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 Soil Cover GTr GTr GS GS GMb GMpp 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. Similar monitoring system than in Montreuil was used (metallic cables attached on every geosynthetic layer at different points), to evaluate the displacements along the slope.
Short term behaviour (end of construction tc to tc+ 72h): SShort term Short term behaviour just after completion of the cover soil on the geosynthetics was observed (Villard et al, 2000). Figures 10 and 11 show the distribution of the tangential displacement of the different components of (P1) and (P2) along the slope (distance L from the top ), between the time corresponding to the end of construction (tc) and time (tc + 72 hours): in both cases, a “critical interface” was noted depending on which the sliding (difference in displacement between the two geosynthetics in contact) was strongest: for (P1), between the geomembrane GmPP and geospacer GS (φg = 7°, interface with the least friction) and for (P2) between the geospacer GS and the geotextile for reinforcement GtR (φg= 18°, same condition).
400 GTr 350 GS 300 GMpp Clay 250 200 u (mm) 150 100
displacement Tangential 50 L(m) 0 01020304050
Figure 10. Trial P1 - tangential displacements in the different components at tc
90 GTr 80 GS
70 GMb 60 Clay 50 40
u (mm) 30 20 10 L(m) Tangential displacement 0 -10 01020304050
Figure 11. Trial P2 - tangential displacements in the different components at tc
121 The comparative behaviour of the two trials (P1) and (P2) clearly illustrates how it is possible to modify the whole deformation of a GLS only by changing the friction properties of one interface (here geomembrane/ geospacer).
Long term behaviour after construction (2 years):
The main studies available in the literature are limited to the short term behaviour of the geosynthetic liner on slope. For the present study, the two trials were monitored during two years. Long-term monitoring is more complex, additional measures of the settlements of the waste body beneath the cap GLS should be needed because the waste embankment, consisting of domestic and non hazardous waste, is compressible and mechanically viscous: the profile of the slope is deformed, and more seriously, the monitoring table at the top of the slope settles, following the waste body deformation. Figure 12, which plots the vertical settlement ( s ) versus elapsed time confirms the similarity of behaviour for the two profiles (P1 and P2) over time. The settlements are significative, due to the compressibility of waste. In these conditions the GLS should accommodate and conform to its support without ruin of its functional properties. The results collected from this case historie are remarkable since for the two trials, the tangential displacements of all the components of the GLS are of the same order than those in the cover soil (Figure 13). So there is practically no additional relative displacement between the different sheets after the construction stage (tc+ 72h).So no additional tensile mobilization in the GLS can be expected.
tc(end of construction) + 72h
0 3 6 9 12 15 18 21 24 0 (t-tc) months 100
200
300 P1 (L=30m) P2 (L=30m) 400 P3 (L=30m) settlement (mm) P1 (L=5m)
500 P2 (L=5m)
P3 (L=5m) 600
L= 5m L=30 m
s
s
L
Figure 12. Torcy landfill: Vertical settlements along the slope (L= 5m and L = 30 m from the top )
122
200
150
cover soil 100 GTr
u (mm) GS 50 GMpp clay 0 Tangential displacement Tangential 0 3 6 9 12 15 18 21 24
(t -to) months
200 cover soil GTr 150 GS
GMb clay 100
u (mm) 50
Tangential displacement 0 0 3 6 9 12 15 18 21 24 ( t -to ) months
Figure 13. Torcy landfill: Evolution during 2 years of the tangential displacements along the slope for the different components of the GLS (Trials P1 & P2)
2.3 Conclusions
The observation of the actual behaviour of GLS on slope demonstrates that their mechanical behaviour is complex and really difficult to modelize: extreme sensitivity to the interface friction properties, influence of the construction conditions, influence of the compressibility of the waste body for cap liners. In the next Chapter 3, assessment of interfaces friction will be considered and in chapter 4 ,a decisive boundary condition for the GLS, the anchorage strength of the geosynthetics at the top of the slope will be analysed.
3 USE OF INCLINED PLANE TO ASSESS STRESS MOBILIZATION OF LINER ON SLOPE
GLS stability
Figure14. Assessment of the interface properties of the different components of the GLS
123
The large scale experimentations presented above have demonstrated behavioral sensitivity to small modifications in, for example, the friction interface relationship. The complete friction interface relationship (shear stress τ vs. tangential displacement δ and, ultimately, vs. time t) often proves necessary in explaining distribution of tensile forces and relative displacements in the Geosynthetic Lining System. Hence, thorough understanding of the complete friction interface relationship (τ /σ' = f( δ, t) at a fixed normal stress σ'), and not just friction limit values (φsg and φgg with τlimit /σ' = tan φ, φ threshold value) of the soil-geosynthetic and geosynthetic-geosynthetic, is required to the stability analysis of sloped systems (Gourc et al ,2004). These interface relationships are determined using devices of either the shear box or inclined plane type and such equipment is currently undergoing standardization (European Standard final draft prEN ISO 12957, 2001: Article 1 for the Shear Box test, Article 2 for the Inclined Plane test , Gourc et al , 1996). The inclined plane test is commonly used when studying the stability of sloping geosynthetic liner systems under conditions of low normal stress. The inclined plane offers the dual advantage of enabling testing at low normal stresses at the interface and allowing for test condition modulation. The rational minimum normal stress is σ’ = 25 kPa for the Shear Box test which is higher than the actual stresses induced by a layer of cap soil cover .So Inclined Plane test is generally preferred for the design of cap liner. Both the shear box (SB) and the inclined plane (IP) tests are presented on Figures 15 and 16 respectively. Shear box device is initially based on a large direct shear equipment. In addition the present box has been adapted to fit the need of a uniform distribution of the normal stress σ', thanks to an hydraulic bag set under the compression plate. Inclined plane device is specially designed for tests on soil- geosynthetic or geosynthetic-geosynthetic interfaces. This is the adaptation of the two devices to this configuration which is presented on Figures 15 and 16. In addition, adaptation of the device for simulation of water flow at the interface is also possible (Gourc et al,2001, Briançon et al , 2002) .
Figure 15. Shear Box Device ( SB ) .Adaptation to Geosynthetic / Geosynthetic interfaces
Displacement monitoring