Interface and composite behaviour of geosynthetic-reinforced soils

Abstract

Reinforcing soils with geosynthetics can significantly improve their engineering behaviour and performance, mainly by increasing their bearing capacity, providing additional confinement, reducing vertical settlement and lateral deformation, and increasing overall stiffness. Reinforcement is achieved by the tensile forces that the geosynthetics develop under load, which contributes to the overall stability of the reinforced soil composite. Geotextiles and geogrids are the two geosynthetic products commonly used as reinforcement elements in a soil mass. In particular, the apertures within geogrids provide additional interlock embedded soil particles. Due to the increasing use of geosynthetics in geotechnical engineering for soil reinforcement, stabilisation and ground improvement, a deeper understanding of the interface and composite behaviour of geosynthetic-reinforced soils is required.The interface parameters, such as interface friction angle, adhesion and pull-out resistance, are essential for the design and stability analysis of geosynthetic-reinforced soil structures. The two commonly used experimental techniques to determine these interface parameters are large-scale direct/interface shear tests, and pull-out tests. However, the interaction mechanisms between soils and geosynthetics in these two testing modes remain poorly understood, as does the theory used to describe them. In particular, geogrids have complex and varying aperture shapes, sizes and thicknesses. The contribution of each shear strength component at the soil-geogrid interface has not been well established in the literature. To investigate the interface behaviour of geosynthetic-reinforced soils, laboratory large-scale direct/interface shear and pull-out tests were carried out on different types of materials, including roadbase, crusher run aggregate, sand, and four types of geosynthetics. The applications of geosynthetics in base reinforcement and subgrade stabilisation were experimentally modelled in the large-scale interface shear tests. Theoretical frameworks were then proposed to interpret the experimental results by quantifying the contribution of different shear strength components mobilised along the interface between the soil and geogrid. Furthermore, the effects of scalping on the direct and interface shear strengths of aggregate were investigated. The particle breakage caused by loading and shearing in the direct and interface shear tests on aggregate with different degrees of scalping was assessed and discussed, based on sieving analyses and inferred particle breakage indices.In terms of testing techniques, the conventional single-stage testing method was found to be very labour intensive and time consuming for large-scale direct/interface shear and pull-out tests. This is because single-stage tests require at least three specimens tested under three different normal stresses, so a large amount of soil is needed, with sample preparation required for each test. This makes single-stage large-scale direct shear and pull-out testing very expensive in commercial laboratories. Based on the limited references to multi-stage direct shear tests in the literature, a multi-stage testing method was attempted for the large-scale direct/interface shear and pull-out tests. The feasibility, reliability and applicability of the multi-stage testing method were evaluated, including its advantages and disadvantages, and an optimum multi-stage testing procedure is recommended.Anchored geosynthetics are able to withstand higher tension and provide higher anchorage capacity. The influence of anchorage angles on the pull-out resistance of geotextile wrap around anchorage was investigated both experimentally and theoretically. The three-stage mobilisation of the pull-out resistance of a geotextile wrap around anchorage was investigated, and theoretical equations were derived to predict the pull-out resistance of the geotextile wrap around anchorage for varying anchorage angles.Finally, the composite behaviour of clay reinforced with ordinary sand column (OSC) and geotextile encased sand column (GESC) was experimentally investigated using direct shear, triaxial and oedometer tests. The effects of OSC and GESC on the shear strength, consolidation characteristics, pore water pressure, Mohr circle, failure envelope and stress path, are discussed and analysed, in terms of both total and effective stresses.