Uncertainties of thermal boundaries and soil properties on permafrost table of frozen ground in Qinghai-Tibet Plateau

In the permafrost regions of the Qinghai-Tibet Plateau(QTP),the permafrost table has a significant effect on the stability of geotechnical engineering.The thermal boundaries and soil properties are the key factors affecting the permafrost table.Complex geological environments and human activities can lead to the uncertainties of thermal boundaries and soil properties.In this paper,an array of field experiments and Monte Carlo(MC)simulations of thermal boundaries and soil properties are carried out.The coefficient of variation(COV),scale of fluctuation(SOF),and autocorrelation distance(ACD)of uncertainties of thermal boundaries and soil properties are investigated.A stochastic analysis method of the probabilistic permafrost table is then proposed,and the statistical properties of permafrost table on the QTP are computed by self-compiled program.The proposed stochastic analysis method is verified with the calculated and measured temperature observations.According to the relationship between ACD and SOF for the five theoretical autocorrelation functions(ACFs),the effects of ACF,COV,and ACD of soil properties and the COV of thermal boundaries on the permafrost tables are analyzed.The results show that the effects of different ACFs of soil properties on the standard deviation(SD)of permafrost table depth are not obvious.The SD of permafrost table depth increases with time,and the larger the COVs of thermal boundaries and soil properties,the deeper the SD of permafrost table;the longer the ACD of soil properties,the shallower the SD of permafrost table.This study can provide a reference for the stability analysis of geotechnical engineering on the QTP considering the uncertainties of thermal boundaries and soil properties.

Determination method of permafrost table in seasonal frozen soil areas under“Water-Heat-Salt”coupling

The permafrost table is an important index for the design and construction of roads in cold regions,so it is necessary to find a convenient,accurate and fast judgment method to determine the permafrost table.In this study,a three-field coupled model was established based on the hydrothermal salt coupling within the permafrost and the similarity theory,and the changes of the permafrost table under different temperature,moisture and salt conditions were numerically simulated by considering the transient temperature change and the influence of the permafrost layer on the seasonally thawed layer.In addition,an accelerated permafrost table test method was designed based on the time-domain variation and hydrothermal salt coupling by the similarity theory,which rapidly simulated the permafrost table change under different temperatures,moisture,and salts in the natural environment.Comparing the simulation and test results with the measured values in the field,the errors are less than 3%,which verified the feasibility of the method for determining the permafrost table,and the simulated results are better than the test results.Results show that the results of determining the permafrost table with a single index have different degrees of deviation,and the permafrost table obtained by the temperature index is the most accurate in general,and it is more accurate to use the average value of the three indexes as the permafrost table compared with a single index.

Study on thermal-state variation of high-grade highway embankment under different pavement conditions

The research shows that the selection of pavement type is very important for the thermal stability of high-grade highway embankment in permafrost regions because of the different solar absorption rates between asphalt concrete and asphalt concrete pavement.In this paper,the common embankment of high-grade highway in permafrost regions is selected as the research object to study the influence of asphalt concrete and cement concrete pavement on the embankment temperature,freeze-thaw cycle process and the change law of the permafrost table,which provides a basis for the use of reasonable pavement materials in permafrost regions.

CHARACTERISTICS OF THE ACTIVE LAYERS ON FILDES PENINSULA OF KING GEORGE ISLAND, ANTARCTICA

From the data of the pitting, geoelectrical prospecting, temperature measurement, salt content analysis and detection by layering frost-heaving instruments, the authors discuss firstly the structural features of sediments in the active layers in this region, and proves the presence of the bowl-shaped frost table in the stone-circles area, and then analyse the regulatities of temperature distribution in the active layer, effect of salt content on electric resistivity, thaw-settlement and frost-heaving, and their control on periglacial land-form development. It suggests that the five layers should exist in the subsurface structure , namely, active layer, frost sand and gravel layer, frost volcanic rock permeated by sea water, frost volcanic rock unperme-ated by sea water, and unfrost ancient continental basement. Finally, the permafrost table and its vertical gradient are deduced.

Permafrost degradation along the Qinghai-Tibet Highway from 1995 to 2020

Permafrost degradation significantly affects engineering infrastructure,hydrologic processes,landscape and geomorphic processes,ecosystems and carbon cycling in cold regions.The permafrost degradation along the Qinghai–Tibet Highway(QTH)on the Qinghai–Tibet Plateau,China,introduces an adverse effect on the deformation of the highway subgrade.At present,observation of a long series of ground temperatures is lacking.From 1995 to 2020,a monitoring system of ground temperature in 10 natural sites along QTH was built and maintained.Ground temperatures at different depths were continuously observed semi-monthly.In this study,permafrost changes along QTH were quantitatively investigated based on these records.The main results showed that both the permafrost table depth(PTD)and ground temperature at different depths exhibited an increasing trend from 1995 to 2020 with widespread spatiotemporal differences.The higher the annual mean and range of PTD were,the higher the increase rate in PTD.The increase rates in PTDs in the warm permafrost regions were 6.18 cm per year larger than those in the cold ones.Overall,the increase rates in ground temperature decreased with the increase in depth at each site.At different depths,the smaller the mean annual ground temperature(MAGT)was,the larger the increase rate in the permafrost temperatures.The larger the range of ground temperatures was,the bigger the increase rate in the permafrost temperatures.At a depth of 6.0 m,the increase rate in the ground temperature in cold permafrost regions was twice that in warm permafrost regions.Information on the magnitudes and differences in permafrost degradation along QTH is necessary for the design of effective adaption strategies for engineering construction and environment protection in permafrost regions under climatic warming.

Thermal and deformational repairing effect of crushed rock revetment acting as reinforcement along Qinghai–Tibet railway in permafrost regions

Due to the particularity and complexity of permafrost subgrade,research on its long-term maintenance and reinforcement under climate warming and engineering activities is of great significance.To mitigate subgrade diseases caused by thermal disturbance during the engineering construction and operation in the initial stage,the crushed-rock revetment(CRR)was additionally paved with a thickness of 1.5 m and 1.0 m on some sunny and shady shoulders of the traditional embankments along the Qinghai-Tibet railway,respectively.The improving effects for thermal and deforming stability are evaluated based on observation data of ground temperatures and embankment deformations at two sites from 2002 to 2014.The results show that a larger uplifting magnitude in the artificial permafrost table(APT),greater ground temperature decreasing amplitudes and reduction ranges of settling rate appear under the shady embankment shoulder in warm permafrost region,and both sides in the cold permafrost region when reinforcing with CRR.However,in warm permafrost region,the laying of CRR on the sunny slope of subgrade may cause considerable thermal disturbance to the underlying permafrost foundation,combined with the resulting additional stress,induce the further expansion of differential settlement.Moreover,the thermal stability strengthening effect of the CRR is closely related to the variation of the APT thickness in the earlier stage,convection intensity inside the CRR,‘cold energy reserve’in the deeper permafrost,and amount of solar radiation received by the CRR.More effective reinforcements should be implemented to alleviate the potential threaten beneath sunny embankment slope in warm permafrost regions.