Reliability-based stability analysis considering GCL shear strength variability (original) (raw)
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Interface shear strength variability and its use in reliability-based landfill stability analysis
Geosynthetics International, 2006
Failure of modern landfills by slippage of lining materials and waste bodies is not uncommon. The majority of failures are controlled by slippage at interfaces between lining components. Information on variability of interface shear strength is required both to carry out limit equilibrium stability analysis using characteristic shear strengths and to analyse the probability of failure. Current practice is to carry out a limited number of site-specific tests, and this provides insufficient information on the variability of interface strength for design. A summary of measured strengths and an assessment of variability are presented for seven generic interfaces common in landfill lining systems. This combines values from the international literature, from an internal database, and from the results of repeatability testing programmes. The implications of variable shear strength are examined though failure probability analysis for two common design casesveneer and waste body slippage -and this adds to the small number of studies published previously. The reliability analyses show that relatively high probabilities of failure are obtained when using variability values from the literature and an internal database, even when factors of safety > 1.5. The use of repeatability data produces lower probabilities for typically used factors of safety, although they are still higher than recommended target probability of failure values.
International Journal of Engineering Sciences & Research Technology, 2014
Now a day’s failure of modern landfills by slippage of lining materials is common. The majority of failures are controlled by slippage at interfaces between lining components. Information and variability of interface shear strength is required to carry out both limit equilibrium stability analysis using characteristic shear strengths and probability of failure analysis. Current practice is to carry out a limited number of site specific tests and this provides insufficient information on the variability of interface strength for design. The implications of variable shear strength are examined though probability of failure analysis of two common design cases: veneer and waste body slippage. The reliability analyses show that relatively high probabilities of failure are obtained when using variability values from the literature and an internal database even when factors of safety ≥ 1.5. The use of repeatability data produces lower probabilities for typically used factors of safety, although they are still higher than recommended target of probability failure (Pf) values
Large-Scale Shear Tests on Interface Shear Performance of Landfill Liner Systems
Geosynthetics in Civil and Environmental Engineering
Interface shear performance of various landfill liner systems were evaluated for landfill stability by conducting large scale shear tests. Testing program covers the interfaces between (1) geosynthetics (geomembrane (GM) sheet (HDPE and PVC) and non-woven geotextile) and subsoil, (2) geosynthetics and compacted clay liner (CCL), and (3) GM and geotextile. The focus of this paper is placed on interface shear performance under both as installed condition (dry for geosynthetics and optimum moisture content for CCL or subsoil) and saturated / wet condition, since landfill liner system is often subjected to saturated / wet condition due to the higher water retention capacity of CCL as well as the contact to leachate and/or groundwater. For geotextile-GM interface, there is no significant effect on the interface shear strength. The saturated CCL-GM interface had lower shear strength compared to the interface under as installed condition, although the shear performances of CCL-geotextile interface under both conditions are similar to each other. For the interfaces between geosynthetics and subsoil, the frictional resistance of HDPE with textures surface had a significant drop from 23 to 15 degree in the saturated / wet condition.
E3S Web of Conferences
The design of a competent basal lining system is crucial in ensuring a long-lasting and functional engineered municipal solid waste (MSW) landfill. However, due to the inclusion of numerous geosynthetics and geomaterials forming a multi-layered lining system, there rises an uncertainty on determining the critical or weakest interface. This is exacerbated by the different properties offered by these lining materials and their inter-crossing functions in landfills. According to ASTM D5321-20 standard, the interface shear strengths used in design of bases and side-slopes of lining systems are determined through a single interface testing configuration. However, minimal research has been done to evaluate the consequences of multi-interface testing configurations on the minimum factors of safety (FoSmin). The present study was thus conducted to further investigate this phenomena while establishing the appropriateness of double interface testing configuration using large direct shear equi...
The Evolution of Geotech - 25 Years of Innovation, 2021
Geomembrane asperities are surface protrusions which distinguish smooth ge omembranes from textured geomembranes. Asperities possess geometrical features such as height and concentration and are hypothesised to develop high interface shear strength, resist sliding and increases stability. To date, many textured-geomembranes with different asperity geometries have been manufactured and used in landfill linings together with geosynthetics like geotextiles. Previous studies have considered the effects of asperity geometries to geomem brane/geotextile interface shear characteristics. However, limited studies have considered the effects of asperity height and concentration on the landfill side-slope liner factor of safety (FoS) using the geomem-brane/geotextile critical interface as the point of reference. Thus, this study was aimed at investigating the influence of asperity geometries on liner stability. This study utilized experimental results from direct shear test (i.e. friction angle and adhesion) and performed probabilistic stability analysis using SLIDE2. Available results indicated that FoS increased as both asperity concentration and height increased. However, asperity-height increased beyond 1.2 mm mobilized FoS reduction. Therefore, obtaining an optimised liner stability factor is hinged on selecting the appropriate geomembrane asperity geometry at the critical geomembrane/geotextile interface.
IFCEE 2015, 2015
This paper discusses the probabilistic approach in landfill veneer cover design as well as the impact of variability associated with the design parameters. A framework for the reliability based design optimization methodology as reported in the literature is considered. Conventional deterministic design uses a factor of safety approach that do not cater for these uncertainties that arise due to heterogeneity of Municipal Solid Waste (MSW). The formulation considers a two-part wedge mechanism with the soil-geosynthetic interface acting as the plane of weakness. Random variables considered in the present study are unit weight, cohesion and friction angle of the cover soil, adhesion between cover soil of active wedge and geosynthetic, and long term design strength of the reinforcement. Generally the long term design strength of the reinforcement (T D) can be derived from the ultimate value of geosynthetic reinforcement strength by applying reduction factors for chemical/biological degradation, creep, and installation damage. As opposed to the use of conventional reduction factors, in the present contribution, the geosynthetic force required to stabilize the cover soils for the stability is treated as random variable. The mean value of required geosynthetic force is computed for different coefficients of variation of soil-geosynthetic interaction coefficient and ultimate tensile strength of the geosynthetic reinforcement.
Long-term shear strength of geosynthetic clay liners
Geotextiles and Geomembranes, 2008
Geosynthetic clay liners (GCLs) often have a sandwich-like multilayer structure, e.g. bentonite encased between two geotextile layers connected by fibers or yarns, either by needle-punching or stitch-bonding. Therefore, the internal shear strength of the GCL depends on the strength of reinforcing fiber bundles or yarns and their anchoring strength in the cover and carrier geotextiles. When used on long and steep slopes and covered with thick soil layers, the GCL is permanently exposed to a combined action of compressive and shear stress. Such load conditions are characteristic for landfill covers and the slope stability of the overall cover system in the long run strongly depends on the long-term internal shear strength of the GCL. A new test method was developed to study this long-term shear behavior. The focus was not only on creep, as it is normally done, but on aging effects. The shear test devices allow the measurement of creep curves and times-to-failure at elevated temperatures in different media (tap water and de-ionized water). In this publication, the main findings of the experiments on needle-punched GCLs with and without thermal treatment are summarized. Tap water as a test medium was essential to ensure sodium to calcium ion exchange in the bentonite layer. Under this condition extremely long test durations without failure were achieved. Sliding failure occurred when de-ionized water was used. Two failure modes were observed: brittle failure of the GCLs with thermal treatment and slow disentanglement of fiber bundles for untreated GCLs. Short-term shear strength (e.g. peel strength) is unrelated to the actual long-term shear strength, i.e. to the times-to-failure achieved in long-term shear strength test. Hence, short-term shear strength alone will not provide reliable dimensioning data for product design and choice of resins. Therefore, the often suggested approach, namely, restriction to short-term tests only and application of factors of safety, is challenged by these results.
Effect of Specimen Conditioning on Geosynthetic Clay Liner Shear Strength
Specifications for laboratory shear strength testing of geosynthetic clay liners (GCLs) must replicate field conditions while still accounting for time and cost considerations. A database of 414 GCL in- ternal and 534 GCL-geomembrane (GM) interface shear strength results has been assembled. Specifically, the results of large-scale direct shear tests conducted by a single independent laboratory are evaluated to de- velop guidelines on specimen conditioning. It was found that both the GCL internal and interface peak shear strengths decreased with increasing time of hydration (th). However, the GCL internal shear strength did not change for th beyond 48 hs and the GCL-GM interface shear strength did not change for th beyond 24 hs. The normal stress used during hydration affected significantly the peak shear strength due to bentonite swelling. Hydration under low normal stress followed by consolidation led to similar GCL internal peak shear strength as hydration under high normal stress...
State-of-the-art report: GCL shear strength and its measurement
Geosynthetics International, 2004
This paper presents a comprehensive source of information on the shear strength and shear strength testing of geosynthetic clay liners (GCLs). Essential concepts of shear stress–displacement behavior and shear strength interpretation are presented, including long-term performance issues, followed by detailed discussions on the laboratory measurement of the shear strength of GCLs and GCL interfaces. The paper also provides recommendations for the selection of design failure envelopes for stability analyses and checklists to assist users in the specification of GCL shear testing programs. North American practice is emphasized and discussions are focused primarily within the context of landfill bottom liner and cover systems. Conclusions are drawn with regard to GCL shear strength behavior and current GCL strength testing practice, improvements for GCL strength testing and reporting are suggested, and future research needs are identified.