Study On Excess Pore Pressure And Inrush Of Coastal Deep Excavation Subjected To Confined Water Fluctuation

Fully exploiting and utilizing urban underground space is an inevitable outcome of promoting urban modernization and sustainable urban development,and this situation directly promotes the development of foundation pit excavation in a larger and deeper direction,which is faced with increasingly complicated engineering environment and technical conditions,if the treatment is unreasonable,it will lead to the increasing risk during foundation pit construction.Among these risks,water inrush is an important issue that cannot be ignored in the design and construction of foundation pits,and this accident is particularly prominent for the impact of the foundation pit in the coastal area where confined water level shows dynamic characteristic.Based on this,this paper systematically studies the stability of confined water foundation pits in coastal areas from three aspects:experiment,theory and numerical analysis.The main research contents and innovations are as follows:1.A test equipment for pore pressure response of aquitard under fluctuation of confined water has been developed.The effects of fluctuation period,amplitude and initial water level of confined water on the distribution of excess pore pressure in heterogeneous aquitard were studied in this paper,then,the variation characteristics of pore pressure in aquitard under fluctuation of confined water,meanwhile,the influence of fluctuation factors of confined water are analyzed.The results shows that the amplitude of oscillatory excess pore pressure decayed rapidly along the seepage path,and the distribution of the residual pore water pressure along the depth deviated from the linear relationship.The fluctuation period and amplitude of confined water have little influence on residual excess pore pressure,while significantly affect the oscillating excess pore pressure attenuation speed and propagation range in soil.The capacity of confined water pressure in heterogeneous soils has significant difference from that in homogeneous soils.2.Considering seepage consolidation and based on the truth that clay seepage deviating from Darcy flow in low velocity,the Hansbo non-Darcy seepage theory is introduced into the one-dimensional saturated soil consolidation equation of Terzaghi,Finite difference method is employed to calculate the change of excess pore pressure in aquitard caused by the fluctuating confined water,and the reliability of this method is verified by comparing with the existing analytical solutions.Then,the effects of non-Darcy parameters,confined water fluctuating period and initial water level on the variation of excess pore water pressure in aquifer are analyzed.Finally,the application of the project example indicates that considering non-Darcy factor could reduce designed drawdown depth of the confined water level.3.Based on the fact that water inrush accidents are mostly caused by the existence of vertical seepage passages in the soil at the bottom of deep foundation pits,a new method for exploring the mechanism of water inrush in foundation pit is put forward.In the finite element analysis,the weak defects of the foundation pit are considered,and the critical hydraulic gradient of the seepage failure of the aquitard soil layer is defined.Then,the effects of the depth of the weak surface,the width-thickness ratio of the base and the position of the weak surface on the hydraulic gradient distribution at the bottom of the pit are studied,and the results were fitted to instruct engineering prevention Finally,two methods adopted to decide the design values of confined water head in excavation with fluctuation of confined water are compared,it shows that applying the average water level of dynamic confined water to evaluate the possibility of water inrush in excavation can’t take into account the most dangerous situation,while the maximum water level method can,but overestimates the maximum hydraulic gradient,resulting in waste in engineering construction.When considering the weak defect in the excavation base,in addition to the traditional plastic failure point of view,the problem of foundation pit inrush should also be analyzed from the perspective of seepage failure.