Research On Controlling Factors For The Excavations In Expansive Soil Of Chengdu

Expensive soil,with features of swelling and shrinkage,multi fissure and strength-decaying,seriously damages construction projects.Some accidents such as cracking and collapse often appear in the swelling soil foundation pit in Chengdu area.There is no clear conclusion for the cause of these accidents.Is it caused by swelling of soil,strength-decaying or fissures? Based on laboratory tests and numerical calculation,the main factors controlling the stability of Chengdu expansive soil foundation pit are studied in this paper.The paper mainly research and content are:(1)Physical and mechanical parameters of Chengdu expansive soil and its fillings are tested in the laboratory in order to study on the relationship between the strength and the water content.It is found that the soil strength parameters under saturated state are much lower than those of natural water content,and the natural water content of the white clay filler is higher than that of the yellow matrix soil,the strength of the white clay is lower than that of the yellow clay.(2)A finite difference software-FLAC3 D is used to simulate the response of foundation pit displacement,soil pressure and surface settlement under the condition of soil swelling and strength attenuation,using the temperature field to simulate the humidity field.The results show that the larger the displacement of the retaining structure is,the smaller the expansion force.When the strength of the soil decreases,the displacement of the structure will reach a certain extent,and the expansion force will not be effective.The expansion of soil is only a critical factor for the final instability when the displacement of foundation pit is small.(3)A discrete element software-FLAC3 D is used to model Chengdu expansive soil cantilever foundation pit with fractures,to study the relationship between displacement of foundation pit and fissure dip angle,spacing of cracks and strength of fracture surfaces.The results show that the existence of cracks seriously reduces the integrity of the soil and causes the stability of foundation pit to become worse.The fissure between 30° and 60° is the most dangerous to the stability of foundation pit,and the inclination near the horizontal or vertical fissure has no effect on the displacement of foundation pit.The decrease of the gap between cracks causes the soil to be more broken and the displacement of the foundation pit is increased and the stability is reduced.The sensitivity of the foundation pit model with cross combined fissure to the crack spacing is higher than that of the single fissures.The influence of the cohesive force on the stability of the foundation pit is huge.When its value falls to a certain boundary value,the fracture surface begins to destroy,the displacement of the foundation pit increases rapidly and then may be unstable.The internal friction of fissure surface is also closely related to the stability of foundation pit.The displacement of foundation pit slowly increases with the decrease of internal friction angle and then increases rapidly.