Research On Hydraulic Fracturing Failure Mechanism Of Excavation Pit Based On Extended Finite Element Method

With the advancement of geotechnical engineering technology and the acceleration of urbanization,underground engineering has gradually encountered geological conditions of large buried depth and high pore pressure,cases of foundation pit damage caused by confined water are common in domestic and foreign countries.Hydraulic fracturing causes the occurrence of water inrush channels in the foundation pit,the phenomenon of water surge on the surface of the foundation pit,and the collapse of the foundation pit support structure,which brings great security risks.Therefore,it is necessary to find the causes and expansion laws of hydraulic fracturing of the foundation pit,obtain the complete water inrush channel path and the location of the inrush point,and find the key factors affecting the hydraulic fracturing of the foundation pit.When the predecessors studied the failure of the foundation pit under the pressure of confined water by continuum mechanics,the expansion of the water inrush channel was generally not considered,and various discontinuities such as crack initiation,expansion,and formation of water inrush channels in the pit cannot be simulated.In order to overcome the shortcomings mentioned above,this paper introduces various fluid-solid coupling failure models based on fracture mechanics and compares the advantages and disadvantages of theoretical solutions and numerical simulation methods for various fracture mechanics models.Finally,the extended finite element method was chosen to study the hydraulic fracturing of the foundation pit.In this paper,an extended finite element model of hydraulic fracturing of foundation pit is established.Dimensional analysis shows that the ratio of the excavation width(B)to the thickness of the underlying layer(D)is the key to the failure mode of the foundation pit.The narrow excavation foundation pit(B/D<4)tends to be damaged by shear,and the critical water pressure at the time of failure is larger than the standard value.The wide excavation foundation pit(B/D>4)tends to be damaged by tension and the critical water pressure approximately equals to the weight of the overlying soil layer.The narrow excavation foundation pit tends to form a gushing channel in the soil behind the supporting wall,and the horizontal displacement caused by hydraulic fracturing around the gushing channel gradually decreases as the buried depth becomes shallower.The wide excavation foundation pit tends to form a vertical upward gushing water failure channel inside the foundation pit,and the horizontal displacement caused by hydraulic fracturing around the gushing water channel is evenly distributed along the buried depth.Then,the extended finite element simulation of the hydraulic fracturing case of an on-site foundation pit in Ontario,Canada,was carried out.The calculated water gushing positions were in good agreement with the on-site reported water gushing positions,and the complete expansion process of the water inrush channel was obtained which verifies the feasibility of using the extended finite element method to research the hydraulic fracturing of the foundation pit under the pressure of confined water.The porosity of soils neglected in traditional research is analyzed.It is found that the larger the permeability coefficient and the void ratio of the soil,the more incomplete the development of the water inrush channel,the harder it is to form the inrush point,and the stronger the ability of the foundation pit to resist the hydraulic fracturing.A standard extended finite element model for hydraulic fracturing in soils is established.Analysis shows that the width distribution of the gushing channel is a standard hump distribution,that is,the width of the gushing channel grows along the channel length and gradually increases to the maximum value,then the width of the water channel gradually decreases to 0 within about 7m from the tip of the water channel.The fluid leak-off in the gushing channel increases the pore pressure of the soil within 4?5m around the gushing channel and increases the void ratio of the corresponding region.The study also found that the greater the fluid leak-off coefficient and lateral pressure coefficient of the soil,the stronger the soil's ability to resist hydraulic fracturing.Aiming at the shortcomings of Abaqus in fluid-solid coupling extended finite element simulation,an innovative crack path length algorithm was proposed and implemented by Python and Numpy scientific computing library.An automatic modeling program for the hydraulic fracture model of the foundation pit and a data extraction program were developed.