The Mechanism Of Cutting Slope Freeze-thaw Instability And Its Protection By Vegetation In High-latitude Frozen Regions

For nearly100years, the process of global climate changing continues to accelerate, which cause many engineering geological problems. In Heilongjiang Province, With the rapid development of highway construction, the instability of cutting slope continue to emerge. It is urgent problems to be solved to seek reasonable method of slope protection which could combinate with the ecological environment of the road region, and to give a clear answer on the theory and practice.In high-latitude seasonal frozen areas of Northeast China, water temperatures in shallow slope change largely, which could impact on slope stability directly. Root absorbent and transpiration of slope protection plant not only could be able to adjust the moisture of shallow slope but also could improve the mechanical parameters of the soil for the reason of its roots penetrating into the soil and forming root-soil complex. In addition, from the respects of physiology and adaptability in geographical environment, using kinds of methods, selecting protection plants which both has strong resistance, also can protect beautify the slope is also one of content in this article.Relying on the research project of Heilongjiang provincial Science and Technology Department (GZ07C401), the Key project of Heilongjiang provincial Transportation Department "Highway Slope Stability and Greening by Vegetation in Cold Regions", and the project of International Program on Landslides" Research on vegetation protection system for highway soil slope in seasonal frozen regions"(IPL-132), this article determined its topics, and began to carry out research work.Through the survey of instability cutting slope along the Tongsan Highway from Jiamusi to Harbin, study area and study section was selected. Through on-site sampling and laboratory experiments, the basic physical indicators of the soil, such as density, permeability coefficient, particle composition were obtained. Through triaxial test, for soil and root-soil, the variation of the cohesion, internal friction angle, deformation modulus varing with density, moisture, freezing-thawing cycles was got. The results show that:Both for unfrozen soil and for unfrozen root-soil, their cohesion, internal friction angle, deformation modulus are all increased with the increase of soil density; when soil moisture is smaller than optimum moisture, the cohesion is increased with the increase of soil moisture; when soil moisture is bigger than optimum moisture, the cohesion is decreased with the increase of soil moisture; The internal friction angle and the deformation modulus of the soil is decreased with the increase of soil moisture; After freezing-thawing cycles, both for the soil and root-soil, their mechanics parameters is similar with unfrozen soil, just not have the same changing size. For three kinds of plant:turf, Lespedeza, Amorpha, through the data analysis of the moisture in different depth, using the theory of plant physiology, meteorology, soil physics, both the distribution of root system in cutting slope and the law of moisture migration and distribution under the joint action of plant roots and freeze-thaw was got. The results show that:Through absorbent root and leaf transpiration, shrubby plants (lespedeza, Amorpha)on the slope can effectively reduce the shallow moisture of the slope, especially for the soil boby within the scope of100cm below the surfface. Through shape analysis of root combining with site direct shear tests of root-soil, the mechanical mechanism of plant roots protecting the slope was got. The results show that:shear strength of root-soil within60cm is about2times of shear strength of the soil. Through light penetration testing, combined with temperature measurement data, field distribution of slope body’s temperature and shear strength were got, combining with soil mechanics theory and triaxial test results, the relationship between the shear strength and soil moisture was got. Using the the theory of soil physics, heat transfer, combining with the monitoring data of slope body’s temperature and moisture, in diferrent plant cover condition and different slope depth, the law of moisture distribution changing with the temperature was got. The results show that:During the soil freezing process, unfrozen water will migrate to a frozen zone, and part of them become into ice. During the spring melt period, frozen soil layer began to melt from two directions:the surrface and deep unfrozen soil, the melt water in shallow slope can not penetrate in time, result in the moisture in the shallow layer is very hight. Through the derivation of slope’s safety factor, which is based on the effective stress, the impact of physical, mechanical and geometry parameters on slope stability was analized furtherly. Based on strength Less finite element, the safety factor under different coverage conditions and in the most unfavorable season was got. The results show that:In the melting period of spring, if the turf served as protecting slope plant, two monitoring sections of the slope are all in critical state of instability state, if Lespedeza served as protecting slope plant, they are in stable state Through saturated undrained ring shear test and residual strength ring shear test, the variation of physical parameters in sliding process was studied. The results show that:view friction angle of the slope body is22.2°, view friction angle of the slope body in residual strength is7.6°. Combining with landslide case analysis, correctness of the analysis method and analysis results was verified. Based on remote sensing data and meteorological data, through the study of relationship between vegetation indices and meteorological factors, mathematical model between vegetation index and temperature, precipitation and sunshine was built, then meteorological factors was anti-derivated.Combinning with the terrain elevation data and plant growth characteristics, for Amorpha, Lespedeza and sea buckthorn, evaluation system of plant adaptive growth was established, and conduct space adaptability evaluation. The results show that:in Heilongjiang Province. the selected plants have good growth adaptation.