| Swellng,fissure and overconsolidation are the most representative engineering properties of expansive soil,among which expansibility is the most direct cause of engineering damage in areas related to expansive soil.However,it is still difficult to determine the failure mechanism of expansive soil slope under the influence of expansion,and the classical strength theory and limit equilibrium stability analysis method are not enough to explain the shallow and gentle slope failure behavior of expansive soil slope.The influence mechanism of expansion on shear strength of expansive soil is a key scientific problem to be solved urgently in this paper.The flexible support technology with geogrid reinforcement as the main structure is often used to solve the slope engineering problems caused by the special properties of expansive soil,but the mechanism of the interaction between the reinforcement and soil under the influence of expansion is not clear,and the reinforcement design is often based on experience rather than theoretical guidance.Therefore,how to consider the influence of expansibility in slope stability analysis and reinforcement design is a key technical issue in this paper,and the interaction mechanism between geogrid and expansive soil under the influence of expansion is another key scientific issue in this paper.In this paper,the change rule of expansive soil shear strength under the influence of expansion is studied by improved direct shear test,and the attenuation mechanism of effective shear strength index is revealed.A dry density-dependent strength model is proposed and a method for stability analysis of expansive soil slope considering the change of moisture expansion is established.Considering the effect of lateral expansion,the interaction mechanism of reinforcement and soil is studied and a calculation method of geogrid reinforcement design for expansive soil slope is proposed.The stability of reinforced expansive soil slope is analyzed and evaluated by field monitoring and numerical simulation.Based on the above research,the main achievements of this paper are as follows:(1)The shear strength of expansive soil under constant volume condition is 6.4%～24.6%higher than that of conventional direct shear test.With the decrease of dry density,both peak shear strength and ultimate shear strength decrease.A theoretical formula for characterizing shear strength parameters at different densities is presented.The decrease of cohesion is due to the decrease of intermolecular force caused by the increase of soil particle spacing.On the one hand,the decrease of the internal friction Angle is due to the large particle soil being broken into fine particles under the action of expansion,which reduces the roughness of the shear plane.On the other hand,due to the expansion of soil,the contact between large particles and large particles is replaced by the contact between large particles and small particles,and the friction decreases gradually due to the weakening of the interlocking action of the contact surface.(2)The safety factor of slope considering expansion effect is 28%～33%lower than that without expansion effect.The higher the dry density of expansive soil is,the greater the saturated expansion deformation is,and the lower the slope safety factor is under rainfall condition.Therefore,when dealing with the expansive soil slope,special attention should be paid to the prevention and drainage measures of the high dry density expansive soil slope,and the moisture content of the backfill expansive soil should not be too low to avoid excessive attenuation of strength caused by soil expansion.The influence of soil expansion on the stability of the slope should not be ignored.(3)In the process of humidification,the lateral earth pressure,grid strain and vertical deformation of geogrid reinforced soil model show three typical stage characteristics with time.It is closely related to the release of the expansive potential energy of the expansive soil,and the three changes greatly in stage Ⅰ,then gradually slows down in stage Ⅱ,and finally reaches a stable state in stage Ⅲ.The maximum tensile strain of geogrid reinforced expansive soil is only 1.71%under the most unfavorable condition.During the humidification process of the reinforced soil,the displacement of the grid is basically consistent with that of the soil,which shows that the variation of the lateral displacement is larger than that of the inner displacement,and the variation of the lateral displacement is larger than that of the vertical displacement.The soil of the single-layer reinforced soil is always cooperative with the upper and lower grids.(4)Geogrid can obviously improve the shallow stability of expansive soil slope.After the grid treatment,the slope does not slide under the action of rainfall,and the integrity of the slope is significantly improved.Because of the inverse action of the grid on the slope,the soil is effectively constrained as a whole,so that the distribution of expansion stress is homogenized,and the humidifying expansion deformation is significantly inhibited.The constraint effect of geogrid on expansion deformation decreases with the increase of the reinforcement distance between soil and grid,but for expansive soil slopes with different expansion strength,reasonable reinforcement distance can significantly shorten the construction period and improve the economy of the project.In this paper,a direct shear test device was developed to keep the volume of specimens constant before and after shearing,and the corresponding test method was improved.The attenuation mechanism of the effective shear strength index of expansive soil under the influence of expansion was revealed,and the stability analysis method of expansive soil slope considering the influence of expansion was proposed.The mechanism of interaction between reinforcement and soil of geogrid reinforced soil slope under the influence of expansion is improved and the calculation method of reinforcement design of expansive soil slope is proposed.The results can provide guidance for slope reinforcement and stability analysis in expansive soil area. |
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