Geotextiles

Terram is a market leader in the design and manufacture of innovative geosynthetics, providing a unique range of value engineered solutions that also help minimise the environmental impact of construction.

We are dedicated to the creation of innovative alternative solutions to traditional construction methods when faced with a diverse range of engineering and environmental challenges such as grade changes requiring retaining walls or slopes, or improving soil conditions for better performing and more cost effective roadways, car parks, railways, and building structures.

We combine our unique manufacturing process technologies and extensive industry experience to create a dynamic system research and development programme, designed to extend the range of solutions that Terram can offer.

Intensive pre-manufacture computer based analysis, laboratory testing and site trials are undertaken to determine the critical performance factors of all new systems. Once in production, extensive performance and through life testing regimes are implemented to ensure their long term functionality and durability.

Our unrivalled expertise and experience in the successful design and manufacture of geotextiles has been accumulated over 40 years. We remain committed to staying at the forefront in the development of innovative cost effective geosynthetic solutions.

WHY GEOTEXTILES

Within the construction and landscaping sector the use of geotextiles has become fundamental in solving an increasingly diverse range of global geotechnical and environmental problems.

Specifically the primary benefit of using geotextiles is to enhance the performance, and design life, of development within the built environment. Typical, traditional, applications for geotextiles include soil reinforcement, ground stabilisation, filtration, drainage, protection and erosion control. However, the ongoing impact of climate change and the requirement to deliver sustainable development is increasingly creating the need for more diverse and innovative geotextile applications across all sectors.

Geotextiles are now incorporated into solutions to deal with the increasing occurrence of:

• Flooding

• Temperatureextremes

• Differentialgroundmovementandnon-uniform settlement problems

• Landslipsandlandslides

• Groundheave

Within the transportation, infrastructure and built environment sectors, applications now include pervious paving systems, green roofs, geocellular tank systems, trench soakaways and reed beds.

Through the ongoing commitment to achieve sustainability within the built environment, geotextiles are being incorporated into major infrastructure projects such as renewable energy schemes, on-shore wind farms and alternative forms of transport; such as light rail networks. Their use plays an active role in minimising the quarrying and use of natural aggregates of which there is a limited and finite supply.

In addition to the many sustainability benefits of using geotextiles, one of the primary advantages is the potential to reduce overall construction costs and in some cases having a major influence on the financial viability of the proposed development. This is particularly the case with brownfield regeneration where the benefits of a reduced construction depth are highly significant in terms of capital costs, risk and overall contract periods.

WHY TERRAM

3

GROUND STABILISATION

Non woven geotextiles provide an effective solution to the problem of constructing stable granular layers over soft foundation soils. When a granular layer is constructed directly on a soft subgrade, the imposed loadings such as traffic may cause intermixing of the granular material with the soft subgrade. This results in a loss of bearing strength of the granular layer and premature failure of the structure. This fundamental issue and the solution using a geotextile is shown in diagram 1.

The overall stabilisation function of a geotextile can be divided into three fundamental components-separation, confinement and filtration. These are shown in diagram 2.

When a geotextile is used for ground stabilisation the three components do not necessarily contribute to the same degree forallapplications.Forexample,thecontributionprovidedbytheconfinement component depends on the tensile stresses which can be generated in the geotextile.

These tensile stresses can only be generated if the surface of the structure e.g. pavement, is deformed at each vehicle passage. This has major implications when designing pavements because permanent pavements cannot tolerate the amount of deformation (rutting) required to place the geotextile into tension. Therefore, for permanent pavements, it is not possible to generate the confinement component - although the separation and filtration components are still present.

In all stabilisation applications, both the separation and filtration components contribute to the overall stabilisation function of a geotextile. However, it is only those applications where significant surface deformations are tolerable e.g. access roads, that the confinement component can also be considered part of the overall stabilisation function of a geotextile.

Typical applications include:

• Highways

• Railways

• Carparks

• Accessroads

• Cyclewaysandfootpaths.

Separation component: where the geotextile prevents the intermixing of the granular layer with the soft subgrade. By maintaining its integrity, the granular layer achieves maximum bearing strength throughout its depth over the full design life of the structure.

Confinementcomponent:wherethegeotextilerestrains lateral movement of the material at the bottom of the granular layer.

Filtrationcomponent:wherethegeotextileallowscontrolled passage of excess pore water from the soft subgrade.

Intermixing of granular layer with soft subgrade

WITHOUT TERRAM GEOTEXTILES

Granular layer Soft subgrade

WITH TERRAM GEOTEXTILES

Granular layer

Terram Geotextile

Terram maintains the boundaries between adjacent soil layers

Terram Geotextile

SEPARATION COMPONENT FILTRATION COMPONENT

CONFINEMENT COMPONENT

WITH TERRAM

WITHOUT TERRAM

Granular layer

Terram maintains the boundaries between adjacent soil layers Terram maintains the boundaries between adjacent soil layers

Soft subgrade

Terram Geotextile

Granular layer

Soft subgrade

Terram confines lateral movement of granular material

Terram Geotextile

TerramGeotextile

Granular layer

Soft subgradeSoft subgrade

Effect of separation component on bearing strength distribution within pavement structure.

Bearing strength

Dep

th

Bearing strength

Dep

th

1

2

54

GROUND STABILISATION APPLICATIONS

Terram can be used in a wide range of ground stabilisation applications. Major examples are discussed below.

Access roads

Accessroadsusinggeotextilesarenormallyconstructed to the configuration shown in diagram 3 with the geotextile placed at the base of the pavement structure with the granular material placed directly over it.

In this application, the geotextile is laid directly on the unprepared subgrade in accordance with installation guidelines. If the surface of the access road can tolerate rutting, then all three components shown in diagram 2 contribute to the overall stabilisation benefits. The thickness and type of granular material required to construct the pavement are covered in the installation guidelines.

Geotextiles allow work to proceed during adverse weather conditions and also prevent local shear failures from occurring particularly in soft areas. This situation can occur where heavy construction traffic is operating on a road sub-base of a depth designed for lighter, post-construction traffic. In addition, the use of a geotextile further reduces the cost of access roads by minimising maintenance and the need to continually ‘top-up’ the pavement with extra aggregate.

Rigid Pavements

In the case of rigid pavements, rocking of the concrete slab can create high suction pressures at the subgrade/sub-base interface drawing fines into the granular layer. Moreover, the presence of these dynamic suction forces can break down the integrity of the subgrade soil structure, thus requiring the use of a ‘tighter’ controlled filter for protective purposes. In these instances, geotextile grades with pore sizes finer than 50 microns have been used successfully as filter separators.

Flexible pavements

These are usually designed in accordance with national standards, which are often based on empirically determined criteria. In this environment, geotextiles can act as a long-term stabiliser preventing sub-base contamination from a soft subgrade, both during the construction phase and throughout the design life of the pavement. The general configuration for a flexible pavement utilising a geotextile stabilisation layer is shown in diagram 4. Because permanent pavements cannot tolerate rutting at the surface of the completed pavement, only the separation and filtration components shown in diagram 2 contribute to stabilisation performance. Experience has shown that these separation and filtration components contribute significantly to the long-term performance of permanent pavements.

The use of geotextiles over soft areas, whilst preventing aggregate punching into the subgrade, enables the placement of sub-base layersofincreasingcompactiondensityandstrength.Additionally,this technique typically allows a reduction on the over-excavation of soft material that would otherwise be required to achieve a predetermined surface grade level.

Geotextiles can also control the migration of fines from weather sensitive soils, thereby protecting the sub-base from contamination. This enables the use of coarser, more free-draining materials in the sub-base, which is beneficial from the viewpoint of removal of water from within the pavement construction.

Treatment of very soft ground

Wherethesubgradeisverysoft(CBR<l%,undrainedshearstrength<10kN/m2), it may be prudent to use grades of geotextile which have higher tensile strengths to aid constructability. Typical applicatons include construction over soft peat and sludge capping layers.

Area stabilisation and other applications

Geotextiles can also be used as a stabiliser in the construction of parking areas, sports areas, airfields, and other area earth-fill applications. Here it is again required to prevent the intermixing of the soft subgrade with the granular fill material, and consequently, the same criteria/benefits as for pavements apply.

Subgrade of fine soil

It is important to note that stiff silty clays, clay rich mudstones and chalk can be reduced to a slurry beneath a rail track structure, even though on first inspection they would appear to provide a good formation for rail ballast. It is therefore recommended that the solution shown in diagram 5 is adopted even in such apparently good ground conditions.

Embankments

Extensive use has been made of geotextiles beneath embankments constructed over soft foundation soils. The design often involves the use of a drainage layer under the embankment, shown in diagram 5. In this situation, the geotextile is used to prevent contamination by separating the granular drainage blanket from the soft foundation soil. In addition, the geotextile allows passage of excess pore water from the soft foundation soil not the drainage blanket.

In this instance the geotextile is also used as a permeable separator between the drainage blanket and the embankment fill. This enables a more permeable, or more readily available, granular material to be used in the drainage blanket as shown in diagram 5.

Terram Geotextile beneath embankments

Soft foundation soil

Terram Geotextile Pavement Embankment fill

Drainage blanket

Terram Geotextile stabilising temporary pavements Terram Geotextile stabilising permanent flexible pavements

Soft subgrade

Terram Geotextile

Granular material

Soft subgradeCut off drain

Surface course Base courseTerram Geotextile Granular sub-base

Excavation to suit pavement

grade level

3 4

76

Typical applications include:

• Highways

• Railways

• Carparks

• Accessroads

• Cyclewaysandfootpaths.

5

GROUND STABILISATION DESIGN, SELECTION & INSTALLATION

There are two steps involved in the design of ground stabilisation applications using Terram Geotextiles.

1.Determinethethickness(andtype)ofgranularlayerabove the Terram Geotextile.

2. Select the appropriate grade of Terram Geotextile to be used.

The required thickness of the granular layer is dependent on the particular stabilisation application, the level of external loading to be applied to the structure, and the strength of the subgrade supporting the structure.

Forpavementconstruction,thethicknessofsub-baseand/orcapping layer should be determined from appropriate national design criteria. However, where these do not exist, or are inappropriate, other more simplified procedures may be adopted. Forexample,thenomogramindiagram 8, which has been established for the design of unpaved roads, can be useful in checking initial layer thicknesses for paved roads where the sub-base is used as a construction platform.

Allapplicationsaresitespecificandthefollowinggeneralguidelines should not be used to replace more rigorous designs and specifications, nor the experience of installers familiar with geotextiles. However, where information is scarce, the following may prove useful for the selection and installation of Terram Geotextiles for stabilisation applications.

Subgrade

Constructionislargelydependentonthestrengthofthesubgrade, which is often dictated by its moisture content. In situations where the water table is high, subsurface drainage may be advisable if the topography allows. Ground stabilisation designs require a realistic appraisal of subgrade strength, but in theabsenceofactualfielddatathesubgradeCBRvaluesgivenindiagram 7 can be of guidance.

Before placement of the geotextile, the area should be cleared of any large angular objects, such as stones and tree stumps, whilerutsandsharpundulationsinexcessof100mmshouldbe levelled. In addition, strong perennial weeds, such as thistles, require weedkiller treatment to prevent them from penetrating the completed construction. Other surface vegetation can be left undisturbed to provide additional support, if this is allowable and notdetrimentaltothestructure.Forverysoftfoundationsoils(subgradeCBR<l%andundrainedshearstrengths<10kN/m2),the presence of surface vegetation can actually aid construction.

Terram Geotextile Selection and Installation

The grade of Terram Geotextile selected must be sufficiently robust to resist installation damage during construction of the stabilised structure. The lower the subgrade strength and the larger the stones in contact with the geotextile, the more robust it has to be. Diagram 6 presents guidelines for the selection of appropriate Terram nonwoven geotextile grades based on subgrade strength and stone size.

The Terram Geotextile should be unrolled directly onto the subgrade,andadjacentrollsoverlapped300mm–1000mm(thesofter the subgrade, the greater the overlap). Where subgrade strengths are particularly low, or in other critical situations, a combination of overlaps and sewing may be more economical (forfurtherdetailsrefertotheTerramJointingLeaflet),

Vehicles must not be allowed to run directly on exposed geotextiles.Constructiontrafficshouldberestrictedtoareaswhich have already been covered with aggregate and preferably compacted to the minimum depths required.

Aggregate selection and placement

The aggregate selected must be well-graded, compactable and for permanent works, capable of transporting rising water and resistant to long term degradation. Diagram 9 shows the recommended grading curve for compactable granular materials.

The thickness of the aggregate layer will depend on the level of external stress to be applied to the structure and on the strengthofthesubgrade.Forpavements,thedesigndepthshould take account of the maximum axle load anticipated during construction and in service, and should be increased by 10-20%onbendsorwhereslightlypooreraggregatesareused.The nomogram shown in diagram 8 can be used to calculate the thickness of aggregate required for a temporary access road or the sub-base thickness required for a permanent pavement.

The aggregate should be bladed forward onto the geotextile and graded down to the required uncompacted depth. On permanent pavements over stronger subgrades, the typical practice is to applythesub-baseinlayerswhicharecompactedto150mmusing a vibro-roller. Eight to ten roller passes should generally be adequate, although this will depend on the performance specification of the roller and aggregates.

On soft subgrades it is prudent to place at least 300mm of lightly compacted material in one lift (and 500mm on exceptionally soft soils), before overlaying this with a thinner layer(s) of better compacted material.

Othercircumstances,suchasverylow-CBRsubgrades,heaviertraffic loadings, or poorly graded sub-base, may require differing treatments.Forinstance,onverysoftclaysubgrades,heavycompaction can lead to rutting and heaving, and it may be necessary to increase the initial layer thickness and allow time for consolidation of the subgrade before the placement of thinner, more compacted layers.

Selection limits for grades of Terram nonwoven geotextiles based on stone size and subgrade strength.

Subgrade CBR value (%)

Max

imu

m s

ton

e si

ze (m

m)

0.5 1 2 3 4

50

100

150

200

250

0

Compacted thickness of aggregate required on top of Terram Geotextiles.

Recommended grading curves for compacted granular layers on top of Terram Geotextiles.

Perc

enta

ge

pas

sin

g

mm0

6020620.60.20.02 0.06

Silt fraction Sand fraction Gravel fraction

10

20

30

40

50

60

70

80

90

100

Not recommended M

o i s t u r e s e

n

s

i

t

i

v e

& hard to

com

pact

Recomm

ended g

radi

ng

Rough ridin

g gra

ding

200 300 400 500 600 700 800 900 1000 1100 1200 1300

Aggregate thickness (mm)

Axle load (tonnes)

30 20 15 10 5

Example: Subgrade = 2%,Axle load = 10 tonnesAggregate thickness required = 350mm

43 2

1.5 1.0 0.5

Subgrade CBR (%)

Soil type Plastic index % >600mm <600mm

Heavyclay 70 2 1 60 2 1.5 50 2.5 2 40 3 2

Silty clay 30 5 3

Sandy clay 20 6 4 10 7 5

Silt 2 1

Sand(poorly-graded) non-plastic 20 10

Sand(well-graded) non-plastic 40 15

Sandy gravel (well-graded) non-plastic 60 20

CBR% Depth of water table below formation level

Approx CBR values for some typical soils compacted at the natural moisture content

98

T4000

T3000

T2000

T1500

T1000

6

7

8

9

Cylinder Testing

Terram are committed to offering a total specification service within the landfill industry. Working with carefully selected independentUKASaccrediteddependentpartnersTerramcanfacilitateCylinderTestingfacilitiesinaccordancewiththeEnvironmentAgency’sStandard–‘Amethodologyforcylindertesting of protectors for geomembranes on landfill sites’, (available at www.terram.com).

Undertaking these tests allows contractors to assess the cost effectiveness of using a particular grade of Terram NP and drainage aggregate combination. This allows the contractor to choose a local supply of stone and then select the appropriate grade of geotextile to protect the geomembrane liner thus reducing the environmental impact of the project by minimizing transport miles.

LANDFILL ENGINEERING

Landfill Engineering

Needlepunched Geotextiles are used extensively in landfill construction. The primary application is when used above and below the geomembrane liner as a protection layer preventing damage occurring during stone placement and filling operations.Terram NP products provide excellent protection to the geomembrane and maintain its integrity by helping minimize environmentalstresscracking(ESC)andpreventingpuncturefrom the in-situ materials and from the stone drainage layer placed on top.

Using our laminator, Terram NP products can also be combined with both drainage and lining materials to form specialised geocomposite products designed specifically for your application. Typical materials that can be combined are geonets, geomembranes and band drains; utilising a combination of products means that protection materials can be offered with multiple functions thereby reducing installation times, and costs on site.

Typical applications include:

• Basallinerprotection

• Caplinerprotection

• Settlementlagoonlinerprotection.

Typical anchor trench detail

In-situ materialstone drainage layer

Compactedsoil

Terram Needlepunch protection Geotextile

Geomembrane Liner

The information contained in this chart is for guidance only. It is not intended as a design tool and technical support should be sought from Terram before specifying a material grade. No warranty is given or implied for the use of this information for design or installation.

NP40

NP35

NP30

NP25

NP22

NP19

NP17

NP14

NP11

NP9

NP8

NP7

NP6

NP5

5m 10m 15m 20m 25m 30m 35m 40m 45m 50m

Depth of Fill

20-40mm

Stone Grade

20-30mm

10-20mm

5-10mm

1110

MEASURING THE LEAD SHEET DEFORMATION

Applied load

Upper steel plate

Sand

Separator geotextile

Drainage aggregate

Geotextile specimen

1

2

3

4

5

6

Geomembrane

Lead sheet

Dense rubber pad

Lower steel plate

Load cells

Cylinder

7

8

9

10

11

12

CYLINDER TESTING RIG SET UP

Applied load

6

7

8

5

4

3

12

10

11

9

2

1

COASTAL AND RIVER CONTROL

Althoughnaturalvegetationcanprovideacertainlevelofsoilerosion protection, it is often inadequate on steep slopes or areas prone to wave run-up or intermittent high velocity flows. This can occur in coastal areas, rivers, lakes, reservoirs, drainage ditches, flood bunds and overspill ponds.

Riverandcanalbanksaresubjecttowaveactioncreatingreverseflow conditions, it is this dynamic erosive force that undermines bank stability and affects channel flow and navigability.

The hydraulic and filtration properties of Terram Geotextiles allows them to be highly effectively when used in place of traditional filter layers. Typically, a single layer of geotextile fabric canreplaceasuccessionofstonefilterlayers.Asinglebeddinglayer of stone is laid on the geotextile, to carry the rock armour that resists erosion caused by variable hydraulic forces.

By using Terram Geosynthetics, the filter system can be assembled above the water and lowered into position, making it easier to install, with less time being lost through bad weather or difficult water conditions. They can be pre-sewn or site-sewn into large sheets, reducing the number of overlapping joints, which is a particular benefit in tidal or fast flow conditions.

The defensive elements that require protection are:

• Thein-situsoil

• Thefilterunitprotectingthesoil

• Therockarmourprotectingthefilter.

The problems

Riverandcanalbanksaresubjecttowaveaction,i.e.reversingflow conditions. It is this dynamic erosive force that undermines bank stability and affects channel flow and/or navigability.

On coastlines, the three key agents of erosion are:

• Wavesandtideserodingtheshoreline

• Wavesandtidalscourtakeerodedsedimentawayfromthecoast

• Currentssweepthedisplacedmaterialouttosea. Traditional solutions

Defencestructuresforerosioncontrolalwaysrequireafiltertoprevent soil washout. Traditional filters usually consist of several layersofstoneaggregate.Aggregatemaybegradedorsingle-sized but installation is always time consuming and expensive. Furthermore,conventionallayersareawkwardtoplaceonsteepslopes, cannot always be installed in tidal zones and the laying process demands reliable and expert supervision.

The benefits of using geotextiles for erosion control

The hydraulic and filtration properties of geotextiles allows a single layer of geotextile fabric to replace a succession of some ofthetraditionalstonefilterlayers.Asinglebeddinglayerofstone is laid on the geotextile, to carry the rocks that resist the movements caused by the hydraulic forces.

Other benefits include:

• Geotextileshavebuilt-infactorycontrolledfilterproperties

• Checkingforcorrectinstallationiseasier

• Geotextilesareeasiertoinstall,eitherbyhandormechanically

• Geotextilescanbepre-sewnorsite-sewnintolargesheets

• Workingunderwaterismucheasierbecausethefiltersystem can be assembled above the water and lowered into position

• Lesstimelostthroughbadweatherordifficultwaterconditions.

Geotextile Selection

There are several methods for manufacturing non-woven geotextiles with different methods affecting the performance characteristics of thematerial.Foradviceastowhichtypeofmaterialisbestforyourproject,contacttheTERRAMTechnicalLine.

There are a number of key physical and hydraulic performance characteristics that require consideration when selecting and specifying non woven geotextiles. Some of the principal ones are outlined below.

Survivability

It is important to ensure the geotextile will survive the rigours of the construction process. Whilst it may offer the performance you require, if it doesn’t survive the site, then it’s wasted time and money. Ensure your geotextile is strong to resist damage during the installation process.

Durability

The geotextile needs to have the right level of resistance to attack from UV and chemical and biological exposure according to the specific requirements of the project.

Puncture Resistance

The geotextile will experience potentially damaging loads during the installation and during its working life. It therefore must be sufficiently robust to withstand these loads. Isotropic properties will aid this by spreading the load evenly in all directions.

Extensibility

To accommodate the irregular shapes of the overlying material the geotextile must have sufficient extensibility to remain in contact with the rock whilst at the same time not puncturing or compromising the hydraulic properties of the geotextile.

Permeability

Permeabilityistheratethatliquidspassthroughthegeotextile.Acorrectly specified geotextile will allow the free passage of liquids without causing hydrostatic pressure to build up.

Filtration

Correctspecificationofthefiltrationpropertiesofthematerialallows the free flow of liquids whilst preventing the transit of soil particles across the geotextile.

Typical applications include:

• CoastalProtection

• DamsandFloodDefenceBunds

• RiverandCanalBankProtection

• RiverRevetments

• CulvertHeadWalls

• StructuredCauseways

• CliffProtection

• Bridgeabutments

• Submergedbreakwaters.

Secondary stonefilter layer

Primary rockarmour

Primary stonefilter layer

Tertiary stonefilter layer

Beach material

Primary rockarmour

Beach material

TerramGeotextilefilter layer

GRADUATED STONE FILTER LAYER GEOTEXTILE FILTER LAYER

Typical applications include:

• Artificial islands

• Beaches

• Harbours

• Lagoonslakes&resevoirs

• Landreclamation

• Offshorewindgenerators

• Outfalls

• Rockgroynes

• Scourcontrol.Typical detail of a river revertment with

a geotextile beneath rock armour, stone-filled mattresses or pre-cast blocks.

750mm riprap

2m tidal range

Terram geofabric filter/separator

1312

PROTECTION AND FILTRATION

Permeable Pavements

Increasingly there is a risk of flooding affecting millions of people worldwide. The global occurrence of major coastal and river flooding events requires geosynthetics to play an increasingly important role in creating solutions in the attempt to provide enhanced protection to our fragile environment. Asaconsequence,themanagementofourwaterquality becomes increasingly critical, and there is a growing requirement to develop safe, innovative geosynthetic based filtration solutions, with properties that are pre-determined and consistent through being factory-controlled.

Terram Geotextiles can play a major role in the construction of permeable pavements acting in two discrete locations, with differing functions, within the pavement construction. The first location is a separation layer between the base of the construction and the subgrade soils. We have already discussed the benefits of Terram Geotextiles in this application earlier in this document (pages 4 to 7); however to recap the geotextile in this application has two functions:

1.Toprevent‘pumping’offinersoilsintotheunboundlayersof open graded aggregate, thus preventing siltation.

2. To add tensile resistance to the structure thus helping resist subgrade deformation when under load.

The second application is that of a filtration layer located beneath the permeable surface. Whilst both applications require the use of a geotextile it should be understood that, in many instances, there is not a ‘one size fits all’ solution. The important factors when specifying geotextiles for use in permeable pavements are outlined in the design considerations.

There are a number of pollutants that are of concern in highway or carpark run-off including:

• Sediments

• Metals(zinc,copper,cadmium)

• Hydrocarbons(oilandfuel)

• PesticidesandHerbicides

• Chlorides(de-icers).

(FromCIRIAGuideC582–SourceControlUsingConstructed Pervious Surfaces)

Within the Terram range there are geotextiles specifically developed to retain pollutants in surface water run-off to prevent them washing into local water courses or the underlying soils. ActingpurelyasafilterTerramGeotextiles,ifspecifiedcorrectly,willalsoremovesedimentsfromtherun-offwater.Furthertothis within the Terram Geotextile range are products with proven performance in terms of hydrocarbon entrapment and treatment. This is backed up with extensive, and on-going, research undertaken with leading universities.

Design Considerations

•Poresize

•Permeabilityand breakthrough head

•Punctureresistance

•Tensilestrength.

RAIL

Typical applications include:

• SteepenedEmbankments

• DamsandFloodDefenceBunds

• RetentionBunds

• GreenWalls

• CulvertHeadWalls

• SoundBarriers.

Railways

Terram Geotextiles have been successfully used for more than 25 years in the construction of permanent way. Their primary use is as a filter separator layer situated between the ballast and the sub-ballast

Where the subgrade consists of coarse soils e.g. sands and gravels, the geotextile acts as a separator between the subgrade and the ballast, shown in diagram 10. The subgrade, under the effect of pressure and vibration from trains passing overhead, is prevented from working its way up into the large voids between the rail ballast stones and conversely, the ballast stones are prevented from working their way down into the soil subgrade. Without a geotextile in place, progressive settlement of the track could occur resulting in loss of track alignment and increased maintenance requirements. The pores in the geotextile are small enough to block the passage of fine sand and very coarse silt but large enough to allow the transmission of groundwater for pore water pressure relief.

In areas where the subgrade consists of finer soils, a modified solution is required, shown in diagram 11.Clayandsiltparticlesare microscopic and thus small enough to pass through the pores of any geotextile (diagram 11). Normally in ‘static’ drainage conditions the cohesive nature of clays allows them to ‘bridge over’ the pores in the geotextile.

Water is often present at the interface below rail ballast and subgrade because rainwater is free to percolate down through the ballast to subgrade level.

Silts and clays tend to disintegrate in water when aided by the vibration from trains and abrasion from ballast.

Under the pressure and deflection created by a train passing overhead the resultant slurry can be ‘pumped’ up into the ballast through any geotextile.

This results in progressive settlement of the track structure due to ground loss into the ballast. In addition the ballast is increasingly contaminated by the slurry and thus performs its functions less well. If unchecked, ‘pumping’ leads to very substantial maintenance requirements.

Asolutiontothisproblemistoplacealayerofsuitablygraded‘blanketing sand’ directly onto the silt clay or mudstone subgrade. This acts as a fine soil filter and is well proven in preventing the ‘pumping’ of fine soils. The geotextile is placed on top of the sand blanket to maintain separation between the sand and ballast and retain the integrity of the filter system.

Terram PW1 Geotextile used as a filter separator over granular soil

Subgrade - silty sand and gravel

Terram Geotextile

Ballast

Pore water pressure relief

Terram Geotextile

Blanketing sandBallast

Terram PW1 and blanketing sand over subgrades susceptible to ‘Pumping’

1514

10

11

Typical applications include:

• Carriageways

• PermeableParkingAreas

• GrassPavedAreas

• GreenRoofs.

A Fiberweb plc company

Terram Limited

Mamhilad, Pontypool, Gwent NP4 0YR, United Kingdom

tel +44 (0) 1495 757722 fax +44 (0) 1495 762393

email info@terram.com www.terram.com

Registered office as above Registered in England no 2254236 VAT no GB 888 1788 50

FM 22730

TECHNICAL SUPPORTAfullrangeofproductdatasheets,casestudiesandprojectinformation request forms are available from Terram technical support or as downloads from the Terram website. www.terram.com.

DISCLAIMERThe information contained herein has been prepared in good faith and is, to the best of our knowledge, accurate in all material respects. However, it does not purport to be comprehensive and since the circumstances and conditions in which such information and the products discussed herein can be used may vary and are beyond our control, no representation or warranty, express or implied, whether of merchantability, fitness for purpose or against patent infringement or otherwise, is or will be made

and no responsibility or liability is or will be accepted by Terram Limited,anyofitsaffiliatesoritsortheirrespectivedirectors,officers, employees or agents in relation to the accuracy or completeness or use of the information contained herein or any such products and any such liability is expressly disclaimed. The information contained herein is offered free of charge and we give no undertaking to provide any additional, updated or corrected information.

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