| China's urban rail transit construction now has entered a period of accelerated development.During the process of blasting excavation of metro tunnel,the area that urban metro tunnels pass beneath is full of high-rise buildings.The explosion source is close to the foundation and the harmful effect of blasting on the foundation and the superstructure is inevitable.The stability of the foundation is the prerequisite for the safety of buildings.Based on the background of urban metro tunnels blasting excavation projects and considering that the pile foundation is commonly adopted by high-rise buildings,research on the blasting vibration effect of pile foundation is of great practical significance for controlling the blasting vibration,ensuring the stability of buildings and ensuring the safe and efficient construction of urban underground projects.Based on the blasting excavation projects of large-section tunnel between Hongshan Station and Xiaohongshan Station of Wuhan Metro Line 8,the propagation law of blasting vibration in strata,the dynamic response characteristics of pile,the blasting vibration safety criteria of pile and the blasting safety explosives charge of pile are studied by the combined method of field test,theoretical analysis and numerical simulation.The main contents and achievements are as follows.(1)Study on the vibration propagation law of tunnel blasting excavation.The vibration propagation law in civil air defense tunnel which is above the large crosssection tunnel is analyzed through field monitoring.Based on the monitoring data,the propagation law of vibration in strata induced by the blasting excavation of the large-section tunnel is investigated using numerical simulation.In order to further clarify the propagation law of blasting vibration in strata,the stress wave theory is used to analyze the distribution of blasting vibration in strata.Finally,through dimensional analysis,the prediction model of blasting vibration velocity considering the influence of elevation difference is derived.Research results indicate that:(a)Along the axis of the large-section tunnel,the distribution law of blasting vibration in different depth from the surface is similar.Within a certain depth from the surface,due to the superposition and the reflection of the stress wave,the blasting vibration velocity of the surface particles is greater than the blasting vibration velocity of the particles at corresponding positions at other depths.Within a certain range behind the working face,due to the influence of the excavated area,the particle vibration velocity increases continuously with the increase of the distance from the working face(cavity effect).There are some complex effects of stress wave transmission and reflection and superposition on both sides of the interface of plain fill and clay.(b)Perpendicular to the axial direction of the large-section tunnel and within a certain depth from the surface,the vibration velocity of the surface particle is greater than the blasting vibration velocity of the particle at the corresponding position at other depths.In plain fill layer,due to the reflection and superposition of stress waves on the free surface and the interface of plain fill layer and clay layer,the vibration velocity of the corresponding monitoring point does not have a strict corresponding relationship with the increase of the depth from the surface,but in the clay layer,with the increase of the distance from the surface(the distance to blasting source decreases),the particle vibration velocity at the corresponding position at different depths tends to increase.(c)Along the depth direction,at different distances from the center of the working face,the vibration velocities distribution shows a similar pattern.In the range of 0-4.69 m,the reflection of the stress wave on the ground surface and the transmission and reflection at the interface of plain fill layer and clay layer make the vibration velocity fluctuate and the vibration velocity at the surface is the largest.When the distance from the surface is greater than 4.69 m,the vibration velocity attenuation law is consistent with the conventional attenuation law.In the plain fill layer,as the distance from the center of the working face increases,the fluctuation of particle vibration velocity decreases continuously.At different distances from the center of the working face,the influence depth of the surface and the interface of plain fill layer and clay layer on the particle vibration velocity variation law remained basically the same.(d)Based on the derivation results of the stress wave theory,it can be known that under the same soil layer thickness and different incident wave frequencies,when the soil layer thickness h is an odd times of ?/4(? is the incident wave wavelength),as the incident wave frequency increases,the surface vibration specific velocity(the ratio of particle vibration velocity to ground particle vibration velocity under specified conditions)decreases continuously and rapidly,and when h is an even times of ?/4,as the incident wave frequency increases,the surface vibration specific velocity decreases continuously but slowly;When the thickness h of the soil layer is different,the specific velocity at the ground surface fluctuates as the thickness of the soil layer increases,but the overall trend is attenuated.As the angle of the incident wave increases,the vibration velocity in the depth direction shows an attenuation trend,and the attenuation amplitude continues to increase.(2)Study on dynamic responses of pile under blasting loads of tunnel.Based on the blasting excavation of a large-section tunnel,the dynamic responses of piles of an adjacent building under blasting loads are studied by numerical simulation.The peak particle velocity(PPV)distribution on different sections and along the axis of pile is analyzed systematically.Results show that:(a)It can be found that the PPV along the axis of pile sections meets the law of y-direction PPV> x-direction PPV> z-direction PPV except at the section at the top of pile;(b)The PPV distribution curves at outer boundary of different sections of pile show that the x-direction PPV curves at the section of the pile located at the interface of plain fill layer and clay layer and the sections of the pile near the interface of plain fill layer and clay layer deviate to the explosion side,while the x-direction PPV curves at other sections are approximately circular.It indicates that the existence of interface makes the x-direction PPV distribution curve deviate;Except section at the top of pile,the maximum y-direction PPV is at point H2(0°)on the explosion side and that on the other sections is at point H1(180°);the z-direction PPV reaches the maximum at points V1(270°)and V2(90°)or the nearby locations.(c)Along the axis of the pile,the x-direction PPV distribution is similar.From the whole point of view,the maximum x-direction PPV is at point H2 in the four monitoring positions;The resultant PPV distribution presents a similar rule along the axis.Overall,in the range of 0-1.8m,the resultant PPV at H2 at the explosion side is the largest.In the range of 1.8m-10.0m,the resultant PPV at H1 at the back-explosion side is the largest.(3)Study on dynamic response of pile under different parameters.Based on the numerical simulation method,the dynamic response characteristic of pile with different pile head boundary conditions,in different strata,at different distance from explosion source,of different length and different diameter is analyzed.The results show that(a)under the four cases of the pile head with 800 KN on it,1250 KN on it,free condition and fixed condition,the distribution curves of X-direction PPV basically coincide in the range of 3.8m-10.0m,and the boundary condition only affects the x-direction vibration of pile in the range of 0-3.8m;In the range of 5.4m-10 m,the distribution curve of resultant PPV basically coincides in the four boundary conditions of pile top.(b)Compared with the condition of plain fill-clay composite strata,the near-ground fluctuation range of X direction PPV of H2 along the axis of pile under the clay stratum condition is deeper,while the fluctuation range of the resultant PPV is shallower.(c)Although the distance between pile and explosion source is different,the variation trend of resultant and x-direction PPVs is similar.With the increase of the distance from the explosion source,the influence depth of the ground surface,the interface of plain fill layer and clay layer on the distribution of resultant and x-direction PPVs is increasing.(d)The variation trend of resultant and x-direction PPVs along the axis of pile with different length is similar and there is a peak on the upper and lower part respectively.When the pile length increases to 12 m,the peak x-direction PPV in the lower part is greater than that in the upper part.The resultant PPV at the position H2 of pile section reaches the maximum value near the pile tip.(e)The x-direction PPV and resultant PPV of particles in the H2 position of different diameter piles are similar along the pile.The resultant and x-direction PPVs do not change monotonically with the pile diameter.(4)Study on safety criterion of pile under blasting loads of the tunnel.The wave function expansion method and the numerical simulation method are used to study the safety criterion of piles based on the maximum tensile strength theory.Results show that the critical vibration velocity of the pile in the plain fill layer is far greater than that in the clay layer based on the wave function expansion method,which indicates that the pile is more likely to damage in the clay layer.The safe vibration velocity of pile is theoretically determined as 20.44cm/s.Numerically,the statistical relationship between the peak tensile stress and the horizontal PPV is established,and the safe vibration velocity of pile is 7.96 cm/s based on the maximum tensile stress criterion.When the wave function expansion method is used to obtain the blasting vibration safety criterion of pile,the blasting stress wave is simplified into a simple plane harmonic wave,and the medium is considered to be elastic.The theoretical calculation result is larger than the result obtained by numerical simulation.Combining the two calculation results,the blasting vibration safety velocity of the pile is set to be 7.96 cm/s,and the result is more conservatively safe.(5)Study on safety explosive charge of pile under blasting loads of the tunnel.The expression of the horizontal PPV on the pile related with the blasting charge quantity is established by solving the Euler-Bernoulli equation.Combined with the blasting excavation of large-section tunnel,the relationship between the blasting charge quantity and the maximum horizontal vibration velocity of the pile with a diameter of 60 cm,a pile length of 10 m,and a distance of 10 m from the blast source is established based on the derived expression.According to the proposed blasting vibration safety criterion of pile,the maximum charge of 98.2kg in one delay time is calculated to ensure the safety of pile under the blasting excavation of large-section tunnel.The dynamic response of the pile under different explosive charges is calculated based on the numerical simulation method.Therefore,the relationship between the explosive charge quantity and the maximum horizontal PPV of the pile is established and the maximum charge in one delay time should be less than 142.4kg combined with the blasting vibration safety criterion of pile.When analyzing the safety explosive charge of pile based on the Euler Bernoulli equation,the material damping is not considered in the calculation,and the calculated pile blasting safety explosive charge is smaller compared with numerical result.Combining the two calculation results,the pile blasting safety explosive charge is set as 98.2kg,and the result is more conservative. |
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