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.     

Three-Dimensional Nonlinear Analysis for the Cooling Characteristics of Crushed-Rock Interlayer Embankment with Ventilated Duct along the Qinghai-Tibet Expressway in Permafrost Regions

The crushed-rock embankment and duct-ventilated embankment have been used as effective cooling measures to protect permafrost underlying the Qinghai-Tibet Railway from thawing in China. These two cooling techniques are not directly applied to the Qinghai-Tibet Expressway, however, due to the large width and higher temperature of pavement surface. Therefore, considering the heat transfer characteristics of crushed-rock interlayer embankments and duct-ventilated embankments, we designed the crushed-rock interlayer embankment with ventilated duct. For cold regions engineering projects, the thermal regime is the most important factor that determines the stability of construction. To investigate the thermal stability of this new type of embankment, a three-dimensional numerical model was developed based on heat and mass transfer theory. The model includes coupled heat transfer between the airflow and the duct wall, air convective heat transfer within the crushed-rock interlayer, and heat conduction with phase change in the soil layer. The computational results indicated that the numerical model can reasonably solve the coupled heat and mass transfer for the crushed-rock interlayer embankment with ventilated duct. Based on an assumption that the mean annual air temperature will increase by 2.6°C in the next 50years, it was determined that in areas where the mean annual air temperature is currently ?4.0°C, the crushed-rock interlayer embankment with ventilated duct can be an effective measure to decrease the underlying ground temperature and ensure the stability of the Qinghai-Tibet Expressway in permafrost regions.     

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.     

Influence of Route Direction on Thermal Field of Embankments Constructed in High-Altitude Permafrost Area

This paper discusses the effect of route direction, embankment height, and pavement type on the thermal field of embankments built in permafrost regions. A finite-element model (FEM) is adopted to simulate diverse conditions of the embankment. The 30-year meteorological data including the solar radiation, air temperature, and wind velocity are used as the boundary conditions for the Qinghai-Tibet Highway. The results obtained from the FEM calculations are found to be in good agreement with the actual measurements on the thermal field. Further, the results show that route direction has great impact on the equilibrium of the thermal field within embankments in permafrost regions. The thermal imbalance is more obvious for embankments in the east–west direction and less in the north–south direction. In addition, the thermal asymmetry is closely related to seasonal variation and it is more pronounced in winter and less in summer.     

Analysis of the Deformation of Embankments on the Qinghai-Tibet Railway

Temperature changes and deformations were monitored on various embankment types on the Qinghai-Tibet Railway. Some of these embankments utilized permafrost protection techniques such as duct ventilation, crushed-rock embankments, crushed-rock protected slopes, or thermal-insulation treatments. Some embankments were built conventionally without considering permafrost protection. It was found that the majority of the deformations on both the permafrost-protected and the conventionally built embankments were due to deformation of warm frozen layers closely related to the temperature changes in the underlying permafrost. However, it was found that building embankments with permafrost protection reduced the magnitude of the settlements. After 2–3?years, deformation of all the embankments with permafrost protection countermeasures became smaller and smaller, whereas deformation was still increasing in the conventional embankments, where the settlement in the underlying permafrost could reach a considerable level, and could be a potential trigger for embankment failure. This should be taken into consideration in the railway engineering project on the Qinghai-Tibet Plateau.     

Deformation Characteristics of Ventiduct Embankment on Qinghai-Tibet Railway

The Qinghai-Tibet railway, the highest railway in terms of elevation, was built under the harshest of weather conditions in the world. A new technique, called the ventiduct embankment, was used for the first time in railway construction in the Qinghai-Tibet permafrost region of China to overcome the specific difficulties of the permafrost plateau. To evaluate its effectiveness, several full-scale test embankments were constructed at a representative field site, the Qingshuihe experimental section. The effectiveness of the ventiduct embankment structure has been verified by comparing it with the performance of the nearby control embankment in terms of permafrost table, settlement, and thaw depth. These observations are presented in this paper. The ventiduct embankment structure based on this study has been adopted in Qinghai-Tibet permafrost railway construction.     

Design Parameters Analysis of Insulated Embankments in Qinghai-Tibet Plateau Permafrost Region

Based on the problems of application of embankments with insulation in permafrost regions, a finite-element method (FEM) was adopted to analyze the influence on permafrost protection effects of the parameters such as insulation materials and thickness, embedded depth, annual mean permafrost temperature, and construction season timing. The FEM calculated results have shown that, in the long-term view of Qinghai-Tibet plateau with rising temperature, all embankments with insulation can protect the permafrost underneath and delay thawing of the permafrost table. The protection effects are related to insulation materials and thickness, embedded depth, construction season timing, and annual mean permafrost temperature. For the embankment design, the influence of all parameters should be taken into account comprehensively, and the most appropriate parameters group should be adopted to protect the permafrost most effectively.     

Passively Cooled Railway Embankments for Use in Permafrost Areas

Permafrost (permanently frozen ground) underlies approximately 25% of the world’s land surface. Construction of surface facilities in these regions presents unique engineering challenges due to the alteration of the thermal regime at the ground surface. Even moderate disturbance of the preexisting ground surface energy balance can induce permafrost thawing with consequent settlement and damage to roadway or railway embankments. Railway embankments are particularly susceptible to thaw settlement damage because of the need to maintain the alignment and even grade of the rails. The present work examines the heat transfer and thermal characteristics of railway embankments constructed of unconventional, highly porous materials. It is possible to produce a passive cooling effect with such embankments because of the unstable density stratification and resulting natural convection that can occur during winter months. The convection enhances the upward transport of heat out of the embankment during winter, thus cooling the lower portions of the embankment and underlying foundation soil. Numerical results have been obtained with an unsteady two-dimensional finite-element model that is capable of solving the coupled governing equations of pore air flow and energy transport. The numerical results are obtained for conditions typical of those found in railway configurations which allow open exchange of air between the embankment structure and the surrounding ambient air mass.     

Case Study of Degrading Permafrost beneath a Road Embankment

This paper reports on a case study of degrading permafrost beneath a road embankment in Northern Manitoba, Canada. Field measurements of ground temperatures for a three-year monitoring period have been used in calibrating the geothermal model developed to reproduce the conditions and trends in the subsurface thermal regime beneath the embankment. The numerical model was used to investigate the future ground thermal regime of the foundation and evaluate the potential impacts of embankment construction and climate change. Particular interest was paid to the foundation soil near the toe of embankment where relatively rapid permafrost degradation was occurring.     

Qinghai-Xizang Railroad Construction in Permafrost Regions

Construction of the Qinghai-Xizang Railroad (QXR) in permafrost regions presents a number of significant engineering problems. The engineering properties of permafrost can vary greatly, and climate warming, especially warm permafrost with high ice content, must be considered. Permafrost warming could induce ground ice thaw, producing embankment settlement. Consequently, thermal stability is a key consideration for the QXR construction in permafrost regions. In order to ensure permafrost thermal stability under the background of climate warming, ideas of embankment cooling to preventing permafrost change are proposed. Many methods of embankment cooling have been proposed to prevent the thawing of ground ice. For example, block-stone embankment, block and debris slope protected, thermosyphon, and special bridge designs. The amount of engineering practice and observational data testify that measurements of embankment cooling effectively decrease permafrost temperature and heighten the permafrost table beneath embankments.     

Modeling Impacts of Thaw Lakes to Ground Thermal Regime in Northern Alaska

In northern Alaska, the ground is largely underlain by permafrost. Many engineering problems in this region can be attributed to the variations of ground thermal regime. Engineering projects such as construction of gas pipelines must be based on a good understanding of ground thermal regime and its interaction with seasonal climate changes. Numerical modeling is used to simulate a multimedia system with transient heat transfer in this research. The system includes a snow cover on the top, a shallow lake in the middle, and soils beneath the lake. The finite-element method is used for the spatial domain solution, and the finite-difference method is used for the temporal domain solution. The model is applied to three sites in northern Alaska for a nine-month period during the winter of 1995–1996. The result reveals the impacts of thaw lake on the ground thermal regime, the formation of the talik, as well as the formation of ice in the lake. The model is verified against field observations. The difference between the simulated and observed ice thickness is less than 3%.     

In Situ Test on Cooling Effectiveness of Air Convection Embankment with Crushed Rock Slope Protection in Permafrost Regions

An experimental air convection embankment (ACE) was constructed in Beiluhe on the Qinghai-Tibet Plateau during 2001–2003, using coarse (5–8 and 40–50 cm), poorly graded crushed rock fill material on the slope of embankment with thick ground ice permafrost foundation, which should be called the air convection embankment with crushed rock slope protection (ACE–CRSP). The highly permeable ACE–CRSP installation was designed to test the cooling effectiveness of ACE–CRSP concept in an actual railway project. Ground temperature data were collected from test sections on the railway with thermistor sensor strings. The results showed that the mean ground temperature under the layer of the crushed rock with coarse particle diameter of 40–50 cm was lower than that under one with finer particle diameter of 5–8 cm, and the fluctuating range of temperature under the former was bigger than that under the latter. It was obvious that the maximum thaw depth was raised under the layer of crushed rock with coarse particle diameter of 40–50 cm, which resulted from the stronger cooling effectiveness of air convection during the winter. The amount of heat exchange also showed that the absorbed cooling energy of the foundation, under the layer of the crushed rock with coarse diameter, was larger than that with finer diameter.So, we believe that the cooling effectiveness of the crushed rock layer with coarse diameter was stronger than that one with finer diameter.     

Substructure Simulation of Inhomogeneous Track and Layered Ground Dynamic Interaction under Train Passage

The vibrations in track and ground induced by train passages are investigated by the substructure method with due consideration to dynamic interaction between an inhomogeneous track system comprising continuous rails and discrete sleepers, and the underlying viscoelastic layered half space ground. Initially, the total system is divided into two separately formulated substructures, i.e., the track and the ground. The rail is described by introducing the Green function for an infinite long Euler beam both for moving axle loads action from a train and for reactions from sleepers. The ground is formulated by the layer transfer matrix approach for wave propagation along the depth. Subsequently, these substructures are integrated to meet the displacement compatibility and force equilibrium via inertia of sleepers and stiffness of railpad springs. The dynamic equations are solved in the frequency–wave-number domain by applying the Fourier transform procedure. Based on the assumption of a constant train speed, the time domain response is evaluated from the inverse Fourier transform computation. The dispersive characteristics of the layered ground and the moving axle loads lead to significantly different response features, depending on the train speed. The response is classified as quasistatic for a low speed, whereas it is dynamic for a high-speed situation. An illustrative case study is presented for Swedish X-2000 train track properties and ground profile.     

Study of Methods to Control Frost Action in Cold Regions Tunnels

This paper presents a case history study of a tunnel in the cold plateau region of China. It describes the permafrost distribution of the mountain in which the tunnel is located and the observed air and surrounding rock temperatures in and outside the tunnel. By comparing these measured temperatures, it is found that the effect of the thermal insulation doors, installed at both ends of the tunnel, raises the air temperature in the tunnel higher than the antisnow shelter installed at both ends of the tunnel. The thermal insulation doors are thought to be better than the antisnow shelters in preventing the tunnel from being damaged by frost action.     

土壤离子电导率法和地电提取法在青藏高原冻土区寻找隐伏金矿的对比研究

青藏高原冻土区找寻隐伏矿床的难度很大,选用传统的物化探技术手段并未取得找矿突破。本次研究采用土壤离子电导率和地电提取测量法在高寒区--扎家同哪金矿区开展隐伏矿找矿预测工作,在未知区圈定了3处找矿远景靶区,其中,Ⅰ类找矿靶区异常规模较大,位于测区中部,成矿条件好,应作为下一步工作的重点勘探区。此外,选择研究区4号勘探线作为已知剖面,开展了方法有效性和可行性试验研究,结果显示,已知矿体上方有明显的土壤离子电导率异常地电提取Au元素异常,进一步证实这2种方法组合在青藏高原冻土区的找矿应用效果显著,值得推广应用。     

In situ Sea Ice Experiments in McMurdo Sound: Cyclic Loading, Fracture, and Acoustic Emissions

The breakup of first-year sea ice plays an important role in the dynamics and thermodynamics of polar ice covers. A recent research program has studied the in situ mechanical properties of the annual ice in Antarctica to support the development of physically based models of the breakup process. As part of this effort, two field trips were conducted to McMurdo Sound, and the present paper describes the experimental work and presents selected results. The in situ experiments investigated the constitutive and fracture behavior of edge-notched, square plate specimens of first-year ice and involved a detailed characterization of the physical properties and thermal state of the ice. Acoustic emissions, which are generated by microcracking, were monitored in the crack tip vicinity and provide insight regarding the size of the process zone. The paper describes the physical properties and microstructure of the sheet, the cyclic-loading response, and the acoustic emissions activity from an extensive series of experiments conducted on one of the in situ specimens. Varying the cyclic-loading frequency and amplitude provided a means to examine rate effects on the anelastic and viscous components of strain and the extent of microcracking near the crack tip. The viscous deformation rate estimated from the experiments exhibited an increasing power-law exponent with values between one and three. Acoustic emissions monitoring indicated that microcracking occurred in a process zone near the crack tip, and the size of the process zone increased with decreasing cyclic loading frequency. Practical aspects of the experiments are considered, and the results are put into context with the overall modeling goals of the project.     

Permafrost Studies in the Qinghai–Tibet Plateau for Road Construction

Advances have been made in engineering–geological studies of roadbeds in permafrost areas of the Qinghai–Tibet Plateau. The methodology used in engineering–geologic investigations has been greatly improved and provisions and regulations have been devised to guide engineering activities. Special attention has been paid to ground temperature and ice content because they are two of the most important and unique parameters for permafrost. Based on the data collected from recent studies, recommendations are made to enhance the stability of roadbeds. Under climate warming, a “roadbed cooling” approach is suggested for road constructions in “warm” permafrost. Advances have been also made in the prediction of changes in the permafrost engineering–geological environment, as induced by climate warming.     

Field Assessment of the Performance of a Ballasted Rail Track with and without Geosynthetics

Understanding the complex mechanisms of stress transfer and strain accumulation in layers of track substructure under repeated wheel loading is essential to predict the desirable track maintenance cycle as well as the design of the new track. Various finite element and analytical techniques have been developed in the past to understand the behavior of composite track layers subjected to repeated wheel loads. The mechanical behavior of ballast is influenced by several factors, including the track confining pressure, type of aggregates, and the number of loading cycles. A field trial was conducted on an instrumented track at Bulli, New South Wales, Australia, with the specific aims of studying the benefits of a geocomposite installed at the ballast-capping interface, and to evaluate the performance of moderately graded recycled ballast in comparison to traditionally very uniform fresh ballast. It was found that recycled ballast can be effectively reused if reinforced with a geocomposite. It was also found that geocomposite can effectively reduce vertical and lateral strains of the ballast with obvious implications for improved track stability and reduced maintenance costs.     

Seasonal Thermal Displacements of Gravity Dams Located in Northern Regions

Seasonal temperature displacements are an important component of the total displacements recorded by pendulum measurements at gravity dams located in northern regions. A hybrid dam displacement model is presented in this paper to interpret these displacements and extrapolate the response for an extreme thermal event not yet experienced by the dam. The hybrid model uses a simplified deterministic structural dam representation with beam elements in complement to a hydrostatic seasonal time (HST) statistical displacement model. Comparisons are first established between 1D heat transfer analyses of typical gravity dam sections, and 2D finite-element (FE) analyses. Thermomechanical displacements are compared to show the validity of the proposed simplified deterministic beam model for typical dams. A case study of an actual 40?m gravity dam located in Quebec, Canada is then presented. It is shown that the deterministic model can be calibrated using the pendulum displacements and the HST model. The calibrated deterministic model is then used to extrapolate the displacement response for extreme thermal events not yet experienced by the dam. The proposed methodology represents a simple extension of the gravity method, widely used to verify gravity dam stability, as a first step to interpret recorded pendulum displacements and set appropriate warning and alarm levels on a rational basis before developing 2D and 3D thermomechanical FE deterministic dam models that require a lot of resources and expertise to be used effectively.     

Fundamental Parameters for the Stiffness and Strength Control of Artificially Cemented Sand

The treatment of soils with cement is an attractive technique when the project requires improvement of the local soil for the construction of subgrades for rail tracks, as a support layer for shallow foundations and to prevent sand liquefaction. As reported by Consoli et al. in 2007, a unique dosage methodology has been established based on rational criteria where the voids/cement ratio plays a fundamental role in the assessment of the target unconfined compressive strength. The present study broadened the research carried out by Consoli et al. in 2007 through quantifying quantifies the influence of voids/cement ratio on the initial shear modulus (G0) and Mohr-Coulomb effective strength parameters (c′,?′) of an artificially cemented sand. A number of unconfined compression and triaxial compression tests with bender elements measurements were carried out. It was shown that the void/cement ratio defined as the ratio between the volume of voids of the compacted mixture and the volume of cement is an appropriate parameter to assess both initial stiffness and effective strength of the sand-cement mixture studied.