Ice Lens Growth Mechanism And Hydro-Thermal-Mechanical Coupling Research On Frost Heave

Frost heave is mainly caused by the initiation and growth of ice lens.Freezing leads to frost heave of strata both in permafrost and artificial ground freezing regions,and excessive frost heave causes many geotechnical hazards in mining engineering,tunneling,and civil engineering,etc.The underlying physicochemical mechanisms behind frost heave and ice lens growth involve molecular interactions at interfacial contacts,fundamental in our understanding of a wide range of processes of crystallization,nucleation,phase changes,and mineral replacement.Understanding the basic physical properties of frozen soil,revealing the coupled hydro-thermalmechanical behavior during ice lens growth,and improving our understanding of the physical mechanics of frozen soil,will benefit the efficient resource exploitation and provide critical knowledge and guidance for constructions in both permafrost and artificial ground freezing regions.This scientific work focuses on studying the ice lens growth mechanism,and considers the coupled hydro-thermal-mechanical behavior of frozen soil under the influence of specific environment.The intergrated program of laboratory tests,modelling and simulations etc are utilized to study these scientific problems such as frost susceptibility,frost heave mechanism and theory during coupled hydro-thermal-mechanical processes,and frost heave controlling.The main results that have been achieved in this work are the following:(1)The microstructural scanning and molecular structure analysis revealed the different hydrophysical properties between montmorillonite and kaolin,indicating montmorillonite minerals have better hydrophilicity than kaolin and thus cause less frost heave.In addition,the influence of particle size on the ice lens growth mechanism is studied.The frost susceptibility of a given soil is evaluated by considering different particle sizes with different sand-silt-clay fractions in natural environment.Only in medium-grained granular media such as silt we find higher frost susceptibility,while significant frost heave may occur in the coarse-grained soil if the fine-grained soil content significantly increases.(2)In order to study frost heave-frost heaving induced pressure(FHIP)of frost susceptibility soil,a FHIP testing apparatus considering environmental constraint was developed to perform a series of freezing tests under various constraints and thermal gradients.The study reveals the coupled hydrothermal behavior in freezing soil during ice lens growth and obtains the dynamic response between thermal gradient-mechanical constraint-frost heave-FHIP.Moreover,the effects of ice crystals on pore structure and flow properties of freezing soil are investigated,and the evolution characteristics of frost heave-FHIP under mechanical constraints are analyzed.The mechanism explanation of the increased FHIP with constraint is provided,and a mathematical description shows that the maximum heaving pressure is a function of temperature of ice lens.(3)Based on crystallization kinetics and thermodynamic theory of interfacial water film,the effects of the equivalent water pressure are taken into account to determine the phase transition and the permeability in freezing soils,a new ice lens growth model is established using a water activity based chemical potential gradient for the calculation of water flow velocity,and the corresponding mathematical description of the segregation potential is presented here.This research enables a better understanding of phase change and fluid flow in the frozen fringe under the influence of water activity,and describes the growth and evolution of the ice lens.(4)According to analogy with the effective stress principle of unsaturated soils,we have clarified the physical meaning of the effective stress principle of frozen soil.Considering the coupled hydro-thermal-mechanical mechanism in freezing soils,a discrete ice lenses based model for frost heave is established with the focus on segregation and growth of the ice lens.Furthermore,considering the strong dependence of the ice lens growth on the overburden pressure,the FHIP is equivalent to a nonlinear function of mechanical constraint,frost heave,freezing time and other factors under the coupling mechanism of hydro-thermal-consolidation.Combing in-situ freezing heave,segregational heave,and the deformation of the soil column,a FHIP model is constructed in a constrained environment.Based on the numerical results of the FHIP model,the main controlling factor for the phase transition rate in the frozen fringe is discussed,and the influence of phase transition on the negative pore water pressure has been analyzed.The physical mechanism of frost heave with a nonlinear decreasing pattern under mechanical constraints is revealed based on the numerical analysis of cryo-suction,permeability,and cryo-geometry structure of the frozen fringe.(5)After the mechanical equilibrium analysis between ice and granular media,the growth mechanism of the ice lens taking account of the geometry structure of the frozen fringe has been obtained,and the corresponding research reveals that the degeneration of the frozen fringe is the main reason for the decreasing growth rate of the ice lens.An innovative testing method and testing apparatus for frost heave controlling were developed,an automatic freezing depth controlling method for ice lens growth inhibition was proposed.Experimental data demonstrate that decreased controlling freezing depth or increased circulating cold bath temperature can effectively decrease frost heave.