Study On Shear Strength Of Unsaturated Silty Clay Under Freeze-thaw

Freezing and thawing will affect the shear strength of unsaturated silty clay in the seasonal freezing zone.Therefore,it is of great practical significance to consider the two influencing factors of matrix suction and freezing and thawing when selecting soil strength index parameters in foundation pit engineering.The shear strength of unsaturated silty clay changes under the influence of matrix suction.The selection of soil-water characteristic curve can effectively establish the relationship between water content,matrix suction and shear strength parameters.This paper takes unsaturated silty clay in Changchun area as the research object.After the undisturbed soil sample has been treated with different times of freezing and thawing cycles,with the help of nuclear magnetic resonance instrument,pressure plate instrument and GDS unsaturated soil triaxial test instrument,the porosity,soil-water characteristic curve and shear strength index were tested and studied.The test results show that the porosity of silty clay in a saturated state will increase with the increase in the number of freeze-thaw cycles.After 9 freeze-thaw cycles,the soil-water characteristic curve of unsaturated silty clay shows a tendency to move to the lower right of the coordinate axis.After comparing the VG model as the best solution for curve fitting;The increase of matrix suction will increase and decrease the cohesive force and internal friction angle respectively.,the increase in the number of freeze-thaw cycles will not change the test results.In the case of the same matrix suction,the cohesive force and internal friction angle will decrease and increase with the increase of the number of freeze-thaw cycles.Prediction using soil-water characteristic curve shows that after the effect of freeze-thaw cycles 9,cohesion unsaturated silty clay and internal friction angle respectively,with an increase of water content and saturation increase and decrease trend presentation;Use test data as the basis for simulation parameters,use Midas GTS software to numerically simulate the selection of internal support and row pile support for foundation pit engineering in Changchun area.Analyze the influence of matrix suction and the number of freeze-thaw cycles on the settlement around the foundation pit,the uplift of the bottom of the pit,and the horizontal displacement of the supporting pile.The results show that with the increase of matrix suction,the land subsidence value first increases and then decreases and gradually stabilizes with the increase of horizontal distance,the horizontal displacement of the supporting pile increases first,then decreases and gradually stabilizes.When the matrix suction is constant,ground subsidence,pit bottom uplift and horizontal displacement of supporting piles are affected by the increase in the number of freeze-thaw cycles,showing an increasing trend.The significance of the test and simulation analysis is to provide more reliable technical parameters for the design and construction of the unsaturated silty clay foundation pit engineering in the long-spring frozen area.