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Geotextiles or geomembranes in track? British railways' experience
Geotextiles and Geomembranes 3 (1986) 129-142
Geotextiles or Geomembranes in Track? British Railways' Experience D. J. Ayres British RailwaysBoard, Soil MechanicsSection,66 PorchesterRoad, London W2 6ET, UK
ABSTRACT Modern railway track is kept to a fixed geometry by an appropriate design of sub-sleeper layers, good quality ballast, a filter layer over cohesive subgrades and sufficient thickness of construction against fatigue and deformation of the subgrade. Sand of the correct grading can give permanent filtering of cohesive soil under dynamic conditions but no geotextile has been found which will behave similarly, in spite of extensive searches and tests. A geotextile can be used as a separator to permit a thinner sand layer under the ballast. The bearing capacity of subgrades of over-consolidated soils can be improved by waterproofing. This is achieved by a geomembrane of low density polyethylene film which is inserted in the middle of the sand filter for protection. 0 ver 1000 miles of this system has been installed in the past 25 years. The role of geogrids or other reinforcing techniques in the subballast layers is not yet proved to be of benefit.
1. INTRODUCTION The nature of track maintenance has changed over the last one hundred and fifty years from labour intensive methods to those using equipment incorporating the latest technology. This supposedly enables the many years of experience of the local gang of their particular patch to be replaced by a machine which can measure the necessary parameters and adjust the alignment and level accordingly. The use of continuous welded rail increases the possession time 129 Geotextiles and Geomembranes 0266-1144/86/$03"50 O Elsevier Applied Science Publishers Ltd, England, 1986. Printedin Great Britain
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required for repair if the track is removed so that in situ systems of cleaning, reballasting, waterproofing, thermal insulation and filtering have been introduced with various degrees of expense and efficiency. In industrial societies having frequent passenger services with speeds regularly well in excess of 160 km/h ( 100 mph) and allowing heavy freight in timetable intervals, any new product offering an economic remedy for a well-established problem will be welcomed with hope and healthy suspicion. Such was the case with geotextiles, where the early promise of the static filtering tests was expected, in vain, to be reflected under dynamic track conditions.
2. T R A C K B E D DESIGN To carry modern traffic the thickness and component layers of the trackbed are specified according to the design philosophy of the railway administration concerned. In very cold climates the thickness of construction to accommodate frost attack generally exceeds that required for bearing properties of the subgrade. The International Union of Railways (UIC) produced a code of practice ~ on trackbed and earthworks concerned with conventional sleepered track for high speeds and high axle loads. Various design methods exist to establish subgrade depth for a given traffic loading spectrum but, in the absence of a proven rigid mathematical solution, they depend in each case on a substantial proportion of empirical assessment. The basic data from investigations were derived either by testing samples, carrying out CBR or plate tests or by classifying the local soil system and hydrology in relation to empirical charts. Some methods specify the thickness of ballast in relation to the traffic loading and speeds and require a subbase layer giving a standard quality of modulus at its upper interface. Many years ago it was thought that pumping track was connected with the low strength of the subgrade soil; this idea still surfaces from time to time. The confusion was resolved on British Railways in the early 1950s, where many inspection trenches of old and new trackbeds, with varying filter systems or no filter systems, were evaluated. Track movement on slurried ballast seemed to be worst where the subgrade had a very high strength. Sands which could filter silts or clays without themselves becoming polluted by fine particles were sampled from track and it was noted that it
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was not necessary to have this filter layer in contact with the subgrade. As long as there is a continuous layer of the correct sand across the whole width of the track, slurry cannot rise beyond it either from the subgrade or from impregnated layers below the sand. The criterion of pumping track is that a slurry exists at or above bottom sleeper level. Slurry can exist in the ballast below this horizon with little effect, that is to say it is not the effect of lubrication nor of excess pore water pressure. These findings were used with great economy on British Railways. If the slurry derives from the subgrade soil this is called erosion pumping failure (EPF). Slurry can also occur in the upper ballast as a result of the collection of attrition products from mediocre ballast, from wind-blown deposits, brake dust, concrete sleeper erosion and dirt dropping from vehicles. These latter products form a slurry with water whether or not there is a sand blanket layer below and produce sleeper movement of the same nature as above: this is called dirty ballast pumping failure (DBPF). These pumping failures exhibit the same typical quick response to rainfall and to the onset of dry weather and have the same random pattern of sleepers affected. DBP failure is more widespread than EP failure and is being tackled by control of the grading and mineral quality of ballast and by improved methods of maintenance. The pumping failures are not related to the strength of the subgrade soil. Failures related to the bearing capacity of the subgrade have a different pattern of response to rainfall and the movement is invariably cumulative. There is a time-lag of several days after the onset of wet weather before a loss of track level occurs and this is continuous until after at least a week of dry weather. 2 It is an effective stress condition, so that measures to deflect water from the subgrade, e.g. the installation of a suitable waterproof geomembrane, form part of the remedy. As the subgrades which are liable to bearing capacity failure (BCF) are of cohesive soil, they are the same as those which would, in the presence of water, form a slurry. That is to say, in the absence of a dynamic filter layer, they would give rise to EP failure. British Rail (BR) design involves placing a polyethylene film, as a sandwich, in the middle of a layer of blanketing sand to control both EP and BC failures. The thickness of sand below the plastics film is about 100 mm for practical construction purposes but it would still function at a third of this. Above the film the sand acts as a separator to prevent puncturing by the ballast and is again at least twice as thick as the diameter
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of the largest ballast particle (50 mm). Thus, for over 25 years, BR has used a filter plus geomembrane in its standard design with a track mileage exceeding well over 1000 miles of this successful system. It has been suggested recently that sand particles of the blanket embed in the soft plastics film to provide a soil reinforcing effect but this is in doubt, as the dynamic loading and the release of load after the passage of a train would reduce this friction component. There are, of course, other less frequent causes of failure such as frost heave, peat or compressible soil, underground fires and internal erosion, which are not considered here.
3. A R R I V A L OF GEOTEXTILES The availability of geotextiles in commercial quantities about 10 years ago and the optimistic claims made for them interested BR. If the implications were true, fabrics could replace the sand and, over strong cohesive subgrades where only erosion pumping failure occurred, the logistic advantages in placing them offered a permanent remedy at great economy. Unfortunately this was not the case. The British Rail Board (BRB) Soil Mechanics Section started testing at Paddington in 1974 using a pulsating machine, and quickly found that slurry from remouided London clay could pass easily through the fabrics. At this point the possibilities were reviewed. Modern railway track is maintained to a high geometrical accuracy and the use of continuous welded rail now makes it more expensive and time consuming to carry out remedial work in the subballast layers. Such work should last at least 40 years and BR had already established that a suitable sand at any horizon in the trackbed would filter successfully under dynamic conditions for over 100 years~ and would protect the overlying ballast from the rise of slurry. The European suppliers of geotextile filters were approaching the various railway administrations with design suggestions to incorporate a sheet of geotextile sandwiched in a layer of sand. This did not replace sand and was of little advantage unless they were claiming that their geotextiles in combination with inadequate sand of the wrong grading would provide a composite layer equivalent to a good filter sand. No evidence to support such an approach was forthcoming. In North America, meanwhile, where the logistic advantage of placing a fabric quickly by mechanical equip-
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ment without removal of track was appreciated, the geotextile suppliers advocated just that---sometimes with very similar products to those in Europe alleged to need a sand layer. In this case the result would be a layer of fabric at a depth of 200 mm (preferably more) below the bottom of a sleeper resting on old dirty ballast and covered by clean ballast. If the fabric were any nearer the bottom of sleeper level the use of tamping machines (which act on the ballast to adjust sleeper height) would ravel or tear the fabric. At some BR sites the difficulty of access and of obtaining long possession of the track has made it seem worthwhile to use geotextiles as a temporary expedient which would be cost effective if erosion pumping failure could be prevented for only 10 years. Such sites will be inspected over the years to note the effectiveness of geotextiles both as a filter and as a separator. In these conditions resistance to puncturing and abrasion is important, as well as control of the pore size. A fully heat-bonded, non-woven geotextile, ICI Terram, was kept under review in track on BR and formed part of a research programme recently carried out with other railways.
4. I N T E R N A T I O N A L RAILWAY R E S E A R C H INTO GEOTEXTILES The members of the UIC Working Party, 7H14, all geotechnical engineers, carried out research into the various aspects (earthworks construction, frost effects, hydrogeology, trackbed design, drainage, dynamic loading on subgrades and filters, slope stability, maintenance machines, etc. ) under the budgetary control of ORE, the research arm of UIC. O R E subgroup Dl17/7B under the co-ordination of BR collated the laboratory research, site reports and views of all eleven co-operating railways. 3 The general properties of geotextiles, physical, chemical, method of manufacture, dimensions of fibre, etc., were considered. The known general properties can be found in publications outside the scope of this paper. 4 Particular questions such as the use of polyester fibres in alkaline environments in the vicinity of lime-stabilised soil were noted. The filtering qualities were considered both for static and dynamic conditions in relation to the established criteria and to published research.
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Companies marketing geotextiles for trackbed in Europe and North America were contacted.
4.1. Staac drminage The two approaches considered for static filtering were: (a) that based upon the classic Terzaghi filter rule which, however, does not extend to cohesive soils; (b) the application by Schober and Teindl 5 of the Cistin/Zeims criterion for granular filters to geotextiles by a programme of tests. The latter requires data concerning the particle size of the base soil and might indicate a geotextile with very small pores which may not be available commercially. The design method assumes that the uniformity coefficient of the pores of the geotextile is unity and correlates the uniformity coefficient of the soil to the quotient of the average particle size divided by the pore size. The permeability requirement would be satisfied if the permeability of the geotextile is greater than that of the base soil. There are various suggested extensions of the theories to cohesive soils using the Atterberg Limits and moisture content in addition to the particle size distribution. Some of these claim to be valid to control contact erosion, suffosion, clogging and permeability. Without commenting on their validity they require detailed evaluation of the properties of the cohesive soil for each site, whereas the engineer seeks general answers involving a small group of geotextiles. For granular static filters the answer seems to be available 6 in the form of a sand conforming to simple grading limits. It was hoped that a small range of geotextiles might be available, giving protection to most cohesive soils against contact erosion, whilst allowing particles moving as a result of suffosion to pass through without clogging. This implies that the geotextile should not be too thick. The general view of all the railway administrations, in response to a questionnaire on static filters, was that satisfactory track drains could be obtained with commercially available, filtering, non-woven geotextiles in the range 100-200 g/m 2. Higher surface density might be required for fabrics nearer the track. Less information is available for woven geotextiles and, if the Schober and Teindl approach is not possible, then for silty soils a pore size of 200/zm has been suggested.
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5. GEOTEXTILES IN THE T R A C K BED The knowledge that many fine-medium sands will permanently prevent the passage of clay slurry even under track loading led many railway administrations to try geotextiles with or without various sand layers below a thickness of 200 mm or more of new ballast. These systems were: (i) Without any overlying or underlying protective layer of sandy gravel. This case would be the most economic and easy to install but only offers a short-term life. (ii) With an underlying sand or sandy gravel layer. In this case the geotextile acts only as a separator permitting a smaller total thickness of sand. It is adequate for strong subgrades when an impermeable geomembrane is not required. This might be a useful application when there is an existing sand or ash layer of suspect qualities at the base of the old track bed. (iii) With an overlying protectivelayer of gravelly sand. Unless the gravelly sand is an adequate filter, in which case the geotextile is redundant, a permanent remedy cannot be expected but a longer life is expected than in case (i) as the fabric is not punctured or destroyed by abrasion. (iv) Between two protective layers. The expense of providing such layers would be justified if they were of filter materials, which would make the geotextile redundant, but greater benefit would derive from the installation of a geomembrane. There is only the special case where adequate granular filter material cannot be obtained at an economic cost in which this might be considered. So far B R have had few failures reported following the installation of geotextiles in track. The reports of destruction on other railways have been noted, the main site tests of which are by Raymond. Observations reported in this work 7 are on geotextiles taken from US and Canadian track, where the axle loads and maintenance of track geometry are of a different order from those in Europe. Tests on thermally bonded fabrics do not refer to materials such as the 100% bonded materials evaluated on BR (Terram) and on DB, West Germany (Lutradur). Similarly, the depth of placing has been deeper in Europe. It is generally accepted here that needle-punched geotextiles have not been used successfully in track without a protective layer. Raymond finds that a needle-punched geotextile which has been, in addition, bonded with resin, provides an
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o p t i m u m material, 7 and consequently specifies a minimum mass per unit area of 700 g/m 2 and a pore size of 40 ~m. Laboratory tests by BR on a European geotextile conforming nearly to the above have shown that it does not have very different dynamic filter properties from other fabrics and that even the grade of 750 g/m 2 offers little advantage in the control of the passage of clay particles. The B R dynamic tests were carried out on a pulsator apparatus using a 200 m m diameter cell. In order to simulate natural conditions, undisturbed samples of stiff to hard clay were procured from site by rotary coring and were installed still saturated in the cell. Remoulded material was not used. In this way the original flocculated and saturated soil structure was retained and a strong material not liable to plastic deformation was the slurry source. The displacements in the test could then be c o m p a r e d with the loss of material due to erosion as it passed through the geotextile. The geotextile was placed on the 25 mm layer of soil and sealed at the edges. On this was placed a 25 mm layer of pea gravel which was saturated to its top surface with water. A 100 mm diameter loading platen was applied to the top of the gravel and delivered a dynamic force in the range of 0 to 2.2 kN at a frequency of 3 Hz. As the number of cycles increases, the platen progresses into the pea gravel. Displacement gauges measure the progress of the platen and a graph of displacement (ram) can be plotted against some function of time. The test soil used was from the Lower Lias (Jurassic) and was an over-consolidated marine deposited calcareous clay, slightly cemented, and taken from a depth below the weathered zone. The Atterburg Limits were PL = 24, L L = 44 and 95% of material was less than 63/zm in size. Several tests had to be carried out on each fabric to achieve a high standard of repeatability using natural soil. The latter had discontinuities in some soil samples which were invariably reflected in the test behaviour. The remit was to test all geotextiles commercially available in Europe which were claimed to control or filter clay slurries under dynamic conditions. Manufacturers were invited to supply the fabric from their range which they felt would be most successful in these conditions. Fabrics from Austria, France, East Germany, West Germany, Poland, Holland, Canada, U S A and UK were examined. The majority were non-wovens. At the time no manufacturers of woven geotextiles could offer material which they considered suitable for the purpose.
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The presence of very little material greater than 60 p,m in the grading of the test clay meant that negligible sand could be trapped against the geotextile after the passage of some slurry to provide a particulate filter against further slurry movement. At the end of the test the amount of slurry which had come through the fabric was measured. The clay sample was investigated to ensure that the erosion pattern was symmetrical. This step was taken to ensure that the clay sample was not fissured before being placed in the pulsator. In several cases a wax impression of the erosion hole in the clay was taken as a check of the measurement of the weight of clay which had been eroded. The efficiency of the various geotextiles can be compared in either of two ways. For the majority of the geotextiles tested, when the vertical displacement of the platen is plotted against the square root of the number of cycles, a function emerges which approximates to a straight line over much of the graph. The gradient of this graph gives a reasonable measure of the efficiency of the geotextile. If the weight of clay which had been eroded in the course of the test is recorded at the end of the test and divided by the number of loading cycles, this quotient gives a reasonable measure of the efficiency of the geotextile. There is a reasonable correlation between these two measurements of efficiency. The main result of the pulsator test was that none of the commercially available geotextiles tested were able to filter clay under the test conditions employed--at the end of each test, clay had penetrated the pea gravel. However, it was found that the geotextiles tested gave a spectrum of efficiencies. It appears that mechanically bonded (needlepunched) geotextiles are slightly more efficient than other forms of geotextiles for filtering under the test conditions. This difference, however, is of little importance when compared with the fact that all materials failed to provide filter protection. BR is reluctant to commit resources for excavating deep inspection trenches in track to remove fabric samples for test and to make good the disturbance if the track is in good order. This does not prevent shallow trenches being excavated to inspect the rise of slurry over the geotextile when the latter is not disturbed. 5.1. Site test
However, excavations were carried out at Brentingby, Leicestershire, where the natural ground was Glacial Till. ~ Terram 3000 had been
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installed at 370 m m depth below sleeper level, directly beneath new ballast, and was investigated after 31 months in the track. It was found that the subgrade, which contained 67% of material passing the 63/zm sieve, was covered under rail by a layer of crushed stone sand. Where the geotextile was in direct contact with the subgrade, the slurry had risen to a height of 100 mm above it but no slurry was present over the crushed stone sand. Tensile tests were carried out on small samples taken across the width of the track. The results showed a small reduction of strength under each rail, implying that most of the damage sustained can be attributed to dynamic forces from the train loading. This was of the order of ten million tons per year.
6. O T H E R A P P L I C A T I O N S There can be special track conditions from time to time when geotextiles, geomembranes and geogrids may be considered, using the qualities of filtration, separation, drainage, waterproofing and reinforcement. For example, where water under an artesian head springs from a subgrade of mudstone, it would be difficult to place a simple granular filter, as the progressive rise of water coupled with the vibrations from train loading would lead to sorting of the particle sizes in the filter blanket. In this case a geotextile should be placed over the granular layer to control upward suffosion and maintain its properties. A particular problem arose when the BR system of concrete slab track ( P A C T ) was installed in a cutting over soft clay on a dout/le track. The cutting was bounded by vertical retaining walls 10 m high, which had rather shallow footings. Deep excavation to repair track would probably have reduced the passive earth resistance at the toe of the wall to an unsafe level. One of the advantages of BR slab track is that its foundation depth is less than that for conventional track and its use was necessary at this site. It was desirable to achieve a complete and p e r m a n e n t dynamic filter between the concrete slab and the underlying clay within a thickness of 115 mm and to allow construction traffic to move over it during placing within the confines of a 10 m wide site 5(X) m long. The solution adopted used 25 mm of medium fine natural sand covered by a 400 g/m 2 thermally bonded fabric. Upon this was laid 90 m m of well-compacted crushed gravelly sand of high uniformity coefficient. This was covered by a fine mesh geogrid upon which rubber-
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tyred machines could run and on whose mesh debris dropped from traffic would lodge. Immediately before placing concrete, the geogrid with its attendant rubbish was removed, leaving a clean surface of gravelly sand to receive the slab.
6.1. In-plane permeability It is debatable whether this property of geotextiles is of advantage in trackbed design. Comparison of the in-plane permeability of various geotextiles must be on the basis of the fabric having been subject to dynamic compressive loads applied normally before measurement of this property. After the fabric is installed in track beneath clean ballast, fines will migrate downwards after rainfall to cover the fabric; clay fines from below would rise to coat the undersurface and to impregnate it. The permeability would thus fall by one or two orders of magnitude. These conditions are, however, irrelevant as in those cases where prevention of access of water to the subgrade is desirable, a geomembrane will perform this task very much better. The geotextile would improve conditions only to the extent that it becomes less permeable.
7. LONG-TERM VIEW Geomembranes are already judged successful over 25 years. About 15 km of geotextiles are in track and some sites are being monitored both for effect on track and for degradation of different fabrics when used as separators. Some composite geotextiles have also been installed. Even though geotextiles are useless as a permanent replacement for a sand blanket, their delaying effect, if any, on the rise of slurry can only be considered by gathering data over a long period and taking the local hydrogeoiogical conditions into account. This information will be reviewed in a few years. The modulus values of geotextiles and geogrids compared with those of trackbed layers would seem to indicate little theoretical advantage from their reinforcement effect. However, this possibility should not be discounted since there are no adequately instrumented tracks with control sections which would provide information. There are various materials advocated to strengthen the subballast layers based upon increasing resistance to tensile forces. They include random filaments,
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random patches of mesh, strategically located geogrids and threedimensional plastic layers. Research is needed in the behaviour of these systems under saturated conditions. Not only must any reduction of deformation be noted but, also, any effect of the reinforcement in reducing the dynamic filtering properties of blanketing sand if it is e m b e d d e d within it. The development of a successful geotextile filter to work under dynamic loading must be considered in relation to the known successful sands. It has been observed many times in inspection trenches in track and in the same load cell used to evaluate geotextiles that slurry of cohesive material will penetrate from 2 mm to 4 mm into the underside of the sand layer and then stop. The sand layer used in the test cell was 25 m m in thickness. With unsuitable sands it can penetrate well over 100 m m into the layer and sometimes right through it if there is sorting of sizes during placement. Successful sands have voids of the size of 5 to 30 /zm and it is likely that the plate-like particles in the slurry advance into the sand and lock in a limiting position. This advance occurs as the pressure in the slurry rapidly rises and falls under train loading and the particles are repeatedly reorientated and presented to the voids. At the same time, the dynamic seepage forces in the slurry suspension are applied to the sand skeleton and are related to the viscosity of the slurry. These dynamic pressures are attenuated a short distance above the base of the sand layer, while the clean water in sand above the slurry penetration responds at a different viscosity to the dynamic compressive loading from the train on the sand. The thinnest sand layer seen acting as a successful filter in the track, over London clay, was 25 mm but a slurry penetration of 3 mm implies that, subject to placing restrictions, this would seem to be a limiting thickness. For a geotextile manufactured to close tolerances, application of the same argument would predicate the requirement of thickness to prevent slurry passage and that this thickness might be of the order of 10 ram. The pore size would certainly not be greater than the 40 /zm size suggested by Raymond. Double layers of commercially available geotextiles which were inspected on BR, 280 g/m 2, and on Deutches Bundesbahn, 250 g/m", showed the same lack of control of slurry m o v e m e n t but there was no deleterious effect due to slipping. The difference between a skeleton of mineral particles and that of bonded plastics filaments is an important variable and the nature of bonding, mechanical, thermal or chemical, would be as important as that of the plastics material used.
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The thickness of the layer in track will be that after compression under load and such a layer must have the same, or better, resistance to deformation as granular materials. As thickness increases, the cost and logistic advantage of using geotextiles reduces. It is possible that a fabric sufficiently thick to be an effective trackbed filter by itself would not be economic in relation to sand nor capable of being placed as easily. The use of geomembranes in the form of plastics film requires a protective layer of sand and a swift and economic method of placing this system without removal of track which has not yet been developed. If the track is not affected by erosion failure (there may be an old sub-base of thin sandy gravel or ash) but deformation is due to bearing capacity failure, then a bituminous emulsion can be applied during the passage of a ballast cleaner to seal the cut ballast subsurface. British Rail has some 200 km of this installed. 9
8. CONCLUSIONS On a strong granular subgrade with a low water table only good ballast of proper mineral, size and shape qualities is necessary. Where there are problems it is essential to identify the nature of failure of track before the choice of geotextile, geomembrane or sand layer can be made. Each material has a useful function and, in turn, the type and quality and m e t h o d of placing of each material must relate to each specific site. G e o m e m b r a n e s have a greater role than geotextiles in trackbed water control. No geotextile commercially available in the world has been found which can prevent the passage of clay and silt particles under dynamic track loading. As a separator over granular filters or used alone to inhibit the rate of rise of slurry, a fully thermally bonded non-woven geotextile is favoured. Further site inspections will be made at test sections with different materials acting as separators at shallow depths, in order to see whether this view is justified over a long period.
ACKNOWLEDGEMENT The author wishes to thank the Director of Civil Engineering, British Railways Board, for permission to publish this paper.
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1. UIC, Railway earth-works and trackbed, Bulletin No. 719, International Union of Railways, Paris (1985). 2. Ayres, D. J., The treatment of unstable slopes and railway track formations. J. Soc. Engineers, London, 2 (4)(1961) 121-38. 3. ORE, Filtration and drainage: Part 3, Use of geotextiles, Question D 117, Report No. 24, Office for Research and Experimentation of UIC, Utrecht (1983) (Unpublishedmrestricted internal distribution. ) 4. Rankilor, P. R., Membranes in Ground Engineering, J. Wiley & Sons, London, 1981. 5. Schober, W. and Teindl, H., Filter criteria for geotechnics. Proc. 7th European Conf. Soil Mech. Fndn Eng., Brighton (1978) 12 I-9. 6. Spalding, R., Selection of materials for subsurface drains. Report LR346, Transport and Road Research Laboratory, Crowthorne (1970). 7. Raymond, G., Geotextiles for railroad bed rehabilitation. Proc. 2nd Int. Conf. on Geotextiles, Las Vegas, 2 (1982) 479-84. 8. McMorrow, J., The excavation and analysis of Terrain 301~1 after 2 years 7 months in the track, BRB Soil Mechanics Section, London (1983). (Internal report, not distributed. ) 9. Ayres, D. J., Unstable track formations respond to bituminous spray treatment. Railway Gazette Int. (July 1972).
Geotextiles or Geomembranes in Track? British Railways' Experience D. J. Ayres British RailwaysBoard, Soil MechanicsSection,66 PorchesterRoad, London W2 6ET, UK
ABSTRACT Modern railway track is kept to a fixed geometry by an appropriate design of sub-sleeper layers, good quality ballast, a filter layer over cohesive subgrades and sufficient thickness of construction against fatigue and deformation of the subgrade. Sand of the correct grading can give permanent filtering of cohesive soil under dynamic conditions but no geotextile has been found which will behave similarly, in spite of extensive searches and tests. A geotextile can be used as a separator to permit a thinner sand layer under the ballast. The bearing capacity of subgrades of over-consolidated soils can be improved by waterproofing. This is achieved by a geomembrane of low density polyethylene film which is inserted in the middle of the sand filter for protection. 0 ver 1000 miles of this system has been installed in the past 25 years. The role of geogrids or other reinforcing techniques in the subballast layers is not yet proved to be of benefit.
1. INTRODUCTION The nature of track maintenance has changed over the last one hundred and fifty years from labour intensive methods to those using equipment incorporating the latest technology. This supposedly enables the many years of experience of the local gang of their particular patch to be replaced by a machine which can measure the necessary parameters and adjust the alignment and level accordingly. The use of continuous welded rail increases the possession time 129 Geotextiles and Geomembranes 0266-1144/86/$03"50 O Elsevier Applied Science Publishers Ltd, England, 1986. Printedin Great Britain
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required for repair if the track is removed so that in situ systems of cleaning, reballasting, waterproofing, thermal insulation and filtering have been introduced with various degrees of expense and efficiency. In industrial societies having frequent passenger services with speeds regularly well in excess of 160 km/h ( 100 mph) and allowing heavy freight in timetable intervals, any new product offering an economic remedy for a well-established problem will be welcomed with hope and healthy suspicion. Such was the case with geotextiles, where the early promise of the static filtering tests was expected, in vain, to be reflected under dynamic track conditions.
2. T R A C K B E D DESIGN To carry modern traffic the thickness and component layers of the trackbed are specified according to the design philosophy of the railway administration concerned. In very cold climates the thickness of construction to accommodate frost attack generally exceeds that required for bearing properties of the subgrade. The International Union of Railways (UIC) produced a code of practice ~ on trackbed and earthworks concerned with conventional sleepered track for high speeds and high axle loads. Various design methods exist to establish subgrade depth for a given traffic loading spectrum but, in the absence of a proven rigid mathematical solution, they depend in each case on a substantial proportion of empirical assessment. The basic data from investigations were derived either by testing samples, carrying out CBR or plate tests or by classifying the local soil system and hydrology in relation to empirical charts. Some methods specify the thickness of ballast in relation to the traffic loading and speeds and require a subbase layer giving a standard quality of modulus at its upper interface. Many years ago it was thought that pumping track was connected with the low strength of the subgrade soil; this idea still surfaces from time to time. The confusion was resolved on British Railways in the early 1950s, where many inspection trenches of old and new trackbeds, with varying filter systems or no filter systems, were evaluated. Track movement on slurried ballast seemed to be worst where the subgrade had a very high strength. Sands which could filter silts or clays without themselves becoming polluted by fine particles were sampled from track and it was noted that it
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was not necessary to have this filter layer in contact with the subgrade. As long as there is a continuous layer of the correct sand across the whole width of the track, slurry cannot rise beyond it either from the subgrade or from impregnated layers below the sand. The criterion of pumping track is that a slurry exists at or above bottom sleeper level. Slurry can exist in the ballast below this horizon with little effect, that is to say it is not the effect of lubrication nor of excess pore water pressure. These findings were used with great economy on British Railways. If the slurry derives from the subgrade soil this is called erosion pumping failure (EPF). Slurry can also occur in the upper ballast as a result of the collection of attrition products from mediocre ballast, from wind-blown deposits, brake dust, concrete sleeper erosion and dirt dropping from vehicles. These latter products form a slurry with water whether or not there is a sand blanket layer below and produce sleeper movement of the same nature as above: this is called dirty ballast pumping failure (DBPF). These pumping failures exhibit the same typical quick response to rainfall and to the onset of dry weather and have the same random pattern of sleepers affected. DBP failure is more widespread than EP failure and is being tackled by control of the grading and mineral quality of ballast and by improved methods of maintenance. The pumping failures are not related to the strength of the subgrade soil. Failures related to the bearing capacity of the subgrade have a different pattern of response to rainfall and the movement is invariably cumulative. There is a time-lag of several days after the onset of wet weather before a loss of track level occurs and this is continuous until after at least a week of dry weather. 2 It is an effective stress condition, so that measures to deflect water from the subgrade, e.g. the installation of a suitable waterproof geomembrane, form part of the remedy. As the subgrades which are liable to bearing capacity failure (BCF) are of cohesive soil, they are the same as those which would, in the presence of water, form a slurry. That is to say, in the absence of a dynamic filter layer, they would give rise to EP failure. British Rail (BR) design involves placing a polyethylene film, as a sandwich, in the middle of a layer of blanketing sand to control both EP and BC failures. The thickness of sand below the plastics film is about 100 mm for practical construction purposes but it would still function at a third of this. Above the film the sand acts as a separator to prevent puncturing by the ballast and is again at least twice as thick as the diameter
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of the largest ballast particle (50 mm). Thus, for over 25 years, BR has used a filter plus geomembrane in its standard design with a track mileage exceeding well over 1000 miles of this successful system. It has been suggested recently that sand particles of the blanket embed in the soft plastics film to provide a soil reinforcing effect but this is in doubt, as the dynamic loading and the release of load after the passage of a train would reduce this friction component. There are, of course, other less frequent causes of failure such as frost heave, peat or compressible soil, underground fires and internal erosion, which are not considered here.
3. A R R I V A L OF GEOTEXTILES The availability of geotextiles in commercial quantities about 10 years ago and the optimistic claims made for them interested BR. If the implications were true, fabrics could replace the sand and, over strong cohesive subgrades where only erosion pumping failure occurred, the logistic advantages in placing them offered a permanent remedy at great economy. Unfortunately this was not the case. The British Rail Board (BRB) Soil Mechanics Section started testing at Paddington in 1974 using a pulsating machine, and quickly found that slurry from remouided London clay could pass easily through the fabrics. At this point the possibilities were reviewed. Modern railway track is maintained to a high geometrical accuracy and the use of continuous welded rail now makes it more expensive and time consuming to carry out remedial work in the subballast layers. Such work should last at least 40 years and BR had already established that a suitable sand at any horizon in the trackbed would filter successfully under dynamic conditions for over 100 years~ and would protect the overlying ballast from the rise of slurry. The European suppliers of geotextile filters were approaching the various railway administrations with design suggestions to incorporate a sheet of geotextile sandwiched in a layer of sand. This did not replace sand and was of little advantage unless they were claiming that their geotextiles in combination with inadequate sand of the wrong grading would provide a composite layer equivalent to a good filter sand. No evidence to support such an approach was forthcoming. In North America, meanwhile, where the logistic advantage of placing a fabric quickly by mechanical equip-
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ment without removal of track was appreciated, the geotextile suppliers advocated just that---sometimes with very similar products to those in Europe alleged to need a sand layer. In this case the result would be a layer of fabric at a depth of 200 mm (preferably more) below the bottom of a sleeper resting on old dirty ballast and covered by clean ballast. If the fabric were any nearer the bottom of sleeper level the use of tamping machines (which act on the ballast to adjust sleeper height) would ravel or tear the fabric. At some BR sites the difficulty of access and of obtaining long possession of the track has made it seem worthwhile to use geotextiles as a temporary expedient which would be cost effective if erosion pumping failure could be prevented for only 10 years. Such sites will be inspected over the years to note the effectiveness of geotextiles both as a filter and as a separator. In these conditions resistance to puncturing and abrasion is important, as well as control of the pore size. A fully heat-bonded, non-woven geotextile, ICI Terram, was kept under review in track on BR and formed part of a research programme recently carried out with other railways.
4. I N T E R N A T I O N A L RAILWAY R E S E A R C H INTO GEOTEXTILES The members of the UIC Working Party, 7H14, all geotechnical engineers, carried out research into the various aspects (earthworks construction, frost effects, hydrogeology, trackbed design, drainage, dynamic loading on subgrades and filters, slope stability, maintenance machines, etc. ) under the budgetary control of ORE, the research arm of UIC. O R E subgroup Dl17/7B under the co-ordination of BR collated the laboratory research, site reports and views of all eleven co-operating railways. 3 The general properties of geotextiles, physical, chemical, method of manufacture, dimensions of fibre, etc., were considered. The known general properties can be found in publications outside the scope of this paper. 4 Particular questions such as the use of polyester fibres in alkaline environments in the vicinity of lime-stabilised soil were noted. The filtering qualities were considered both for static and dynamic conditions in relation to the established criteria and to published research.
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Companies marketing geotextiles for trackbed in Europe and North America were contacted.
4.1. Staac drminage The two approaches considered for static filtering were: (a) that based upon the classic Terzaghi filter rule which, however, does not extend to cohesive soils; (b) the application by Schober and Teindl 5 of the Cistin/Zeims criterion for granular filters to geotextiles by a programme of tests. The latter requires data concerning the particle size of the base soil and might indicate a geotextile with very small pores which may not be available commercially. The design method assumes that the uniformity coefficient of the pores of the geotextile is unity and correlates the uniformity coefficient of the soil to the quotient of the average particle size divided by the pore size. The permeability requirement would be satisfied if the permeability of the geotextile is greater than that of the base soil. There are various suggested extensions of the theories to cohesive soils using the Atterberg Limits and moisture content in addition to the particle size distribution. Some of these claim to be valid to control contact erosion, suffosion, clogging and permeability. Without commenting on their validity they require detailed evaluation of the properties of the cohesive soil for each site, whereas the engineer seeks general answers involving a small group of geotextiles. For granular static filters the answer seems to be available 6 in the form of a sand conforming to simple grading limits. It was hoped that a small range of geotextiles might be available, giving protection to most cohesive soils against contact erosion, whilst allowing particles moving as a result of suffosion to pass through without clogging. This implies that the geotextile should not be too thick. The general view of all the railway administrations, in response to a questionnaire on static filters, was that satisfactory track drains could be obtained with commercially available, filtering, non-woven geotextiles in the range 100-200 g/m 2. Higher surface density might be required for fabrics nearer the track. Less information is available for woven geotextiles and, if the Schober and Teindl approach is not possible, then for silty soils a pore size of 200/zm has been suggested.
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5. GEOTEXTILES IN THE T R A C K BED The knowledge that many fine-medium sands will permanently prevent the passage of clay slurry even under track loading led many railway administrations to try geotextiles with or without various sand layers below a thickness of 200 mm or more of new ballast. These systems were: (i) Without any overlying or underlying protective layer of sandy gravel. This case would be the most economic and easy to install but only offers a short-term life. (ii) With an underlying sand or sandy gravel layer. In this case the geotextile acts only as a separator permitting a smaller total thickness of sand. It is adequate for strong subgrades when an impermeable geomembrane is not required. This might be a useful application when there is an existing sand or ash layer of suspect qualities at the base of the old track bed. (iii) With an overlying protectivelayer of gravelly sand. Unless the gravelly sand is an adequate filter, in which case the geotextile is redundant, a permanent remedy cannot be expected but a longer life is expected than in case (i) as the fabric is not punctured or destroyed by abrasion. (iv) Between two protective layers. The expense of providing such layers would be justified if they were of filter materials, which would make the geotextile redundant, but greater benefit would derive from the installation of a geomembrane. There is only the special case where adequate granular filter material cannot be obtained at an economic cost in which this might be considered. So far B R have had few failures reported following the installation of geotextiles in track. The reports of destruction on other railways have been noted, the main site tests of which are by Raymond. Observations reported in this work 7 are on geotextiles taken from US and Canadian track, where the axle loads and maintenance of track geometry are of a different order from those in Europe. Tests on thermally bonded fabrics do not refer to materials such as the 100% bonded materials evaluated on BR (Terram) and on DB, West Germany (Lutradur). Similarly, the depth of placing has been deeper in Europe. It is generally accepted here that needle-punched geotextiles have not been used successfully in track without a protective layer. Raymond finds that a needle-punched geotextile which has been, in addition, bonded with resin, provides an
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o p t i m u m material, 7 and consequently specifies a minimum mass per unit area of 700 g/m 2 and a pore size of 40 ~m. Laboratory tests by BR on a European geotextile conforming nearly to the above have shown that it does not have very different dynamic filter properties from other fabrics and that even the grade of 750 g/m 2 offers little advantage in the control of the passage of clay particles. The B R dynamic tests were carried out on a pulsator apparatus using a 200 m m diameter cell. In order to simulate natural conditions, undisturbed samples of stiff to hard clay were procured from site by rotary coring and were installed still saturated in the cell. Remoulded material was not used. In this way the original flocculated and saturated soil structure was retained and a strong material not liable to plastic deformation was the slurry source. The displacements in the test could then be c o m p a r e d with the loss of material due to erosion as it passed through the geotextile. The geotextile was placed on the 25 mm layer of soil and sealed at the edges. On this was placed a 25 mm layer of pea gravel which was saturated to its top surface with water. A 100 mm diameter loading platen was applied to the top of the gravel and delivered a dynamic force in the range of 0 to 2.2 kN at a frequency of 3 Hz. As the number of cycles increases, the platen progresses into the pea gravel. Displacement gauges measure the progress of the platen and a graph of displacement (ram) can be plotted against some function of time. The test soil used was from the Lower Lias (Jurassic) and was an over-consolidated marine deposited calcareous clay, slightly cemented, and taken from a depth below the weathered zone. The Atterburg Limits were PL = 24, L L = 44 and 95% of material was less than 63/zm in size. Several tests had to be carried out on each fabric to achieve a high standard of repeatability using natural soil. The latter had discontinuities in some soil samples which were invariably reflected in the test behaviour. The remit was to test all geotextiles commercially available in Europe which were claimed to control or filter clay slurries under dynamic conditions. Manufacturers were invited to supply the fabric from their range which they felt would be most successful in these conditions. Fabrics from Austria, France, East Germany, West Germany, Poland, Holland, Canada, U S A and UK were examined. The majority were non-wovens. At the time no manufacturers of woven geotextiles could offer material which they considered suitable for the purpose.
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The presence of very little material greater than 60 p,m in the grading of the test clay meant that negligible sand could be trapped against the geotextile after the passage of some slurry to provide a particulate filter against further slurry movement. At the end of the test the amount of slurry which had come through the fabric was measured. The clay sample was investigated to ensure that the erosion pattern was symmetrical. This step was taken to ensure that the clay sample was not fissured before being placed in the pulsator. In several cases a wax impression of the erosion hole in the clay was taken as a check of the measurement of the weight of clay which had been eroded. The efficiency of the various geotextiles can be compared in either of two ways. For the majority of the geotextiles tested, when the vertical displacement of the platen is plotted against the square root of the number of cycles, a function emerges which approximates to a straight line over much of the graph. The gradient of this graph gives a reasonable measure of the efficiency of the geotextile. If the weight of clay which had been eroded in the course of the test is recorded at the end of the test and divided by the number of loading cycles, this quotient gives a reasonable measure of the efficiency of the geotextile. There is a reasonable correlation between these two measurements of efficiency. The main result of the pulsator test was that none of the commercially available geotextiles tested were able to filter clay under the test conditions employed--at the end of each test, clay had penetrated the pea gravel. However, it was found that the geotextiles tested gave a spectrum of efficiencies. It appears that mechanically bonded (needlepunched) geotextiles are slightly more efficient than other forms of geotextiles for filtering under the test conditions. This difference, however, is of little importance when compared with the fact that all materials failed to provide filter protection. BR is reluctant to commit resources for excavating deep inspection trenches in track to remove fabric samples for test and to make good the disturbance if the track is in good order. This does not prevent shallow trenches being excavated to inspect the rise of slurry over the geotextile when the latter is not disturbed. 5.1. Site test
However, excavations were carried out at Brentingby, Leicestershire, where the natural ground was Glacial Till. ~ Terram 3000 had been
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installed at 370 m m depth below sleeper level, directly beneath new ballast, and was investigated after 31 months in the track. It was found that the subgrade, which contained 67% of material passing the 63/zm sieve, was covered under rail by a layer of crushed stone sand. Where the geotextile was in direct contact with the subgrade, the slurry had risen to a height of 100 mm above it but no slurry was present over the crushed stone sand. Tensile tests were carried out on small samples taken across the width of the track. The results showed a small reduction of strength under each rail, implying that most of the damage sustained can be attributed to dynamic forces from the train loading. This was of the order of ten million tons per year.
6. O T H E R A P P L I C A T I O N S There can be special track conditions from time to time when geotextiles, geomembranes and geogrids may be considered, using the qualities of filtration, separation, drainage, waterproofing and reinforcement. For example, where water under an artesian head springs from a subgrade of mudstone, it would be difficult to place a simple granular filter, as the progressive rise of water coupled with the vibrations from train loading would lead to sorting of the particle sizes in the filter blanket. In this case a geotextile should be placed over the granular layer to control upward suffosion and maintain its properties. A particular problem arose when the BR system of concrete slab track ( P A C T ) was installed in a cutting over soft clay on a dout/le track. The cutting was bounded by vertical retaining walls 10 m high, which had rather shallow footings. Deep excavation to repair track would probably have reduced the passive earth resistance at the toe of the wall to an unsafe level. One of the advantages of BR slab track is that its foundation depth is less than that for conventional track and its use was necessary at this site. It was desirable to achieve a complete and p e r m a n e n t dynamic filter between the concrete slab and the underlying clay within a thickness of 115 mm and to allow construction traffic to move over it during placing within the confines of a 10 m wide site 5(X) m long. The solution adopted used 25 mm of medium fine natural sand covered by a 400 g/m 2 thermally bonded fabric. Upon this was laid 90 m m of well-compacted crushed gravelly sand of high uniformity coefficient. This was covered by a fine mesh geogrid upon which rubber-
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tyred machines could run and on whose mesh debris dropped from traffic would lodge. Immediately before placing concrete, the geogrid with its attendant rubbish was removed, leaving a clean surface of gravelly sand to receive the slab.
6.1. In-plane permeability It is debatable whether this property of geotextiles is of advantage in trackbed design. Comparison of the in-plane permeability of various geotextiles must be on the basis of the fabric having been subject to dynamic compressive loads applied normally before measurement of this property. After the fabric is installed in track beneath clean ballast, fines will migrate downwards after rainfall to cover the fabric; clay fines from below would rise to coat the undersurface and to impregnate it. The permeability would thus fall by one or two orders of magnitude. These conditions are, however, irrelevant as in those cases where prevention of access of water to the subgrade is desirable, a geomembrane will perform this task very much better. The geotextile would improve conditions only to the extent that it becomes less permeable.
7. LONG-TERM VIEW Geomembranes are already judged successful over 25 years. About 15 km of geotextiles are in track and some sites are being monitored both for effect on track and for degradation of different fabrics when used as separators. Some composite geotextiles have also been installed. Even though geotextiles are useless as a permanent replacement for a sand blanket, their delaying effect, if any, on the rise of slurry can only be considered by gathering data over a long period and taking the local hydrogeoiogical conditions into account. This information will be reviewed in a few years. The modulus values of geotextiles and geogrids compared with those of trackbed layers would seem to indicate little theoretical advantage from their reinforcement effect. However, this possibility should not be discounted since there are no adequately instrumented tracks with control sections which would provide information. There are various materials advocated to strengthen the subballast layers based upon increasing resistance to tensile forces. They include random filaments,
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random patches of mesh, strategically located geogrids and threedimensional plastic layers. Research is needed in the behaviour of these systems under saturated conditions. Not only must any reduction of deformation be noted but, also, any effect of the reinforcement in reducing the dynamic filtering properties of blanketing sand if it is e m b e d d e d within it. The development of a successful geotextile filter to work under dynamic loading must be considered in relation to the known successful sands. It has been observed many times in inspection trenches in track and in the same load cell used to evaluate geotextiles that slurry of cohesive material will penetrate from 2 mm to 4 mm into the underside of the sand layer and then stop. The sand layer used in the test cell was 25 m m in thickness. With unsuitable sands it can penetrate well over 100 m m into the layer and sometimes right through it if there is sorting of sizes during placement. Successful sands have voids of the size of 5 to 30 /zm and it is likely that the plate-like particles in the slurry advance into the sand and lock in a limiting position. This advance occurs as the pressure in the slurry rapidly rises and falls under train loading and the particles are repeatedly reorientated and presented to the voids. At the same time, the dynamic seepage forces in the slurry suspension are applied to the sand skeleton and are related to the viscosity of the slurry. These dynamic pressures are attenuated a short distance above the base of the sand layer, while the clean water in sand above the slurry penetration responds at a different viscosity to the dynamic compressive loading from the train on the sand. The thinnest sand layer seen acting as a successful filter in the track, over London clay, was 25 mm but a slurry penetration of 3 mm implies that, subject to placing restrictions, this would seem to be a limiting thickness. For a geotextile manufactured to close tolerances, application of the same argument would predicate the requirement of thickness to prevent slurry passage and that this thickness might be of the order of 10 ram. The pore size would certainly not be greater than the 40 /zm size suggested by Raymond. Double layers of commercially available geotextiles which were inspected on BR, 280 g/m 2, and on Deutches Bundesbahn, 250 g/m", showed the same lack of control of slurry m o v e m e n t but there was no deleterious effect due to slipping. The difference between a skeleton of mineral particles and that of bonded plastics filaments is an important variable and the nature of bonding, mechanical, thermal or chemical, would be as important as that of the plastics material used.
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The thickness of the layer in track will be that after compression under load and such a layer must have the same, or better, resistance to deformation as granular materials. As thickness increases, the cost and logistic advantage of using geotextiles reduces. It is possible that a fabric sufficiently thick to be an effective trackbed filter by itself would not be economic in relation to sand nor capable of being placed as easily. The use of geomembranes in the form of plastics film requires a protective layer of sand and a swift and economic method of placing this system without removal of track which has not yet been developed. If the track is not affected by erosion failure (there may be an old sub-base of thin sandy gravel or ash) but deformation is due to bearing capacity failure, then a bituminous emulsion can be applied during the passage of a ballast cleaner to seal the cut ballast subsurface. British Rail has some 200 km of this installed. 9
8. CONCLUSIONS On a strong granular subgrade with a low water table only good ballast of proper mineral, size and shape qualities is necessary. Where there are problems it is essential to identify the nature of failure of track before the choice of geotextile, geomembrane or sand layer can be made. Each material has a useful function and, in turn, the type and quality and m e t h o d of placing of each material must relate to each specific site. G e o m e m b r a n e s have a greater role than geotextiles in trackbed water control. No geotextile commercially available in the world has been found which can prevent the passage of clay and silt particles under dynamic track loading. As a separator over granular filters or used alone to inhibit the rate of rise of slurry, a fully thermally bonded non-woven geotextile is favoured. Further site inspections will be made at test sections with different materials acting as separators at shallow depths, in order to see whether this view is justified over a long period.
ACKNOWLEDGEMENT The author wishes to thank the Director of Civil Engineering, British Railways Board, for permission to publish this paper.
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1. UIC, Railway earth-works and trackbed, Bulletin No. 719, International Union of Railways, Paris (1985). 2. Ayres, D. J., The treatment of unstable slopes and railway track formations. J. Soc. Engineers, London, 2 (4)(1961) 121-38. 3. ORE, Filtration and drainage: Part 3, Use of geotextiles, Question D 117, Report No. 24, Office for Research and Experimentation of UIC, Utrecht (1983) (Unpublishedmrestricted internal distribution. ) 4. Rankilor, P. R., Membranes in Ground Engineering, J. Wiley & Sons, London, 1981. 5. Schober, W. and Teindl, H., Filter criteria for geotechnics. Proc. 7th European Conf. Soil Mech. Fndn Eng., Brighton (1978) 12 I-9. 6. Spalding, R., Selection of materials for subsurface drains. Report LR346, Transport and Road Research Laboratory, Crowthorne (1970). 7. Raymond, G., Geotextiles for railroad bed rehabilitation. Proc. 2nd Int. Conf. on Geotextiles, Las Vegas, 2 (1982) 479-84. 8. McMorrow, J., The excavation and analysis of Terrain 301~1 after 2 years 7 months in the track, BRB Soil Mechanics Section, London (1983). (Internal report, not distributed. ) 9. Ayres, D. J., Unstable track formations respond to bituminous spray treatment. Railway Gazette Int. (July 1972).