Dynamic Behavior Of Pile Foundations Subjected To Urban Subway Tunnel Blasting Excavation

With the rapid development of rail transit construction in major cities,numerous tunneling projects have emerged.Nevertheless,the vibration induced by blasting excavation in rock tunnels,with upper soil and lower rock strata environments,can present a potential threat to adjacent pile foundation structures.The Safety Regulations for Blasting(GB6722-2014)provide a safety criterion for the blasting vibration control of superstructures with a two-factor blasting vibration control standard of dominant frequency(DF)and peak particle velocity(PPV).However,this regulation fails to address the safety control standard for underground pile foundations.In practical pile foundation engineering,safety regulations have considered the vertical loads from the upper structures,but have regrettably overlooked the short-term,high-frequency,and high-amplitude blasting loads that are frequently generated by nearby tunnel blasting excavation.The impact of these blasting loads can potentially result in damage to the pile foundation,thus significantly impeding the normal functioning of the superstructure and putting both the safety of residents and the overall efficiency of the construction project in jeopardy.Therefore,it is crucial to initiate a study on the dynamic response of adjacent pile foundations exposed to tunnel blasting vibration,with the aim of comprehending the dynamic effect and failure mechanism of pile foundations at various standoff distances.In addition,it is imperative to establish safety control criteria for pile foundations to improve blasting safety evaluation methods and standards,and to provide guidance for secure and efficient excavation and blasting construction in urban areas.Such an effort holds tremendous theoretical and engineering value.The main objective of this thesis is to investigate the dynamic effect on adjacent pile foundations under the blasting vibration induced by tunnel drilling and blasting methods.Specifically,this thesis summarizes the characteristics of the tunnel drilling and blasting excavation project,geotechnical conditions,and the structure characteristics of the pile foundation.Comprehensive research methods of scale model tests,numerical simulation,and theoretical analysis are used to investigate the propagation law of blasting seismic waves in the geotechnical strata,the mechanisms and response characteristics of both single and group pile foundations under blasting vibration,as well as the pile foundation failure mechanism and vibration velocity of safety criteria under blasting vibration.The main research content and results are presented as follows.1.Considering the concealment of pile foundation engineering and the difficulty in monitoring the underground geotechnical body,a geological model for tunnel drilling and blasting excavation and pile foundation engineering is analyzed and generalized through literature review and practical investigation,and a dynamic analysis experiment system for pile foundation under blasting loads is established.Specifically,based on the similarity principle,the model design scheme and the physical and mechanical parameters of similar materials are determined.Combined with the generalized engineering geological model,a physical model including the upper soil,lower rock strata,cast-in-place pile foundations,and explosion is established.A model test on the dynamic effect of pile foundations subjected to tunnel blasting excavation is conducted.2.Based on wave propagation theory,an analytical model for the attenuation of stress waves in rock and soil strata is established.The analytical results of the model indicate that when the standoff distance R is less than five times the blasthole radius,the particle displacement and vibration velocity induced by the explosive stress wave attenuate in an R-2 manner.Conversely,when R is greater than five times the blasthole radius,particle displacement and vibration velocity attenuate in an R-1 manner.Additionally,a semi-theoretical and semi-empirical formula for predicting the attenuation law of blasting loads in rock and soil media under rock blasting is established by combining explosion mechanics theory.3.The propagation and attenuation law of blasting stress waves in a semi-infinite rock and soil medium is analyzed using the ANSYS/LS-DYNA finite element method.The influence of the rock-soil interface and free surface on blasting vibration propagation is emphasized.Results indicate that the impact of the rock-soil interface on blasting vibration is determined by the position of the explosion source or the standoff distance between the interface and the explosion.When the distance between the rock-soil interface and the explosion is small,the vibration velocity attenuation caused by the interface is significant and should be considered regarding its effect on peak vibration velocity.When the distance between the rock-soil interface and the explosion is large,the attenuation effect of the soil-rock interface is very limited.The impact of ground-free surface on blasting vibration propagation is determined by particle standoff distance and depth of particle.For standoff distance greater than 40 cm,the soil particle vibration velocity increases with decreasing depth due to the Rwave effect,exhibiting a strong amplification effect with a corresponding R-wave influence depth of 16 cm.When the standoff distance is small,the soil ground-free surface has a limited impact on particle vibration velocity,mainly affected by the reflection superposition effect of the blasting stress wave on the free surface,with an impact depth of about 8cm.4.Based on the model test of pile foundations under blasting vibration,ANSYS/LS-DYNA is utilized to establish a numerical model to calculate the dynamic effect of the single pile foundations subjected to blasting vibration.The results show that:(a)When the standoff distance between the explosion and pile foundation is bigger than the R wave emergence distance(defined as the far field of the explosion),the horizontal vibration velocity/acceleration of pile foundations increase with the decrease of pile depth,exhibiting an amplification effect.Within a certain distance,as the standoff distance increases,the R-wave formation becomes more complete,resulting in a more pronounced amplification effect,a more complex velocity/acceleration response curve at the pile top,and a longer vibration duration.Moreover,due to the high-frequency filtering effect of the rock and soil strata,the dominant frequency of the pile foundation is lowered,approaching the first-order natural frequency of the pile foundation-structure system,which may result in a resonance effect of the pile foundation-structure system.Under the influence of blasting vibration,a single pile foundation exhibits shear failure at the connection between the pile top and the pile cap.For a two-pile foundation structure with a connecting beam,tensile failure occurs at the connecting beam due to the unsynchronized vibration of the two pile caps.(b)When the standoff distance between the explosion and pile foundation is smaller than the R wave emergence distance(defined as the near field of the explosion),as the decrease of standoff distance.The maximum blasting loads occur at the pile bottom.Since the R wave in this distance has not been generated,the horizontal velocity/acceleration decreases with the decrease of pile depth.Because of the short standoff distance,the dominant frequency is significantly increased and decreases with the decrease of pile depth.There is no amplification effect for velocity/acceleration as well as the resonance effect for the pile foundation.Compared to the horizontal velocity/acceleration of the pile foundation in the par field of the explosion,the vibration duration is significantly reduced and the vibration waveform is simpler,the bottom of the pile foundation is the most dangerous point in the whole superstructure-pile-soil system.The pile-soil interface tension failure,punching failure,or buckling failure of the pile,or attentively,concrete spalling failure occurs on the pile-structure system.5.A theoretical model for the vibration response of a monopile foundation under the influence of blasting vibration is developed based on the principles of elastic foundation beam and Timoshenko beam theory.The model takes into account the effects of rotational inertia and shear deformation of the pile foundation on its vibration characteristics.By employing the scattering theory,the dynamic stress concentration coefficient of the single pile foundation is studied in relation to the spectral characteristics of the blast stress waves,the pile diameter,and the physical and mechanical properties of the pile and soil.Additionally,a safety criterion for controlling the blast vibration safety of the single pile foundation is proposed with the consideration of the criterion of relative separation at the pile-soil bond interface surface.The results of the calculation and analysis show that the blasting safety vibration velocity for the single pile foundation of the model is 1 cm/s.6.Based on the model test of pile foundations and the numerical simulation of pile foundations under blasting vibration,ANSYS/LS-DYNA is utilized to establish a numerical model to calculate the dynamic effect of group pile foundations subjected to blasting vibration.The results show that:(a)When the spacing between piles in a group pile foundation is small,compared to a single pile foundation,the multiple scattering effects of stress waves in the pile group cause an increase in the dynamic stress concentration factor of the pile group foundation.Therefore,to ensure the safety of the pile group foundation under blasting vibration,the multiple scattering effects of each pile in the pile group on the blast stress waves should be considered.(b)When the group pile is located in the far field of the explosion,the dominant frequency of the overall vibration of the pile foundation is small,and the vibration velocity of the pile foundation shows the characteristics of maximum at the bottom and small at the top.In these cases,there is no amplification effect in the part of the group pile foundation below the surface,and the dynamic response of the group pile foundation is controlled by the multiple scattering effects of the group pile foundation and the strong restraint of the pile cap.The vibration velocity at the pile bottom of the central pile in the group pile foundation is the largest due to the multiple scattering effects.The overall vibration velocity of the pile foundation shows a tendency to decrease with the decrease of the pile depth due to the strong constraint of the pile cap.The vibration velocity of the pile cap above the surface of the pile group foundation shows a strong amplification effect with the increase of height and the decrease of constraints.Therefore,the superstructure-pile-soil system in the far field of explosion should focus on the response of the superstructure and its resonance damage.(c)When the group pile is located in the near field of the explosion,the vibration response of the pile foundation near the blast side of the explosion(closest to the explosion source side)is the most intense,and the vibration velocity at the bottom of the pile foundation is the largest.As the pile foundation depth decreases,the vibration velocity of the pile foundation gradually decreases.Since the pile foundation is close to the explosion,the dominant frequency of the pile foundation vibration is relatively high,and the possibility of resonance of the superstructure-pile group foundation-soil system under the action of blasting seismic waves is small.Therefore,the focus of the research should be on the dynamic interaction of the pile foundation-soil system,tensile failure of the pile-soil interface,bending moment failure of the front row of the pile group foundation due to the high blast stress,or spalling damage of the concrete at the bottom of the pile foundation.7.By employing the multiple scattering theory,the dynamic stress concentration coefficient of the group pile foundation is studied in relation to the spectral characteristics of the blast stress waves and the pile diameter.Additionally,a safety criterion for controlling the blast vibration safety of the group pile foundation is proposed with the consideration of the criterion of relative separation at the pile-soil bond interface surface.The results of the calculation and analysis show that the blasting safety vibration velocity for the group pile foundation of the model is 0.95 cm/s.