FINITE-ELEMENT FORMULATION FOR THE ANALYSIS OF INTERFACES, NONLINEAR AND LARGE DISPLACEMENT PROBLEMS IN GEOTECHNICAL ENGINEERING

The use of fabric materials in soil construction projects has created an important need to theoretically predict their behavior using experimentally measured material properties. At sites where temporary roadways are constructed by placing stone over existing soft soil, the performance of the roadway is often unsatisfactory due to the development of large deformations and rutting with the application of repeated wheel loadings. The principal objective of the mathematical formulation is to model the behavior under load of a reinforced soil system, where a fabric is placed over a soft soil and covered with stone for use as a temporary haul road. This approach is now widely used to improve the behavior of temporary roadways, particularly where very soft soils are encountered.; The purpose of the theoretical and analytical investigation is to define the stress distribution and the load-deformation characteristics of the soil-fabric system for varying geometries and material properties. Included in the mathematical formulation are such features as: nonlinear behavior of the soil and fabric materials, friction parameters of the interface, tension characteristics of the fabric materials, large displacements in finite deformation, "no tension" conditions of the cohesionless materials, and yielding of plastic materials. The mathematical model is a more complete approximation of the actual fabric-soil system than is presently available.; The principal features that were implemented in the finite element formulation to accurately model the problem are: (a) Eight Node Isoparametric Element. Eight node isoparametric elements are used to model the soil and stone. This type of element permits using curved boundaries and gives accurate representation of the variation in stress and strain through the element. (b) Nonlinear Material Behavior. The nonlinear behavior of the material is described by a uniaxial stress-strain curve. The program computes for the stress conditions, the corresponding elasticity matrix and/or the yield conditions of the elements. (c) Anisotropy. For materials such as stone having different moduli of elasticity in two orthogonal directions, the anisotropic elastic constants are used for an initial elastic analysis. (d) Interface Modeling. With the use of six node spring elements, the interface stress conditions are computed; and the slip or separation condition between interfaces is established. (e) Fabric or Membrane Flexible Elements. A special element is used to represent the fabric at the interface of stone and clay. In this thesis the term fabric element is used replacing a more general term flexible membrane element. The fabric element used can take only tension forces with compression or bending resistance not being permitted in this element. The fabric elements provide the "reinforcing" of the system and can have nonlinear material properties. (f) Large Displacement. Nonlinear strain-displacement relations are included as an option. Hence, the solution is valid as the fabric-soil system undergoes large displacements. (g) Incremental Loading. The load applied on the system is added in small increments to give a complete load-displacement history of the model. (h) No-Tension Analysis. The fact that stone cannot take a significant level of tension is fully considered in the no tension model. The model modifies the stresses for elements in failure and applies equilibrating forces for the introduced modification. (i) Failure Conditions. The plasticity characteristics of each element for each material are taken into account by use of the Drucker-Prager yield criterion for each element. Associated plasticity is used together with a corresponding flow rule. The plasticity formulation is limited to small strains. The present finite element formulation does not include time effects due to viscosity or consolidation.