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Within the rail sector the use of geosynthetics has become fundamental in solving anincreasingly diverse range of geotechnical and environmental problems associated withtrack construction and rehabilitation.

When correctly specified and installed, the use of geosynthetics within permanent-wayconstruction has been proven to enhance trackbed performance and significantly extendoverall design life.

There are four major functions that geosynthetics fulfil when they are employedwithin, beneath and around ballast and sub-ballast layers: separation, filtration,drainage and reinforcement/stabilisation. The functions of separation and filtrationare often considered as singular. Geosynthetics can replace the functionality oftraditional construction materials whilst providing significant construction savingsand increased speed of installation.

The ongoing impact of changing construction and the requirement to deliver sustainabledevelopment is increasingly creating the need for more diverse and innovativegeosynthetics solutions across all aspects of rail construction.

WHY GEOSYNTHETICS?

WHY TERRAM?Terram is a market leader in the design and manufacture ofinnovative geosynthetics, providing a unique range of engineeredsolutions to a diverse range of applications within civilengineering.

We are a leading supplier of approved geosynthetics to the railindustry. Through the knowledge and experience acquired byworking alongside leading designers and contractors within thePermanent Way sector we developed the PW range of products.

A key factor in our long term success within the geosyntheticindustry is that we remain open to new ideas and technologies.Terram are proud of its team’s ability to innovate and developspecific product solutions through jointly discussing problematicconditions with clients.

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TRACK STRUCTURE COMPONENTS

Crib (between sleepers)

Sleeper

BallastSub-Ballast

Made Ground

Natual Ground

ShoulderTop BallastBottom Ballast

Subgrade level

Formation level

The components in Figure 1 that relate to the selection ofgeosynthetics used in the track structure are ballast, sub-ballast(blanket) and sub-grade. These components are brieflydiscussed below:

Ballast Material

Ballast is usually formed from crushed very strong rocke.g igneous granite. The size of particles are controlled bycrushing and sieving – for example, particles are sized andgraded between 13mm and 38mm.

Other required characteristics include high abrasion resistance,crush resistance, impact resistance, frictional surface texture,appropriate shape (e.g. not flat stones), frost resistance, resist-ance to weathering etc.

Ballast is generally an expensive material to buy and replace.

Functions of Ballast

In Permanent Way construction the ballast is required to help witha number of structural, environmental and maintenance issues.

Ground InvestigationTo maintain track alignment it is necessary for the track ballastto resist a number of forces through it including vertical, lateraland longitudinal forces.

As the load is carried the ballast ‘spreads’ the load downwardsand outwards from the sleepers in order to reduce the pressureon the sub-grade soil. The depth of ballast needs to be greatenough, depending on the soil type, to reduce deflection ofthe sub-grade to acceptable levels and thereby avoid bearingcapacity failure.

The ballast helps to absorb the energy from the train passingabove. This is particularly important at joints in the track wherethe impact of wheels on the joint cause shock waves to traveldownwards and possibly cause damage to the sub-grade.

Weather Resistance

The grading of the ballast means there are large voids withinthe structure. These large voids enable the water to be quicklydrained from around the sleepers and track reducing the poten-tial for degradation.

These large voids also mean that clean ballast is not susceptibleto frost heave, in addition to this it also provides thermal insu-lation for the ground below.

Ballast is very slow to deteriorate due to weathering.

MaintenanceThe large, angular, and rough ballast particles facilitateadjustment/realignment of the track by ‘tamping’ therebyextending the life of the Permanent Way structure and reducingpossession time.

The large voids in the ballast provide storage space for the silt,which results from the degradation of the ballast. This prolongsthe life of the ballast.

Other IssuesThe ballast helps to absorb noise from the train running on thetrack, it provides electrical resistance between the rails and inhibitsthe growth of vegetation

Sub-ballast Material

The sub-ballast comprises durable particles of sand and gravel.In order to prevent penetration by fine silt/clay sub-grades, it isnecessary for the sub-ballast to have a fine sand componentand to prevent penetration by the larger ballast it requires thegravel component.

To provide a dense and stable material the particle size distributionshould be ‘well graded’.

As with ballast, sub-ballast can be an expensive material to buyand is not always readily available.

Figure 1: Sectional view through typical rail structure

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Functions of Sub-Ballast

There are a number of functions performed by the sub-ballast thatare in common with ballast. The sub-ballast continues to spreadthe load down and out from the ballast above in order to reducethe pressure on the sub-grade soil.

To avoid bearing capacity failure and to ensure the deflection onthe sub-grade remains within acceptable levels, the combineddepth of ballast and sub-ballast needs to be of sufficient depth.This depth will be dependent on the soil type and local conditions.

The sub-ballast material is not prone to frost heave and adds tothe protection of the sub-grade soil provided by the ballast.

In addition there are number of functions that are specific to thesub-ballast. It prevents sub-grade attrition by the ballast particularlyon ‘hard’ sub-grades such as mudstones, very stiff/hard silty clays etc.

In terms of water control the sub-ballast acts as a ‘filter’ for rainand ground water flow and intercepts the rainwater percolatingdown through the ballast and due to its relatively low permeabilitycompared with the ballast, sheds the great majority of the waterto the side drains.

When the loading on a saturated fine soil (e.g. silts and clay) is in-creased, the ‘pore water pressure’ within the soil increases. If thisload is maintained, the ‘pore water’ in the soil is slowly expelled ina process called ‘pore water pressure relief’. As the ‘pore water’ isexpelled, the fine soil ‘consolidates’.

This is particularly significant in clay-based soils. As the sub-ballastis more permeable than the fine soil below, the relatively smallquantity of water under pressure can escape upwards into thesub-ballast. This ‘pore water pressure relief’ process enhances thestrength and stability of the sub-soil and hence the track structure.It is important to understand for impermeable geosynthetics inPermanent Way (not currently universally accepted as a solution)that the volumes of water flow concerned are very small.

The foundation soil’s ‘engineering properties’ can vary due to theeffect of ground and surface water conditions. For example:

• Excessive ground water in the track foundation reduces soil stability. This can occur if there is a high water table just below ground level. Artesian water pressure can occur if a railway trackis placed in a cutting that goes through a water table. This may result in a continuous flow of water through the track bed.

• Surface water in combination with the abrasive action of ballast, caused by a train passing overhead, can reduce a mud stone sub grade to slurry.

It is highly desirable to have knowledge of the ground soil and waterconditions to avoid problems during construction and operation ofthe track, hence the need for a complete ground investigation.

It is clear that there is unlikely to be a single ‘geosynthetic’ solutionfor all situations. This fact has been the driving force behind thedevelopment of the PW range of products. There are now a numberof geosynthetic products available from Terram. Each one is targetedat a specific combination of soil types and ground water conditionsand it is important to have knowledge of the ground conditions sothat the most appropriate solution can be selected.

Method of Ground Investigation

Methods of Ground Investigation are well documented elsewherebut may include, for example, the following:

There is the traditional method of investigation including a deskstudy of local geology, past track performance, previous nearbyground investigations, and seeking local knowledge.

This can be followed up with trial pitting to determine precise localground conditions perhaps supplemented by boreholes whereproblems at depth may be suspected.

There are further ground investigation techniques increasinglyavailable from specialist firms: Automatic Ballast Sampling can be amore rapid alternative to trial pitting. It is recommended that a wayof sealing boreholes is considered, in order to avoid the possibilityof leaving a series of punctures in any existing sub-ballast orgeosynthetic. It is suggested that sealing holes with a bentonite/cement/sand grout might be considered.

A non-invasive Ground Probing Radar survey can be used to obtaina continuous profile of an existing sub-grade/ballast interface – auseful tool to help analyse and plan maintenance needs.

Field Identification and Description of Soils

Although information on soil type is well documented elsewhere,for ease of reference the following three tables of soils informationhave been included.

Ground Investigation

The need for a Ground Investigation

The most variable and least controllable aspect of a track structureis the foundation soil. The foundation soil dictates the engineeringproperties of the ground. Typical examples are as follows:

Very stiff over-consolidated clay provides a good building platform.Most of the water in its structure has been squeezed out resultingin a material that is hard to indent with your nail.

Contractors dislike soil material that is difficult to compact. Loosewet silty sand is difficult to compact, e.g. Alluvium soil, found inriver flood plains, is a very soft soil that can be squeezed throughthe fingers. This material provides a poor building platform.

Peat is an organic soil material; over time it decomposes andcauses differential settlements. As a result peat provides a verypoor building platform.

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Fig. 3: Basic Soil Descriptions based on BS 5930

(British Standard 5930:1999 Section 6 should be referred to for further detail)

Mixed soil types

Very course soil

Course SoilsOver 65% sand andgravel sizes with siltclay

Can only be seen complete in trial pits

Often difficult to recover from boreholes

Particle shapes can be described e.g. angular typical ofcrushed gravel or rounded typical of dredged gravel.Can be well graded, uniformly graded or gap gradedbased upon particle size distribution.

Particle grading can be similarly described as forgravel above.Individual particles of fine sand are visible to thenaked eye.Sand possesses no cohesion when dry.

Individual particles of coarse silt are so small as tobe barely visible to the naked eye.Silt possesses some cohesion but powders easilybetween the fingers when dry.Silt disintegrates in water.

Clays and silts frequently occur as a mix and laboratoryanalysis is required to determine the proportions.Clay is cohesive. It can be broken into lumps when drybut not powdered. It softens when wetted and sticksto the fingers. It shrinks when drying and usuallyshows cracks.Clay will slowly disintegrate in water.

Fine SoilsOver 35% silt andclay sizes with sandand gravel

Boulders >200

Cobbles 200

course 60

Gravel medium 20

fine 6

course 2

Sand medium 0.6

fine 0.2

course 0.06

Silt medium 0.02

fine 0.006

Clay/Silt

Clay 0.002

Principle soil types Particle SizesMax (mm)

Some simple characteristics to aid identificationin the field

Terram PW1: Pore size, O90wet - 0.085mm

Fig. 4: Strength of Fine Soils

STRENGTH OF FINE SOILS ie: soils containing over 35% of silt and clay sizes.

Soil Strength Description Manual Test for Estimated StrengthsShear StrengthCu - kN/m2

Finger easily pushed into ground up to 25mm.Sample exudes between fingers when squeezed.Very Soft 0 - 20

Finger pushed into ground up to 10mm.Sample moulded by light finger pressure.Soft 20 - 40

Thumb can make an impression easily.Sample moulded by strong finger pressure.Firm 40 - 75

Can be indented slightly by thumb.Sample cannot be moulded by fingers.Stiff 75 - 150

Can be indented slightly by thumb.Sample cannot be moulded by fingers.Very Stiff 150 - 300

Can be indented slightly by thumb.Sample cannot be moulded by fingers.

Hard Clay >300(Alternative rock description:Very weak mudstone)

Fig. 5: Density of Coarse Soils

DENSITY OF COARSE SOILS ie: soils containing over 65% sand and gravel sizes.

Soil Density Description Boreholes with SPT N- values

Very Loose

Loose

Medium Dense

Dense

Very Dense

0 - 4

4 -10

10 - 30

30 - 50

>50

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Geosynthetics in Rail Construction

Geotextiles as an alternative to sub-ballast

Granular sub-ballast, as previously discussed, can be an expensivematerial to buy and is not always easy to obtain. Furthermore, it islabour intensive and time consuming to lay and in today’s pressuredenvironment of ‘track possessions’ engineers are constantly lookingfor cost and time effective alternatives.

PW1 has been used in the rail industry as a filter separator betweenballast and sub-grade, for at least 25 years. Discussions withPermanent Way engineers on the use of geotextiles revealed a wholerange of views. Some engineers found that geotextiles workedeffectively and trial excavations 8 to 10 years after installationrevealed that the geotextile was in good condition and wasexpected to go on working for a good while longer. Other engineersfound that ‘pumping’ had occurred (discussed later) and the soilhad passed through the geotextile and contaminated the ballast.

The explanation for the variety of reported experience is based on soiltypes and water conditions. It is clearly important to select a geosyntheticsolution appropriate to the soil conditions and a rationale for this isoffered below

Over Cohesive Soils, Mudstones and Shales

Reference is made to Table 1, Soil Descriptions based on BS5930,where it is stated that silt ‘disintegrates in water’. It is similarly statedthat clay ‘will slowly disintegrate in water’. Although this applies to lumpsamples the same behaviour will occur in unconfined intact material.

Clean ballast allows rainwater to percolate downwards. If the sub-gradebeneath is a silt or clay then clearly the potential exists for such soilsto disintegrate.

The load and vibration from a train will cause agitation and acceleratethis disintegration. Under the pressure and deflection in the groundcaused by a train passing over head, the ‘slurry’ formed by the siltand clay in water can be ‘pumped’ up into the ballast. This ‘pumping’or ‘wet-bedding’ failure is all too familiar to track engineers.

It might be thought that the clay soils would disintegrate morequickly than weak clay rock sub-grades. In fact it is reported thatthe abrasive action of ballast on clay rich mudstones and shalescan lead to their more rapid disintegration.

The consequences of ‘pumping failure’ are:

I. Ground loss from the sub-grade i.e. subsidenceII. Loss of track alignmentIII. Contamination of ballast leading to a reduction in its functional

propertiesIV. High maintenance costs.

A geotextile used directly over ground susceptible to pumping failurewill not work as effectively as sub-ballast for the reasons given below:

I. The maximum particle size of a silt particle is 0.06mm and a clay particle has a typical diameter of 0.02mm whereas a ‘typical’geotextile will have a mean pore opening size larger than this. Asa result individual particles of silt and clay are small enough to pass through the pores of any geotextile. This can seen in Figure 3.

II. In relatively ‘static’ and dry conditions, for instance under a road sub-base, the cohesive nature of silty clay allows the soil to bridge over the pores in the geotextile. The geotextile can perform itsfilter/separation function well.

III. The situation under rail ballast is a special case because of the ‘pumping’ conditions. Silt and clay particles suspended in water can be ‘pumped’ through the pores in any geotextile.

This ‘pumping’ failure can be prevented by use of a suitable wellgraded sand and gravel sub-ballast because:

I. The sub-ballast prevents rainwater from ‘ponding’ directly on thesub-grade soil. As illustrated in Fig 2.

II. The fine sand component of the sub-ballast forms the basis of a ‘fine soil filter’ preventing the passage of silts and clays.

Excavation into ‘blanketing sand’ has shown that the silt and clay sub-grade soil penetrates no more than perhaps 15mm up into the sand before forming a stable soil filter.

Sub Ballast

Pore water pressure relief

Rain

Figure 2: Sectional view showingsub-ballast in rail structure

Geotextile‘PUMPING’ of microscopicsilt and clay particales in slurry

Figure 3: Illustration of ‘pumping’ through geotextile

Granular Soil

TERRAMPW1/PW2

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An alternative to using a fully graded sub-ballast described in point IIabove is given in the UK Railtrack Line Code of Practice, TrackSubstructure Treatments 2 and 3 whereby a ‘blanketing sand’ ofspecified grading is laid on the sub-grade to act as a fine soilfilter/separator to prevent ‘pumping’ Figure 4.

The sand does not contain the graded gravel fraction of a fullsub-ballast and is thus prone intermixing with the ballast. An approvedgeotextile e.g. Terram PW1 is therefore specified between the sandand ballast to act as a coarse soil filter/separator.

Excavations into track bed constructed 8 to 10 years earlier in thismanner has revealed that this construction has worked successfullyand looked likely to continue so for many more years. It is believedthat as well as the blanketing sand helping to form a soil filter it alsooffers ‘cushion’ support to the geotextile which prevents the ballastfrom readily puncturing or wearing through the geotextile under therepeated train loading.

Similar to sub-ballast, blanketing sand is expensive and not alwayseasy to obtain. It is also labour intensive and time consuming to lay.For an alternative fully geosynthetic solution to the ‘pumping’ problemengineers have looked to impermeable barriers, however and asmentioned earlier, these are not universally accepted as solutionand now there are developed new composite geosynthetics comprisingelements such as engineered bound sand or micro-porous filterssandwiched between geotextiles.

Over Fine Soils (> 35% silt & clay with < 65% sand & gravel)

Fine soils are mixed soils, containing possibly all the grading sizesincluding clay silt, sand and gravel. The clay and silt fractions compriseover 35% of the soil and dominate its engineering properties/behaviour.In the context of sub-ballast replacement by geosynthetics, fine soilscan be regarded similarly to cohesive soils, and clay rich mudrocks above.

Over Granular Soil (i.e. sand & gravel)

Sub-ballast is essentially a granular soil so the first question to ask is‘is the sub-ballast needed at all or can the ballast be placed directlyonto the granular sub-grade soil?’

If the sub-grade contains a significant proportion of gravel, it is unlikelyto be penetrated by the ballast. In this case, neither sub-ballast norgeotextile will be needed.

Soil conditions can vary considerably along a length of track. In somecircumstances engineers may consider it to be cost effective to lay ageotextile for the entire length as a precaution against local adversevariations in soil. This approach is particularly relevant when usingautomatic ballast cleaning machines where the sub-soil may neverbe seen.

On occasion, when a geotextile is placed directly over ‘gravely’ soilthere is a chance of it being punctured. The geotextile can becomepinched between the gravel and sharp ballast due to the weight ofthe train above. However, in these situations, it may be that thegeotextile was not required due to the granular material below.

Alternatively, if the granular sub-grade was a loose, slightly silty, fineto medium sand with occasional gravel, with the presence of a highwater table, then an approved geotextile would be a very effectiveaid to construction over this difficult ground, as well as a replacementfor sub-ballast.

The pores of Terram PW geotextile would physically prevent thepassage of sand whilst allowing easy passage for the ground waterwhen expelled from the soil under load.

An appropriate detail which does not require blanketing sand is setout in the Railtrack Line Code of Practice, Track Sub-structure Treatment 1.

Blanketing Sand

Pore water pressure relief

Rain

TERRAMPW1

Figure 4: Terram PW1 and blanketing sand over subgrades susceptibleto pumping

Figure 5: Terram PW1 geotextile used as a filter separator over granular soil

Terram PW1 - Standard filter/separator should be usedwhere there is an existing formation with a small percentageof coarse particles, i.e. less than 10% by weight <14mm.

Terram PW2 - Robust filter/separator should be used in anexisting formation where there is a an existing formation witha larger percentage of coarse particles, i.e. greater than 10%by weight >14mm.

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Fine sand sized particles

Bridge ofGranular soils

Soil filterbecoming finer

Undisturbed soil

TERRAMPW1

Figure 6 below shows how Terram PW1 or PW2 can be used as analternative to sub-ballast to filter a coarse soil.

Under pumping conditions, there will be an initial washing out ofsilts and clays from the coarse soil immediately beneath the geotextile.To an extent, the silts and clays ‘pumped’ through the geotextile willcontaminate the lower ballast.

Stable conditions will become established when fines have washedout of the local soil pores leaving a bridge of granular soil immediatelybelow the geotextile. Subsidence will be minimal in a coarse soildominated by sand and gravel because the fines can be washed outof the pores without collapsing the soil structure.

If there is upward ‘artesian’ seepage of ground water, a natural soilfilter can form below the granular bridge thus preventing fines beingcontinually removed from the coarse soil.

In confirmation of the above, there have been examples reported ofsilts and clays only contaminating the bottom 25 to 50mm of ballast,after which further contamination has ceased.

The engineer needs to decide if ‘pumping’ contamination of thelower ballast is acceptable. If not, then an option is to use blanketingsand and Terram PW1 geotextile as outlined previously as for cohesiveand fine soils.

If some contamination can be accepted, perhaps in order to facilitatequick work during a line possession, the use of a geotextile directlyon the coarse soil without blanketing sand may be considered valid.It is recommended that this solution is only adopted if the percentageof silt and clay fraction in the coarse soil is less than 15%.

Fig.6: Terram PW1 geotextile used as a filter separator over coarse soil

Over Coarse Soil (< 35% silt & clay with > 65% sand & gravel)

Coarse soils are mixed soils containing possibly all the grading sizesincluding sand, gravel, silt and clay. The sand and gravel fractionscomprise over 65% of the soil and dominate its engineering proper-ties/behaviour.

One of the most common sub-grades to be encountered in trackmaintenance and renewals is obviously old track-bed material. Thistypically comprises worn ballast, silt from the degraded ballast, sand,ash, and perhaps some silt and clay contamination, and can fall inthe category of coarse soil.

Subgrade Reinforcing Composites (PW4-LA)

The inclusion of geogrids in Permanent Way contruction is recognisedto improve both track life and performance by stiffening the ballaststructure over weak ground. It has not been as widely used in UK railwaysas it might have been because previously it has been necessary to placethe reinforcement separately to the geotextile thus requiring an extraoperation.

The advantage of reinforcing composites (such as Terram PW4-LA) isthe speed at which the geotextile and reinforcement can be laid ‘asone’ with the beneficial effect on construction times. This is of coursethe basic aim of all the Terram composites.

An extensive, full-scale, independent research programme was carriedout by British Rail and this clearly showed that over soft sub-grades:

I. Geogrids can help extend maintenance intervals by minimising settlement.

II. When using geogrids in ballast, the rate of settlement approachesthat of tracks on firm foundations.

III. The geogrid has a stiffening effect and will reduce the elasticdeflections.

IV. The use of geogrids with high profile ribs can limit the lateral creep of ballast particles, reducing settlement and, therefore,the rate of deterioration of the vertical track geometry.

Other benefits of the use of reinforcing composites are:

I. They provide a genuine alternative to increasing depth of ballast, or the application of chemical stabilisation.

II. An improved and more uniform rail ballast performance allows consistent high speeds to be achieved.

III. They help avoid necessary excavation and replacement with thick layers of imported fill.

Low bearing capacity can exist in most soil types including organicsoils. Susceptibility to pumping problems may or may not also bepresent and the decision as to whether or not to use underlying‘blanketing sand’ is as discussed previously.

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A full range of product data sheets, case studies and projectinformation request forms are available from Terram technicalsupport or as downloads from the Terram website. www.terram.com.

TECHNICAL SUPPORT

The information contained herein is, to the best of ourknowledge, accurate in all material respects. However, sincethe circumstances and conditions in which such informationand the products mentioned herein can be used may varyand are beyond our control, no representation or warranty,express or implied, of any nature whatsoever is or will be

made and no responsibility or liability is or will be acceptedby us, any of our affiliates or our or their respective directors,officers, employees or agents in relation to the accuracy orcompleteness or use of the information contained herein orany such products and any such liability is expressly disclaimed.

DISCLAIMER

Fiberweb Geosynthetics LtdBlackwater Trading Estate

The Causeway, MaldonEssex CM9 4GGUnited Kingdom

tel +44 (0) 1621 874200fax +44 (0) 1621 874299

email info@terram.comwww.terram.com