SELECTION OF PROTECTIVE CUSHIONS FOR GEOMEMBRANE PUNCTURE PROTECTION

Transcription

1 SELECTION OF PROTECTIVE CUSHIONS FOR GEOMEMBRANE PUNCTURE PROTECTION Eric Blond, CTT Group / SAGEOS, St-Hyacinthe, Québec Martin Bouthot, CTT Group / SAGEOS, St-Hyacinthe, Québec Olivier Vermeersch, CTT Group / SAGEOS, St-Hyacinthe, Québec Jacek Mlynarek, CTT Group / SAGEOS, St-Hyacinthe, Québec ABSTRACT Geomembranes are often used in civil engineering applications to create an hydraulic barrier between two medias. Their basic function is to remain impervious over the entire design life of the project. However, mechanical stresses induced by the confined materials could produce a deformation of the membrane and, in critical situations, could ultimately puncture it. Protective cushions, typically heavy weight geotextiles, are used in geosynthetics engineering to reduce occurrence of local stresses and thus to control potential puncture of the liner by the material installed immediately above (typically the drainage layer). Various non-woven geotextile cushion design methods are presented and discussed in the first part of the paper. It is shown that inappropriate understanding of the basic hypothesis used to apply these methods could lead to the inappropriate selection of a cushion material and thus to the failure of the lining material. Laboratory evaluation of protective cushions efficiency, based on a large-scale performance test (modified ASTM D5514 procedure), is described in the second part of the paper. Experience gained throughout laboratory testing of different systems confirms that analytical methods can be considered as good approximations for the preliminary steps of a project, i.e. conceptual choices / pricing). However, they should not be used as the single basis to specify the properties of a protective cushion. This was shown to be even more evident when a protective cushion is designed for installation over a composite lining system instead of a single liner. The third part of the paper presents authors modifications to the existing practice for geosynthetic protective cushion laboratory evaluation. Specific guidelines for conducting a performance test are proposed, as well as a criteria for evaluation of the protection efficiency. RÉSUMÉ Les géotextiles de protection sont utilisés couramment pour limiter la concentration de contrainte sur un point d une géomembrane, et ainsi contrôler cette source de poinçonnement par le matériau situé immédiatement au dessus, typiquement le matériel constituant la couche de drainage des lixiviats. Différentes méthodes de conception de cette couche de protection sont présentées dans la première partie du document. Il est montré qu une mauvaise compréhension des conditions aux limites considérées pour le développement de ces méthodes peut résulter en une mauvaise sélection du matériau de protection. En conséquence, l inefficacité du géotextile de protection pourrait conduire à l apparition de fuites dans la géomembrane, réduisant de ce fait son efficacité comme barrière hydraulique. La seconde partie du document présente une méthode d évaluation en laboratoire de l efficacité des géoprotecteurs, dérivée de l essai ASTM D5514. L expérience issue de l analyse de la performance de nombreux systèmes d étanchéité à confirmé que l utilisation exclusive de méthode de conception théoriques ou empiriques peut constituer une bonne approximation de ce qui devra être installé pour des fins budgétaires ou pour la sélection conceptuelle du système d étanchéité, mais qu elle devrait être complétée de l évaluation systématique de la performance du système par le biais d un essai de laboratoire. Cet aspect s est avéré particulièrement critique pour les systèmes d étanchéité complexes comme les systèmes à double étanchéité. Des modifications à la procédure généralement suivie pour la sélection des géoprotecteurs sont finalement proposées par les auteurs. Des recommandations précises pour la réalisation d un essai de performance des géotextiles de protection sont proposées, ainsi qu un critère d analyse de la performance. 1. INTRODUCTION 1.1 Potential sources of puncture of lining materials A lining material can be punctured in three different ways: during construction, under operations stress, or because of material interaction, i.e. inappropriate selection of the protective cushion. The two first causes of puncture can be handled by appropriate inspection techniques and supervision guidelines. However, the third one is design related and must be addressed while designing the project. These three puncture mechanisms are described below.

2 Construction Construction damages cannot be completely avoided. These can essentially be caused by accidental knife cuts in the vicinity of the seams, or during installation of the granular drainage layer, or by trucks turning over a too thin layer of backfill installed over the geomembrane, or accidental drop of heavy stone or tool over the membrane, or many other construction accidents. Stress Small deformation (< 3 %) Yield Elastic deformation Plastic deformation Break Rupture (short term puncture) Operations Penetration of large pieces of waste (i.e. long pieces of metallic materials or large solid industrial waste) through the drainage layer down to the geomembrane, or circulation of heavy compactors over a too thin layer of waste are examples of potential sources of puncture caused by operations. Materials interaction The third puncture mechanism of geomembranes is caused by material interaction. In that case, the granular material typically used to build the leachate drainage layer (LDL) produce localized deformations on part or the entire surface of the liner, if the protection material is not adequately selected. 1.2 HDPE liners puncture behavior Essential parameters controlling the puncture behavior of HDPE geomembranes are: - Normal stress level: a high normal stress will induce high localized stresses to the geomembrane. - Particle size distribution of the granular drainage layer: a large particle will favor stress concentration and larger stress level in the geomembrane. - Nature of the substratum directly below the considered liner: The softer it will be, the less resistance it will offer to the penetration of large particles through the system. - Properties of the protection layer / cushion: a heavier or stronger material will help dissipation or absorption of the stress and thus reduce the stress level in the geomembrane. Because of the stress-strain properties of typical HDPE geomembranes, granular particles of LDL will not fully pass through the liner or puncture them in a short term service. However, the related deformation can lead to the development of stress cracking on a long term prospective if the polyethylene material stress level, or localized elongation, is too high (figure 1). Strain Figure 1: HDPE stress-strain behavior Local Elongation Thickness Reduction Figure 2: influence of a protrusion on a geomembrane On the other hand, the localized thickness reduction in the vicinity of a particle puncturing the liner reduces availability of anti-oxidants, and thus reduces locally the durability of the liner (figure 2). 2. CURRENT DESIGN CRITERIA In North America, the most common design criteria was developed by Narejo and Koerner (1995). They proposed three different classes of protection level, depending on the type of landfill. For each type, an acceptable level of deformation of the geomembrane is proposed. This criteria is summarized in Table 1. Narejo & Koerner s design criteria is performance based: the higher are critical conditions of the application, the lower will be the tolerated deformations of the liner. 3. DESIGN METHODS 3.1 Theoretical design method (Koerner & Narejo, 1996) This design method is based on a theoretical analysis of Table 1: Narejo & Koerner design criteria Protection Description Applications Criteria level I Critical Hazardous waste landfills; Hazardous waste surface impoundments. Localized geomembrane deformation is less than 0.25 % II Intermediate Municipal solid waste landfills liners; Municipal waste surface impoundment liners; Heap leach pads; Dams. No yield of the geomembrane after 100 hours when submitted to a performance puncture test under a load equal to 1.5 to 2 times the design normal load. III Noncritical or temporary protection Municipal waste landfills covers; Heap leach pads; Dams. Yield of the geomebrane; No holes.

3 the deformation of a geomembrane under a protrusion (particle of the drainage layer). It is associated to the design criteria presented in Table 1. A correlation is proposed between the particle size of the granular materials, the normal load expected on top of the lining system, the mass per unit area of a non-woven geotextile and various safety factors. Design tables are also provided for lanfills designed following Subtitle D classical configuration. Applied Load Settlement Gauge > 300 mm Diameter Test Cylinder Load Plate Sand Levelling Off Layer Geotextile Separator p allow 1 M = 450 FS H A 2 1 MFs MF PD MF A FS CR 1 FS Where: P allow: polymer allowable pressure FS: global factor of safety; H: cone height = ½ d 50; M A: mass per unit area for the protection geotextile; MF S: partial safety factor for the protrusion shape; MF PD: partial safety factor for packing density; MF A: partial safety factor for soil arching; MF CR: partial safety factor for creep; MF CR: partial safety factor for chemical / biological degradation The last step of this design method is to conduct a laboratory test (ASTM D5514) to verify the compatibility of the liner, geotextile and drainage layer. 3.2 Experimental design methods - German approach German philosophy for the design of protective cushions gives an emphasis on Stress-Cracking Resistance and long term performance of the whole system. The associated design criteria reduces the allowable elongation in the geomembrane to 3% local elongation and/or 0.25% arch deformation (can be related to level I in Narejo & Koerner design criteria). Design is based on a performance test (cylinder test shown in Figure 3), conducted with the materials to be installed on-site. Being associated to a very low allowable deformation of the geomembrane, this strategy favors heavy weight geotextiles (up to 3000 g/m²) or sandfilled geobags. The specific test method is normalized by the German Federal Institute for Material Research and Testing (BAM). In Germany, the test is typically conducted using a 200 mm-thick drainage layer, with generally 16 to 32 mmdiameter gravel, and 2.5 mm-thick HDPE geomembrane. A 0.5 mm soft metal plate, made of organ pipe method (40 % lead & 60 % tin), is placed beneath the geomembrane to monitor the geomembrane deformations occurring during the test. The material supporting the lining system is a standard elastomeric pad. Test conditions, proposed by Seeger & Müller (Seeger & Müller, 1995), are presented in Table 2. These conditions CBD Three Load Cells Figure 3: Cylinder test Proposed Drainage Material Geoprotector HDPE Geomembrane Soft Metal Plate Elastomer Pad were defined in order to simulate the system puncture behavior over the typical service life of the geomembrane. Table 2: Recommended test conditions for the Cylinder Test according to Seeger and Müller (1995) Temperature Duration Load 40 C 1000 hours 1.50 x Design load 23 C 1000 hours 2.25 x Design load 23 C 100 hours 2.50 x Design load After completion of the test, the geomembrane is inspected for damages of its upper surface (cracks or nicks), sharp angled deformation and maximum permissible local strain using the metallic plate. The average longitudinal strain in the geomembrane is obtained by fitting a circular segment to the indentations in the lead sheet, selecting the most crucial segment, and then calculating the arch elongation. However, it has been demonstrated that this approach typically under estimates the peak strains in the geomembrane due to the indentation of a gravel particle (Seeger & Muller). Guidance table for geoprotector selection are available from the German regulations (Table 3). 3.3 Empirical design methods - UK approach This method uses a correlation between CBR puncture resistance of geotextile (ASTM D6241 / ISO 12236) and results of a performance test similar to the German cylinder test. The differences between the UK test and the German test are as follows:

4 Table 3: Geo-protector selection according to German regulations (after Saathof & Sehborck, 1994) Waste height (m) Vertical stress (kpa) Suggested geoprotector Mass per unit area (g/m²) < 2 < 30 Nonwoven geotextile Nonwoven geotextile Wovennonwoven composite GCL 4200 > 25 > 375 Sandfilled geosynthetic > CBR puncture resistance (kn) mm mm mm 5-10 mm Waste height (m) Figure 4: Guidance curves derived from Shercliffe (1996) - A 1.3 mm-thick lead plate is used instead of organ metal plate; - Average local strain calculation is different; - Site specific drainage material is used instead of a similar drainage material. The required performance, proposed in this design criteria, is also 0.25 % local strain. From this design criteria, guidance curves for geoprotector selection according to EA regulations are proposed in Figure 4 (after Shercliffe). The method is based on the assumption that CBR puncture value (ASTM D6241 or ISO 12236, involving a 50 mm cylindrical flat probe) is a most relevant parameter for geotextile protectors specification. As a consequence, this also suggests that the geo-protector tensile stiffness is crucial for geomembrane puncture protection. This observation is supported by Tognon, Rowe & Moore (2000) who observed that a reinforced rubber mat would provide a better protection performance than an unreinforced one. 3.4 Contractor design Many project require verification of the puncture behavior of the geomembrane using a test section constructed onsite. In that case, a lining system is reproduced and loaded on site, the performance of the lining system being based on visual observation of the puncture of the liner. This method is relevant for evaluation of construction damages, but is seen as totally inappropriate for evaluation of long term puncture performance of geoprotectors: creep and micro-deformation of the geomembrane are not considered, material handling is typically poor, test parameters are not controlled (i.e. temperature of the liner, applied load, etc.), test duration is not sufficient and analysis is very superficial. Although it is still often used in some areas of the world, this method should absolutely be avoided for long term performance evaluation of geo-protectors as long as it completely excludes any long term consideration, among numerous evident reproducibility and general relevance problems. 3.5 Discussion There are essentially 3 acceptable design methods for puncture protection of geomembranes: One theoretical (US approach), one purely experimental (German approach) and one empirical (UK approach). Based on numerous experimental data gathered within previous landfill design projects, it appears that the US (theoretical) method typically provides conservative protective cushions compared to empirical / experimental methods if the same performance criteria is to be respected. A question arises which method to use? The fully experimental (German) design method can be defined as a safe method, as long as it is based on the systematic verification of the protection efficiency of a geo-protector. However many engineers would prefer an appropriate analytical approach. It should be noted that the design parameter provided by the empirical (UK) or theoretical (US) method are not the same: Narejo & Koerner s method is based the design on geotextile weight, while UK design is based on CBR puncture resistance. Both approaches were confirmed using experimental data, but in different market area. The specific products used to generate them were thus different. It is thus important to correlate the design method used in a project to type of product used to develop this method: all geotextiles are not the same and do not behave the same way. This is specially true with the development of non-conventional geo-protectors (i.e. composites including non-woven geotextiles and other products, i.e. bark, sand, tire shred, etc.) First important issue is the type of granular material to be used on site. If an analytical design method is to be used without experimental verification of the performance of the system, a high safety factor should be proposed to include granular material potential variability (shape,

5 angularity). This question is even more critical if the potential contractors bidding on a project can gather the materials from different pits. Secondly, the other major parameter influencing the performance of geo-protectors is the substratum. If a subtitle D lining system is to be installed (clay / 1.5 mm HDPE / geotextile), the analytical design methods can typically provide acceptable design values. Narejo & Koerner (1996) even provide design tables for this type of system, depending on properties of granular material. However, if another system is installed (i.e. double lining, GCL, geonet of geocomposite in direct contact with the membrane, etc.), the experimental conditions used to develop or valid the above mentioned analytical design method are not met anymore. In that case, they will typically provide unconservative results, as long as the substratum does not have the expected stiffness. 4. SUGGESTED DESIGN APPROACH The relevant aspects of the above discussion for design of protective cushions can be summarized as follows: - There is no consensus on the property to be used to specify a protective cushion at this time (weight or CBR puncture), as long as the various study already conducted are market / product specific. - The entity controlling the type of material to be installed on a project is the contractor. As long as he is the one who buys the geomembrane, geoprotector and granular material based on engineer specification. This specification should thus be prepared in such a way that it will provide the maximum flexibility to the contractor in order to reach the required cost for the project, and assured that the expected performance will be met. - Analytical design methods provide interesting values for subtitle D lining systems, but are not appropriate for other types of lining systems. Considering these elements, it appears that the best approach is to use analytical methods as a guidance for the pre-qualification of a product, but that the geoprotector final selection should be associated to the results of a laboratory test involving specific candidate materials, not generic materials (as suggested in the above described analytical methods). The laboratory verification is particularly critical if the lining system to be installed is not a subtitle D design. The proposed laboratory test to be conducted for geoprotectors evaluation is presented in the following paragraph. 5. LABORATORY EVALUATION OF PROTECTIVE CUSHIONS 5.1 Equipment The laboratory test suggested for evaluation of protective cushions is derived from ASTM D5514. It involves the same equipment (600 mm pressurized vessel) as presented in Figure 5. Figure 5: Laboratory test apparatus for protective cushion evaluation In this test, a lining system is completely reproduced in a vessel 600 mm in diameter x 750 mm in height. Air pressure is used to apply normal load in the upper chamber. For 'subtitle D' type lining systems, the system is installed upside down, the air pressure being applied directly on the geomembrane. This set-up models conditions of a very soft sub-base, and is thus critical with regard to puncture of the liner. For complex lining systems (i.e. double lining), the full 750 mm height is available for materials installation, including sub-grade and drainage layer. A friction reduction system is installed on the inside walls of the equipment. 5.2 Recommended Testing Parameters The testing parameters presented in Table 4 are adapted from the work conducted by Seeger & Müller, 1995 for HDPE liners: - A minimum test load is introduced to consider the most critical construction loads applied on the liner. - The temperature and duration of the test is maintained at 100 hours. According to Seeger & Müller, 1995, the material interaction observed under these conditions are equivalent to what would be observed on a typical geomembrane service life.

6 Table 4: Test Conditions Test load Temperature / Duration Configuration maximum of 2.5 x design load or 700 kpa 21 C / 100 hours Complete lining system reproduced in the vessel - The standard elastomer pad used to simulate the sub-grade is replaced either by air pressure or by the actual soil to be installed on-site. 5.3 Test analysis Considering Narejo & Koerner criteria for typical municipal solid waste landfills, no yield deformation should be observed on the geomembrane. Thus, analysis of the test consists in the numeration of the plastic deformations in the geomembrane and identification of visual defects 24 hours after completion of the test. For critical projects, other technical analysis can be applied to quantify geomembrane deformation caused by material interaction. 5.4 Example of a test result The picture presented on Figure 6 shows a geomembrane tested in the equipment described above. Design of the cushion was conducted considering Narejo & Koerner s analytical method, which typically provides very conservative results for subtitle D designs. Global safety factor for the selected geotextile and drainage material was 3.0. However, in that particular case, the lining system was a double liner with a geonet (not geocomposite). The stress distribution in the liner and on drainage material contact points was thus tremendously different than assumed by the basic hypothesis used by Narejo & Koerner to develop the analytical method. Analysis of the test results has shown that: - The liner would have been punctured in several points if no laboratory verification had been conducted (circles drawn on Figure 6); - The geonet alone without any geotextile laminated on top and bottom of it significantly concentrates stress on the geomembrane which could be damaging for the geomembrane under the selected normal stress. (see regular pattern observed on Figure 6) The systematic verification of the material interaction of this complex lining systems has thus been very helpful to avoid improper use of a design method, as long as the stress distribution in the system was totally different than considered by the creators of the design method. 6. CONCLUSION Leaks caused by construction activities can be significantly reduced if an appropriate quality assurance Figure 6: Example of a poor geomembrane behavior program is followed during installation of the liner. This program should include material conformance verification and seam integrity testing to certify the conformance of these issues, as well as any method developed for the detection of knife / razor cuts or accidental puncture (i.e. electrical leak detection). It should be noted that application of an appropriate CQA program, including all of the above techniques, could potentially lead to the reduction of the normal leak rate considered in the design of landfill liners. Leaks caused by operation of the landfill can be significantly reduced with installation of a sufficient thickness of the drainage layer combined with the use of a high weight protection geotextile. Appropriate supervision of the operations and installation of selected waste on the first lifts will also help in the reduction or suppression of this source of leakage. Protective cushions intended to protect a geomembrane from puncture caused by long term material interaction should always be designed considering the methods presented above and not selected arbitrary upon realization of a pseudo field test or past habits. Unless the lining system is a subtitle D, this design should always be verified by the laboratory test described above, this test being conducted with the materials to be installed on site. 7. REFERENCES ASTM D (2001) Standard Test Method for Large Scale Hydrostatic Puncture Testing of Geosynthetics ASTM D Standard Test Method for the Static Puncture Strength of Geotextiles and Geotextile- Related Products Using a 50-mm Probe Blond, Bouthot, 2003: Design of Geomembranes Protection Layers. Proceedings Forum 2003: Design and regulations of geosynthetics in Landfill engineering, SAGEOS ed., March 26, 2003, Toronto EPA regulations, Subtitle D Gallagher, 1999

7 ISO 12236:1996 Geotextiles and geotextile-related products -- Static puncture test (CBR test) Koerner, Wilson-Fahmy, Narejo, 1996: Puncture Protection of Geomembranes: Part III: Examples, Geosynthetics International, Vol 3, n o 5, pp Narejo, 1995: Three Levels of Geomembrane Puncture Protection, Geosynthetics International, Vol 2, n o 4, pp Narejo, Koerner, Wilson-Fahmy, 1996: Puncture Protection of Geomembranes: Part II: Experimental, Geosynthetics International, Vol 3, n o 5, pp Saathof & Sehborck, 1994: Indicators for selection of Protective Layers for Geomembranes. Proc. 5 th Int. Conf. On Geotextiles, Geomembranes and Related Products, Singapore, pp SAGEOS, Puncture protection of geomembranes, SAGEOS internal report. Seeger & Müller, 1996: Limits of Stress and Strain: Design Criteria for Protective Layers for Geomembranes in Landfill Systems. Geosynthetics: Applications, Design and Construction. De Groot, Den Hoedt & Termaat Eds, Balkema Shercliffe D.A., 1996: Optimisation and Testing of Liner Protection Geotextile Used in Landfills, EuroGeo 1, First European Geosynthetics Conference, Maastricht Wilson-Fahmy, Narejo, Koerner, 1996: Puncture Protection of Geomembranes: Part I: Theory, Geosynthetics International, Vol 3, n o 5, pp