Dynamic Response Analysis Of Micro Steel Pipe Piles In Rock Foundation Pit Considering Blasting Damage Effect Of Jointed Rock Mass

Blasting is an efficient and fast rock excavation method widely used in urban foundation pit engineering.However,the disturbance and damage caused by blasting to the support structure of foundation pit has always been a hot topic in the field of blasting engineering during the excavation of rock foundation pit.The dynamic response of micro steel pipe piles to stress waves generated by blasting is particularly important,and improper support can have a negative impact on the stability of the foundation pit.In addition,natural rock formations usually contain joints,and different combinations and distributions of joint spaces can also affect the blasting damage effect of the rock mass.Therefore,it is particularly important to further consider the action mechanism of micro steel pipe piles in the blasting damage effect of jointed rock masses.In order to investigate the effect of joints in rock mass on the propagation of explosion stress waves and the localized damage evolution of jointed rock mass,a RHT constitutive model was established using finite element numerical method to explore the mechanical behavior of rock crack propagation and the stress field distribution around joints.This paper takes the foundation pit project of the second phase of Qilu Hospital in Qingdao as the background,and combines field data collection with numerical simulation analysis to analyze the mechanical mechanism of rock damage and deformation caused by blasting excavation in foundation pit,and study the dynamic effects of rock foundation pit during excavation,including the localized damage characteristics of jointed rock mass,the evolution law of foundation pit damage under pile-rock combination,and the dynamic response of micro steel pipe piles.The main conclusions and achievements are as follows:(1)Using the RHT rock damage constitutive model,the effect of different joint combinations and blast hole spacing on rock crack damage was analyzed.The spatial arrangement of joints and blast hole spacing will affect the path of rock damage,including the propagation of circular cracks,the generation of microcracks,and the extension of joint wing cracks.Taking the joint end as the measuring point,the general law of blasting on the dynamic response of joint ends under joint spatial combination was revealed: with the increase of explosion distance L,the displacement,velocity,acceleration,and maximum stress of the joint wing end show an inverse S-shaped attenuation,which can be well described by the logistic function.The total energy of the rock is independent of L,while the kinetic energy first increases and then decreases;as the angle of joint A increases,the displacement,velocity,and acceleration of the wing end show a step-like decrease,while with the increase of the distance between the blast hole and joint A,the displacement,velocity,and acceleration show a nonlinear increase.(2)Based on the conditions of single-hole blasting with dual joints,the typical destruction characteristics under the action of explosion load were analyzed,including the main crack and fragmented zone,annular crack zone and short radial cracks,as well as wing cracks,main cracks and sub-cracks.Under the action of explosives and joints,the direction and quantity of the main crack extension will change.The main crack will deviate in the direction of intrusion according to the inclination angle of the previous joint before the shock wave.Although most of the energy is absorbed or reflected by the previous joint,there is still some stress wave reaching the later joint,causing secondary cracks.In addition,due to the shear stress at the end of the joint,the wing cracks propagating along the blasting hole and the tip direction will start from the tip of the joint.These conclusions are based on numerical simulation results and have been verified,so they can be used as important references for analyzing the destruction process of granite under explosion load.(3)Simulating the excavation process of a deep foundation pit under blasting,the stress cloud maps generated by the blasting and the final damage results of the rock are displayed for the four excavation stages of the foundation pit.A comparative analysis is conducted on the dynamic response of micro steel pipe piles to the blasting effect with and without interbedded joints in the foundation pit at different stages.As the excavation depth of the foundation pit increases,the area of damage caused by explosives to the rock gradually becomes relatively concentrated and smaller.During the blasting operation,the top vibration velocity response of the first-level pile gradually becomes more prominent as the excavation depth increases,and the amplitude of the velocity change increases significantly.In contrast,the velocity changes in the middle(7.6m)and bottom(15.2m)of the pile become increasingly gentle,with a significantly reduced amplitude.Therefore,the excavation depth has a greater impact on the top vibration velocity response of the first-level pile.In order to ensure construction safety,corresponding measures should be taken to reduce the amplitude of the vibration.(4)The correlation between excavation-induced deformation and the main influencing factors such as piles,explosives,and joints,as well as the support mechanism of pile-anchor protection in blasting excavation were investigated.Three factors,namely pile stiffness,explosive dosage,and joint angle,were selected for the study.By changing the parameters,the comparisons and analyses of slope top displacement,maximum shear stress,maximum principal stress,bottom displacement,and dynamic damage characteristics were carried out.Stiffness changes have a significant effect on the settlement displacement of the foundation pit.Increasing the stiffness of the pile is mainly to reduce the deformation of the pile and further improve the stability of the foundation wall.Increasing the amount of explosives leads to more microcracks and annular cracks in the rock,resulting in secondary damage.Within the allowable range,an appropriate increase in the amount of explosives will have a good blasting effect.When the joint angle is 45 degrees,there is a peak value of 0.08 μm/μs,which is more than twice the peak value of other cases,and the disturbance at the top of the pile is more significant in this case.Under this case,the plastic strain of the rock on the left side of the joint increased significantly,and the rock strain was larger.