Interaction behaviour of soil and geosynthetic reinforcement

The research described in this thesis concerns the use of transparent soil in physical modelling to better understand theoretical and analytical analyses of a geotechnical engineering problem. One of the more recent evolutions in the field of geotechnics is the use of geosynthetic materials as reinforcement to improve the shear resistance of soil, and ultimately provide reinforcement to earth structures. Their application in engineering earthworks has increased significantly in recent years. When designing reinforced earth structures, a vital aspect is to understand the interaction between the reinforcement and the compacted soil as this governs the overall stability. The main function of the reinforcement is to redistribute the stresses within the soil structure in order to enhance the internal stability of the reinforced soil structure. The reinforcement undergoes tensile strain as it transfers loads from unstable to stable zones of the soil. The most common example of soil-geogrid interaction research is to investigate pull-out capacity. The lack of knowledge of interaction mechanics between soil and reinforcement has considerable impact on the ability to implement rigorous analytical solutions, or to assign suitable parameters for interface elements in numerical modelling. By using classical pull-out, previous researchers have indicated that the interface factors vary between 0.6 - 0.8 (FHWA-NHI-00-043, 2001); hence, it is likely that many designs over predict the possible resistance that may be generated. Furthermore, in the absence of field validation, there is uncertainty as to how representative small scale pull-out tests reflect the likely behaviour that would prevail in the prototype structure. The transparent soil utilised here is representative of coarse soil and allows nonintrusive measurement of soil displacement on a plane highlighted by a sheet of laser light, captured by a digital camera. This enables the measurement of the displacement of the soil on the target plane by using the image process technique “Particle Image Velocimetry”. This technique allows the observation of the interaction between soil and geogrid, and the shear and pull-out boundary which is mobilised around the geogrid. The principal aim of this research is to investigate the detailed interaction between granular soil and geosynthetics, and to provide a better understanding of the interaction both analytically and numerically. To achieve this aim, this research is separated into two key areas: 1. Analytical modelling of the interaction between soil and geogrid to assess the degree of uncertainty inherent in the methods; 2. Advanced visualisation element tests using transparent soil technology and Particle Image Velocimetry (PIV) to directly observation of the patterns of strain between the soil and reinforcing material.