Wenbing Yu

Laboratory investigation on cooling effect of coarse rock layer and fine rock layer in permafrost regions

Instability and failures of railway/roadway embankment are mainly caused by thaw settlement of permafrost in cold regions. To protect foundation soils beneath the embankment from thaw settlement is the primary construction principle. In the present study, the heat transfer characteristics of two cases of porous media were examined experimentally, with periodic temperature boundary at the top surface of models, while the bottom and sides were impermeable and insulated. Wind velocity over the model top kept a constant. Case 1 was coarse rock layer, with mean particle size 22 cm. Case 2 was fine rock layer (mean particle size 7 cm). The results for both cases are analyzed and compared. Results show that both the two cases have cooling effects. They can cool the soil underlying them. The cooling effect of Case 2 is better than that of Case 1. At the end of the test, temperature at the bottom of Case 2 is 3.5 °C colder that of Case 1. At the end of the negative and positive temperature stages, temperatures within coarse rock layer change monotonously from top to bottom, but it is no-monotonously within fine rock layer. The temperature of rock particle surface is warmer than the pore-air temperature in coarse rock layer during the positive temperature stage. A critical thickness for rock layer is necessary if they can yield cooling effects. The cooling characteristics of rock layer can be used to prevent embankment from thawing settlement and forming longitudinal cracks in permafrost regions.

NON-LINEAR ANALYSIS FOR THE FREEZING-THAWING SITUATION OF THE ROCK SURROUNDING THE TUNNEL IN COLD REGIONS UNDER THE CONDITIONS OF DIFFERENT CONSTRUCTION SEASONS, INITIAL TEMPERATURES AND INSULATIONS

In this paper, based on the governing differential equations of the problem of temperature field with phase change, the finite element formulae of this problem are obtained from Galerkins method. An illustrative example of the temperature field in cold region tunnel under the conditions of the different construction seasons, different initial temperatures, different thicknesses and thermal conductivity of insulation material is provided. Then some significant results are presented, which can provide a reference for actual tunnel engineering designs in cold regions.

Forecast analysis for the re-frozen of Feng Huoshan permafrost tunnel on Qing-Zang railway

According to the in-situ measured data of the air temperature and ground temperature inside of Feng Huoshan Tunnel on Qing-Zang railway, China, the thawed range of the permafrost surrounding the tunnel is very large, which is caused by the hydration heat of the cast-in-situ concrete lining, insulation measurement during winter construction and other artificial activities. In this paper, taking the coupled effects of moisture transfer and heat conduction into account, the finite element formulae of this problem with phase change are derived from the governing differential equations and moisture transfer equations using Galerkins method and the software for computers is edited. Using the software, forecast analysis for the re-frozen of Feng Huoshan permafrost Tunnel is made. The analysis results indicate that the vault, left wall and right wall of Feng Huoshan Tunnel without the thermal insulation material will be refrozen on 30 September 2004, and the arch bottom will be refrozen on 30 September 2005. However, if the tunnel is fitted with the thermal insulation material whose thermal conductivity is 0.0186(W/m K), the vault of the tunnel will be refrozen on 30 September 2005, and the left wall, the right wall and the arch bottom will be refrozen on 30 September 2006. If thermal conductivity of the thermal insulation material is 0.03(W/m K), the vault of the tunnel will be refrozen on 30 September 2004, and the left wall, the right wall and the arch bottom will be refrozen on 30 September 2005. So, the thermal insulation material will prevent the thawed range of Feng Huoshan permafrost tunnel from being refrozen. The thawed range of the permafrost surrounding the cold region tunnel caused by construction must be taken into account in design and the time of the field observation should be extended, or observational results, such as their temperature and stress field, will greatly be different from results in stability.

In situ determination of mechanical properties of frozen soils with the pressuremeter

Abstract An in situ test was conducted to investigate the short-term mechanical behavior of frozen soils along the Qinghai–Tibet Highway, in China, using the pressuremeter. This paper uses the plane elasticity theory and the radial expansion equation of cylindrical hole in infinite elastic medium provided by Lame to interpret the test data. The initial horizontal pressure, the critical plastic load and the limit pressure are reduced from the pressuremeter testing curves, and then, the pressuremeter modulus and the shear modulus are worked out. This paper also analyses the relationships between the mechanical parameters and the soil temperature and the water contents, respectively, and gives out the approximate formula. At the same time, some inevitable problems encountered during the operating of test are discussed.

Heat Effect Analysis of Buried Oil Pipeline in the Qinghai-Tibet Plateau

The Geermu-Lasa oil pipeline was located in the Qinghai-Tibet Plateau permafrost regions. The building and operating of pipeline will change the temperature field of soil around it, which can lead to changes of frozen soil mechanic properties, and this will induces deformation or even fracture of pipeline. These phenomena will affect the normal transportation of oil. In this paper, temperature field around the pipelines were analyzed due to different pipe diameters and different insulation layer thicknesses in the way of finite element method. The rule of thawing and freezing of soil around the pipeline in an annual cycle was obtained. Artificial permafrost table variations under the pipeline were also obtained due to different operating conditions. For 30cm diameter pipeline with 7cm insulation layer, its artificial permafrost table depth change value is just 0.48m after 30-year running. These analysis results can provide references to the construction of the new Geermu-Lasa oil pipeline.

Cut-Slope Icing Prevention: Case Study of the Seasonal Frozen Area of Western China

AbstractIcing caused by the construction of cut-slope roadways is a severe problem in cold regions and is difficult to mitigate. This study proposes a method for preventing icing on cut-slope roadways. A new structure for icing prevention was designed and constructed in the field. The key points of the proposed structure are the layered insulation drainage ditch and insulated facing wall. This structure is designed to ensure that water outcrops located on cut slopes and the groundwater drainage ditch remain unfrozen during winter. Surface water is drained through an upper open gutter during the warm season, and groundwater from the cut slope is drained through the insulated blind ditch. The air temperature, ground temperature under the pavement, and temperature of the stone blind ditch are presented and analyzed. The field experiment proved that the proposed structure performs well and effectively mitigates icing on cut-slope roadways.

Numerical Simulation of Pile Foundations of Qinghai-Tibet Power Transmission Line: Influence of Temperature Region

To analyze the thermal effect of the pile foundation of permafrost, a two-dimensional transient finite element model of the thermal fields with phase change were established. The developments of heat influence limit and maximum thawed depth with and without climate warming were predicted and analyzed. Results indicate that (1) the heat influence limit and maximum thaw depth in permafrost regions enlarge with time elapse, while the global climate warming will have a greater influence to full-space pile foundation compared with the cone-cylinder pile foundation; (2) Considering the global climate warming, heat influence limit (Lmax) and maximum thaw depth (Hmax) in 50th year for full-space pile foundation, cone-cylinder pile foundation are 10.1m, 5.2m, 4.1m, 3.7m, respectively; the maximum thaw depth of full-space pile foundation during the operation will have exceeded the depth of structure (2.5m), which might put the structure at risk; (3) the structure of cone-cylinder pile foundation could effectively preserve permafrost and avoid pile foundations failure; (4) the spacing of cone-cylinder pile foundations is reasonable and the interaction of temperature distribution among cone-cylinder pile foundations can be negligible.

Effects of Framed Embankment for High-Grade Expressway in Permafrost Regions

Construction of high-grade highways is an important action to meet the requirement of communication and transportation in permafrost regions. Frame embankment is proposed to reduce the scale effects caused by wide pavement. Numerical simulation method is employed to analyze the improvement of frame embankment for thermal stability of roadbed, with the consideration of global warming. Compared the response of permafrost at symmetric position of each embankment to construction, the consistency degree of response is taken as decision fundament. Two indexes are selected, that is, the difference of permafrost table between the embankment and that at natural side, and the change of mean annual geothermal under the embankment. Additionally, the convenient for construction and wind-blown sand hazard are considered to determine reasonable frame space. So, numerical simulation of flow field of wind around embankments is carried out. It is concluded that the minimum space between two embankment is 6 m. The aim of this study is to provide scientific guidelines on construction of major permafrost engineering in the future.