Technical Paper by A. Fakher and C.J.F.P. Jones WHEN THE ...

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FAKHER & JONES • When Bending Stiffness of Geosynthetic Reinforcement is Important 1 INTRODUCTION A clay with a very high water content behaves neither like a liquid nor like a solid, it has very little shear strength and can be termed a super soft clay. In the current paper, super soft clay is defined as a disturbed cohesive soil whose water content is higher than its liquid limit; such materials display extremely low yield stresses and represent difficult construction conditions (Fakher et al. 1999). An established technique used to enable construction on super soft clays is the introduction of a primary construction stage that is used as a working platform on which the main construction can be founded. A working platform can be produced by placing a layer of geosynthetic reinforcement over the super soft clay and covering this with a layer of cohesionless fill (Figure 1) (Yamanouchi and Gotoh 1979). Although it has been demonstrated that this construction technique is successful, there is no general agreement with respect to the reinforcement mechanism and how the reinforcement improves the bearing capacity of working platforms. The current paper is aimed at providing insight into the reinforcement mechanism associated with a thin layer of cohesionless fill supported on geosynthetic reinforcement layered on the surface of super soft clays. The numerical modelling presented has been undertaken in parallel with a number of physical model studies into construction over super soft clay (Fakher and Jones 1996a; Fakher and Jones 1996b; Fakher et al. 1996; Zakaria 1994). A major advantage of numerical modelling is that it enables parametric studies to be undertaken that enhance the results and findings of physical models. 2 DETAILS OF THE ANALYSIS 2.1 Idealisation and Boundary Conditions A representative super soft clay, overlain by a layer of sand with/without a layer of 0.5 B Sand D Super soft clay Geogrid reinforcement 10 B 7 B 10 B Figure 1. The geometry of the problem. 446 GEOSYNTHETICS INTERNATIONAL • 2001, VOL. 8, NO. 5
FAKHER & JONES • When Bending Stiffness of Geosynthetic Reinforcement is Important reinforcement, was simulated in the current study. A surface load was applied by a rigid, rough footing of width, B, placed on the surface of the sand layer (Figure 1). The super soft clay and cohesionless fill were idealised as construction materials and the reinforcement considered to be either formed as a cable or a row of simple beam elements. The analyses were performed using the finite difference program FLAC (Itasca 1991). Details of the analysis grid and the boundary conditions used are shown in Figure 2. Because of the symmetrical geometry, each analysis was performed on half of the model. 2.2 Behaviour and Mechanical Parameters of Soils Leighton Buzzard sand was used as the cohesionless soil in the parallel experimental studies on construction over super soft clays, and the properties of this material were assumed in the analytical study (Fakher and Jones 1996b). A friction angle, φ = 35°, was determined from laboratory tests, and a value for Poisson’s ratio, ν = 0.37, was chosen based upon the suggestion by Stroud (1971). A unit weight of 16 kN/m 3 was assumed for the sand. The elastic modulus, E, of sand depends upon the confining pressure and is not constant within a mass of sand. However, to simplify the analysis the elastic modulus, E, of the cohesionless fill was assumed to be 32 MPa based upon general values suggested for quartz sands (Lambe and Whitman 1979). The values for the shear modulus, G, and the bulk modulus, K, of the cohesionless fill were derived using the following relationships: Figure 2. The grid pattern used in the current study. GEOSYNTHETICS INTERNATIONAL • 2001, VOL. 8, NO. 5 447
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