Lai Yuanming

Numerical analysis for cooling effect of open boundary ripped-rock embankment on Qinghai-Tibetan railway

The heat convection of fluid inside the ballast layer and ripped-rock layer, which are regarded as porous media in railway embankment, is a process of heat and mass transfer. At present, the ripped-rock embankment, as a new type of embankment structure, has widely been used in the construction of Qinghai-Tibetan railway. However, because its ripped-rock layer is almost open in two bilateral boundaries and closed at top and bottom, and air can flow into/out of the ballast layer and ripped-rock layer, the convection and transfer heat patterns are very complicated in the embankment. Therefore, based on the temperature and geology conditions of the Qinghai-Tibetan Plateau, a numerical approach of the unsteady two-dimensional continuity, momentum (non-Darcy flow) and energy equations of heat convection for incompressible fluid in porous media is provided to analyze the velocity and temperature characteristics of the ripped-rock embankment with different embankment heights under open boundary condition for the coming 50 years in this paper. The calculated results indicate that, due to the influence of the outside wind, the convective heat transfer mainly relies on the forced convection in the open ripped-rock embankment. Even if the air temperature will be warmed up by 2.6°C in the coming 50 years, it still has a better cooling effect on the underlying soils and a low temperature frozen-soil core is formed in the permafrost below it if the embankment is constructed in the regions whose present mean annual air temperature is −4.0°C. Furthermore, the cooling effect of high ripped-rock embankment is better than that of low embankment. This is because the wider bottom of high embankment has a more influence dimension on the underlying frozen soil. However, cardinal winds on the Qinghai-Tibetan Plateau disturb its convection pattern, so that an asymmetric temperature distribution occurs under high embankment and it is possible to induce a transverse uneven deformation of embankment, but no similar situation occurs under low embankment. This asymmetric temperature field problem should be considered when ripped-rock embankment is designed and constructed.

Nonlinear analysis for the three-dimensional temperature fields in cold region tunnels

In this paper, based on the governing differential equations of the problem on temperature field with phase change, the finite element formulae of three-dimensional temperature fields are obtained from Galerkins method. An illustrative example of the three-dimensional temperature fields in cold regions tunnel both with insulation and without insulation is presented on the basis of the air temperatures surveyed in the inside of the tunnel. The example shows that it is necessary to analyze the three-dimensional temperature fields in cold regions tunnels, and that it is effective to lessen the frozen or thawed areas of the rock surrounding the tunnels by constructing insulation, which can provide reference and numerical method for actual engineering design in cold regions.

Analysis of shear lag and shear deformation effects in laminated composite box beams under bending loads

A method for analyzing the shear lag and shear deformation effects on symmetrically laminated thin-walled composite box beams under bending load is presented. The method is based on the theory of composite laminated plates and is deduced by means of the principle of minimum potential energy, which makes the procedure simple and practical. The formulas given by this method not only satisfy the equilibrium conditions on the cross-section of the thin-walled composite box beams but can be simplified to the known formulas of thin-walled box beams with isotropic materials as well. Finally, an example is given and its numerical results are analyzed and discussed. The values of vertical displacements and ply normal stresses obtained by this paper are compared with those obtained by the finite element method (FEM) or previously published experimental and numerical results, respectively. A comprehensive analysis on the effects of shear lag and shear deformation is given for a simply supported thin-walled composite box beam subjected to a centralized load at mid-span.

Approximate analytical solution for temperature fields in cold regions circular tunnels

According to the practical situation in permafrost regions, the governing differential equations of heat transfer in frozen zone and unfrozen zone are simplified. Then, using dimensionless and perturbative method, the approximate analytical solution for simplified heat transfer equation has been obtained. The research of the freezing process of circular tunnel with initial temperature near 0 °C has been made. Comparing the approximate analytical results with finite element results, it can be seen that the approximate analytical solution is of higher precision and this solution can meet the precision requirement of practical engineering. This solution can be used in both engineering computation and the examination of numerically calculated results.

Effects of freeze-thaw cycles on soil mechanical and physical properties in the Qinghai-Tibet Plateau

Extreme freeze-thaw action occurs on the Qinghai-Tibet Plateau due to its unique climate resulting from high elevation and cold temperature. This action causes damage to the surface soil structure, as soil erosion in the Qinghai-Tibet Plateau is dominated by freeze-thaw erosion. In this research, freezing-thawing process of the soil samples collected from the Qinghai-Tibet Plateau was carried out by laboratory experiments to determinate the volume variation of soil as well as physical and mechanical properties, such as porosity, granularity and uniaxial compressive strength, after the soil experiences various freeze-thaw cycles. Results show that cohesion and uniaxial compressive strength decreased as the volume and porosity of the soil increased after experiencing various freeze-thaw cycles, especially in the first six freeze-thaw cycles. Consequently, the physical and mechanical properties of the soil were altered. However, granularity and internal friction angle did not vary significantly with an increase in the freeze-thaw cycle. The structural damage among soil particles due to frozen water expansion was the major cause of changes in soil mechanical behavior in the Qinghai-Tibet Plateau.

NONLINEAR ANALYSIS FOR FROST-HEAVING FORCE OF LAND BRIDGES ON QING-TIBET RAILWAY IN COLD REGIONS

ABSTRACT A mathematical mechanical model and the governing differential equations of the coupled problem of temperature, moisture migrating, and stress fields with phase change, are first derived from the theory of heat transfer, the theory of moisture migration, and frozen soil mechanics. Then the finite element formulas of this problem are obtained by using Galerkins method. Using the finite element formulas, a nonlinear analysis for the frost-heaving process of the land bridge in the next 20 years is made. The calculated results show that only soil at the bottom of the pile generates plastic deformation, and the deformation is quite small; therefore, the studied land bridge is safe.

EFFECT OF CLIMATIC WARMING ON THE TEMPERATURE FIELDS OF EMBANKMENTS IN COLD REGIONS AND A COUNTERMEASURE

The heat convection in ballast mass and broken rock mass in railway embankments is the problem of heat convection in porous media. In order to calculate the temperature distribution of the Qing-Tibet railway embankment, from the governing equations used to study forced convection for incompressible fluids porous media, the finite-element formulas for heat convection in porous media are derived by using Galerkins method. The temperature fields of the traditional ballast embankment and the broken rock mass embankment, constructed on July 15, have been analyzed and compared under the case that the air temperature on the Qing-Tibet plateau will warm up by 2.0 °C in the next 50 years. The calculated results indicate that the permafrost 5 m below the traditional ballast embankment will be thawed in the regions in which the air yearly average temperature is higher than −3.5 °C or the yearly average temperature at the native surface is higher than −1 °C. The embankment will undergo large thawing settlement and even liquidize. The railway embankment will be damaged by permafrost degradation. The broken rock mass embankment not only can resist the effect of climatic warming up on it but also can provide cool energy for the permafrost under it. It can assure permafrost stability and not be subject to thawing. Therefore, it is highly recommended that the broken rock mass embankment be used for Qing-Tibet railway embankment structure in high-temperature permafrost regions so that permafrost embankment can be protected as well as possible.

NONLINEAR ANALYSES FOR THE SEMICOUPLED PROBLEM OF TEMPERATURE, SEEPAGE, AND STRESS FIELDS IN COLD REGION RETAINING WALLS

In this article, a mathematical mechanical model and the governing differential equations of the semicoupled problem of temperature, seepage, and stress fields, with phase change, are first derived from the theory of heat transfer, the theory of seepage, and frozen soil mechanics. Then the finite element formulae of this problem are obtained from Galerkins method. Finally, an illustrative example is provided that shows the effect of a seepage field on the temperature field of cold region retaining walls is large and the influence of a frost-heaving force on the stresses of cold region retaining walls is very large. So the effects of these factors on cold region retaining walls should be taken into account in cold region engineering design. Comparisons of the results of this approach with the measured data in the field have been made. The agreement is very good.

Mechanism of evolution on winter-time natural convection cooling effect of fractured-rock embankment in permafrost regions

Mechanism and evolution of the pore-air natural convection cooling effect in fractured-rock embankments in permafrost regions are studied using a numerical representation of the non-dimensional governing equations in variable permeability porous media. The analyses show that winter-time natural convection in fractured-rock embankments begins to occur in the side slope portions and gradually develops from the left and right side slope portions to the middle portion of embankment. The more significant distortion of isotherms from their initial orientations due to heat conduction alone is that the cooling effect of winter-time natural convection on the fractured-rock embankment is stronger. The minimum critical Rayleigh numbers triggering natural convection of the pore-air in the side slope portions and the middle portion of embankment were obtained. The factors of influence on triggering winter-time convection cooling effect in fractured-rock embankments were analyzed. Finally, the assertion that the techniques of the fractured-rock revetment and berm are the effective measures to maintain the heat stability of the roadbed in permafrost regions is theoretically demonstrated.

Experimental study on the micro-fabric of frozen sediment using triaxial deformation and computerized tomography

Abstract The micro-fabric of frozen sediment was studied using a triaxial deformation machine with computerized tomography (CT) to describe a quantitative relationship between microstructure and mechanical behaviour at different temperatures and confining pressures. Micro-fabric changes were described by CT images and CTvalues. Relationships between CT values and the stress–strain curves were analyzed. It was shown that CTvalues can describe the deformation quantitatively.