An introduction to geosynthetics (3.1)

Transcription

1 TM An introduction to geosynthetics (3.1) The key functions associated with geosynthetics Manufacturing processes Measuring the properties of geotextiles - index tests Specifying geotextiles - matching required functions to types of geotextile Other properties to consider Quality Assurance - manufacturing and independent testing About GEOfabrics Limited GEOfabrics Limited Skelton Grange Rd, Stourton Leeds LS10 1RZ United Kingdom Telephone: +44 (0) Facsimile: +44 (0) info@geofabrics.com Website: 1/8

2 Geosynthetics are usually sheet materials supplied in roll form and they are used in many geotechnical applications. There are five categories - geogrids, geomembranes, geonets, geotextiles (subdivided into woven and non-woven) and related products (materials such as erosion mats & cuspates) that do not fall naturally into one of the other four categories. There are six main functions that these materials can provide and many products provide one of more of these, particularly the geocomposites which, as the name suggests, are made up of multiple components. The functions are: Separation Drainage Prevent intermixing Collect/convey Preventing intermixing of soil types or soil/aggregate to maintain the integrity of each material yet still allow the free passage of liquids/gases. Commonly used in between sub-base/subgrade and around drainage materials. Allowing fluids and gases to flow both through the plane of the material. Commonly used as components in geocomposites used for surface water runoff or for gas collection under membranes. Filtration Protection Prevent movement of fill particles Prevent puncture Restraining soil particles subject to hydraulic forces whilst allowing the passage of liquids/gases. This function is often partnered with separation e.g. in coastal defence applications or wrapped drains. Preventing or limiting localised damage to an adjacent material, usually a geomembrane used to line a lagoon or a landfill. Thick geotextiles prevent puncture or excessive strain in the membrane. Reinforcement Provide additional strength Providing additional strength to soils to enable steep slopes and soil structures to be constructed, and allow construction over weak and variable soils. Isolating one material form another. The most frequent use of this function is in landfills where impermeable linings prevent contamination of surrounding soils. 2/8

3 Manufacturing processes Needlepunched, non-wovens geotextiles Needlepunching is a mechanical process which, rather than using heat, fixes the fibres relative to each other by entanglement. Reciprocating banks of barbed needles compact loose fibre into a labyrinth of interconnected fibres. The use of continuous filament fibres creates geotextiles with the separation/filtration functions. Using staple, crimped fibres enables the production of thick geotextiles that are suitable for heavy-duty filtration/protection and they also provide the additional function of protection. Heat-bonded, non-wovens geotextiles Continuous filament fibres are extruded from spinnerets to form a swirling pattern of fibres across a web. The web passes through a pair of heated rolls or an oven, where the fibres are bonded together to form a uniplanar geotextile. This method generates low-cost products that are used in sub-base/subgrade separation. Geogrids Three processes. The first heats and stretches polymer that has been pre-punched with a regular pattern of holes. The second comprises bundles of polymer fibres in a mesh pattern that are coated with bitumen or a PVC (polyvinyl chloride). The third takes sheathed bundles of fibres that are then welded. Woven and knitted geotextiles Manufactured by weaving or knitting yarns of drawn polymer. These yarns may be flat tape, mono-filament, multi-filament, and fibrilatted (flat tape slit and twisted). The yarn type dictates the properties of the finished geotextile. Geonets Polymer mesh which is extruded in a tubular form & slit in the machine direction to create a sheet. Essentially a layer of rods overlaying a second layer at an angle. A third layer can be introduced to increase thickness and, thus, flow capacity. Geomembranes Polymer sheet that is extruded flat or as a tube to be slit in the machine direction. The textured (roughened) versions are for use on slopes where higher levels of interface friction angles are required. Geocomposites Heat and/or adhesives are used to create single components by bonding barriers, drains, filters, protectors and reinforcement in different combinations. The objective is to produce materials which are multi-functional and are faster to install than the individual components. Interface friction becomes an issue when geosynthetics are placed on slopes and bonded materials address this potential problem. 3/8

4 Measuring the properties of geosynthetics - some index tests Index tests enable a direct comparison to be made between different geotextiles. They are also used for quality control during manufacturing. Most UK specifications either reference a product or set a performance specification based on three or four tests with required values. Tensile strength, pore size, water flow, CBR puncture resistance and cone drop perforation are the most common properties to be listed in a specification. Mass per unit area is also frequently specified though this is not necessary, as it is not a performance characteristic. Direct comparison of published data is possible when the same test methods are used which in the UK should be EN standards. Any supplier of geotextiles to a UK contract is required to submit the measured properties of their materials to the latest published European Standards. Engineers should not accept data from individual member countries or non-member countries. For instance, the USA publish data from different non-comparable tests (ASTM standards), often in non SI units. Wherever an ISO test standard has been published the USA, European and all other countries belonging to the International Standards Organisation have agreed the test standard internationally using SI units (e.g. kn and m - not lbs and ft). As a member of the European Union, engineers in the UK are required to specify geotextile test methods as published by the Comité Européen de Normalisation (CEN). These test standards are published and ratified by the British Standards Institute (BSI) and given a BS designation. Many of these standards were originally part of BS6906 which have now been adopted at European level with EN designations. Some test standards have international recognition and are ratified by the International Standards Organisation (ISO). CE marking CE marking (part of the Construction Products Directive - 89/106/EEC) is intended to remove technical barriers to trade within European states. Standardisation of testing methods and procedures has been driven by the need to simplify specifications and, in turn, designing with Eurocodes. The CE mark will provide a guarantee that, for a particular application, the properties for a product match those published by its manufacturer. This should not be confused with ISO 9001 certification, which applies to a management quality system. All manufacturers of building and construction products in the designated European states, are required to comply with the CE marking requirements. This became mandatory for geotextiles as of October 2002 and product packaging should reflect this. Not all of the 84 potential tests for geosynthetics apply to every product. The ultimate application dictates which tests are appropriate. The CEN TC189 committee, comprising representatives from across the industry, including consultants, manufacturers, testing laboratories and universities, was responsible for establishing this criteria. The DTLR are the awarding authority in the UK and, following a factory inspection by independent auditors to verify procedures, together with further inspections of records and equipment calibration, GEOfabrics Limited has obtained CE marking for the majority of their HP and MP geotextiles. 4/8

5 Tensile strength BS EN ISO (replacing BS6906 Part 1) Characteristic opening size BS EN ISO (replacing BS6906 Part 2) Water flow or permeability BS EN ISO (replacing BS6906 Part 3) CBR puncture resistance BS EN ISO (replacing BS6906 Part 4) Tensile creep BS EN ISO (replacing BS6906 Part 5) Cone Drop Perforation BS EN (replacing BS6906 Part 6) In-plane flow capacity BS EN ISO (replacing BS6906 Part 7) Shear resistance BS EN ISO (replacing BS6906 Part 8) A sample is clamped between two jaws and strained at a constant rate until failure. Maximum load and extension are recorded. A sample is placed in a sieve shaker (as used for soil particle size analysis). O 90 graded sand is flushed through the geotextile using a water. The rate at which water flows through the geotextile at a 100mm hydraulic head is measured and quoted in litres per square metre per second. A sample is clamped in a ring and a 50mm diameter plunger is pushed against the centre of the sample extending it until failure. Maximum load and plunger displacement are recorded. Very rarely used and only where soil reinforcement is involved A sample is clamped in a ring and a 45" cone allowed to free fall half a metre on to it. The diameter of any resulting perforation is measured. This is not often used. It measures the water flow within the plane of the sample under various pressures and hydraulic gradients. Again very rarely used. Shear characteristics are measured in a 300mm box with the sample fixed to one half. Wide-width tensile testing Specifying geotextiles matching required functions to types of geotextile When specifying geotextiles it is important for the Engineer to match the best features of any one type of geotextile to the application for which it is intended. Then to specify only those index values that are relevant. Example: Select a geosynthetic for use in a road construction where the underlying soils are a fine silt and a minimum amount of sub-base is required. Functions required in order of priority: reinforcement, separation, some filtration Relevant index tests to specify: tensile strength, tensile elongation, (possibly creep if loaded for long period i.e. even after consolidation), puncture resistance, puncture extension, cone drop perforation, pore size, permeability Relative values to specify: (see chart) high tensile strength, high puncture resistance, low extension, medium cone drop, low pore size, medium to low permeability (specific values depends on calculation beyond the scope of this guide) Geotextile type to investigate: woven flat tape Example: Select a geosynthetic for use under rock armour in a coastal defence scheme. The maximum armour weight is 4t, the underlying beach material is a coarse sand and ideally the armour should be placed directly onto the geosynthetic. Functions required in order of priority: filtration, separation, some reinforcement Relevant index tests to specify: permeability, pore size, tensile strength, tensile elongation, cone drop perforation, puncture resistance, puncture extension, thickness, in-plane flow 5/8 CBR puncture testing.

6 Relative values to specify: (see chart) high permeability, low pore size, medium tensile strength, high tensile elongation, low cone drop perforation, high puncture resistance, high extension, high thickness (specific values depends on calculation and can be found in the GEOfabrics' Handbook for Coastal Geotextiles available on request) Geotextile type to investigate: needlepunched, non-woven (using staple fibres) Mechanical comparison Tensile load Tensile extension Puncture resistance Puncture displacement Cone drop perforation Woven tapes High Low High Low Medium filaments Medium Low High Low Medium Non-woven heat bonded Low Medium Low Medium Low continuous fibres Low Medium Medium High Medium staple fibres Low High High High High Net Low High Low High n/a Grid High Low n/a n/a n/a Hydraulic comparison Permeability Pore size In-plane flow (related to thickness) Woven tapes Low Low None filaments High High None Non-woven heat bonded Medium Medium None continuous fibres High Low Low staple fibres High Low Medium Net High High High Grid n/a n/a n/a Other properties for consideration Whilst most geosynthetics are manufactured from polymers, which are relatively inert materials, some are more susceptible to chemical, biological or mechanical damage than others. Durability must be considered for both installation and in service. To quote a leading consultant a layer of cling film has a lower permeability than a metre of compacted clay. Areas to consider are: UV resistance The performance of most polymers is degraded, to different extents, by ultra violet light (UV). The polymer bonds breakdown and this can result in a loss in properties. If geosynthetics are to be exposed for more than 30 days in the UK, it is recommended that they should contain a well-dispersed UV inhibitor that protects the polymer chains. Carbon black is the most cost-effective agent for these purposes. Specifications should therefore include an accurate description of the type of UV protection and the concentration by weight - 1% is typical. This should be the concentration of the carbon black and not the weight of carbon black dispersion that is added to the polymer. Carbon black comprises very fine particles that are difficult to handle. It is normal to mix the particles with a carrier to make a dispersion that is easier to handle. So, 2.5% by weight of the dispersion could be added but this equates to 1% carbon black. 6/8

7 Chemical and biological resistance Some geosynthetics are used in aggressive environments such as in the containment of landfills and contaminated land. As the rate of chemical attack relates directly to the surface area available it is important for Engineers to request proof of stability with the specific chemicals present. This information, generated by the polymer manufacturers, should be available from the geosynthetic manufacturer. In some instances it may be necessary to carry out a specific immersion test at elevated temperatures using the actual mix of chemicals. A long-term leachate immersion test has been running in GEOfabrics' laboratories for four years. The samples are removed periodically for examination. Fire resistance Geosynthetics are used in applications where they are accessible by the public (e.g. rock face cladding) and in applications such as tunnel linings, where flammability can be a consideration. There are specific tests to measure flammability. There are inflammable polymers and others that can by made inflammable, to varying degrees, by the inclusion of additives. Mechanical damage resistance The rigours of installation can often be more demanding than the ultimate in-service requirements. Site damage tests can be specified such as rock drop tests for coastal applications. Laboratory tests have been developed to closely simulate in-service conditions. One of these is the Cylinder test that evaluates the performance of geotextile protector, liner and drainage aggregate combinations. Toxicity Geosynthetics are frequently used where surface or ground water regulations apply. Evidence must be provided to confirm that no materials will migrate or be extracted from the geosynthetic. Alternatively, that the nature or the levels of any extracts do no present a risk to the environment. Quality Assurance manufacturing and independent testing It is important for the Engineer that the product supplied consistently meets the specification. The specification should include references to the frequency of testing and which key test standards are required. Some tests show greater sensitivity than others for different geosynthetics e.g. the puncture resistance test is a very good indication of quality for a needlepunched, non-woven staple fibre geotextile whereas a permeability test will show very little change with varying strength. It would be irrelevant and a waste of resources to check permeability regularly in this example. The answer would always be the same! Samples should be taken from production at intervals specified in the QC procedures and tested by the manufacturer. QC Certificates relating to the roll numbers delivered to site should be supplied to the Engineer. In addition, the Engineer should require the Contractor to supply samples from geosynthetics delivered to site and supervise their testing, either with the manufacturer or an independent accredited laboratory. Accreditation in the UK is usually achieved under the UKAS NAMAS scheme which sets stringent standards and procedures. 7/8

8 About GEOfabrics GEOfabrics Limited is an ISO 9001-accreditated manufacturer based in West Yorkshire. The Company manufactures geocomposites and needlepunched, non-woven geotextiles based on staple fibres. Services offered include: Advice from Chartered Civil Engineers with long experience in the use of geosynthetics. Solutions to specific problems - geocomposites are often manufactured that are project-specific NAMAS accredited laboratory guaranteeing consistent quality and site-specific testing facilities such as the cylinder testing for landfill design. Roll widths to a maximum of 6m (widest in Europe) for minimum overlap costs and fast installation. A comprehensive design-guide library together with a technical library of published papers and case studies to deal with specific questions. Please do not hesitate to telephone, fax or visit the GEOfabrics' web site if you require advice. GEOfabrics Limited is a Corporate Member of the International Geosynthetics Society (IGS) which exists to promote good practice in the industry and encourage innovation in this field. For more information, or if you would like to become a member of the UK Chapter of the IGS, go to There has been a vast amount of geosynthetics research carried out and there is an immense wealth of practical experience acquired over the last 30 years. In many instances there is more knowledge on their use than with traditional materials. To quote Dr Malcolm Bolton, Senior Soil Mechanics Lecturer at Cambridge University, We now know more about the forces in a soil block reinforced with a geotextile than we do for a gravity solution 8/8