Study On The Mechanism Of Behaviors Of Foundation Pit In Chengdu Clay

Chengdu clay is one of fissured clay distributed in Chengdu.The engineering practices showed that the accidents such as collapse,excessive displacement are more prone to occur in the foundation pits built in Chengdu clay than the foundation pits built in other types of soils.In order to find out why these happen and what are the key factors that affect the behaviors of foundation pit in Chengdu clay,following research work was carried out:(1)The accidents of 24 foundation pits in Chengdu clay were investigated and analyzed.It was found that most of the accidents occurred after continuous rainfall,so it can be inferred that infiltration of water into the soil is one of the key causes of the accidents of foundation pit in Chengdu clay.(2)Compressive triaxial tests of 61 specimens of Chengdu clay extracted from two foundation pits,which are yellow-brown,red-brown and gray-white colored clay respectively,were carried out to study the shear strength of Chengdu clay in natural state and saturated state.The results showed that the cohesion in natural state be as higher as 130 k Pa,while the cohesion will decrease sharply after saturation.For example,the cohesion of yellow-brown colored clay decreased by 81.81% after saturation.(3)Direct shear tests of 48 specimens were carried out to study the shear properties of the fissure surface.The results showed that the shear strength of fissure surface is much lower than that of the soil,and its shear strength will decrease when there exists water in the fissure.(4)Expansion force tests of 15 specimens were carried out on the three types of Chengdu clay.The results showed that the expansion force of gray-white colored clay is about 4 times that of the other two types of clay.(5)Based on the strength properties from the compressive triaxial test,the influence of softening characteristics on the deformation of foundation pit was studied by using Abaqus.The results showed that under the most unfavorable conditions,the horizontal displacement on the top of pile will increase by 44.26% and the earth pressure will increase by 21.78%.Moreover,the plastic zone will appear in the soil behind the pile,and the overall stability of the foundation pit will decrease.(6)Based on the strength properties from the direct shear test,the influence of fissure on the deformation of foundation pit was studied by using Abaqus.The results showed that when the angle of the fissure is 30°+ 120° and in the most unfavorable case,the horizontal displacement on the top of pile will increase by 89.88% and the earth pressure will increase by 42.26% compared with the case without fissure,and a large plastic zone will appear in the soil behind pile.(7)Based on the results from the expansion force test,the influence of expansibility on the deformation of foundation pit was studied by using Abaqus.The results showed that when the clay is the gray-white colored clay,the horizontal displacement on the top of pile will increase by 102.78% and the earth pressure will increase by 49.10%.