Study And Engineering Application Of Shear Strength Of Unsaturated Seasonal Frozen Soil

The soil in the seasonally frozen area is generally in the unsaturated state of melting in winter and freezing in summer.Under normal temperature,unsaturated soil is more difficult to obtain its shear strength index than saturated soil.In addition,in the winter season,the soil is frozen within a few meters below the surface,the soil shear strength and its index after freezing are much different from those before freezing.The overwintering foundation pit may cause potential safety hazards due to the frost heave of the soil.Based on this,this paper analyzes and fits the experimental data obtained by the triaxial compression experiment of unsaturated soil under normal temperature and negative temperature conditions and numerical simulation analysis of deep foundation pits at different temperatures based on actual engineering,as follows:1.Using the Geo-Experts pressure plate instrument to measure the soil-water characteristic curve of the sample and from the fitting results of the three soil-water characteristic fitting curves given in the article,it can be known that the Fredlund &Xing model has the best fitting effect on the experimental data;The physical parameters of unsaturated soil obtained by each fitting model are consistent with the actual values.2.According to the results of the triaxial compression test of unsaturated soil:when the matrix suction is constant,the shear strength of unsaturated soil increases with the increase of confining pressure;when the confining pressure is constant,the shear strength of unsaturated soil increases with the increase of matrix suction The increase is obvious.When the matrix suction is 57 k Pa,the shear strength formula of unsaturated soil based on the two fractal models of pore volume and massdistribution has a good prediction effect on the experimental value,but with the increase of the matrix suction,the prediction effect is getting worse(matrix When the suction force is 400 k Pa,the prediction deviation is slightly reduced,which may be caused by the instability of the undisturbed soil.The predicted value is generally smaller than the experimental value,and the difference between the prediction effects of the two fractal models is small.When the shear strength theory based on the unified effective stress of the two fractal models is used to predict the experimental results,most of the experimental values are larger or similar to the predicted values.In order to make up for the shortcomings of poor prediction effect under high matrix suction,a fitting curve related to matrix suction or water content is given.3.According to the experimental results of triaxial compression of unsaturated frozen soil,it can be seen that a small confining pressure has little effect on the stress-strain curve of frozen soil.When the confining pressure and matrix suction are constant,the partial stress increases with decreasing temperature;when the temperature and confining pressure are constant,the partial stress increases with decreasing matrix suction.Under the same volumetric water content,as the temperature decreases,the cohesion of frozen soil gradually increases,and the internal friction angle of the frozen soil gradually decreases;under the same negative temperature,as the volumetric water content decreases,the cohesion of frozen soil gradually decreases;The friction angle in the frozen soil gradually increased.When the dry density is constant,the volumetric ice content has a strong linear relationship with the unsaturated frozen soil shear strength index.4.The results of numerical simulation show that: under the condition of constant overload around the foundation pit,as the temperature decreases,the settlement around the foundation pit gradually decreases;from the simulation resultsof the uplift of the foundation pit,it is concluded that when the temperature drops to0°C The frost heaving of the bottom surface of the foundation pit is the main factor of the uplift phenomenon.When the temperature continued to drop to-15°C,the bulge rebounded slightly due to the substantial weakening of the overload.Due to the freezing effect of the soil,the horizontal displacement,maximum shear force and maximum bending moment of the pile all increase with the decrease of temperature,which is the embodiment of the effect of soil frost heave.Comparing the alarm value of foundation pit deformation monitoring given by the specification with the simulation value,it is concluded that foundation pit uplift is the focus of monitoring under simulated conditions.