Geosynthetics

The coupled effect of using geosynthetic reinforcement and randomly distributed fibers on the stability of slopes was evaluated using finite element modeling and limit equilibrium methods by analyzing a case study in Oslo, Norway. The main objective was to simulate the failure condition of the original slope and quantify the improved stability of a hypothetical reinforced slope constructed with geosynthetic layers and distributed discrete fibers. The stability of the slope was evaluated in both the short-term condition with its' undrained shear strength parameters, and the long-term drained condition. Results indicate that the combination of the techniques was found to have a possible increase of about 40% in the short-term condition and about 60% in the long-term condition of the factor safety associated with the slope.

A numerical investigation was conducted to evaluate the geotechnical safety and slope
stability of Municipal Solid Waste (MSW) landfills, considering the effects of
geosynthetic reinforcements, biodegradation of the waste, and associated changes in
material properties, and extreme wind force simulating hurricane conditions. Three
different landfill slopes, 1:1, 1:2, and 1:3 having the height of 122m and width of 2134m,
were analyzed using Limit Equilibrium Method (SLOPE/W) and Finite Element
Modeling (ANSYS). Techniques developed in this study were used to analyze a case
history involving a geogrid reinforced mixed landfill expansion located in Austria. It was
found that few years after construction of the landfill, there is a significant decrease in the
FS due to biodegradation. Extreme wind loading was also found to cause a substantial
loss in the FS. The geosynthetic reinforcement increased the slope stability and
approximately compensated for the damaging effects of biodegradation and wind
loading.

Landfilling, by all indications, will continue to be the predominant method of solid waste disposal. Traditional civil engineering drainage medium (i.e. sand or gravel) are being replaced by geosynthetics which are much thinner in an effort to create more usable volume for waste. This study examines the effect of compressive creep of geonets as used in leachate collection and detection systems, and how it affects in-plane drainage. HDPE geonet was subjected to a compressive load of 110 psi. The in plane flow rate of municipal solid waste leachate was measured, as well as the change in thickness, for 120 days. In addition, geonet samples were placed between two pieces of HDPE geomembrane. These samples were subjected to a normal load of 140 psi for 120 days. The samples were then inspected for sign of geonet imprint into the geonet, or for strand layover.

Many modern landfills are constructed with double liner systems. Leachate leakage rates through double liner systems are calculated using recently developed formulations which are theoretically correct for leakage detection system (LDS) materials that have unrestricted lateral flow properties. But their applicability to geonets, the most commonly used LDS material, has yet to be determined. In double liner systems, the leakage through the primary liner, the properties of the LDS material, and the slope of the LDS determine the flow patterns in the LDS. These flow patterns are then used to determine the amount of leachate, if any, which leaks through the bottom liner into the ground. This thesis describes the experimental determination of the flow patterns in the geonets and their relationships to established design formulations.

The dissertation is an experimental and analytical investigation of the long term performance of mechanically stabilized earth (MSE) walls with geosynthetics, with particular focus on rational methods to enable the determination of the applicable factors for use in Load Resistance Factor Design (LRFD). An overview of current issues concerning MSE walls is followed by an extensive literature review addressing MSE walls, pullout strength, creep and creep rupture, durability and degradation, design methodology, analytical prediction, and field evaluation of MSE walls. The experimental tasks comprise: (i) creep and creep rupture, (ii) durability and degradation, (iii) small scale testing of MSE walls with a model prototype ratio of 1:5.5, and (iv) construction of prototype MSE wall and instrumentation for long-term performance. The analytical work comprises finite difference modeling using the Fast Lagrangian Analysis of Continua (FLAC) software, (i) For creep up to 10,000 hours accelerated exposure for HDPE and PET geogrids, with super-ambient temperatures and soil water conditions related to soil conditions in Florida, the significant part of creep was due to temperatures and not solution exposures, with creep rupture occurring primarily for HDPE. (ii) For durability, performance at ambient temperatures was extrapolated, based on the Arrhenius method. The variation in degradation between the different solutions was minimal, indicating hydrolysis as the main cause for PET at elevated temperatures. (iii) Two HDPE and two PET reinforcement small scale (1:5.5) MSE walls were tested, with different surcharges each for 72 hour periods. Panel movements, strains in the reinforcement, and wall settlements were measured, indicating values smaller than the predicted, mostly for the smaller surcharges due to distortion caused by scaling neglecting the gravity effect. (iv) For analysis with FLAC computer software, two correction factors "a" and "b" were applied to correct the discrepancies between the model and the test values. The PET MSE small scale wall showed more deviation because the material has a low modulus of elasticity. (v) A preliminary comparison of the small scale and the prototype MSE wall behavior indicated discrepancies due to distortion scaling related to the lack of gravity simulation.