Nonlinear Thermal Analysis for Qing-Tibet Railway Embankments in Cold Regions

Heat convection in ballast mass in railway embankments is a 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, detailed finite-element formulas for heat convection in porous media are derived using Galerkin’s method. The temperature distributions on central lines of the traditional railway embankment, the ripped-rock embankment, and the ripped-rock revetment embankment that were constructed on July 15, 2002 have been analyzed and compared on July 15, October 15 in the 24th year after construction, and January 15 in the 25th year after construction under the climatic and geological conditions on the Qing-Tibet Railway. The calculated results indicate that the traditional railway embankment will raise the permafrost temperature under the embankment base and make the permafrost embankment thermally unstable. The ripped-rock embankment and the ripped-rock revetment embankment will reduce the permafrost temperature under the embankment base in cold regions, therefore maintaining the thermal stability of permafrost. However, the ripped-rock embankment needs more rock mass while the ripped-rock revetment embankment need less rock mass, and its construction cost is lower than that of the former. Therefore, it is highly recommended that the ripped-rock revetment embankment be used for the Qing-Tibet railway embankment structure in high temperature permafrost regions so that the permafrost embankment can be protected as much as possible.     

Effect of Anisotropy and Destructuration on the Behavior of Murro Test Embankment

This paper investigates the influence of anisotropy and destructuration on the behavior of a test embankment on soft clay. The test embankment at Murro, Finland, was commissioned in 1993 by the Finnish Road Administration and has been monitored for over 10?years. The construction and consolidation of Murro test embankment is analyzed with finite element method using three different constitutive models to represent the soft soil. The results are compared with field observations. The constitutive models used include two recently proposed constitutive models, namely S-CLAY1 that accounts for initial and plastic strain induced anisotropy and its extension, called S-CLAY1S. The S-CLAY1S model accounts, additionally, for interparticle bonding and degradation of bonds. For comparison, the test embankment is also analyzed using the isotropic Modified Cam Clay model. The simulations demonstrate that for this type of problem, it is important to account for the anisotropy, whereas destructuration appears to have less influence on predicted deformations. However, only a model incorporating destructuration can explain the decrease in undrained shear strength during consolidation that was measured in field.     

Experimental Study on the Ventiduct Embankment in Permafrost Regions of the Qinghai-Tibet Railroad

This technical note presents an experimental study on the ventiduct embankment in permafrost regions of the Qinghai-Tibet Railroad. Two experiments were conducted in a test box and they were designed with similarity theory. Some preliminary results are given to describe the temperature regime properties of the embankment, and the temperature differences between the ventiduct embankment and the normal embankment are compared. Results showed that the ventiduct embankment has a good cooling effect; it has a faster cooling rate than the normal embankment; the range of influence of a single duct can reach one duct diameter; the temperature distribution of the ventiduct embankment is asymmetric along the wind direction.     

Numerical assessment of the seismic response of an earth embankment on liquefiable soils

This study presents a numerical assessment of the seismic behaviour of an earth embankment founded on liquefiable foundation soils during earthquake loading. Analysis was carried out using an effective stress-based, fully coupled, finite element method. The behaviour of the sandy soil is described by means of a cyclic elastoplastic constitutive model which was developed within the framework of the Armstrong–Frederick type non-linear kinematic hardening concept. The numerical method and the analysis procedure are briefly outlined and as an example, the seismic response of an earth embankment on a saturated sand foundation is assessed. Based on the numerical results, the distinctive patterns of seismic response of the embankment are discussed. Special emphasis is given to the computed results of excess pore water pressures, co-seismic and post-seismic deformations, and accelerations during the seismic excitation. It has been found that the numerical model can capture fundamental liquefaction aspects of the embankment foundation system and produce preliminary results for its seismic assessment.      

Unsaturated Infinite Slope Stability Considering Surface Flux Conditions

A slope stability model is derived for an infinite slope subjected to unsaturated infiltration flow above a phreatic surface. Closed form steady state solutions are derived for the matric suction and degree of saturation profiles. Soil unit weight, consistent with the degree of saturation profile, is also directly calculated and introduced into the analyzes, resulting in closed-form solutions for typical soil parameters and an infinite series solution for arbitrary soil parameters. The solutions are coupled with the infinite slope stability equations to establish a fully realized safety factor function. In general, consideration of soil suction results in higher factor of safety. The increase in shear strength due to the inclusion of soil suction is analogous to making an addition to the cohesion, which, of course, increases the factor of safety against sliding. However, for cohesive soils, the results show lower safety factors for slip surfaces approaching the phreatic surface compared to those produced by common safety factor calculations. The lower factor of safety is due to the increased soil unit weight considered in the matric suction model but not usually accounted for in practice wherein the soil is treated as dry above the phreatic surface. The developed model is verified with a published case study, correctly predicting stability under dry conditions and correctly predicting failure for a particular storm.     

Thermal Interaction between Permafrost and the Qinghai-Tibet Railway

Measures were taken to protect the thermal regime of the roadbed embankment after construction and to lower permafrost temperatures in the rich-ice and warm permafrost regions along the Qinghai-Xizang Railway. However, these measures were taken only for some sections of the railway, leaving many sections unprotected. This article addresses those areas where no measures were taken and presents analysis of the variation of soil temperatures under the embankment in seasonal frozen soil areas, degrading permafrost areas, and warm and cold permafrost areas. The results show that soil temperatures, maximum seasonal freezing depth, and the permafrost table under the embankment differ according to the different frozen soil areas after embankment construction. In seasonal frozen soil areas and degraded permafrost areas, the seasonal frost layer remained frozen the next year under the shaded shoulder of the embankment. In degrading permafrost areas, a thaw layer between the permafrost table and the bottom of the seasonal frost formed under the embankment. In warm permafrost areas, the permafrost table under the embankment was unstable and soil temperatures near the permafrost table showed an obvious increasing trend. In cold permafrost areas, the permafrost table under the embankment was clearly raised and temperature lowered in the soil near the permafrost table, which is advantageous to permafrost thermal stability under the embankment. In particular, the differ-ence in solar radiation from the slope exposed to the sun to the shady slope of the embankment is responsible for the difference in the soil thermal regime and the permafrost table, which potentially can affect roadbed stability.     

Embodied Energy and Gas Emissions of Retaining Wall Structures

The embodied energy (EE) and gas emissions of four design alternatives for an embankment retaining wall system are analyzed for a hypothetical highway construction project. The airborne emissions considered are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulphur oxides (SOX), and nitrogen oxides (NOX). The process stages considered in this study are the initial materials production, transportation of construction machineries and materials, machinery operation during installation, and machinery depreciations. The objectives are (1)?to determine whether there are statistically significant differences among the structural alternatives; (2)?to understand the relative proportions of impacts for the process stages within each design; (3)?to contextualize the impacts to other aspects in life by comparing the computed EE values to household energy consumption and car emission values; and (4)?to examine the validity of the adopted EE as an environmental impact indicator through comparison with the amount of gas emissions. For the project considered in this study, the calculated results indicate that propped steel sheet pile wall and minipile wall systems have less embodied energy and gas emissions than cantilever steel tubular wall and secant concrete pile wall systems. The difference in CO2 emission for the retaining wall of 100?m length between the most and least environmentally preferable wall design is equivalent to an average 2.0?L family car being driven for 6.2?million miles (or 62 cars with a mileage of 10,000??miles/year for 10?years). The impacts in construction are generally notable and careful consideration and optimization of designs will reduce such impacts. The use of recycled steel or steel pile as reinforcement bar is effective in reducing the environmental impact. The embodied energy value of a given design is correlated to the amount of gas emissions.     

Centrifugal and Numerical Modeling of a Reinforced Lime-Stabilized Soil Embankment on Soft Clay with Wick Drains

A centrifuge test was performed on a reinforced embankment backfilled with lime-stabilized soil on soft clay installed with wick drains. The finite-element program PLAXIS was employed to simulate the centrifuge test on the basis of the test dimensions. Verifications of the numerical model were taken by comparing the numerical results with the measurements obtained from the centrifuge test, and it was found that both were in good agreement. Three cases of unreinforced, one-layer reinforced, and two-layer reinforced embankments were simulated and compared on the basis of the numerical model. A centrifuge similarity drainage relationship was also proposed in this paper on the basis of the equal degree of consolidation between the model and prototype drains.