Assessment of Geosynthetics Interface Friction for Slope Barriers of Landfill J.P.Gourc , R.Reyes-Ramirez & P.Villard Lirigm, University Grenoble 1, France [email protected] ABSTRACT: Stability of Geosynthetic Lining Systems is, for a geotechnical standpoint, a complex matter. Three geomechanical questions were identified: sliding of the geosynthetic lining system on slope, pull-out strength of the geosynthetic anchorage at the top of the slope, rain erosion of the cap cover. Research programmes carried out in France on these topics, , are presented. Use of laboratory facilities (mainly different Inclined Planes) and large scale experimentations on actual slopes is especially emphasized. The observations derived from the tests and their detailed interpretation are really fruitful, as they highlight specific local interaction behaviour between soil and geosynthetics, which are not taken into account in design methods, more particularly wrinkles and real relative displacements of geosynthetics along the slope (Chapter 2), realistic value of interface friction angle (Chapter 3 ), pull-out strength of a “L-shape” anchorage (Chapter 4), identification of the mechanisms of control of erosion by geosynthetics (Chapter 5). 1 INTRODUCTION Preservation of the lining barrier of domestic and industrial waste disposal is important firstly for environmental reasons. One of the modern strategies for waste disposal is the concept of “bioreactor” (Figure 1): the landfill is now considered as a center of energy recovering , which requires still more care specifically for the implementation of the cap cover ( Olivier et al, 2003), since: - biogas should be collected with the minimum loss through the barrier - leachate could be recirculated in order to accelerate the waste degradation without uncontrolled water supply from the cap cover. The control of flow through the cap cover induced serious geotechnical problems, because: - cap liner are more and more composite structures, Geosynthetic Lining Systems (GLS), with interface matters - slopes of cap cover are steep, in order to make more profitable the disposal site, with the maximum dumping volume. - the cap cover is supported by a waste body which is often extremely compressible. Figure 1. The “bioreactor” new concept for an updated domestic waste landfill (from Environmental French Agency Ademe ) 116 This Lecture is the opportunity to summarize several research works carried out at the Lirigm of the University of Grenoble in France, related to the different stability problems arising in landfill applications. Three questions were identified (Figure 2): - sliding of the geosynthetic lining system on slope (chapters 2 and 3) - pull-out strength of the geosynthetic anchorage (chapter 4) - rain erosion of the cap cover This research programme includes theoretical and experimental development, but in the framework of this Lecture, use of laboratory facilities (mainly Inclined Planes) and large scale experimentations is emphasized. Anchorage GLS stability Erosion Control Figure 2. Main issues related to the stability of Geosynthetic Lining Systems on landfill slopes 2 STABILITY OF GEOSYNTHETIC LINING SYSTEMS (GLS) ON SLOPE In the framework of large research programmes, sponsored by the French Environmental Agency ADEME and Industrial Companies, two large scale experimentations were carried out on actual landfill sites (Montreuil and Torcy in France). Main results are presented below. The sealing systems used for the sloping sides of waste storage centres are made up of different geosynthetic and mineral components. The distribution of forces within each component is complex and results mainly from the deformability and frictional interaction between components. One of the aims of the Geosynthetics Lining System implemented is to separate the functions of the different items: stability is guaranteed by the geotextile of reinforcement (GtR) while the geomembrane (Gm) acts as the sealing layer and must be subjected to the minimum possible tensile stress. In addition a geospacer (GS) for transmissivity of water to drain is possibly inserted between the geomembrane and the geotextile of reinforcement. The (GtR) has also a filter function to avoid the clogging of the geospacer. Different design methods based on simple limit equilibrium method are available (Giroud, 1989, Soyez et al, 1990, Koerner et al, 1991 (Figure 3), Poulain et al 2004). However, as demonstrated by a Finite Element Method approach (Villard et al, 1999), selected assumptions for the interface properties and boundary conditions are questionable. Several unfortunate failures have resulted from soil sliding down the slick liner/drainage system interface. An accurate design of the (GtR) is required because this sheet is in charge of providing high frictional strength to the cover soil , but this condition is not sufficient: there is a high sensitivity of the relative displacements and mobilisation forces of the different components of the GLS to the interface friction relationship and the mode of construction. In these conditions, case histories are needed to present a comprehensive view of this issue. 117 Figure 3: Typical stability design method (Two Wedges method) for GLS (Koerner et al, 1991) 2.1 Tensile mobilization in geosynthetics during construction (Montreuil landfill experimentation): The geosynthetic lining system (Figure 4), (Gourc et al,1997,Villard et al, 1999), supported by a clay base layer, consists of an HDPE geomembrane (Gm: J = 458 kN/m), a non-woven geotextile (GtR : J= 65 kN/m) and a cover soil 0.30 m thick granular soil layer. The friction angles φg are 9° for Gm/support interface, 12° for GtR/Gm interface and 29° for the granular soil/GtR interface. The slope is 2H/1V. Fastening post Cable-type measuring Granular material Toe stop 1 2 9m Gt Gm Clay support Figure 4. Monitoring on a slope barrier of the Montreuil landfill The forces acting on the geosynthetics at top of the slope were measured by force sensors positioned between the geotextile sheet clamps and the fastening posts anchored at the top of the slope. The displacements of the geosynthetics and the cover granular layer were monitored by means of cable-type displacement sensors linked to the fastening posts and regularly spaced on the geosynthetic sheets and in the granular soil layer. The full experimental programme consisted of four successive implementation stage, but only the first one is presented here: The layer of granular material is placed on the slope (up to a length along the slope and Lc = 6 m from the bottom). This experimental stage involves monitoring the forces and displacements in the various GLS components while loading the granular material layer metre by metre on the slope over a total loading length, Lc, of 6 m. 118 Figure 5 illustrate the displacements in the granular material and in the geotextile during loading of the cover soil layer for increasing length (Lc). The displacements in the geotextile are far greater than those in the geomembrane. The relative downward displacement between geotextile and geomembrane induces a frictional force towards the bottom of the geomembrane. In agreement with the concept of separation of functions, the elongation due to tensile force in the geomembrane is very low. Figure 6 presents the evolution of the (extreme) tensile force in the geotextile and geomembrane sheets measured at the top of the slope with lifting of soil cover (Lc). Figure 7 is especially interesting, seeing that it presents the strains in the geotextile sheet for increasing length (Lc). It is worth noting that the geotextile, acting as a reinforcement, is subjected to positive tensile strains (elongations) at the top of the slope whereas it is in compression at the bottom of the slope. Given that the slope length is constant, the elongation of the sheet at the top is compensated at the bottom by the formation of wrinkles (Figure 8).This complex behaviour is generally not taken in consideration in design and numerical calculations. Displacement (cm) Phase I Lc = 6 m 7 Lc = 5 m 6 Lc = 4 m 5 Lc = 3 m 4 Geotextile 3 2 1 0 0 1 2 3 4 5 6 7 8 9 L(m) Displacement (mm) Phase I Lc = 6 m 5 Lc = 5 m 4 Lc = 4 m Lc = 3 m Granular soil 3 2 1 0 0 1 2 3 4 5 6 L (m) Figure 5. Montreuil landfill slope: displacements of the granular soil cover (down) and of the geotextile (up) Tension (kN/m) Phase I 1.6 Geomembrane 1.4 1.2 1 Geotextile 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 Lc (m) Figure 6. Montreuil landfill slope: tensile forces in the geotextile and geomembrane 119 2 1 L (m) 0 0123456789 -1 Strain (%) Strain Lc = 6m -2 Lc = 5m Lc = 4m -3 Figure 7. Montreuil landfill slope: Distribution of strains in the geotextile (elongation positive) Tension Compression Figure 8. Mechanism inducing wrinkles at the base of the slope 2.2 Long term survey of a geosynthetic cap lining system (Experimentation of Torcy) The site of Torcy is a landfill of domestic and non hazardous domestic waste. This experiment differs from the previous one by the far larger length of the slope (50 m instead of 9 m) , the inclination of the slope (3H/1V) , the long term monitoring of the movements of the GLS after construction (2 year) and the 4 different lining systems tested (Figure 9) (Villard et al,2000, Feki et al, 2002). Soil Cover GTr Soil Cover GTr GS GS GMpp GMb Clay Clay P1 P2 Soil Cover 0.3 Soil Cover GTr GS GTs Clay Clay 1 m P3 P4 Figure 9. The 4 different geosynthetic lining systems on slope (Torcy) 120 The present presentation focus on (P1) and (P2) trials, which could be distinguished by the type of geomembrane: For (P1) it is a polypropylene geomembrane (Gm PP) and for (P2), it is a bituminuous one (Gm B). The differences in the mechanical characteristics are significant, related to the tensile stiffness (tensile modulus J) : GtR ( J= 580 kN/m), GmB (J = 80 kN/m) , GmPP ( J = 51 kN/m) and the friction propertie φg : Trial (P1): clay/GmPP (13°8) GmPP/GS (7°) GS/GtR (18°) Trial (P2): clay/GmB (18°) GmB / GS (31°) GS/GtR (18°) To attach the geosynthetics at the top of the slope, a large trench of anchorage (1 m deep and 1 m wide) was used to bury the geosynthetics in such a way that no significative sliding of geosynthetics could be observed at the top edge.