Research On Change Mechanism Of Strength And Microstructure Of Loess Under Wetting-drying Cycle

Large area of loess is distributed in the Loess Plateauin which located in northwest China.Due to drought and climate change periodically, the loess is always in a state of saturated and non-saturated alterations under the effect of rainfall and evaporation of dry-wet circulation. So the change of loess’ s physical and mechanical properties is also a dynamic process.The decrease of the strength and the increase of deformation due to the repeatedly dry-wet circulation function, which has important effect on the long-term stability of the buildings foundation, road slope and embankment engineering in loess area. Hence it has important theoretical and practical significance to research the change of strength and microstructure of loess under the dry-wet circulation.Based on the Luochuan shallow loess as the research object, through indoor physical and mechanical test and microstructure analysis, this paper studies the different moisture content loess change rule of the uniaxial tensile strength, the unconfined compressive strength and triaxial shear strength and its change rule of microstructure under the dry-wet circulation, and analyzes the relationship between microstructure characteristics and macro strength,finally discusses the intensity of loess expression.The main research results are as follows:(1)Analyze the influence of moisture content and dry-wet cycling times on tensile strength of undisturbed and remolded loess through uniaxial tensile test and establishes the relationshipbetween tensile strength of the undisturbed and remolded loess and the moisture content or the dry-wet cycling times. Then study the attenuation law of tensile strength and mechanical mechanism under dry-wet circulation undisturbed and remolded loess, which finds that the dry-wet circulation function breaks the reinforce cohesive force of undisturbed loess and causes the loss of its structural strength, also gives rise to the undisturbed loess has the similar mechanical properties to remolded loess after the dry-wet circulation.(2)Carry out unconfined compression test, and study the relationship between the unconfined compressive strength of undisturbed and remolded loess and the moisture content or the dry-wet cycling timesunder dry-wet circulation function, and establish the corresponding formula. Besides, it analyzes the relationship between the compressive strength attenuation value of undisturbed loess and the compressive structure strength.(3)It obtains undisturbed and remolded loess stress-strain curve under dry-wet circulation by using undrainedtriaxial test, and analyze the relationship between loess shear strength parameters and water content,also with the dry-wet cycling times, then study the relationship between the loess compressive structure strength and confining pressure, moisture content,finally reveals the inner link between the shear strength attenuation values of undisturbed loess under wet-dry cycles and shear strength of structure.(4)Analyze the change rule of particles unit body and pore microstructure parameters of undisturbed and remolded loess with water moisture and the dry-wet cycling times quantitatively by using the SEM photograph and image analysis software, furthermore describe the reason of loess microstructure parameters change under dry-wet circulation and different water moisture.(5)Obtain the correlation degree between 9 microstructure parameters and the loess strength based on the grey correlation theory, discovered that the structure parameters of pore and particle and the correlation degree of loess strength had changed obviously under the dry-wet circulation. Analyze the correlation degree between the structural parameters of pore and particle and the loess strength before and after the dry-wet circulation, regard the average diameter and average shapeof pore, directional probability entropy and average diameter of the particle had a critical impact on cohesion of loess, while the average shape factor and average diameter of pore, the average abundance and directional probability entropy of particle had decided the internal friction angle of loess.(6)Analyze the matrix suctionof undisturbed and remolded loess changing with the moisture content, and study the relationship between the matrix suction and the strength of undisturbed and remolded loess, then discuss the relations between loess tensile, compressive and shear strength, and try to establish a formula for tensile and shear strength of unsaturated conventional test.