For the design and performance analyses of geosynthetic-reinforced soil structures under repeated loadings, such as those induced by compaction, traffic and earthquakes, the understanding of cyclic soil-geosynthetic interface behaviour is of great interest.Nevertheless, the experimental data concerning this type of behaviour are very scarce.

Abstract:18 Installation damage of geosynthetics occurs during their handling, positioning on the ground

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Soil-geosynthetic interface shear strength is an essential parameter for the design and stability analysis of geosynthetic-reinforced soil structures. Economic and environmental reasons have led to increasing use of locally available residual soils with a significant percentage of fines and lower draining capacity, when compared with the traditional good-quality backfill materials. This paper describes an extensive laboratory study carried out using a large-scale direct shear test device, in which the influence of soil moisture content, soil density and geosynthetic type on the direct shear behaviour of the soil-geosynthetic interface was evaluated. The study involved a locally available granite residual soil and four geosynthetics: two geogrids (one uniaxial and the other biaxial), one geocomposite reinforcement (high-strength geotextile) and one geotextile. Test results have revealed that the increase in soil moisture content can measurably reduce the soil-geosynthetic interface shear strength. Regardless of soil moisture content, soil density proved to have a remarkable influence on interface shear strength, particularly when geogrids were used. Among the different geosynthetics tested, the biaxial geogrid was found to be the most effective reinforcement for this particular type of soil, concerning the direct shear mechanism. For soil-geogrid interfaces, the coefficients of interaction ranged from 0.71 to 0.99. For soil-geotextile interfaces, the coefficients of interaction varied from 0.54 to 0.85.

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In this paper, data from field installation trials of geosynthetics and laboratory tests are presented and analysed.The influence of several factors was assessed, namely nominal strength and type of geosynthetic, soil, compaction energy and method used to induce installation damage. Visual observations using a scanning electron microscope were performed. From the data collected, reduction factors for installation damage were derived using tensile strength values (traditional approach) and stiffness modulus (for 2% strain). Relative to the stiffness approach, the results obtained indicate that the traditional approach can be conservative. The reduction factors, determined using the traditional approach, were also compared with interval estimates from the literature. To contribute to supporting a shift from a factor of safety approach to a limit state design, bias statistics to correct the deterministic predictions were determined from the results. Different correlations were also established to enable using these results to interpolate reduction factors for similar installation conditions and/or geosynthetics from the same family of products. Installation damage reduction factors should be used in limit state design (ultimate and serviceability). Nevertheless, the stiffness approach can only be used for limit states where tensile failure of the geosynthetics will not occur.

This paper presents the parameters of municipal solid waste shear strength determined in the laboratory (triaxial tests) and by in situ tests: standard penetration tests (SPT) and cone penetration tests (CPT). The results analyzed here are part of a study carried out on the Santo Tirso landfill (north of Portugal) between 2001 and 2007. The influence of the strain levels, waste composition, and waste age on the shear strength parameters is presented, as well as an attempt to establish some correlations between the SPT and CPT tests and to estimate municipal solid waste (MSW) friction angles from the SPT tests. The results indicate that the aging of the waste, which is characterized by a decrease in fibrous and organic materials and an increase in inert materials and fine fraction, leads to an increase in frictional resistance and to a decrease in cohesion. The results of the SPT and CPT tests indicate higher penetration resistance in older and deeper waste. Estimating the frictional resistance from the SPT test seems to obey an empirical relationship expressed by a power function, which depends on the strain level.

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