Proceedings of the 3rd Biot Conference on Poromechanics, 24-27 May 2005, Norman, Oklahoma, USA, 2005
Back analysis of well instrumented excavations showed that the short term behavior of excavations... more Back analysis of well instrumented excavations showed that the short term behavior of excavations is best expressed in terms of effective stress analysis. The three-dimensional consolidation theory, introduced by Biot (1941), assumes solely a one-dimensional loading, namely a change of stresses in vertical direction but drainage of the pore water in all directions. The complex relationships between the threedimensional change of stresses due to the excavation and the resulting wall movements are not yet considered in the current available computation methods. Therefore, the simultaneous influence of the stress relief due to excavation and the stress-path-dependent displacements on the consolidation process of an excavation has to be yet quantified. The paper introduces the research work currently running at our institute that investigate this problematic based on small scale model tests and controlled stress path triaxial tests. Beside the vertical deformations, horizontal deformations can be induced, which result from the wall deformation due to the redistribution of stress as a result of excavation. The permeability of the soil depends on the volume change, i.e. on the deformation behavior of of the soil. The relief of stresses due to excavation is to be understood as a negative load, which causes a volume increment of the soil and an increase of the pore volume. Depending on the permeability and the stiffness of the soil, a negative pore pressure can be developed. The unloading stiffness of the soil is taken in this case as governing stiffness. A question may however arise that to what extent will the permeability of the soil be affected by the heave of the soil due to the excavation The assumed linear elastic material behavior does not consider the irreversible deformations and the possible coupling between shearing and volume change. Moreover, the linear elasticity theory is independent of the stress paths. To date no theoretical approach is available that reasonably describes the consolidation process in excavations. The three-dimensional consolidation theory of Biot (1941) considers the same assumptions and conditions as Terzaghi’s one-dimensional theory. The differential equations of the three-dimensional consolidation analysis are based on the equilibrium and continuity conditions of a volume element of a soil. The continuity condition describes the relationship between the volume change of the soil particles of a given volume element and the amount of water flowing out of the element. It should be noted, however, that the volumetric deformations due to the drainage of the pore water in soil in all directions in three-dimensional consolidation theory are caused by loading in one direction only, namely the vertical direction. The increase in volume is described here by the Poisson’s ratio ν. The complex behaviour of a three-dimensional change of stresses due to excavation cannot therefore be described on the basis of the Biot theory.
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Books by Patrick Becker
The research work is organised in three core areas which are developed one up on the other. Using excavation model tests, a numerical parameter study of three idealized excavations, and stress path controlled element tests, the above mentioned factors are examined. The time and stress path dependent material behaviour can be generally analysed and validated using a numerical analysis of three practical case studies based on a method that considers the stress path dependent stiffness. This method is devolped and explained within this thesis. Two of the practical projects had been accompanied by the author in the framework of this research work during the construction phase and one starting from the design phase.
Papers by Patrick Becker
The research work is organised in three core areas which are developed one up on the other. Using excavation model tests, a numerical parameter study of three idealized excavations, and stress path controlled element tests, the above mentioned factors are examined. The time and stress path dependent material behaviour can be generally analysed and validated using a numerical analysis of three practical case studies based on a method that considers the stress path dependent stiffness. This method is devolped and explained within this thesis. Two of the practical projects had been accompanied by the author in the framework of this research work during the construction phase and one starting from the design phase.
based on empirical values for „EA-Pfähle“. The basics of the
derivation of axial pile capacity based on empirical values and
selected results are presented, which are already integrated in
the national German recommendations for piles „EA-Pfähle“.
The study provides to a large extent a secured range of empirical
values for the base resistance and the skin friction for different
pile systems as a function of the soil strength. Based on comparative
statistical analysis of pile load tests of different pile systems,
it becomes possible to derive a consistent analysis of bearing
behaviour of pile systems, which provides a safe and an economical
pile bearing capacity depending on the expense of preliminary
soil investigations.