Study On The Influence Of Bottom Reinforcement On Deformation And Stability Of Deep Foundation Pit Supported By Diaphragm Wall

Due to poor soil conditions and large excavation,excavating deep and wide pits often accompanied by large deformation,which seriously affects the stability of foundation pits.The engineering practice shows that the bottom reinforcement can effectively inhibit the deformation and improve the stability of foundation pit.The influence of bottom reinforcement on deformation and stability of foundation pit were studied systematically in this paper with the support of Dalian sea-crossing traffic engineering and the aid of PLAXIS 3D,which provides a reference for the design of similar foundation pit engineering.(1)The reinforcement effect of the strip reinforcement is related to the area replacing ratio,and has little relation with the strip width.The area replacement rate has a limited value.When the area replacement rate is over 66.7%,the reinforcement effect has no obvious improvement.The stability coefficient of foundation pit increases with the area replacement rate as "S" type.When the reinforcement replacement ratio is 33.3%-66.7%,the stability coefficient of foundation pit increases faster.The rate of replacement is better and more reasonable in this area.(2)In skirt reinforcement,due to the different modulus between reinforcement area and unreinforced area,there is bottleneck effect at the bottom of foundation pits,namely the soft soil under the reinforcement area is squeezed out of the unreinforced area,which aggravates the deformation of the bottom of foundation pits.Skirt reinforcement is not recommended in similar projects.(3)In grid reinforcement,when the area of pit axis was reinforced,the deformation of the bottom of foundation pits showed "M" type,and when the area of pit axis was not reinforced,it showed "W" type.The heave reinforcement effect is only 18.56% of the project whose longitudinal displacement rate is 40%,however,the effect of another project is 25.49%,whose displacement rate is just 30%.So,the distribution of the reinforcement area is important in the grid reinforcement.(4)There is a significant limit of reinforcement depth for strip reinforcement,grid reinforcement and full hall reinforcement.When the depth of reinforcement is more than 0.8H,increasing the depth of reinforcement can not effectively restrain the deformation of foundation pit,but it can improve the stability of foundation pit.(5)The strip reinforcement,grid reinforcement,step reinforcement and full hall reinforcement have obvious improvement on the deformation and stability of the foundation pit.Full hall reinforcement can better restrain the deformation of the foundation pit and improve the stability of the foundation pit with same amount of reinforcement.(6)The deformation value of foundation pit decreased linearly with the increase of the internal friction angle of reinforced soil,and the foundation stability coefficient showed parabolic growth.The cohesion of reinforced soil has no obvious effect on deformation of foundation pit in a certain range,while foundation pit stability coefficient grows linearly with increase of cohesion.The deformation value of foundation pit is logarithmic function with compression modulus.With the increase of compression modulus of reinforced soil,the deformation decreases,and the stability of foundation pit showed parabolic growth.(7)The main factors affecting the deformation of the foundation pit are the internal friction angle and the compression modulus,and the influence of the internal friction angle is about 4 times as much as that of the compression modulus.The main factors affecting the stability of the foundation pit are internal friction angle and cohesive force,and the influence of the cohesive force is 1.4 times as much as that of the internal friction angle.