Research On The Soil Slope Stability In Seasonal Frozen Areas Based On Strength Deterioration Of The Freeze-thaw Surface

Since the reform and opening up more than 30 years, the economic construction of China reaches rapid development stage. All kinds of modern construction such as water conservancy, hydropower facilities, high-speed railway and highways and others have been developed unprecedentedly. During the project construction process, a large number of slope engineering is inevitably set up. Soil slope is one of the main parts of the road engineering. The stability of the slope is an essential prerequisite to ensure the safe operation of road transport. In seasonal frozen regions, shallow landslide hazards in soil slopes usually happen, which pose a serious threat to road safety operations. During the melting process, there forms stagnant water lubrication between the melting soil and freezing soil interface, which constitutes the weak interface of landslide hazard. This paper designs special methods to form the freezing– thawing (F-T) surface, and takes consider the interface strength as the foundation of experimental tests, then opening to research the soil slope stability. The conclusions of the paper present certainly well theoretical and applied value.This paper is one important part of the National High Technology Research and Development Programs (863 Project). The main content of the paper is as follow:1. Theoretical analysis on soil slope stability based on damage of F-T surface strength.Combined with slope stability limit equilibrium analysis methods, put forward the soil slope stability analysis, which based on the mechanical properties test of the silty clay of seasonal frozen regions, considers shear strength of the F-T surface soil under changes in F-T cycles as well as water content. The region that meet the seasonal frozen soil slope stability factor of safety of modified methods, give regional stability analysis approach. Coefficient amendment of the aim soil slope stability safety factor is put forward, which presents a new approach to the soil slope stability analysis in seasonal frozen regions.2. Experimental test analysis based on the F-T surface(1) Improved testing methods, and obtained the F-T surfaceIn view of the physical properties that the freezing point of salt water is below the ordinary water, based on which designing the special model tests. Select the same soil and add 1% ratio salt water and normal water separately. Then the compacting specimens are produced by soil volume ratio 1:1. At the temperature of ? 3°C , we get the F-T surface in the middle of the specimen, and achieve the innovation on the experiment method. Focused on the F-T surface, the shear strength parallel tests are carried out. The experimental tests results demonstrate that, the shear strength of the F-T surface is lower than the normal soil surface of the specimen, and the data ratio is about 70%. This indirectly proves that the F-T surface is a weak interface soil slope.(2) Combined with the affecting parameters to the soil slopes stability, carry out the research on strength deterioration of the freeze-thaw surface soil.The main affecting parameter of the soil slopes is the shear strength of the soil. Considering the affecting factor of shear strength is water content and F-T cycles times, so the shear strength of F-T surface soil with the factors is analyzed. The test results illustrated that the shear strength of freeze - thaw surface soil deteriorates with the increasing of water content and F-T cycles times significantly. Further more, the water content of the soil reflects more deteriorating affect. With the accumulation of the F-T cycles times, its impact to the shear strength is tends to ease. Considering the specific climate characteristics in seasonal frozen regions, the F-T surface shear strength deterioration model is put forward against the F-T cycles times. It can reflect true state of the melting soil slope, the interface shear strength deterioration within the F-T cycles. The results could present data support to the slope stability analysis in seasonal frozen regions.(3) Based on the shear strength parameters test results, put forward the experimental research on the shear strength parameters between the F-T surface of soil slope in seasonal frozen regions.Parameter reduction method is usually adopted in the soil slope stability analysis. Based on the results of the shear strength tests about the F-T surface, concluded that the shear strength parameters c and ? decrease with the increasing of water content and F-T cycle times. The impacts with the water content increases flatten gradually. At the same time, generalize the coupling number function of the water content and the freeze-thaw. The function shows that water content of the soil affects more significantly. Then put forward the parameter reduction models on the F-T surface of cohesion c and friction angel? , separately, which could present the data support to the analysis of the same type soil slope by parameter reduction method. (4) Based on the shear strength reduction models of F-T surface, put forward the experimental research on the critical depth deterioration of the soil slope in seasonal frozen regions.Combined with the F-T surface shear strength deterioration model, design the critical depth deterioration model test of the soil slope. Research the failure mode and the critical slide depth under different F-T times. The results demonstrate when the soil slope gets instability occurred during the spring melt period of destruction, the critical depth of specimens that contains F-T surface is lower than the normal soil slope. It demonstrates when failure occurred in the area, the slope slip surface is the F-T surface, and the type is shallow landslide.When slope sliding occurs, the slide surface is different from the normal curve style, but it is a region such as approximately rectangular body, and slip surface is approximately a plane. The slide surface is just the F-T surface. Test load - displacement curve compared reveals the damage process soil specimen well. The results reveal that as the more water content, and the smaller of the critical slip depth. As water content ranges from 16% to 21%, the depth of the critical slip decreases 43.2%, considering the seepage and water added, the injuries will be further intensified. Based on test results, the deterioration model of the critical sliding surface depth is put forward, which could provide a basis to a seasonal frozen regions roads slope for monitoring and optimization.3. Numerical simulation analysis of the soil slope stability during the melting process.(1) Based on the Finite Difference Method, set up the numerical simulation model, and analysis the soil slope stability.Apply Fast Lagrangian Analysis of Continua methods, combined the interaction of temperature, soil gravity and seepage, set up the numerical simulation model of seasonal frozen soil slope model.Self-compiled Fish programming to achieve the internal temperature gradient and set the seepage gradient into the slope model, simulate the melting process of the slope. Adopt the Mohr - Coulomb Model as the constitutive model of soil. The simulation results show that in the limit state, the failure modes of melting sliding soil slope of the seasonal frozen regions are as follows: near the top of the slope the slip surface is approximately parallel to the slope of the plane, just the bottom slip surface of the slope is as a circular surface. If the temperature in the slope is distributed well, the soil slope could adopt the unlimited slope analysis methods. The slope possesses big ratio between surface length and critical slide surface.According to the plastic zone distribution and shear strain increment of the development trend of the soil slope inside, determine the failure mode of the seasonal frozen regions slope and the position. Sliding occurs in 0℃soil region, the soil temperature under the sample is lower than 0℃. The sliding occurred in the F-T interface area. Verify initial judge of the F-T surface is the weak interface of slope in the melting process. The simulation results could provide numerical reference for the soil slope stability analysis in the regions.(2) Comparative simulate various factors influencing on the slope stabilityConsider F-T cycles and soil moisture as the main factors affecting slope stability. Pull in F-T surface shear strength of the test as a strength reduction factor input of damage parameter. Through analyzing the safety factors of interface under the critical state in F-T soil slope, simulate the F-T cycles and water content affect on the slope stability. The results demonstrate that, as the F-T cycle times and water content increasing, the safety factor of the slope get bigger. It illustrates the strength of the soil changed, resulting in a higher factor of safety conditions, has been formed inside the plastic shear strain through the damaged area and instability occurs. Comparing simulation results with the experimental results to verify the F-T cycles and water content increased, soil structure, slope stability was reduced, and the influence of water content is more apparent.The content of the research plays an important role in exploration results in both theoretical and experimental methods of the seasonal frozen soil slope stability analysis. The results of the research have some practical significance, and lay the foundation for designing and monitoring of the future practical engineering.