Mechanism Of Shallow Slide On Highway Cutting Slope During Spring In Cold Region

Most of the cold regions in China are seasonal frozen regions where shallow slide of highway cutting slope is a popular phenomenon in spring. Attentions have been paid to the issue in the areas of road engineering and geotechnical engineering in cold region for many years. In winter, the soil moisture migrates upwards as the ground frozen from the surface to the deep. In spring, the ground thawing starts from the surface to the deep, the water at the shallow layer permeates downwards by gravity and cannot infiltrate into the frozen soil beneath, and then it accumulates in a thin soil layer just above the interface. This results in the increase of the water content and the decrease of the shear strength of the soil layer, and making the original homogeneous slope become inhomogeneous, sometimes which makes the shallow slide. Obviously, the water migration in soil during freeze and thaw process plays a key role.The soil moisture migration in the freeze-thaw process is taken as the core point in this dissertation; Combining with soil sample and large scale model experiments, two mathematical models are built up separately for the migrations in freeze and thaw processes for engineering application purpose. The instability mechanism of highway cutting slope during freeze-thaw process is studied with also numerical analysis.The study begins with soil sample experiment, which conditions are well controlled. A close system with watertight boundary at the bottom of the soil sample is adopted, the samples are frozen and thawed in one-way to the practical situation of low water content and deep ground water table in all cutting slope. The freezing rate is handled through the temperature of the top plate, temperature variation with depth is observed regularly time to time from the thermal sensors set at a side of the soil sample, and then the frozen or thaw depth is controlled. Undisturbed soil test is to simulate the natural state, while reshaped soil test is to distinguish the effects of freezing and thawing, the both are exploring the soil moisture migration rules in the two processes.Temperatures at different depths were measured and recorded simultaneously during the test, and the distribution of water content with depth were measured after the sample frozen or thawed at a given depth, and compared with the original water content. All the data acquired in the tests are building a firm base for the mathematic models.In the experiments, 5 temperature conditions for samples with the same initial water content were adopted to measure the moisture migration amount under different temperature gradients; while 5 reshaped samples with different given initial water contents were adopted under the same temperature condition also to measure the water migration amounts. The water content distributions were measured when the samples completely frozen or thawed from the top to a given depth afterwards from the initial uniform distribution. The position of the soil layer with the richest water content above the freeze-thaw interface and the variation of the water content at that depth were studied for the thawed cases.The test results show that some transverse and longitudinal cracks appear in unthawed portion after the sample completely frozen and then thawed to the given depth, a distinguish freeze-thaw interface is also observed; the temperatures at various depths show the similar trend with different top temperatures even with different water contents; the water content curves after freeze-thaw are the similar on the whole, i.e. the water content increases in the upper soil layer and decreases in the lower layer; the more initial water content, the more potential heat consumption in freezing, the slower freezing rate, the more moisture migrated and the more water aggregated above the interface; the effect of temperature condition on moisture migration is quite obvious, the higher freezing temperature at top plate, the more water aggregates above the interface; the inhomogeneous ice content, even segregated ice, appear in reshaped silt-clay samples with initial homogeneous water content after frozen from top to bottom; a thin soil layer with the richest water content appears just above the interface after the sample frozen completely from top and then thawed to the given depth.There are still queries now to the existing water migration mechanism theories: the moisture migration models are too complicated for engineering practical purpose, values of some parameters in the models could not be obtained from experiment directly, engineering characteristics of soil mass is not taken into account enough, almost of the models are lack of validation, application results of these models are not satisfactory in some cases. From this state of the art, the modeling is not in pursuit of the comprehensive description of real solid, liquid and gas phase coupled with heat in porous media, and the feedback effect of moisture migration on temperature field is not taken into account temporarily in this paper. The importance of moisture diffusion in water migration during freezing is emphasized, and the relationship between water migration amount and temperature gradient is handled in addition, from the engineering point of view. The water migration mechanisms for two processes of freezing and thawing are considered separately, and two models were worked out by modification of the Fick law and the Darcy law.(1) Assuming the moisture in unsaturated soil migrates in the way of diffusion during freeze process. In this process, liquid water takes part in the migration by transformed into moisture, and the detailed transformation is not considered. The concentration equation of the moisture in the soil beneath the frozen front is presented, and a moving coordinate system is established for the concentration-depth relationship with the freezing front development. In order to take the effect of temperature gradient into account, temperature gradient is multiplied to the right side of the Fick law, thus an improved Fick law for unsaturated soil is obtained as the water migration model of freeze process. A set of calculation procedure and program are developed at the same time.A comprehensive diffusion term is adopted in this paper, instead of adding a diffusion term caused by temperature gradient. From physics point of view, it describes the fact that temperature gradient influent the diffusion rate directly, not adding an additional amount on the original moisture diffusion. From mathematics point of view, eliminating one parameter is convenient to parameters estimation.(2) Assuming the water migration in thawed soil is mainly the vertical permeation from the gravity. An additional term of water permeation caused by the temperature gradient is added to the Darcy law for unsaturated soil, as a water migration model for the thaw process.Furthermore, values of parameters in the two models are inversed by means of the Generic Algorithm, where the objective function is taken as the curve of water content– depth after the frozen and thawed processes respectively obtained from the experiments, and the forward calculation modules is built from the programs for the two models. The water content - depth curves are then calculated from the inversed parameter values and their mean respectively. The resulted curves are compared with those from experiments, thus the reliability of the model and the feasibility of the approach of this paper are validated.A large scale slope model experiment for instability after frozen-thaw is designed as a link between the soil sample experiment and the in situ experiment in this dissertation, with well-controlled boundary conditions and certain volume to eliminate the effect of boundaries in soil sample test. The designs of similar model tests both at home and abroad are summarized, the similarity law for model is determined. A slope model with 60 centimeters high is designed and made in laboratory, and the temperature condition is preset. Thermal sensors and sensors for water content are buried densely in the upper soil layers of the model. The data are acquired every 5 minutes during the experiment. Time domain reflector (TDR) is adopted in the experiment, which is developed from electromagnetic pulse reflection method, and the measured values are scaled to eliminate the shifting of readings with temperature. During 35 days, 840 hours which consists of 5 freeze-thaw cycles, the experiment shows the phenomenon such as water migration, frozen crack, segregated ice, and local slides on the slope, etc. The analysis on the recorded data shows that the temperature field is well-controlled and the water content variation is complex. The detailed data analysis of the first freeze-thaw cycle indicates rules that the moisture moves upwards by diffusion during the freezing, and permeates downwards by gravity during the thawing. From the results, it can be concluded that the main reason of shallow slide on slope is the significant decrease of shear strength in the thin soil layer just above the freeze-thaw interface resulted from the large increase of water content.In order to deal with the effect of freeze– thaw on shear strength of soil, to reveal the degree of decrease of soil shear strength from the increase of the water content quantitatively, soil samples with different water contents are reshaped, and are further frozen and then thawed, the shear strength parameters of the soil are tested by means of triaxial undrained shearing experiment. The result shows that the values of shear strength parameters decease certainly to a large extent, thus the above conclusion is validated that the main reason of shallow slides on the model slope is the large decrease of the shear strength of the shallow soil resulted from the increase of water content there.Finally, numerical analysis of the slope model for stability in the freeze-thaw experiment is carried out. A second development is made by means of ABAQUS software for finite element analysis, a finite element model is constructed for the model in the experiment, and the boundary conditions and element parameters are assigned. In the depth of 3.0 meters vertical from the surface, grids with 0.3 meters distance are freeze-thaw grids whose parameters vary with water content during the test, and others are not. The input data files with"inp"type are generated by ABAQUS/CAE. The shear strengths of freeze– thaw grids are modified from the statistical relationship with water content from the triaxial test mentioned above. The water content data measured in the experiment, the deformations and stresses of elements after frozen and then thawed to 5 depths are calculated respectively. The results show the potential of shallow slide on the slope.