GB2258874A - Method of forming an impervious barrier beneath a thoroughfare - Google Patents

Abstract

A method of forming an impervious barrier within or below material supporting a thoroughfare comprises applying a liquid composition to the porous material beneath the thoroughfare. The composition penetrates as a liquid and then its components react to form a substantially impervious barrier within or below the layer of porous material. Typically the composition comprises a chemical grout, miscible with water, whose components react by polymerisation into an impervious gel. The composition may be applied by simply flooding it directly onto the porous material or by controlled injection through pipes driven into the porous material at pre-determined intervals and to the required depth to provide diffusion. Primarily for use on railway tracks crossing bridges, viaducts or other supporting structures, it may be applied to structures such as aqueducts and road bridges where necessary to control water permeation from above.

Description

METHOD OF FORMING IMPERVIOUS BARRIER WITHIN OR BELOW A THOROUGHFARE This invention relates to a method of forming an impervious barrier within or below the material supporting a thoroughfare. The method is primarily for use on railway tracks crossing bridges, viaducts or other structures supporting a railway line but may be applied to structures such as aqueducts and road bridges where it is necessary to control water permeation from above.

The purpose of such an impervious barrier is to prevent or reduce water, ie rain water or in the case of aqueducts, waterway water, from penetrating into the supporting structure.

For the objective of explanation, the method will be described in relation to railway tracks but the typical, manner of installation is common to other applications.

Conventionally it has been necessary to remove existing railway tracks and ballast in order to expose the substructure and/or surface beneath the track to which a waterproofing material is to be applied. The exposed surface has to be prepared, a waterproof membrane applied thereto and then the ballast and track reinstated. Such removal and replacement of materials incurs considerable costs and involves disruption of traffic and/or work in specified periods when traffic can be diverted or when the track is not under operation. Moreover some heavily trafficked routes require lengthy lead times for work organisation and delays in contract completion cause considerable additional expense and inconvenience. Also many conventional waterproofing materials cannot be satisfactorily applied in wet weather or to a wet surface.

Conventional waterproofing materials used in such situations are provided as heavy duty sheet materials which have to be rolled into place onto the prepared surface once the track and ballast has been removed.

Alternatively a layer or layers of bituminous material may be spread onto the prepared surface -and then covered by the ballast before the railway track is replaced.

Other systems include laying rigid and semi-rigid sheets or boards and diverse coatings applied by hand or spray techniques to the prepared surface prior to reinstatement of ballast and railway lines.

The invention seeks to overcome the disadvantages of such prior art by providing a method of forming an impervious barrier beneath a thoroughfare without the need to remove railway tracks or other structures provided for carrying traffic along the thoroughfare.

According to the invention there is provided a method of forming a substantially impervious barrier within or below material supporting a thoroughfare comprising the step of: applying a composition in liquid form to the said material such that the composition penetrates into the said material, the composition comprising components which react together to form a substantially impervious barrier within or below the said material.

Preferred features of the invention will be apparent from the following description and from the subsidiary claims of the specification.

The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a cross section of typical single railway track and material supporting the track and illustrates a preferred method according to the invention of forming an impervious barrier beneath the track; and Figure 2 shows a corresponding cross section of a twin railway track when laid on concrete planks, or other structural components.

Figure 1 shows an arrangement in which a layer of ballast 1 supports a railway track 2. The layer of ballast 1 is typically about 600mm deep and is laid over a substructure or stratum la, such as the ground or other material forming a foundation for the track.

Figure 2 shows a twin railway track 2 on a bridge or other structure 3. In this case, a layer of ballast 1 supporting the track is laid on the bridge deck 3a, which may be in the form of concrete planks and which in turn are supported by girders 4.

The method comprises applying a composition in liquid form to material such as ballast 1, supporting the railway track 2. The composition penetrates into the ballast 1, whilst in liquid form and then its components react to form a substantially impervious barrier 5 within or below the layer of ballast 1.

Ballast typically comprises graded hard aggregate, which in use may become contaminated with aggregate particles ground away buy movement within the ballast layer, other fines and with fuel, oils etc. from passing traffic and so forth. The contamination will percolate to low levels within the ballast.

Under certain conditions, for example where the ballast layer 1 lacks depth and/or overall width, the liquid composition may be injected directly into the substructure or stratum la (where this is permeable) underneath the ballast 1.

Typically the composition comprises a chemical grout which is miscible with water and whose components react by polymerisation, to form an impervious barrier. This reaction process is commonly referred to as "Gelling".

In this context grout is a fluid medium capable of being- introduced into spaces, voids, interstices or fissures and which subsequently stiffens, gels or solidifies.

Gelation is defined as a point where a continuous structure with shear strength is formed.

The chemical grout should preferably be resistant to hydrocarbons to withstand spillage of diesel fuel, lubricants etc. A suitable type of chemical grout is that manufactured by Allied Colloids Ltd and marketed by Peter Town Associates under the trade name HydroGel.

HydroGel chemical grouts typically comprise separate grout and catalyst components together with accelerator and retarder additives to control gel time. The various components are mixed in pre-determined ratios to provide the appropriate gel time for the particular circumstances.

One particular composition suitable for use in this application is that marketed by Peter Town Associates under the trade name HydroGel NT3. This is a low toxicity chemical grouting system which comprises a mixture of vinyl monomers in an aqueous solution. When the individual components of the system are mixed in appropriate ratios, a polymerisation reaction occurs to produce a flexible, cohesive gel. The system is made up from the following components: PRODUCT GENERIC NAME CHEMICAL DESCRIPTION NT3A Grout (liquid) Aqueous monomer solution NT3B Accelerator '(liquid) Triethanolamine NT3C Catalyst powder Ammonium persulphate NT3D Catalyst solution Ammonium persulphate solution NT3E Retarder (solid) Potassium hexacyanoferrate 111 The HydroGel grout (NT3A) is activated prior to use by adding the appropriate amount of accelerator (NT3B).

Accelerator (NT3B) is required to initiate the reaction process whilst the addition of further appropriate amounts will accelerate gelation. Retarder (NT3E) is only added when the reaction process (gelation) is required to be significantly delayed ie typically beyond 5 minutes at 200C. NT3A is stable after the addition of accelerator, but the mix must be used within 1 to 2 days.

The HydroGel catalyst solution (NT3D) is supplied premixed or for bulk applications may be prepared from catalyst powder (NT3C).

Essentially equal volumes of activated HydroGel resin (NT3A plus NT3B and where required NT3E) and catalyst solution (NT3D) are used to form the composition. The gel reaction commences when the activated resin and catalyst solutions are mixed. Thorough blending and complete dissolution of all components is required to achieve consistent results.

With the appropriate selection of.accelerator and retarder ratios, HydroGel NT3 formulations can be designed to provide controlled gelation times from less than 1 minute to several hours. The selection of setting time is dependent on the application or stage of application. Gel time is temperature sensitive and samples will usually be mixed after addition of NT3B and/or NT3D to verify appropriate reaction times; Hydrogel NT3 is particularly suitable for this application it being a very low viscosity grout with inherent low surface tension so enabling it to freely flow through the more permeable medium and also penetrate into fine fissures and micro-porous materials. It is also of low toxicity whereas other similar compositions tend to suffer from toxicity problems.

Whatever liquid composition is used, it may be pre-mixed and applied as a single component or partially mixed and applied as separate components which mix at the point of introduction or application.

The composition may be applied by simply flooding it directly onto the ballast 1 or, preferably, by controlled injection through injection pipes 6. These are formed of steel or other sufficiently substantial material to withstand being driven into the ballast i or stratum la beneath the railway track 2 to the required depths.

The injection may be from above, below or adjacent to the track 1 as conditions dictate.

Injection pipes 6 may be temporarily or permanently provided at intervals along the track 1 appropriate to the degree of flow through the ballast 1 or sub-base la, the depth of barrier required and other conditions pertinent to the particular installation. If the pipes 6 are left in place, they may be used for "topping up" or repairing an impervious barrier formed at an earlier date.

The injection times and quantity of composition used should be sufficient to allow for dispersement of the composition through the ballast 1 or sub-structure la to form the barrier at the required depth in accord with the gelling time and other conditions appropriate to the individual installation.

Details of the method such as the ratio of components used to form the composition, the gel time, the method of applying the liquid composition to material beneath the railway track, the number and spacing of injection points, the rate of injection and quantity of composition injected may be varied to suit the particular circumstances. It will usually be necessary to conduct a number of tests beforehand to ensure the ratio of components used provides the required gel time and to ensure that the other parameters are such as to enable a satisfactory impervious barrier to be formed beneath the railway track.

The gel time should be long enough to allow sufficient diffusion of the composition throughout the ballast or sub-structure at the required level but not so long that the composition spreads more widely than required or is lost through a permeable sub-base. Gel times would typically be in the range of 30 seconds to 5 hours and probably in the range of 5 minutes to 2 hours.

Where the ballast or sub-base is physically contained within a structure, ie by bridge parapets, and where the composition will not be lost to the surrounding area and the ballast or sub-structure is suitably porous, the composition should be formulated with sufficiently retarded reaction time to allow all the calculated volume to be introduced from a single, or restricted number of points, before gelation occurs. Conversely, where excessive leakage occurs and/or there is no physical containment, the composition should be formulated with a prompt gel time to allow local, limited volume introduction, into areas of dissipation and so provide a rapid coagulation with the ballast to prevent further leakage.

These methods may be combined as dictated by site conditions, also the composition may be initially formulated for a prompt gelation and as application proceeds modified to a formulation with longer gelation periods. This will allow initial sealing of localised leaks followed by subsequent flooding of larger areas.

When using injection pipes 6 with single bottom outlets (as shown in figure 1) they may need to be withdrawn in sympathy with the rise in composition levels to aid continuity of flow.

The ballast 1 or sub-structure la must be sufficiently permeable to accept a grout. Those having greater permeabil accept larger volumes and/or thicker grouts.

Conversely, those with low permeability accept less and/or lower viscosity grouts. By employing established principles of grout and fluid flows, various theoretical criteria for the injection or impregnation process can be determined.

Methods of calculating these criteria and the formulae used will be well known to those familiar to the application of grouts to particulate mediums in other situations. However some of the factors to be considered are discussed below: Permeability is a coefficient defined by resistance to flow. Permeability of the ballast or sub-structure may be approximately calculated from the Hazen formula, equation (1), an established facility for assessing permeability of particulate soils, sands, gravels etc: DO2 K 100 (1) where K = permeability D102 = average aggregate or particle size of the ballast expressed in millimetres.

A simple, practical alternative method of assessing the extent of voiding in uncomplicated site situations is to fill a container of known volume with a representative sample of the medium to be grouted, vibrate or shake well to produce compaction, top up if necessary, then carefully add water to saturate the sample and continue adding water to completely fill the container. The water may then be carefully drained off and the amount of water recorded against the known volume of the container so providing a ratio of voiding to overall volume. This procedure can be effectual if the sample of media is reasonably dry and free from excessive contaminants and fines.

Permeability of the medium governs grout flow paths as on emergence from the confines of the injection pipe, the only potential application control is pressure, flow rate and gel time. Fluid grout will follow the line of least resistance and in variable permeability media areas of larger void size will be penetrated at the expense of finer zones.

It is typically found that from 0.5 to 300 litres of the composition is required to be applied per square metre of the area to be treated to form an impermeable barrier having a thickness of between 50 and 300 mm.

In uniform media, injected grout assumes a spherical contour radiating from the injection point and for spherical flow in uniformly porous media the time to penetrate to radius R is given by:

where n = porosity a = radius of equivalent sphere K = permeability of medium to water h = driving head Q = ratio of grout viscosity to water R = radius (based on Raffles & Greenwood 1961) From this equation charts may be prepared for differing parameter values to indicate injection time and consequently economic spacing of injection pipes.

Accurate calculations are not essential except on large projects where critical placement is required and factual information on ballast/sub-base medium is available.

When injecting into a permeable sub-structure beneath the ballast, it is necessary to assume continuous voiding with a typical, assessed, void size. The permeability may be used with a spherical flow model, for example to give penetration limits for known shear strength of grout and pressure: Radius or penetration I Yew gas (3) 2S where a = radius of mean tubular pore with same permeability as ballast h = driving head yw= density of water g = acceleration due to gravity S = shear strength (yield value) (based on Raffles & Greenwood 1961) However, where ballast or sub-structure porosity is anisotropic or variable, grout spread can differ to the calculated radius. In these circumstances continued application of large grout volumes via a single point ,of introduction may not be prudent.It may be preferable to inject small amounts and wait for gelation before repeating the process. This procedure will build up areas of impregnated matrix, as consecutively finer zones are permeated by successive applications. Repetitive use of modest grout volumes may be necessary under some conditions and initial spacing of inject'ion points may only determined by site circumstances.

The foregoing formulae relate to spherical flow as predicted in permeable media and may be applicable when injecting grout composition into permeable sub-structure la. However the concept of injecting grout composition into ballast 1 is to produce horizontal radial dispersion through the ballast with comparatively little depth This pattern may be considered as a pancake or cylindrical section.

Examples for cylindrical flow in horizontal media are devised from Darcy's Law, the law of physics relating to fluid flow rates through a permeable medium.

In the context of introducing grout component into relatively clean open ballast the following simple calculations can estimate radial grout flow from an injection point within the level of the required membrane:

where m = gel time (minutes) V = efficiency variable according to site conditions p = maximum pump output (litres/ minute) d = required membrane thickness (mm) k = % of voiding in ballast Al = cross sectional area (m2) (The variable V considers overall site factors that cannot be embraced in simple formulae. It allows for reduction of pump output to compensate for slow diffusion of grout composition through the ballast, reduction of overall pumping time to permit flushing out system to avoid grout gelation in pump and lines, it can also involve an empirically assessed factor.Under some conditions efficiency variable V will be of a low order and significantly affect working patterns) or for limited, contained areas of clean open ballast the following will evaluate gel time if the estimated volume of component can be continually pumped without loss: In X 2 X (5) p (here a variable factor X is primarily necessary to allow additional flow time for the component to permeate through the ballast before gelling. Again this factor is dependent on site conditions and is the inverse to factor V in equation 4.

The following will indicate appropriate distance between injection points where circumstances require more than one injection point

where D = distance between injection points Al cross sectional area (m2) as above (the multiplicand 0.8 is a theoretical compensation factor to provide overlap from adjacent injection poInts, and hence avert risk of grout starvation to localised zones on the periphery of the radial grout flow pattern. This value may be changed asWsite conditions dictate.

Following this formula, the spacing between injection points would typically be in the range of 0.3 to 10 metres and more often in the range of 0.5 to 3 metres.

Examples of application would be as follows: Dimensions of bridge deck (Figure 2) width 8 metres length 20 metres Extent of voiding 15% (k) Required membrane thickness 100mm (d) Extent of known leakage moderate Pump output 50 l/min (p) Gel time 20 min (m) Therefore by applying Equation 4

hence Al w 33m2 in this equation the variable V was taken as 0.5, ie an efficiency factor of 50%.

From the above calculated area of 33m2 grout spread, application of Equation 6 will indicate spacing of injection points:

hence D w 5.2 metres in this example of a bridge deck of 160m2 some 5 major injection points will be required when adopting these calculations.

A further example for an identical bridge where grout loss by leakage was insignificant and therefore longer pumping times could be used is illustrated by equation 5: m ( 50 ) 2 hence m g 96 minutes Where site tempreture or other conditions preclude accurate control of long gelation periods, reduced reaction times should be set and the number of injection points increased. The preceding formulae will establish working Preliminary verification of the calculated parameters may be proven on site by substituting water, with or without a tracer dye, for the grout composition and observing flow. Any variation of the compensation factors, or other adjustments, can then be made before actual impregnation commences.

These latter, elementary formulae only relate to fundamentally free grout flow at pumping pressure below overburden pressure. The efficiency variable can be utilised to compensate for individual site factors but should not necessarily be substituted for additional calculations that may be required for heavily contaminated, saturated, waterlogged and/or congested mediums, extended pumping distances/times, differing gel times and temperature variations.

The level of membrane formation is dependant on circumstance at each individual location but in a typical situation should preferably be formed over a compacted sub-structure la (as shown in Figure 1), a base layer of consolidated ballast or fines (not shown) or a structural member (as shown in Figure 2). The thickness of the membrane will also relate to conditions but typically will be in the range of 50mm to 300mm and preferably approximately 100mm thick.

The choice of application equipment is largely dependent on the gel time of the chosen formulation. Short gel times require twin component pumps for individually dispensing the activated resin and catalyst solutions to a mixing chamber 6a (see Figure 1) immediately prior to the point of injection/application whereas a single component pump may be used for pre-mixed compositions with long gelation times. Accurate metering of the two components is essential to ensure consistency of the final gel. Formulations with long gel times may be fully prepared in batches, batch size being determined by pumping equipment capacity and the porosity of the medium to be impregnated.

The method described also provides a much easier way of introducing an impervious or impermeable medium within or beneath areas of ballast or other material which are susceptible to water penetration than prior art methods.

In particular, the method obviates the necessity of removing railway track following the breakdown of an existing membrane or where a membrane was not originally incorporated in the construction of the track.

The composition may also be applied in wet weather or to wet material without difficulty.

Claims (1)

1) A method of forming a substantially impervious barrier within or below material supporting a thoroughfare comprising the step of: applying a composition in liquid form to the said material such that the composition penetrates into the said material, the composition comprising components which react together to form a substantially impervious barrier within or below the said material.

2) A method is claimed in claim 1 in which the composition comprises a chemical grout which is miscible in water and whose components react by polymerisation to form a gel.

3) A method as claimed under claim 2 in which the composition comprises a chemical grout sold under the trade name "HydroGel".

4) A method as claimed in claim 2 or 3 in which the composition comprises the following components: a resin in the form of an aqueous monomer solution, an initiator/accelerator, such as triethanolamine, for activating the resin and a catalyst, such as ammonium persulphate.

5) A method as claimed in claim 4 in which the composition comprises a retarder, such as potassium hexacyonoferrate 111 or for retarding the polymerisation reaction.

6) A method as claimed in any of the claims 2 to 5 in which the composition is arranged to have a gelation time within the range of 30 seconds to 5 hours and preferably in the range of 5 minutes to 2 hours.

7) A method as claimed in claim 6 in which a composition having a relatively short gelation time is initially applied followed by a composition having a longer gelation time.

8) A method as claimed in any preceding claim in which the composition is applied to the said material by flooding areas to be treated so that the composition soaks into the material.

9) A method as claimed in any preceding claim in which the composition is injected into the said material.

10) A method as claimed in claim 9 in which the composition is injected through pipes inserted into the said material and spaced from each other by a distance of between 0.3 and 10 metres.

11) A method as claimed in any preceding claim in which the composition comprises a plurality of components which are mixed together before the composition is applied to the said material.

12) A method as claimed in any claim 1 to 10 in which the composition comprises a plurality of components which are applied separately to the said material 60 as to mix together within the said material.

13) A method is claimed in any preceding claim in which from 0.5 to 300 litres of the composition is applied per square metre of the area to be treated so as to form a substantially impermeable barrier with a thickness of between 50mm and 300mm 14) A method of forming a substantially impervious barrier with or below a thoroughfare substantially as hereinbefore described and with reference to the examples and accompanying drawings.

15) Any novel feature or combination of features disclosed herein.