Effect of randomly distributed geofibers on the piping behaviour of embankments constructed with fly ash as a fill material

Today, fly ash is being used as a structural fill material in highway and railway embankments, for ash pond bunds, levees, filling low-laying areas etc. Seepage induced failures in the form of piping can weaken and affect the performance of an embankment constructed with fly ash as a structural fill material. This paper presents a study to examine the effect of randomly distributed geofibers on the piping behaviour of fly ash. In this study, a number of experiments were carried out for determining the seepage velocity and piping resistance of fly ash, mixed randomly with flexible polyester fibers having various dosages and lengths. Seepage velocity of flow of water through fly ash is calculated for each case and compared with an unreinforced fly ash. It was observed that fibers reduced the seepage velocity, increased the piping resistance of fly ash and delays the attenuation of piping phenomena considerably. Finally the mechanism by which discrete and randomly distributed fibers restrain piping of fly ash is explained with the fly ash fiber interaction.     

Stabilization of soft clay using short fibers and poly vinyl alcohol

In this study, the effect of the combined addition of fibers and a nontraditional polymer on the mechanical behavior of a clay was investigated. Poly vinyl alcohol, PVA, used as a solution with concentrations of 0.1%, 0.3%, 0.5%, 1.0% and 1.5% and 1,2,3,4 Butane Tetra Carboxylic Acid, BTCA was added as a crosslinking agent at concentration rates of 0.1%, 0.3% and 0.5%, respectively. Short polypropylene fibers were added to the clay at proportionate quantities of 0.25% and 0.50% of the dry weight of the soil. Clay samples were prepared for unconfined compressive strength (UCS) tests at two different initial void ratio values, denoting relatively stiff and markedly soft states. UCS tests were conducted on both 1-day and 14-day cured samples. The results confirmed significant UCS improvements with combined fiber reinforcement and PVA-BTCA stabilization when samples were cured for 14 days. It was also observed that fiber reinforcement outperformed PVA-BTCA stabilization for clays with the lower initial void ratio. PVA-BTCA stabilization was however found to be superior to fiber reinforcement in clays with a relatively higher initial void ratio. The effect of fiber reinforcement and PVA-BTCA stabilization on the stability of soils subjected to excessive wetting was also evaluated using soaking tests. Stabilization with PVA and BTCA was found to enhance the stability of soaked samples significantly. The results of soaking tests proved that BTCA made PVA-stabilized samples more durable when exposed to soaking.     

Shear strength behavior of clayey soil reinforced with polypropylene fibers under drained and undrained conditions

The fundamental mechanisms controlling shear strength and deformability behavior of clay-fiber mixtures have still not been well established, nor the constraints that may affect their performance of shearing under different drainage conditions. This study aims to understand the behavior of a clay soil mixed with polypropylene fibers using results from drained and undrained triaxial compression tests, and to provide necessary calibration data for a shear strength prediction model. In drained tests, shear strength increased with fiber inclusion for a given mean effective stress, represented by an increase in apparent cohesion. In the undrained tests, the shear strength was not affected by pore water pressure generation. Results from the drained and undrained tests indicate that the fiber content had a greater influence on the apparent cohesion than on the friction angle. Drainage affected the improvement in the peak shear strength of fiber-reinforced soils, with superior improvement in the drained tests. As the percent improvement in shear strength decreased with increasing effective confining stresses for both tests, the difference in behavior in the drained and undrained tests was attributed to the strain at failure, with failure occurring at large strains in the drained tests but at smaller strains in the undrained tests.     

Determination of three dimensional hydraulic conductivities using a combined analytical/neural network model

Analysis of groundwater flow through fractured rock masses is an essential step in many engineering and environmental problems, such as in safety assessment of radioactive waste storages, hydrocarbon storage caverns and hydropower projects. The most important hydrological parameter in groundwater flow analysis is the hydraulic conductivity which is anisotropic and heterogeneous in the fractured rock masses. To analyze the groundwater flow correctly, some site investigations through boreholes must be carried out. One of the challenges in seepage analysis for an engineering project is how to determine the anisotropic and heterogeneous hydraulic conductivities of the fractured rock masses using the limited in situ investigation data. In this study, a new practical approach for the determination of three dimensional hydraulic conductivities of fractured rock masses is presented. Starting from rock fracture properties surveyed in six boreholes, the anisotropic hydraulic conductivities are estimated using the in situ injection test results and Oda’s theoretical model. A neural network method is then utilized to generate the three dimensional heterogeneous hydraulic conductivities based on the anisotropic hydraulic conductivities along the six boreholes. In order to evaluate the reliability of this approach, a 3D numerical seepage model using code FLAC3D is performed for a real project. The inflow values in a shaft obtained with the 3D numerical analysis are compared with the in situ measured flow. The result indicates that the derived hydraulic conductivity is acceptable.