Determination of strain parameters of soils for elastoplastic models

The city of Munich forms the core of a fast growing urban region in the Alpine foreland of Bavaria, in southern Germany. The subsurface is composed of Neogene and Quaternary formations made up of loose alluvial, fine- to coarse-grained sediments. Today's landscape is known as the Munich gravel plain, which comprises sander terraces formed during the Pleistocene glacial periods, as well as the modern floodplain of the River Isar. Caused by a major unconformity and hiatus, these rather young gravel terraces overlie Neogene Molasse deposits of the Alpine foreland basin, which in contrast comprise fine-grained fluviatile and lacustrine facies. The geology of Munich is well known due to numerous underground structures (e.g. subway tunnels), which were built in the past decades. Nevertheless, the multiphase geological history and the complex sedimentary architecture in combination with new construction methods constantly raises new geotechnical problems and challenges. Requirements for geological site investigation and ground modelling thus are very high as it is the case in many other urban areas worldwide, which face similar ground conditions, e.g. cities in major floodplains, coastal areas or forelands of mountain belts. The use of the finite element (FE) analysis has become widespread and popular in geotechnical practice as means of controlling and optimizing engineering tasks. However, the quality of any stress-strain prediction depends on the adequate model being adopted in the study. In general, a more realistic prediction of ground movements requires using the models which account for pre-failure behaviour of soil, i.e. a non-linear stress-strain relationship before reaching the ultimate state. Such behaviour, mathematically modelled with non-linear elasticity, is characterized by a strong variation of soil stiffness, which depends on the magnitude of strain levels occurring during construction stages. Pre-failure stiffness plays a crucial role in modelling typical geotechnical problems such as deep excavations supported by retaining walls or tunnel excavations in densely built-up urban areas. An important aspect by the construction of urban tunnels is the control of surface subsidence to minimize any disturbance to nearby buildings and services. The available empirical, analytical solutions coupled with numerical modelling of strain of soils allow sufficient predictions of surface settlement trough due to tunnelling. This paper presents the results of determination of the modules of deformation from compression, K0-triaxial and dilatometer tests and calibration of their values by numerical simulation of these tests. Numerical simulations are carried out using elasto-plastic model with hardening (PLAXIS Hardening Soil). Only the HS model could describe deformations under loading, unloading and reloading realistically. Simulations using the MC model only represent the initial loading well. The module of deformation is undoubtedly the geomechanical parameter that best represents the mechanical behaviour of soils. The characteristic of model with the list of the parameters entering into it is given. In detail it is shown how to find nonlinearity parameter of a compression curve. Design values of deformability of soils are established.

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