Research On The Continuous Collapse Mechanism Of The Excavation Of The Internal Support And Row Pile Support System

With the improvement of urbanization rate in China,the importance and value of underground space development have also steadily increased.The safe development of underground space often cannot be separated from the application of deep excavation support systems.Among many deep excavation accidents,continuous collapse is undoubtedly one of the most serious engineering accidents.Once it occurs,it will cause various effects on the deep excavation and the construction of subsequent buildings.In this paper,PLAXIS 3D was used to numerically simulate a deep excavation engineering example of an internal support and row pile support system in Hainan,and the component removal method was used to study the continuous collapse performance of the support system in the deep excavation.Design suggestions for continuous collapse were proposed.The main research work is as follows:1.Using Hardening Soil with small-strain stiffiness constitutive law,the finite element model of a excavation in Hainan with an internal support and pile support system was founded.The numerical calculation results were compared with actual construction monitoring data and a benchmark finite element model was established.2.The deformation and internal force distribution of horizontal internal support beams,crown beams and waist beams of the excavation supporting system with internal support and row piles were analyzed and studied.It is concluded that the position of the column pile acting on the horizontal internal support is the maximum of the positive bending moment of the horizontal internal support beam.The maximum bending moment of the supporting pile perpendicular to the excavation wall decreases with the position of the supporting pile extending from the middle of the excavation to both sides,and the maximum bending moment of the supporting pile near the inner support is significantly larger.At the bottom of the excavation,the bending moment of the supporting pile is usually a maximum value.The bending moment of the top of the supporting pile near the inner support is larger than that far from the inner support.3.Through the method of component removal,the influence on the adjacent components was studied by the failure of columns,horizontal supports and supporting piles at different positions and numbers.It is demonstrated that the failure of column piles will cause the peak value of bending moment of the connected horizontal internal support beams and adjacent support piles to increase significantly.Moreover,if multiple internal support beams are connected near the support pile at the same time,the failure of single internal support beam has little effect on the mechanical characteristics of the support pile,and the failure of two internal support beams at the same time will cause the peak value of the bending moment of the support pile to rise obviously.The failure of single pile will cause the peak value of bending moment of nearby piles to increase.In addition,the support pile near the excavation angle of the excavation is generally less affected by the failure of the adjacent pile.The design suggestions are put forward to improve the safety of the excavation supporting system and prevent its continuous collapse.4.The component failure factor of the internal support system（K）,namely the ratio of the maximum bending moment(M_max)of the horizontal internal support or supporting pile to the limit value of vertical bending capacity（M_u）,was proposed to judge whether the horizontal internal support or supporting pile was damaged.Failure occurs if K>1 and no failure occurs if K≤1.The investigation demonstrates that the increase of the failure factor K of the adjacent members can be observed due to the failure of the column pile and horizontal internal support component.The failure of the column will also increase the risk of damage to the supporting piles near the internal supports that are not directly connected.