Study On Thermal Stability And Thawing Settlement Prediction For Patchy Permafrost Subgrade

The area of permafrost regions accounts for about 1/5 of the land area in our country, it distributed mainly in Qinghai-Tibet Plateau and Xing an Mountains region of Northeast China, the domestic studies on permafrost engineering mainly focuses on the western region represented by Qinghai-Tibet Plateau in past, but the permafrost in Xing an Mountains region of Northeast China is significantly different from that in Qinghai-Tibet Plateau and other Western Regions. The permafrost is mainly controlled by altitude in western region, and the most is continuous, the body mass is large, the differential deformation of subgrade is small; However, the permafrost has obvious Dimension zone characteristics in the northeast of China, the body mass is small, the continuity is poor, the distribution present dispersed islands like, and the most permafrost belongs to the warm permafrost category, the thermal sensitivity is high, the degradation rate is fast, in engineering practice, there are great differences between the subgrade diseases of different regions in occurrence time, development speed, damage process and the degree, etc. Therefore, it is necessary to enhance the research of engineering technology and theories accorded with the characteristics of high latitude permafrost, but the thermal stability of patchy permafrost subgrade is a very complicated engineering problem that has not been completely resolved yet, especially the high-grade highways with wide pavement in width, because of its strong thermal disturbance on the subsoil, the engineering problems caused by it are also more complicated.This paper is based on the scientific technology program of "Study on The Subgrade Stability of Permafrost Melting and Mountain Landslide Sections in Widening Extension Highway Project" supported by Ministry of Transport (No.2011318223630), and the key scientific research program of "Study on The Design and Construction Technology of Highway Subgrade in The Patchy Permafrost Region of High Latitude Zone" supported by Heilongjiang Provincial Department of Transportation(No.2009-019), and based on the intensive studies of variation characteristics of ground temperature field and reflect process of displacement field by experiment comparison, theory analysis and numerical simulation, it proposed the annual periodic response law of displacement field along with ground temperature, the "thermal-shrink effect" and the "mutagenic effect" of subgrade deformation in patchy permafrost regions, and a new and more effective prediction method is also put forward by applying support vector machine to the thaw settlement prediction of permafrost subgrade. The main research contents and innovative achievements are summarized as follows:1. Through experiments and analysis of the factors for thermal stability of permafrost subgrade, it is discovered that there are obvious stage characteristics for the annual periodic change law of temperature field and displacement field for patchy permafrost subgrade, according to the measured data, the annual periodic response process of displacement field along with ground temperature is divided into the freezing stage, the transition stage and the melting stage.2. Through the analysis of subgrade thermal regime, phase transformation rate, uniaxial elongational property and lateral deformation properties for the permafrost, and the phase transition and moisture transfer process were taken into account, the control equation of ground temperature and the theoretical calculation models were constructed for vertical displacement and lateral displacement; The numerical simulation of ground temperature field and displacement field in transient for patchy permafrost subgrade was done by using finite element analysis software.3. The numerical simulation of slope-directive effect for patchy permafrost subgrade was done by applying different thermal flux conditions on both subgrade slopes, and the results shows that the asymmetry of ground temperature field and displacement field is more obvious with the increasing of temperature difference between both subgrade slops, according to the isoline distribution diagrams under the different thermal differences, the slope-directive characteristics of ground temperature field, change characteristics of displacement field and the subgrade diseases induced by them were analyzed in-depth under the conditions of freezing and melting respectively.4. Based on the analysis of measured data, the annual periodic variations and characteristic differences were obtained for the ground temperature and displacement of the key nodes in the patchy permafrost subgrade, and the deviation reason was analyzed between measured data and numerical simulation result, so the feasibility of theoretical calculation method was verified; And then based on the grouping comparisons of annual periodic changing curves of ground temperature and displacement for the different soil layers of different covering soil thickness and different ice content, it is concluded that the influencing effect of covering soil thickness, ice content and foundation reinforcement measures to the thermal stability of patchy permafrost subgrade.5. Through experiments and analysis of the "mutagenic effect" of vertical displacement and the "thermal-shrink effect" of lateral displacement for patchy subgrade deformation, it is pointed out that the vertical displacement mutation contains two processes of soil strength attenuation and soil moisture transfer in the freeze-thaw course, and the lateral shrinkage is becaused of the lateral discharge of a large amount of soil pore water, according to the occurrence mechanism of each effect, the corresponding formula of displacement increment was presented respectively, and the vertical mutation displacements of some sections were verified as soil strength attenuation caused by the change of ground temperature.6. Based on the analysis of application effects of some existing prediction theories and methods, the principle of support vector machine(SVM) was applied to the thaw settlement prediction of patchy permafrost subgrade, and seted up the corresponding SVM prediction model based on practical project by using the data analysis software MATLAB, compared with the measured data and the results of other prediction models, it shows that the SVM prediction model occupies preferable predication performance, and the prediction difficulties caused by more effect factors and smaller sample size for patch permafrost subgrade is solved better by it.