Finite Element Modell für die Bodenvereisung im Rahmen des Tunnelbaus
Zusammenfassung der Projektergebnisse
In geotechnical applications of artificial ground freezing, safe design and execution require a reliable prediction of the coupled thermo-hydro-mechanical behavior of soils subjected to freezing. In this research project, a three-phase finite element model of freezing soils has been developed in the context of thermo-poro-plasticity, considering solid particles, liquid water and crystal ice as separate phases, and mixture temperature, liquid pressure, and solid displacement as primary field variables. Through three fundamental physical laws and corresponding state relations, the model captures the most relevant couplings between the phase transition, the liquid transport within the pores, and the accompanying mechanical deformation. In addition to the 9% volume expansion of water transforming into ice, the contribution of the micro-cryo-suction mechanism to an even greater frost heave phenomenon was described in the model. By connecting the theory of poromechanics with the theory of premelting dynamics, the liquid-crystal equilibrium equation was first used to interpret the micro-cryo-suction effect in the frozen fringe. Through a comparison study on 1D freezing test, it has been concluded that the latent heat released during freezing process is a prerequisite for the observed frost accretion, whereas the volume expansion, resulting from the water-ice density difference, turns out to be relatively insignificant. Besides, drainage in the unfrozen zone provides a continuous external water supply for the ongoing frost accretion, whereas in an undrained system frost accretion occurs as well however with less heave. To describe the mechanical behaviour of freezing soils, a novel critical state constitutive model was developed within the funding period by extending the currently most advanced phenomenological model for freezing soils, i.e. the enhanced Barcelona Basic Model by Prof. Gens’ group at UPC, to a microstructure-based approach. A novel two-step strength upscaling strategy was proposed by extending the concept of the so-called Linear Comparison Composite (LCC) method. This research on strength upscaling originally was not planned. However, adapting the research direction was leading to surprisingly good results as showcased by several validation tests. The generalization of the proposed extended LCC methodology potentially also allows for a robust multi-scale strength homogenization of other complex hierarchical composites, such as concrete, geological materials, and fibre-reinforced composites. Through an extensive study on the application of developed model to AGF for temporary support of the soil during tunnel construction, the influence of the seepage flow on the formation of frozen arch was investigated. It was shown in the numerical simulations that a large seepage flow can delay or even prevent the closure of the frozen soil body. In order to improve the freezing efficiency, an optimization model using the Ant Colony method was adopted to optimize the freeze pipe arrangement. It has been demonstrated that, without increasing the number of the freeze pipes, the optimized flow-adapted placement of freeze pipes can significantly reduce the freezing time and hence the operating costs. As was shown in one of the analyses, for a seepage flow of 1.0 m/day a reduction of the required freezing time from 140 to 17 days can be obtained. The potential energy and cost savings obtained from numerical simulations of soil freezing in tunneling in association with an optimization of the pipe arrangement underline the transfer potential of the computational model developed within another research project funded by the DFG.
Projektbezogene Publikationen (Auswahl)
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A three-phase finite element model of water-infiltrated porous materials subjected to freezing. In M. Papadrakakis, E. Onate, and B. Schrefler, editors, Coupled Problems 2011, pages 1083–1094. CIMNE, 2011. CD-Rom, ISBN: 978-84-87867-59-0
M.-M. Zhou and G. Meschke
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Computational mechanics of multi-phase problems - modeling strategies at different scales. Computer Assisted Mechanics and Engineering Sciences, 18:73–89, 2011
G. Meschke, D. Leonhart, J. J. Timothy, and M.-M. Zhou
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A three-phase THM coupled FE model for freezing soils. International Journal for Numerical and Analytical Methods in Geomechanics, 37(18):3173– 3193, 2013
M.-M. Zhou and G. Meschke
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Strength homogenization for partially frozen soil using linear comparison composite approach. In Poromechanics V, pages 560–568. American Society of Civil Engineers, 2013
M.-M. Zhou and G. Meschke
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Computational simulation of soil freezing: multiphase modeling and strength upscaling. PhD thesis, Ruhr University Bochum, 2014
M.-M. Zhou
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Strength homogenization of matrix-inclusion composites using the linear comparison composite approach. International Journal of Solids and Structures, 51:259–273, 2014
M.-M. Zhou and G. Meschke
