Study On Moisture Characteristics Of Pore Pressure In Unsaturated Sandy Soiland Its Effect On Strength

In road engineering,the strength of the subgrade is one of the main reasons that affect whether the road can always have good stability during its service life.The subgrade soil is constantly changing in moisture under natural conditions and is usually in a non-saturated state.As a result,there will be dynamic changes in the negative pore pressure within the soil,and have an impact on the strength.At present,the strength analysis of soil layers in engineering often uses the Terzaghi saturated soil strength theory.Although this approach is easy to operate and has high safety,it neglects the contribution of negative pore pressure to strength in unsaturated soils,resulting in excessive waste of resources.It is therefore necessary to investigate the moisture characteristics of pore pressure in unsaturated soils to explore its effect on soil strength and provide theoretical guidance for practical engineering.In this paper,I first used the group’s self-researched pore pressure monitoring device to conduct pore pressure humidity characteristics tests on three kinds of sandy soils with different fineness modulus to explore the effects of water content and water table height on pore pressure during water table rise and fall,and further reveal the change law of pore pressure in unsaturated soil mechanics.Then,the relationship between matric suction and shear strength parameters and pore pressure in unsaturated soil is investigated through the matric suction test and direct shear test,and the shear strength model of unsaturated sand soil based on pore pressure is constructed.Thus,the strength of the subgrade soil can be solved by the pore pressure measured by the device in real-time,avoiding the disturbance damage to the soil layer during the sampling process by the traditional method.The pore pressure strength model is validated using Geo-Studio finite element software,and a shear strength calculation method for sandy soil subgrade based on this model is established.The main conclusions of the paper are as follows.(1)In the pore pressure moisture characteristics test,the measured pore pressure varies with the changes in water content and water table height.The test data show that the water content gradually increases during the water injection stage,the negative pore pressure increases first and then decreases,and the positive pore pressure increases continuously after the soil is in the saturation state and reaches the peak after the end of water injection.In the drainage stage,the positive pore pressure decreases continuously,and after the soil is unsaturated,the water content gradually decreases,and the negative pore pressure increases continuously and tends to a constant value before the end of the drainage.And the negative pressure of the drainage section is greater than that of the injection section under the same water content except for the high-water content,and the positive pressure of the hydrostatic pressure and injection section under the same water level height is smaller than that of the drainage section.(2)The larger the fineness modulus of the soil layer and the smaller the burial depth,the smaller the maximum negative pressure in the injection and drainage stage.And the capillary water in the injection stage of the soil layer rises faster,the faster the process of negative pore pressure increases and then decreases,the critical water content is large;the drainage stage pore water discharge speed is faster,the time to dissipate the remaining positive pressure above the water table line and the time required to increase the negative pressure to the maximum are shorter,the water content at the maximum negative pressure is smaller.(3)With the decrease in water content,the matric suction force,cohesion,and internal friction angle all show a gradual increase.However,the increase of matric suction force is increasing,the increase of cohesion force is decreasing,and the overall change of internal friction angle is small.All three increase with the increase of dry density,but when the fineness modulus increases,the matric suction and cohesion decrease,while the internal friction angle will increase.(4)The relationship between matric suction and negative pore pressure and the relationship with the increment of cohesion is similar,showing that the transformation degree of matric suction to both of them decreases significantly with the decrease of water content.The curves of pore negative pressure and cohesion increment were better fitted,and the percentage of cohesion increment to pore negative pressure was always above 80%.According to Lu Ning’s suction stress theory,the effect of negative pore pressure on strength is considered from the perspective of effective stress,and the suction stress shear strength model of unsaturated sand soil based on measured pore pressure is established.Through software simulation and analysis of the subgrade condition,it is found that the pore pressure strength model has good applicability in the analysis of subgrade stability and shear strength calculation.