Disease investigation along the Qinghai-Tibet Highway shows that permafrost is sensitive to geometric effect of the roadbed (such as width, height, slope ratio and etc.). Change of geometric effect could directly modify the natural energy balance in underlying permafrost foundation and thus cause a series of engineering diseases. Based on this idea, an "energy-balance design theory" is proposed. In this theory, the energy change in permafrost foundation due to engineering construction is taken as research foundation. The guideline of this theory is to seek a balance between the "harmful" energy input caused by natural environment change and artificial engineering construction and the "harmful" energy output caused by cooling measures in highway. The energy balance process of highway engineering in permafrost regions is analyzed in dimensions of space and time, which will be taken as design basis for the highway engineering in permafrost regions. The energy-balance design theory would provide theoretical support for the scientific design of the Qinghai-Tibet Expressway.
In this article, a numerical model is established in the background of permafrost in Madoi. Using the model, the initial temperature field and degradation characteristics of permafrost under different upper boundary conditions are simulated. It is found that the ground temperature of shallow layer permafrost simulated by using a 60-year averaged temperature is obviously different from that by using observed temperature series. It is predicted that within the next 100 years, under the same heating rate and initial temperature conditions, the permafrost degradation process will be less influenced by whether using the 60-year measured temperature. When the initial heating temperature value improves and meets the trend-line, the starting time of permafrost degradation will be in advance from the 45th year to the 20th year. When the 60-year measured temperature and initial heating temperature both improves and meets the upper boundary of initial temperature field, the starting time of the degradation will be in advance form the 20th year to about 15th year. Permafrost degradation will last about 25 years from the beginning of decay to disappear entirely under the series of boundary conditions.
Climate warming could aggravate the frequency of thaw settlement hazards along the Qinghai-Tibet Engineering Corridor (QTEC), and bring negative impacts on the infrastructures. In this paper, the volumetric ice content and the change of active-layer thickness are taken as indexes and applied to the thaw settlement model supported by ArcGIS software to zone the thaw-settlement hazards along QTEC. The results show that in the next 50 years the thaw settlement hazards along QTEC will be moderate and high-risk categories under A1B and A2 scenarios, but low and moderate risk categories under B1 scenario. High-risk areas mainly distribute in warm and ice-rich permafrost regions, such as Chumarhe high plain, Wudaoliang and Kaixinling regions.
Molding temperature and hydration heat of concrete significantly affect the thermal stability of cast-in-place pile foundation in permafrost regions during the construction period. In this paper, aiming at this problem, numerical method was used to research the impacts on refreezing process, thawing depth under pile bottom and temperature field of pile foundations, which were buried in permafrost along the 400 kV Qinghai-Tibet DC Transmission Line (QTDCTL). The results show that concrete temperature at pile center might reach to the highest after three days. The highest temperature at pile bottom might delay about one day. Concrete temperature at pile foundation surface has not been seriously affected by hydration heat, but mainly affected by ambient temperature. The maximum thawed depth under pile bottom appears after 24 days and increases with the increasing molding temperature. The maximum thawed depths with 6℃ and 15℃ molding temperatures are 34 and 55 cm, respectively. The higher the molding temperature, the longer the refreezing time. Refreezing time of pile foundation with 6℃ molding temperature is 52 days, while it delays one week for that of 15℃ molding temperature. Thawed depth decreases with ice content. The mean annual ground temperature is an important factor affecting thawed depth under pile bottom. The maximum thawed depth under pile bottom is 38, 34 and 25 cm in warm permafrost (-0.52℃) and cold (-1.5 and -2.5℃) permafrost areas, respectively. So the reasonable molding temperature range and the thickness of sandy gravel cushion under the pile are recommended to be 6-8℃ and at least 40 cm, respectively, according to this study.
The stability of embankment in permafrost regions depends on the thermal stability of frozen soil under it largely. With the pondings caused by snowmelt, rainfall or other artificial factors beside the embankment for a long time, the water of the ponding can aggravate or induce the decreasing and loss of the thermal stability through infiltration with convection conduction, thermal boundary erosion, providing water for frost heaving, etc. In this paper, the related literatures from domestic and abroad are summarized to understand the research status of the influence of the pondings on the stability of permafrost embankment. These literatures involve the factors of affecting the stability of permafrost subgrade, the influence of water from ponding on the active layer and the thermal effect of the water from thermokarst lake, the seepage water from rainfall, snowmelt or other ponding on permafrost embankment. Based on this, the further outlook and research about this problem are proposed.
In seasonal frozen regions, as the internal structures of soil changes periodically under freezing-thawing cycles, the soil pore and skeleton will change inevitably. Through particle analysis, it is found that soil particles tend to be smaller and concentrate in the range of 0.01~0.05 mm after freezing-thawing cycles, and the particle size tends to be stationary after tenth freezing-thawing cycles. Mercury injection experiment was carried on the loess after freezing-thawing cycles, which demonstrates that the large aperture of soil decreases first, and then increases with freezing-thawing cycles increasing; while the condition is contrary to the small aperture of soil. At last, concentrating on the range of 5～10 μm. In this experiment, the porosity changes with freezing-thawing cycles. Comparing with unfrozen loess, the porosity of intact loess decreases about 9% after five freezing-thawing cycles and reaches to the minimum; it increases about 1.5% and reaches to the maximum after ten freezing-thawing cycles. After ten freezing-thawing cycles, the porosity of intact loess fluctuates at 40% and reaches to steady state. The remolded loess decreases about 6% relatively and reaches to the minimum after three freezing-thawing cycles; it increases about 3% and reaches the maximum after five freezing-thawing cycles. After five freezing-thawing cycles, the porosity of remolded loess fluctuates around 40% and reaches to steady state. With freezing-thawing cycles increasing, the porosity of intact loess fluctuates between 32.5% and 42.6% while the remolded loess fluctuates between 34.8% and 43.3%. The changing range of intact loess's porosity is greater than that of remolded one. The changing curves of the permeability coefficient are similar with the porosity which obtains from the mercury injection experiment. It also verifies the changing law of porosity.
Block stone embankment is a most widely used roadbed form in permafrost regions. To study the impact of newly constructed embankment on the underlying permafrost in cold regions, data monitored at three monitoring sections along the newly constructed Gonghe-Yushu Expressway in Qinghai Province are selected, and the initial temperature regions are analyzed. The results show that the thermal insulation effect of block-stone embankment is closely related to mean annual ground temperatures of the permafrost. The lower the temperature is, the more efficient the embankment will be. Due to the sunny-shady slope effect, the temperature of the left shoulder/foot is always higher than that of the right shoulder/foot. The permafrost tables under the left shoulder, the right shoulder and the central of the embankment are all raised; the magnitude of the uplift is mainly affected by the height of the embankment, having little thing to do with the mean annual ground temperature of the permafrost.
The influence of vegetation change on the soil moisture is one of the key subjects in study of eco-hydrology and hydrology. In this paper, based on soil moisture in the active layer, plant biomass, soil physical and chemical properties in permafrost regions on the source regions of the Yangtze River, the response of soil moisture to different alpine ecosystems was studied. The results showed that the biomass and nutrient in the alpine meadow are higher and the response to precipitation is more intense than those in alpine steppe, which subsequently result in smaller soil moisture variability in the alpine meadow. In the soil completely thaw phase, soil moisture content is relatively lower at the depth of around 0.5 m, but higher at 0.2 m and 1.2 m depths for alpine meadow. However, the soil moisture gradually increases from the surface to the bottom of the active layer in alpine steppe. In the freezing process, the first day of alpine meadow freezing is 3-15 days' lag behind, as compared with that in alpine steppe. In the thawing process, the alpine steppe, which is rich in ice, needs more latent heat for thawing as compared with alpine meadow. Meanwhile, the water holding capacity in alpine meadow is more than that in alpine steppe in the surface layer from 0 to 0.2 m depth, but it is opposite in the middle and bottom. In conclusion, the succession of different alpine systems may change the processes of heat-moisture migration.
Hydrological effects of glaciers, snow and permafrost changes on the downstream water supply have result in a significant impact, showed a significant trend of increasing flood in recent decades, especially in the southern Xinjiang. Take the Huangshuigou River and Qingshui River Basins which located on the southern slope of Tianshan Mountains as the study areas, by the analysis of the extreme hydrological events, and combined with the meteorological data of the Baluntai Meteorological Stations in the upstream mountainous, the response characteristics of the extreme hydrological processes as annual peak flow occurred time, annual peak discharge and annual minimum flow in alpine cold watershed in the context of climate change was studied. The results showed that, year 1986 is the turning point of hydrological processes and climate change, from the beginning of 1986 with the increases of precipitation and temperature, the runoff showed an increasing trend; annual peak discharge occur from time postponed until mid-June to late July; have a positive correlation between the peak discharge and summer rainfall, while the winter and spring temperatures close to the minimum annual runoff. Since 1986 as the temperatures rise, hydrological effects of permafrost degradation, result in the winter runoff increased significantly, but also the minimum annual runoff increased significantly. Changes in precipitation results in the annual runoff increases since 1986, occurred time of the annual maximum runoff appear in the summer and the increasing magnitude of annual peak discharge. Overall, increased peak discharge, flood volume, and the magnitude of interannual peak discharge since the mid-year of 1980s, resulting in a more serious disaster for downstream. Therefore, the strengthening impact assessment of climate change on hydrological processes and flood disaster in cold watershed, so that science and technology play a leading role in disaster reduction.
