Study On Mechanical Property Degradation Of Backfill Soil Under Water-induced And Air-induced Action

There are a lot of backfill areas in China,and more and more foundation pits are excavated in those backfill soil areas.However,some of the foundation pits deform after dewatering or DTH construction,so the water-induced degradation and air-induced degradation on backfill soil are studied in this paper.Through the laboratory test,the equation of water-induced degradation and air-induced degradation on backfill soil is studied,the influence of backfill soil degradation on support system is studied by theoretical analysis,and the harm of backfill soil degradation is studied by practical engineering.The main work and research results of this thesis are as follows:(1)The influence of moisture content and dry-wet cycle on the performance of backfill soil was studied through laboratory tests.With the increase of moisture content and wetting and drying cycles,the cohesion,internal friction Angle and shear strength of backfill soil gradually decrease,the performance of backfill decreased most obviously in the first two dry and wet cycles,and then gradually stabilized.The relationship between cohesion,internal friction Angle and water content of backfill soil approximately satisfies logarithmic function,and there is an exponential relationship between deterioration coefficient and the number of dry-wet cycles.2)The effect of air pumping on the performance of backfill soil was studied through laboratory tests.The cohesion,internal friction Angle and shear strength of backfill soil increase firstly and then decrease with the increase of air pumping,and the cohesion of backfill soil is more sensitive to air pumping than the internal friction Angle.The cohesion and internal friction Angle of backfill soil are normally distributed with air pumping.3)Based on static equilibrium and Mohr Coulomb theory,it is concluded that the arch foot of soil arch is the most dangerous section in support system,and the critical point of arch span equation of soil is derived.Basic on the equation of induce degradation on backfill soil in the second Chapter,such as the relation between the cohesion and the induce deterioration,relation between the internal friction Angle and the induce deterioration.Through analyzing the influence between water-induce deterioration & gas-induce deterioration and the soil arching effect,We get a conclusion that: As the moisture content of the backfill soil increased,the critical point distance of the soil arching is decreasing,and disappeared as if it increased continuously.The critical point distance of soil arching is decreasing as the number of times of the dry-wet cycles increases.when the number of times of the dry-wet cycles reaches more than twelve times,The critical point distance of soil arching tends to be stable gradually.With the increase of air pumping,the critical point distance of soil arching increases initially and decreases afterwards,and then gradually becomes stable.4)Based on the foundation pit supporting engineering of Jinyu Taoyuan Project in Changsha,finite element method was adopted to analyze the influence of backfill soil deterioration on the safety of foundation pit and treatment measures was put forward,which provided reference for other similar foundation pits excavated in backfill soil.