such as the separate stripes of a bar-code type arrangement) or there could be some limited electrical interconnection between the areas as long as that does not diminish too much the edges that are needed to detect the velocity through the overburden and the depth of burial.

According to a fourth aspect of the present invention, there is provided a set of geosynthetic structures each in accordance with the third aspect of the invention, wherein the geosynthetic structures have different patterns of AC electrically- conductive material.

Often, each pattern is non-regular. Preferably, each pattern comprises a series of stripes arranged side by side and having different widths and/or spacings.

BRIEF DESCRIPTION OF THE DRAWINGS Some preferred embodiments of the methods and product according to the invention will now be described in greater detail with reference to the accompanying drawings in which: -

Fig. 1 is a side view showing two different versions of a geosynthetic structure in accordance with the present invention; Fig. 2 is a perspective view showing a geosynthetic structure installed underneath a railway line;

Fig. 3 is a perspective view showing a geosynthetic structure installed underneath a road or a runway;

Fig. 4 is a perspective view showing a geosynthetic structure installed underneath a railway line;

Fig. 5 is a perspective view showing a geosynthetic structure installed underneath a railway line;

Fig. 6 is a diagram along a mock-up of a geosynthetic structure buried underneath ballast and showing real radar data underneath the buried geosynthetic structure;

Fig. 7 is an interpretation of the situation shown in Fig. 6; Fig. 8 shows four different geosynthetic structures having different coded patterns of electrically-conductive material;

Fig. 9 is a perspective view showing geosynthetic structures installed over buried utilities;

Fig. 10 is a perspective view showing a geosynthetic structure wrapped around a buried utility; and

Fig. 11 is a diagram explaining the calculation of velocity through ballast and the calculation of burial depth.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The figures are diagrammatic in nature and should be viewed on that basis. Fig. 1 shows two geosynthetic structures 1. The upper geosynthetic structure 1 comprises a base layer 2 of geosynthetic material on the upper surface of which are printed or painted transverse marker stripes of electrically-conductive material 3. Thus the electrically-conductive material 3 is integral with the underlying geosynthetic layer 2.

The second, lower version of the geosynthetic structure 1 shown in Fig. 1 has an intermediate carrier layer 4 on which the stripes of electrically-conductive material 3 are painted. The carrier layer 4 is substantially the same size and shape as the geosynthetic layer 2. Although the layers 2, 4 are separate from one another and are not integral, the carrier layer 4 is installed on top of the geosynthetic layer 2 when the geosynthetic structure 1 is buried underground. Thus the electrically-conductive material 3 is co-located with the geosynthetic material of the layer 2. The geosynthetic structure 1 is usually in the form of a strip which may conveniently be delivered to the installation site as a roll. The two layers 2, 4 could be delivered as separate rolls, or else could be spirally rolled up together as alternating layers in a single roll.

Fig. 2 shows how a geosynthetic structure 1 in accordance with the present invention may be placed between a lower subgrade layer 5 and an upper, overlying layer of ballast 6. The ballast layer 6 functions as the track bed for a railway track 7 comprising a pair of spaced-apart rails 8 and transverse sleepers 9. In this embodiment, the geosynthetic structure 1 has diagonal parallel marker stripes 31 of electrically-conductive material. Thus the stripes 31 are orientated obliquely to the direction of the railway track 7.

During track construction or refurbishment, the sub-grade layer 5 is prepared. Then, the geosynthetic structure 1 is unrolled and laid as a strip on top of the subgrade layer. The ballast layer 6 is then placed on top of the geosynthetic structure 1. Finally, the railway track 7 is installed.

When the subgrade layer 5 is fine silty clay and becomes wet, a train passing overhead on the railway track 7 may cause a "pumping" action to occur in which fine soil from the subgrade layer 5 in the form of a slurry passes upwards through microscopic holes in the geosynthetic structure 1 so as to enter the ballast.

If this continues over a period of time and more and more soil is pumped up into the ballast, a phenomenon known as "pumping" failure can occur. This is when so much soil has passed up into and has contaminated the ballast that the structural integrity of the subgrade layer 5 is damaged and subsidence eventually will occur, with the railway track loosing its correct alignment.

Ground-probing radar apparatus may be passed along the railway track 7 to detect the existence, extent and depth of the geosynthetic structure 1 (as discussed later on in this specification). Additionally, the radar interrogation of the subsurface construction can detect the top of the clay from the subgrade which has passed up through the geotextile and now exists as a layer within the ballast. Thus, it is possible to obtain early warning of track "pumping" failure. Specifically, it is possible to see from analysing the received radar signal that soil is present above the geosynthetic structure, which is known to be the boundary between the subgrade layer 5 and the ballast layer 6. The depth and position of the geosynthetic structure 1 will show up clearly because of the strong radar reflection from the electrically-conductive material of the stripes 31. Even though the radar reflection from the clay which has been pumped up above the geosynthetic structure 1 will be a relatively weak reflection, its existence may be detected and the fact that it is above the geosynthetic structure may be observed, and be taken as a warning that pumping has occurred and that pumping failure with subsidence may occur in the near future.

In the installation arrangement shown in Fig. 3, the geosynthetic structure 1 is covered with a tarmac layer 10 of a road or a runway.

The arrangement shown in Fig. 4 differs from that of Fig. 2 in that longitudinal stripes 32 of electrically-conductive material are used rather than diagonal stripes. There are three stripes 32. The central stripe runs generally along underneath the median line of the railway track 7. The outer pair of the stripes 32 run generally along underneath the rails 8 of the railway track 7.

In the embodiment of Fig. 5, the difference relative to Fig. 2 is that transverse stripes 33 are used rather than diagonal stripes.

Fig. 6 diagrammatically shows a mock-up of a geosynthetic structure 1 buried below a surface 11. The intervening fill material 12 is 50cm of ballast. Electromagnetic detection apparatus 13 is ground-probing radar and it passes from side to side as viewed in Fig. 6 and emits downwardly-directed electromagnetic radiation 14 which interacts with the electrically-conductive material 3 of the geosynthetic structure 1.

Electromagnetic radiation 15 is returned upwards by the electrically-conductive material and is detected by the apparatus 13. Radiation paths 14 and 15 are merely illustrative and will vary for each stripe 34 as the radar antenna 13 moves. Real radar data from the mock-up is shown at 16 positioned (for the sake of illustration) below the geosynthetic structure 1 to show the correspondence between the features of the detected radar output and the constructional features of the geosynthetic structure 1. It may be seen that each of the stripes 34 is readily apparent as a generally-hyperbolic feature or signature and is thus detectable in the radar output 16.

The apparatus 13 is moved along a line across the site and detects the radar output at a plurality of different positions, in order to build up the radar depth profile 16 of the site.

Each edge of a conductive stripe 34 repeatedly imaged by the radar antenna 13 before the antenna is directly above the edge and as the antenna passes over the edge. This builds up the generally-hyperbolic signature (approximately one side of the hyperbola per stripe edge) in the radar data 16. The apex of the hyperbola is approximately central relative to the stripe 34. The shape of the hyperbola is determined geometrically and is related to the velocity through the fill material 12. Fig. 7 shows the interpretation that may be applied to the radar data 16. The depth of burial of the geosynthetic structure 1 may be calculated. It is also possible to calculate the spacing between the stripes 34 and the approximate width of each stripe.

Fig. 11 explains in more detail how the velocity and depth values for the situation shown in Fig. 6 are calculated. When the radar antenna 13 is offset a horizontal distance X from a particular conductive stripe 34 and if the propagation velocity is V and the depth of burial is D, the two-way travel time T is:-

V

In general, both V and D are unknown. The problem is solved by measuring T at a number of different values of X, and then both V and D can be calculated. The velocity of electromagnetic propagation varies with the overburden (such as railway ballast) as it mechanically degrades. Hence, repeated measurement of the velocity over time allows an estimate to be made of the mechanical condition of the ballast. Fig. 8 shows four different versions (A-D) of geosynthetic structures 1 in accordance with the present invention. Each one bears electrically-conductive material in a respective pattern. In particular, each pattern resembles somewhat a bar code of the type that is commonly applied to food products and the like. Digits or other information may be encoded by using variable widths for the individual stripes and variable spacings between successive stripes. The electrically-conductive material is shown in black. Those portions of the underlying geosynthetic layer 2 which do not bear any electrically-conductive material are shown in white.

By using the general teaching of the technique discussed in relation to Figs. 6, 7 and 11, it may be seen that passing detection apparatus 13 longitudinally along each strip of geosynthetic structure 1 would enable the spacing between the stripes 34 and the stripe widths to be determined, so that the coded information could be read.

The coding could convey many different types of information. For example, the stripes could be used to code the date of manufacture, the type of material used for the geosynthetic layer 2 of the geosynthetic structure, the thickness of the geosynthetic layer 2 or (as discussed later on) the kind of underground feature adjacent to which the geosynthetic structure is intended to be positioned when buried. For example, version A of Fig. 8 could be encoded to indicate the presence of an adjacent water pipe. Version B could be encoded to indicate the presence of an adjacent gas pipe. Version C could be coded to indicate the adjacent presence of a fibre optic cable. Version D could be encoded to indicate the presence of an adjacent electricity cable. In this way, by scanning down from the surface, it would be possible to obtain a warning that some underground feature is buried below and exactly where it is buried, so that excavations will not accidentally damage the underground feature.

As shown in Fig. 9, respective geosynthetic structures 1 bearing appropriate encoded patterns of electrically-conductive material may be laid over the respective underground features to which they correspond. Thus, when for example installing the gas pipe 17 as shown on the left-hand side of Fig. 9, the gas pipe would firstly be laid in position and then a length of geosynthetic structure 1 with the appropriate encoding of stripes 34 would be laid along on top of the gas pipe and then completely buried beneath the fill material 12.

In relation to the right-hand side of Fig. 9, a water pipe 18 would be buried, then overlain with a geosynthetic structure 1 bearing the appropriate coding on its stripes 34, prior to being covered with the fill material 12.

Passing a radar scanner over the surface would reveal the presence and nature of the buried underground utilities, namely the gas and water pipes. Preferably, the geosynthetic structure 1 is run along the full length of the relevant utility so as to provide a warning of the presence of and an identification of the nature of the utility along its full length.

Fig. 10 shows how, as applied to a structure such as a fibre optic cable 19, the geosynthetic structure 1 may be wrapped as a sheath around the cable 19. Thus the coded stripes 34 which indicate the presence of a fibre optic cable have the form of rings. The stripes 34 are shown at an exaggerated scale for reasons of clarity. The geosynthetic structure 1 may be added to the fibre optic cable 19 in the factory so as to produce a composite structure. Then the composite structure is simply buried in the ground 20 as shown in Fig. 10. Preferably, the geosynthetic structure 1 is present along substantially the full length of the fibre optic cable 19 so as to warn of the buried presence of the cable 19 along its full length as well as indicating that the underground feature is actually a fibre optic cable rather than some other utility. The geosynthetic structure 1 could perhaps be stripped away at the ends of the cable 19 to facilitate connection of the cable to other items of communication equipment.

Hitherto, it has been a common and expensive occurrence for buried fibre optic cables to be accidentally severed during roadworks and the like. This problem may be overcome with the remotely-detectable fibre optic cable shown in Fig. 10.

With the above embodiments, it may be seen that the electrically-conductive material enables the remote interrogation of the geosynthetic structure from the surface, without the need to connect electrical wires or the like down from the surface to the buried geosynthetic structure. Thus installation underground is easy and there are no wires that need to be installed running up to the surface which might be difficult and expensive to install and which might be prone to being damaged over time. The detection apparatus, such as ground-probing radar or electromagnetic survey, is able to look down underneath the surface so as to remotely detect the presence of the geosynthetic structure, together with preferably also determining other useful characteristics such as the horizontal extent of the geosynthetic structure, the depth of burial and the nature of any adjacent underground feature with which the geosynthetic structure is closely associated. Regular surveys over time will enable any movement of the geosynthetic structure to be monitored.

If the geosynthetic structure is buried sloping at an angle, such as for providing transverse drainage under a railway, the depths of different areas of the pattern of electrically-conductive material may be accurately and individually measured, so as to ensure that the slope is correct, for satisfactorily shedding into the side drains water that has passed down through the ballast.

Claims

1. A method of remotely detecting a geosynthetic structure which is buried beneath a surface and comprises a geosynthetic layer and AC electrically-conductive material co-located with the geosynthetic layer and provided as a pattern having well- defined edges, the method comprising:- using a geophysical ground-probing technique to look beneath the surface and produce electromagnetic output from the AC electrically-conductive material; and detecting and processing the electromagnetic output in order to determine that the geosynthetic structure is present.

2. A method according to Claim 1, wherein the pattern having well-defined edges comprises a series of stripes.

3. A method according to Claim 2, wherein the stripes have regular widths and spacings.

4. A method according to Claim 2, wherein the stripes have different widths and/or spacings.

5. A method according to any preceding claim, wherein processing the electromagnetic output comprises determimng the extent of the geosynthetic structure.

6. A method according to any preceding claim, wherein processing the electromagnetic output comprises determining the velocity of the electromagnetic output through the overburden layer of material under which the geosynthetic structure is buried.

7. A method according to Claim 6, further comprising repeatedly determining said velocity in order to detect any change in said velocity over time.

8. A method according to any preceding claim, wherein processing the electromagnetic output comprises determining the depth of burial of the geosynthetic structure.

9. A method according to any preceding claim, wherein using the geophysical ground-probing technique comprises moving geophysical ground-probing apparatus along a path on the surface, and detecting the electromagnetic output comprises detecting the electromagnetic output at a plurality of positions along the path.

10. A method according to Claim 9, wherein moving along the path comprises moving an antenna along a strip of the surface.

11. A method according to any preceding claim, wherein processing the electromagnetic output comprises comparing the electromagnetic output against a plurality of different electromagnetic output signatures corresponding to types of geosynthetic structures having respective patterns of AC electrically-conductive material, and identifying the type of geosynthetic structure that is buried below the surface.

12. A method according to Claim 11, wherein the geosynthetic structure is buried in proximity to an underground feature and the type of geosynthetic structure identifies the kind of underground feature.

13. A method according to Claim 12, wherein the geosynthetic structure overlies the underground feature.

14. A method according to Claim 12 or 13, wherein the underground feature is linear and the geosynthetic structure is also linear.

15. A method according to any one of Claims 12 to 14, wherein the underground feature is part of a transmission system for a buried service.

16. A method according to any preceding claim, wherein the geosynthetic structure extends along between a layer of ballast and a layer of underlying subgrade of a railway line.

17. A method according to any preceding claim, wherein the geophysical ground- probing technique is ground-probing radar.

18. A method according to Claims 16 and 17, wherein the geosynthetic structure is scanned by radar apparatus carried by a railway vehicle moving along the railway line.

19. A method according to any preceding claim, wherein the AC electrically- conductive material is integral with the geosynthetic layer.

20. A method of installing a geosynthetic structure which comprises a geosynthetic layer and AC electrically-conductive material co-located with the geosynthetic layer and provided as a pattern having well-defined edges, the method comprising:- burying the geosynthetic structure beneath a surface without providing an electrical connection from the AC electrically-conductive material to the surface.

21. A method according to Claim 20, wherein the geosynthetic structure is selected from a plurality of such geosynthetic structures each having a respective pattern of AC electrically-conductive material.

22. A method according to Claim 21, wherein each pattern is non-regular.

23. A method according to Claim 21 or 22, wherein each pattern comprises a series of stripes arranged side by side and having different widths and/or spacings.

24. A method according to any one of Claims 21 to 23, wherein there is an underground feature in situ, the plurality of geosynthetic structures have patterns coded to identify different kinds of underground features, the particular geosynthetic structure for burial is selected according to the particular kind of the underground feature in situ, and the selected geosynthetic structure is buried overlying the underground feature.

25. A method according to Claim 24, wherein the underground feature is part of a transmission system for a buried service and the geosynthetic structure is buried extending therealong.

26. A method according to any one of Claims 20 to 25, wherein the geosynthetic structure is a strip and the strip is positioned and then buried.

27. A method according to Claim 26, wherein a plurality of the strips are positioned and buried generally in series and/or in parallel.

28. A method according to Claim 27, wherein the strips have marginal edge portions which overlap.

29. A method according to any one of Claims 20 to 28, wherein the geosynthetic structure is delivered to site wound up as a roll and lengths thereof are unrolled and cut off for burial.

30. A method according to any one of Claims 20 to 29, wherein the AC electrically-conductive material is integral with the geosynthetic layer.

31. A geosynthetic structure remotely detectable when buried by ground-probing radar or electromagnetic survey, the geosynthetic structure comprising:- a geosynthetic layer; and

AC electrically-conductive material co-located with the geosynthetic layer and provided as a pattern having well-defined edges.

32. A geosynthetic structure according to Claim 31, wherein the geosynthetic structure is formed as a strip and the AC electrically-conductive material is integral with the geosynthetic layer.

33. A geosynthetic structure according to Claim 31 or 32, wherein the pattern having well-defined edges comprises a series of stripes.

34. A geosynthetic structure according to Claim 32, wherein the electrically- conductive material defines a plurality of planar regions having first marginal edge regions which extend generally longitudinally of the strip and second marginal edge regions which extend generally transversely of the strip.

35. A set of geosynthetic structures each according to any one of Claims 31 to 34, wherein the geosynthetic structures have different patterns of AC electrically- conductive material.

36. A set of geosynthetic structures according to Claim 35, wherein each pattern is non-regular.

37. A set of geosynthetic structures according to Claim 35 or 36, wherein each pattern comprises a series of stripes arranged side by side and having different widths and/or spacings.

38. A method of remotely detecting a buried geosynthetic structure, substantially as herein described with reference to the accompanying drawings.

39. A method of installing a geosynthetic structure, substantially as herein described with reference to the accompanying drawings.

40. A geosynthetic structure substantially as herein described with reference to, or with reference to and as illustrated in, the accompanying drawings.