Hypoplasticity of Structured Geomaterials

Among the group of engineering materials, geomaterials exhibit the largest variability (cf. soft soils - hard granite) not only due to different genesis but also due to their long history in the course of which they have been exposed to various agents such as tectonic forces, earthquakes, weathering, fracturing, percolation of liquids, extremely large deformations etc. All these actions leave behind some traces which affect the mechanical behaviour of geomaterials imposing to them several sorts of internal structure. Tunnellers and other practical engineers having direct contact to the underground well know that geomaterials posses a fascinating internal structure. Also geologists are aware of the internal structure of geomaterials having introduced many notions such as "texture" and the like. In mathematical models of the mechanical behaviour of geomaterials, however, it is up to now scarcely taken into account. This is due to the fact that it is difficult enough to introduce satisfactory constitutive models even for unstructured materials undergoing irreversible deformation. The urgent necessity for realistic models of geomaterials follows from the society demands on reliable engineering constructions and high safety standards (e.g. reduction of hazards caused by excavation or tunnelling activities, prediction of ground motion due to earthquakes and/or landslides, stability assesment for slopes, mitigation of land subsidence, etc.). Hypoplasticity is a novel framework for mathematical models of the mechanical behaviour of ge-omaterials. Having its roots in Rational Mechanics hypoplasticity is characterized not only by its mathematical rigor but also by easy application: The main field of application of hypoplasticity up to now was the mathematical description of soil behaviour in geomechanical problems. This field is not only fascinating from a scientific point of view but it is also of high importance for a series of techno-logical applications like geotechnical engineering, tunnelling, chemical engineering, environmental engineering, offshore and petroleum engineering and mining. In regards of some fascinating effects of structural and engineering geology the project aims to make them better accessible to numerical simulations providing more realistic models than it is at present the case. Consequently, the applicability of modern engineering methods in geologically difficult areas (e.g. in mountainous and coastal regions) will be improved. A unified approach to soil and rock is laid down, as the gradual transition from one to the other is considered as being mainly a change of the internal structure. The envisaged main contribution of the proposed project will be a unification of important structural effects of geomaterials in a rigorous framework of hypoplasticity.

Within this research project models were developed which allow to simulate the mechanical behaviour of structured geomaterials. E.g., the alteration of the structure of clay due to preconsolidation could be modelled with a hypoplastic model with structure tensor. This tensor depends on the previous stress, by which the material was loaded, and allows therefore the description of physical effects depending on the loading history. This material law allows the simulation of normally and overconsolidated samples with only one set of parameters. In the simulation of simple shear tests could be obtained with this hypoplastic version a realistic value for the cohesion due to overconsolidation. Furthermore, it has been developed also a visco-hypoplastic model which allows to reproduce quite well time- dependent (=viscous) phenomena. The approach for this material law consist in the definition of an anisotropic tensorial creep rate which describes the time dependent increase of deformation with constant stress. With this material law can be simulated time dependent material behaviour (e.g. so-called jump tests with variable deformation velocities) of anisotropic materials and it is also possible to simulate time-dependent behaviour such as creep and relaxation. This material law enables therefore to carry out calculations with a significant variation of the stress level and the overconsolidation ratio. This is the case at excavations and in tunnelling. With this material law the estimation of settlements can be improved and therefore a higher reliability of the calculations in foundation engineering and in tunneling can be achieved. The prediction of movements due to earthquakes or landslides can be improved and therefore the safety level can be increased in the case of such extraordinary events.