Permafrost Engineering Research Articles

Pot‐Cover Effect in Permafrost Embankment: In Situ Experiment Evidence and Mechanism Simulation

ABSTRACTUnder the influence of large temperature differences in an impermeable pavement layer of wide embankment in permafrost regions, liquid water accumulates at the bottom of the impermeable cover. The phenomenon is known as the pot‐cover effect and leads to an increase in soil water content and a reduction in bearing capacity of wide embankments. At present, water vapor and liquid water migrations and their effect on embankment thermal‐moisture stability have not been fully confirmed. To better understand the moisture transport and accumulation process within embankments, hydrothermal field monitoring was conducted from 2009 to 2011 on an asphalt concrete layer highway in Beiluhe, central Tibet Plateau. The field monitoring results show that soil moisture content between 50 and 250 cm below the pavement continuously increases with the number of freeze‐thaw cycles, with the largest increase during the 2 years being 6.4%. Then, a coupled hydro‐vapor‐thermal transport model was established and verified. Furthermore, the model was used to analyze the numerical recurrence of the pot‐cover effect. The simulation indicates that the upward migration of liquid water during the freezing period is less than the downward migration during the thawing period, while vapor migrates downward during the thawing period but upward during the freezing period. The migration of water vapor within the embankment during the freezing period is the main cause of the pot‐cover effect in permafrost regions. In addition, the research results can provide new ideas for understanding the internal mechanism of thermal‐moisture dynamics of the embankment and the stability prediction of permafrost engineering.

Study on the lateral thermal disturbances of three parallel embankments on permafrost along the Gonghe-Yushu Engineering Corridor in China

The mutual thermal disturbance of built structures along narrow permafrost engineering corridors has become an evident influencing factor on the thermal stability of frozen soil engineering. To analyse the mutual thermal disturbance of the built structures along a permafrost engineering corridor in the Qinghai-Tibet Plateau, three parallel permafrost embankments (along the G214 Highway and the first- and second-stage projects of the Gonghe-Yushu Highway in the Gonghe-Yushu Engineering Corridor) were chosen as the research sites. These three highways were built at different times following different standards; thus, complicated lateral thermal disturbances occurred among the three parallel embankments. The embankment distress investigations, typical section drilling, and ground temperature observations were used for analysis. Then, the thermal distress and service status were analyzed under the mutual thermal disturbances of the three embankments. The continuous wave settlements and inclination deformations were relatively developed due to the remarkable mutual thermal disturbances from the shorter distances among the three embankments. Moreover, missing or poor drainage systems further aggravated the mutual thermal disturbances. The monitored data revealed that the transverse thermal disturbances weakened the cooling effects of several special structures to a certain degree; these structures included the ventiduct embankments and crushed-rock embankments. A thermal disturbance simulation model for the three parallel permafrost embankments was proposed and included the assignment of the general influence, core influence, and severe influence zones of the thermal disturbance. The computed results showed that the general influence zones and core influence zones under the three embankments were linked together after 3 and 10 years of construction, respectively, on the Gonghe-Yushu Highway. After 50 years, a vast thermal influence area almost 100 m wide and 20 m deep would form under the three embankments. The thermal disturbances at different distances between embankments were also analyzed using this model; the distances between G214 and the first-phase project of the Gonghe-Yushu Highway were inferred to be more than 15 m and 20 m between the first- and second-phase projects of the Gonghe-Yushu Highway, respectively.

Analysis of soil behavior under freezing and thawing conditions

Understanding of the thermal, hydraulic and mechanical behavior of soils under freezing and thawing is important in solving problems of permafrost engineering and artificial ground freezing. Accurate mathematical descriptions of heat and moisture transport and the associated mechanical deformations is critical for developing such understanding. Phase change and transition during freezing and thawing are characterized by a strong non-linearity due to the rapid variations of thermo-physical properties of the material and the release of the latent heat of solidification. Such processes cannot be easily and accurately modelled via the use of local (differential) formulations [1,2]. A non-local formulation, developed within the framework of peridynamics [3–5] is proposed here for ground freezing and thawing. It expands the domain of applicability of the first work on non-local modelling of heat transfer with water flow in saturated porous media [6] into modelling the thermo-hydro-mechanical behavior of unsaturated soils.
 The mathematical description of unsaturated soil behavior is based on three governing equations of conservations of water mass, energy and linear momentum, which are complemented by constitutive relationships that represent the change of soil properties.
 Using the local mathematical model of unsaturated soils [7,8], a peridynamics mathematical framework is formulated. Following [6], the bond-based peridynamics considers a soil body as a collection of particles, see Fig. 1 for the 1D case. The term `bond' refers to the interactions between two points, located at depths and . The point interacts with all other points within a certain finite region , called the horizon of z.
 The peridynamic form of the water mass conservation is:
 where is the water pressure head; is time; is the volumetric content of a soil component; is the density of a soil component; and is the vertical coordinate. The micro-diffusivity function can be defined following [9]. Here, the subscripts i, l, a and s refer to ice, liquid water, air and soil particles respectively.
 The peridynamic form of the energy conservation is:
 where is temperature; is the latent heat of water solidification; is the average thermal conductivity of soil; is the vector of water flux; is the average heat capacity of unsaturated soils and is the specific heat capacity of liquid water. Here, is a micro-conductivity function and is a micro-velocity function, for more details see [6,10].
 The peridynamic form of the linear momentum conservation is:
 where is the acceleration of the peridynamics particle ; is a pairwise force density vector between particles and that depends on its initial and deformed positions Z and Z’; is the horizon volume associated with the particle ; is the body force on the particle . The elastic stress-strain relation for unsaturated soils can be stated as:
 where is the stress increment tensor; is soil compressive modulus for unsaturated conditions, and is the strain increment tensor due to the elastic deformation of the soil matrix, that can be find from the relation for the total strain increment tensor of unsaturated soils . Here is the strain increment tensor corresponding to the change of soil volume due to the frost heave; and is the strain increment tensor corresponding to the plastic deformations of soils.
 The numerical implementation of the peridynamic formulation is verified by comparing the results of simulations with analytical solutions and the formulation is validated by comparing these results with experimental data under the condition of one-dimensional heat and water transfer obtained by laboratory experiments. In the experiments, soil cylinders were cooled from one surface by a source of negative temperature. As a result, the freezing front propagated along the cylinder. The temperature profile in the column was changing, and water migration was observed. The developed model is shown to predict the results of these experiments well.
 The proposed peridynamic model is the first to combine the flow of liquid water with heat transfer and mechanical deformations in the presence of a change of the water phase state in unsaturated soils. It is demonstrated that for the soils with relatively high initial water content, the developed approach describes effectively the change of liquid water content and predict soil deformation. The set of integral-differential equations are solved by a simple numerical scheme.

Applicability analysis of thermosyphon for thermally stabilizing pipeline foundation permafrost and its layout optimization

Rapid permafrost thawing triggered by heat release from the buried warm-oil pipeline would result in the instability of the pipeline, posing a potential risk of an oil spill in permafrost regions. Therefore, how to precisely control the ground temperature to mitigate pipeline foundation permafrost from thawing is of significant importance. Thermosyphon is a widely-accepted and extensively-used countermeasure against permafrost thawing in permafrost engineering. This paper presents 3-year (2015–2018) monitored data of ground temperature and geophysical survey results conducted by electrical resistivity tomography in April 2018, to investigate the thermal stabilizing effect of thermosyphons installed nearby the China-Russia crude oil pipeline (CRCOP) and the influencing factors. Furthermore, numerical simulation tests were conducted to evaluate their long-term cooling applicability and to optimize their layout parameters. Field observations show that the thermosyphons can cool down and even freeze the thawed soil layers around the CRCOP, prohibiting the infiltration of water into the thawing front to some extent and effectively mitigating the rapid thawing of permafrost beneath the pipeline. The performance of the thermosyphons is greatly influenced by the method and time of thermosyphon installation, as well as layout parameters including the number and longitudinal spacing. Two pairs of thermosyphons with a shorter spacing are unable to control the thawing of the underlying permafrost due to the non-anticipated higher oil temperature, the thermal erosion of water ponding, and climate warming. The simulated results indicate that thermosyphons perform well in the first 5 years after installation. Thermosyphon lowers the mean annual temperature of soils surrounding it by about 2 and 1.4 times when the evaporator section length is increased by 50% and the longitudinal spacing is decreased by 0.5 m, respectively. The application of thermosyphons has proven its strength for thermally stabilizing the pipeline foundation permafrost along the CRCOP.

Changes in Unfrozen Water Contents in Warming Permafrost Soils

Climate warming in the Arctic, accompanied by changes in permafrost soil properties (mechanical, thermal, filtration, geophysical), is due to increasing unfrozen pore water content. The liquid component in frozen soils is an issue of key importance for permafrost engineering that has been extensively studied since the beginning of the 20th century. We suggest a synthesis and new classification of various experimental and calculation methods for the determination of unfrozen water content. Special focus is placed on the method of applying measurements to the water potential, which reveals the impact of permafrost warming on unfrozen water content. This method was applied to natural soil samples collected from shallow permafrost from northern West Siberia affected by climate change, and confirms the revealed trends. The obtained results confirm that unfrozen water content is sensitive not only temperature but also particle size distribution, salinity, and the organic matter content of permafrost soils.

Compressive strength of cemented soil in pre-bored grouted planted piles under negative temperature environment

As a new type of pile, if the pre-bored grouted planted nodular (PGPN) pile can be successfully applied to the permafrost engineering, it will have significant economic and ecological values. However, the law of strength development of cemented soil of PGPN pile under warm permafrost environment is still unclear. In this study, the strength characteristics of cemented soil were examined through unconfined compressive strength test, and factors such as the curing time at negative temperature, the types of cementitious materials, and the dosages of cement and early strength agent (ESA) were considered. The experimental results indicate thatthe cemented soil with sulpho aluminate cement (SAC) and Ordinary Portland cement (OPC) + ESA as cementitious materials show high strength and stable performance. In addition, the Reasonable dosages of the cement and ESA were suggested, respectively.

Freezing point and unfrozen water contents of permafrost soils: Estimation by the water potential method

The freezing point and unfrozen pore water content are important parameters of frozen soils monitored for various geotechnical and permafrost engineering purposes. The current paper summarizes experimental data using a novel water potential method for determining a freezing point and unfrozen water content. To examine this method, the results of freezing point determination were compared with direct freezing point measurements and data on unfrozen water content — with the contact method and NMR results. Studies were conducted on natural soils of various particle size distribution, mineral composition, salinity, and total organic carbon (TOC) content. The obtained freezing point and unfrozen water content estimates showed good agreement with the results of direct measurements. For example, the freezing point data indicates a difference of about ±0.05 °C. The average data misfit of water potential with NMR and contact methods is about 0.3 °C in temperature and not more than 0.5 wt% in unfrozen water content. Additionally, the article discusses the advantages and limitations and the practical application of the water potential method. In summary, based on the study results, the water potential method can be recommended to determine the freezing point and unfrozen water content for different types of soils.

Experimental study on the startup and heat transfer behaviors of a two-phase closed thermosyphon at subzero temperatures

In permafrost regions, two-phase closed thermosyphons (TPCTs) can be used to release heat from permafrost. TPCTs are thus embedded in foundations of infrastructures for preventing the permafrost thawing. The installation of a TPCT, however, greatly influences the efficiency of releasing heat, which has not been fully understood. To find an optimum installation for TPCTs, we experimentally studied the temperature differences and thermal resistances of a TPCT that was sequentially installed at an inclination angle of 0, 10, 20, 30, 40, 50, 60, 70, 80 and 90° at subzero temperatures. It is found that the inner startup temperature difference varied irregularly with inclination angle. It is also found that at evaporator and condenser sections, the thermal resistance of the wall contributes negligibly to the thermal resistance of the corresponding section. Furthermore, the thermal resistances vary with inclination angle in the evaporator, adiabatic, and condenser sections. The adiabatic section has a thermal resistance that is comparable to the thermal resistance of the evaporator and condenser sections except the inclination angle of 90°. The TPCT has the minimum total thermal resistance when the inclination angle is about 20°. The results could be helpful for the optimal application of TPCTs in permafrost engineering.

An integrated observation dataset of the hydrological and thermal deformation in permafrost slopes and engineering infrastructure in the Qinghai–Tibet Engineering Corridor

Abstract. Across the Qinghai–Tibet Plateau (QTP) there is a narrow engineering corridor with widely distributed slopes called the Qinghai–Tibet Engineering Corridor (QTEC), where a variety of important infrastructures are concentrated. These facilities are transportation routes for people, materials, energy, etc. from inland China to the Tibet Autonomous Region. From Golmud to Lhasa, the engineering corridor covers 632 km of permafrost containing the densely developed Qinghai–Tibet Railway and Qinghai–Tibet Highway, as well as power and communication towers. Slope failure in permafrost regions, caused by permafrost degradation, ground ice melting, etc., affects the engineering construction and permafrost environments in the QTEC. We implement a variety of sensors to monitor the hydrological and thermal deformation between permafrost slopes and permafrost engineering projects in the corridor. In addition to soil temperature and moisture sensors, the global navigation satellite system (GNSS), terrestrial laser scanning (TLS), and unmanned aerial vehicles (UAVs) were adopted to monitor the spatial distribution and changes in thermal deformation. An integrated dataset of hydrological and thermal deformation in permafrost engineering and slopes in the QTEC from the 1950s to 2020, including meteorological and ground observations, TLS point cloud data, and RGB and thermal infrared (TIR) images, can be of great value for estimating the hydrological and thermal impact and stability between engineering and slopes under the influence of climate change and engineering disturbance. The dataset and code were uploaded to the Zenodo repository and can be accessed through https://zenodo.org/communities/qtec (last access: 23 June 2021), including meteorological and ground observations at https://doi.org/10.5281/zenodo.5009871 (Luo et al., 2020d), TLS measurements at https://doi.org/10.5281/zenodo.5009558 (Luo et al., 2020a), UAV RGB and TIR images at https://doi.org/10.5281/zenodo.5016192 (Luo et al., 2020b), and R code for permafrost indices and visualisation at https://doi.org/10.5281/zenodo.5002981 (Luo et al., 2020c).

Impact process and mechanism of summertime rainfall on thermal–moisture regime of active layer in permafrost regions of central Qinghai–Tibet Plateau

Rainfall can potentially change upper thermal–moisture boundary conditions and influence the hydrological and thermal state of the active layer in permafrost regions. Studying the relationship between rainfall and ground temperature represents an emerging issue in permafrost engineering and environment but the interactive mechanisms of rainfall and the active layer are not well understood. This study aims to analyze the effects and mechanisms of summertime rainfall on the thermal-moisture dynamics of the active layer by field observations and simulation. The observation data demonstrated that frequent light rainfall events had a minor impact on the active layer, whereas consecutive rainfall events and heavy rainfall events had significant effects on soil temperature and water content. Moreover, the soil temperatures were more susceptible to summertime rainfall events. These rapidly cooled the shallow ground and delayed the temperature rise. Summertime rainfall significantly increased the surface latent heat flux, but decreased the net radiation, sensible heat flux, and soil surface heat flux. Rainfall also enhanced the amount of downward liquid water and water vapor, but the impact of rainfall on the increase in the convective heat transfer of the liquid water was lower than the decreases in the heat conduction flux, latent heat flux by vapor diffusion, and heat flux by convection of vapor. Thus, the reduction in the total soil heat flux caused by rainfall directly leads to a cooling effect on the soil temperature and delays the increase in soil temperature. The cooling effect of rainfall events may mitigate the warming rate and maintain the active layer at a relatively low temperature. The results provide new insights into understanding the inner mechanisms of the effect of rainfall on the active layer and on the possible long–term change trends of permafrost under increasing precipitation in the central Qinghai–Tibet Plateau.

Investigation of permafrost engineering geological environment with electrical resistivity tomography: A case study along the China-Russia crude oil pipelines

The electrical resistivity tomography and vegetation surveys were conducted for investigating the distribution of frozen ground and other engineering geological characteristics at four representative sites (BHP, XFB, WLG, and XAZ) along the China-Russia Crude Oil Pipelines (CRCOPs) I and II. The comprehensive analysis shows that CRCOPs play an important role in forming and connecting supra-permafrost subaerial taliks and in facilitating thermokarst landforms, further accelerating permafrost degradation. The survey results indicate that on 15–29 August 2019, the permafrost base under the CRCOP at the XFB site was 21.5 m in depth, that of WLG site was 21.5 m (32.8 m at the undisturbed sites), and that of XAZ site, 37.3 m; the permafrost table of XFB site was 6.72 m (4.97 m on 20 m away from the pipeline), that of WLG site was 8.64 m (1.94 m at the undisturbed sites), and that of XAZ site was 6.72 and 3.38 m on the southern and northern slopes; the hydrothermal influences of CRCOPs extended horizontally to about 60 m away from the pipelines, but the vertical hydrothermal impacts were largely limited to about 10–15 m in depth at the BHP site in the zone of deep (>1.5 m) seasonal frost. The extent of engineering influences from the pipelines and associated engineering activities in patchy and island permafrost are evidently larger than those in seasonal frost. The sites with substantial changes are found in pipeline foundation soils underlain by warm (> − 1 °C; thermally unstable) and ice-rich permafrost. This study can timely help decision makers mitigate frost hazards in a proactive manner.

An Extended Finite Element Method for Heat Transfer with Phase Change in Frozen Soil

This paper deals with a finite element analysis of heat transfer with phase change in frozen soil in terms of the transient temperature field and the thermally induced solid-solid phase transformations. By analyzing the shortcomings of the traditional finite element method in solving the phase change problem and the characteristics of the freezing process in frozen soil, the extended finite element method was proposed to solve the heat transfer problem with phase change for permafrost engineering. Taking a quadrilateral element as an example, the extended finite element method interpolating function and solution format of the temperature field with phase change was derived. According to studies on constructing and solving the level set function, building of the stiffness matrix, and the subdivision of the elements, a program for heat transfer with phase change was proposed. The numerical solution is in good agreement with the measured value.

Effect of length ratios on the cooling performance of an inclined two-phase closed thermosyphon under negative temperature conditions

Two-phase closed thermosyphons (TPCTs) have been widely used in solar energy utilization. The work presents an optimization of the TPCT used in cooling foundation in permafrost regions. A series of experiments under negative temperature conditions were conducted to investigate the effect of length ratios (LRs, length ratio of the evaporator section to the condenser section) on the cooling performance of the TPCT. Experimental results show that there exists a best LR for each angle, and it varies with the inclination angle. As an example, for the TPCT with 5.0-cm inner diameter, the maximum efficiency can be decreased by 17.3% for the TPCT with the LR of 1.00 compared with the TPCT with the best LR of 1.65. Under the best LR, the efficiency can be raised by 30.2% for the TPCT with the inclination angle of 20° compared with that of 90°. Besides, the best LR also changes with the pipe inner diameter, e.g. the best LR varies from 1.65 to 1.27 when the inner diameter changed from 5.0 cm to 8.1 cm. The results suggest that the equivalent length of the evaporator section should be appropriately lengthened to enhance the cooling performance of TPCTs in permafrost engineering.

Analysis of the cooling effect of a concrete thermal pile in permafrost regions

The bearing capacity of a pile is significantly sensitive to ground temperature in permafrost regions. However, the cast-in-place pile foundation may disturb the thermal regime formed in a long term during construction and operation. A two-phase closed thermosyphon (TPCT) performs well in cooling permafrost engineering. This paper proposed a concrete thermal pile consisting of a concrete pile and TPCTs to control the temperature of a pile shaft. The results of an experiment conducted to analyze the cooling effect of the concrete thermal pile were presented. The influence of the length of the condenser section on the cooling effect has been discussed based on a 3-D heat transfer model of the thermal pile. The experiment results indicated that the concrete thermal pile could effectively cool the surrounding foundation soil, and the cooling rate of the thermal pile can reach 2.5 times that of the pile without the TPCT. To enhance the cooling efficiency of the thermal pile, the numerical simulation results suggested the optimal length of the condenser section should account for 44% of the total length of the TPCT. The conclusions summarized from this paper will provide valuable references and lessons to the design of the pile foundation in permafrost regions.

Work conjugate stress and strain variables for unsaturated frozen soils

In general, most of the foundations of structures in the cold regions rest on frozen soils which are often under unsaturated state and subject to mechanical and thermal loads. The thermal loads will induce water migration and water-ice phase change, as a result, the dynamic phase transition between unfrozen water and ice occurs and degree of saturation in frozen soils is affected. These complicate the understanding of the strength and deformation characteristics of frozen soils under mechanical and thermal loads. Furthermore, it leads to a necessity to develop a coupling thermo-hydro-mechanical model for practical permafrost engineering. Up to date, there are limited studies to quantify this complicated problem by a framework with proper stress and strain variables for unsaturated frozen soils. This study derives the expression of input work rate for unsaturated frozen soils based on the multiphase porous media theory. Work conjugate stress and strain variables have been determined based on the expression of input work rate. This set of variables not only can smoothly transit from unsaturated frozen state to unsaturated unfrozen state when temperature is above freezing point, but also can smoothly transit from unsaturated state to saturate state. The proposed framework can be consistently applied to multi-phase soils with unsaturated frozen soils, unsaturated soils and saturated soils. Moreover, the application of the framework can be further extended to other porous mediums. Additionally, the selected variables in this framework are validated against published experimental results. This framework can subsequently be used to establish the coupled thermo-hydro-mechanical models for unsaturated and saturated frozen soils.

Cryogenic deformations of pumping station building at Zapolyarnoye field

The subject of the study was to determine the causes of long-term deformation of the structures of technological equipment inside the building of the pumping station that supplies water to the industrial facilities and residential buildings of the infrastructure of the Zapolyarnoye oil and gas condensate field, located in the permafrost zone of Western Siberia, near the Arctic Circle. The main objectives of the study were to identify the mechanism of these deformations and develop practical recommendations for stabilizing structures and preventing their further destruction based on the analysis of the results. The survey was carried out using a set of methods provided for by the current legislation (visual and measurement control, geodetic works, thermometric observations in wells). The current situation at objects of such importance at the time of our study had no analogues. Analysis of the results of the structure inspection leads to the conclusion that the cause of the deformations were miscalculations in the design, construction and operation of the structure. The construction should be done with participation of specialists in the field of permafrost engineering.

Permafrost Deformation Monitoring Along the Qinghai-Tibet Plateau Engineering Corridor Using InSAR Observations with Multi-Sensor SAR Datasets from 1997-2018.

As the highest elevation permafrost region in the world, the Qinghai-Tibet Plateau (QTP) permafrost is quickly degrading due to global warming, climate change and human activities. The Qinghai-Tibet Engineering Corridor (QTEC), located in the QTP tundra, is of growing interest due to the increased infrastructure development in the remote QTP area. The ground, including the embankment of permafrost engineering, is prone to instability, primarily due to the seasonal freezing and thawing cycles and increase in human activities. In this study, we used ERS-1 (1997–1999), ENVISAT (2004–2010) and Sentinel-1A (2015–2018) images to assess the ground deformation along QTEC using time-series InSAR. We present a piecewise deformation model including periodic deformation related to seasonal components and interannual linear subsidence trends was presented. Analysis of the ERS-1 result show ground deformation along QTEC ranged from −5 to +5 mm/year during the 1997–1999 observation period. For the ENVISAT and Sentinel-1A results, the estimated deformation rate ranged from −20 to +10 mm/year. Throughout the whole observation period, most of the QTEC appeared to be stable. Significant ground deformation was detected in three sections of the corridor in the Sentinel-1A results. An analysis of the distribution of the thaw slumping region in the Tuotuohe area reveals that ground deformation was associated with the development of thaw slumps in one of the three sections. This research indicates that the InSAR technique could be crucial for monitoring the ground deformation along QTEC.

An experimental research on damping ratio and dynamic shear modulus ratio of frozen silty clay of the Qinghai-Tibet engineering corridor

Abstract Strong earthquakes occurred frequently in Qinghai-Tibet Plateau. Major national infrastructure facilities, including existing and proposed projects, are located densely in the permafrost engineering corridor of Qinghai-Tibet Plateau. Based on dynamic triaxial test under the effect of cycle loading for remolded frozen soil taken from Qinghai-Tibet engineering corridor, experimental studies on the influence of confining pressure, temperature, moisture content and frequency on damping ratio and dynamic shear modulus ratio have been carried out systematically. The results are summarized as follows: The damping ratio of frozen Qinghai-Tibet silty clay increases with increasing in confining pressure, temperature and water content, and decreasing in frequency. In addition, the dynamic shear modulus ratio reduces with increasing in confining pressure, temperature and water content, and decreasing in frequency. The relationship equation between the maximum dynamic shear modulus of frozen silty clay and temperature, water content and confining pressure is obtained by using multivariate non-linear fitting method, which shows a better fitting effect. The results of this research can provide basic data for the design of major engineering and seismic fortification along Qinghai-Tibet engineering corridor.

Impact of climate change on allowable bearing capacity on the Qinghai-Tibetan Plateau

Climate change has a substantial impact on infrastructures in the permafrost on the Qinghai-Tibetan Plateau (QTP). In this study, the mean annual ground temperature (MAGT) and permafrost evolution were investigated in both the historical (1950–2005) and projected (2006–2099) periods. Then, an allowable bearing capacity model was used to discuss the allowable bearing capacity change on the QTP. Results show that the MAGT increased by 0.36 °C during 1950–2005. The MAGT will increase by 0.40 (RCP2.6), 0.79 (RCP4.5), 1.07 (RCP6.0), and 1.75 (RCP8.5) °C during 2006–2099. In addition, the permafrost area has decreased by 0.195 × 106 km2 in 1950–2005. The permafrost area will decrease by 0.232 × 106 (RCP2.6), 0.468 × 106 (RCP4.5), 0.564 × 106 (RCP6.0), and 0.803 × 106 (RCP8.5) km2 during 2006–2099. With the degradation of permafrost, the allowable bearing capacity in permafrost zones would decrease accordingly. The decreasing trend is 6 kPa per 10 years in 1950–2005, and will be 0.6 (RCP2.6), 5 (RCP4.5), 7 (RCP6.0), and 11 (RCP8.5) kPa per 10 years during 2006–2099. The most remarkable trend would be observed under RCP8.5. Meanwhile, some scientific advices for the design, construction, operation and maintenance of permafrost engineering in the context of climate change were provided.

Experimental study on heat transfer of upright pipes in cold regions

Laboratory experiments are conducted to investigate the heat transfer characteristics of upright pipes under periodic ambient temperatures. The results provide evidence to support the use of upright pipes in permafrost engineering. This paper presents two models with different inlet pipe diameters. The results indicate that the mean periodic temperatures at the bottom of the pipe and soil decrease with increasing freeze-thaw cycles and ultimately remain in a frozen state. Natural convection is the dominant mode of conducting heat exchange between the upright pipe and air. The inlet diameter greatly affects both the cold energy accumulation at the pipe bottom and the cooling efficiency. Furthermore, the lateral thermal disturbance distance of the upright pipe is approximately more than one fold of the inlet diameter. Therefore, the upright pipe enhances the cooling effect to the surrounding soil and is recommended for the cooling of permafrost foundations.