Dynamic Response And Interaction Mechanism Of Local Topography And Tunnel Under Stress Waves

With the development of the economy and society,many infrastructures will unavoidably be constructed in mountainous areas with complex terrain,due to the mountainous areas covering about 2/3 of the Chinese total landmass.The stress waves caused by blasting excavation and natural earthquakes are important dynamic loads that threaten the safety of infrastructure.Local topography has a significant influence on stress wave propagation,which may reduce or amplify ground motions.Utilizing the vibration reduction by local topography and preventing the adverse effects caused by local topography are of great value to ensure the safety of infrastructure.Except for above-ground structures,many tunnels are also under construction or proposed,and they are important channels for water,gas,and traffic transportation.Due to the limitation of terrain,the dynamic response of tunnels to blast loads and earthquakes is not only related to stress wave characteristics and tunnel characteristics but also affected by nearby local topography.In this paper,a series of research is conducted through field tests and theoretical analysis to explore the dynamic response of local topography and tunnels and the interaction mechanism between them during blasting or earthquakes.The research contents and achievements are as follows:(1)The cylindrical charge is commonly used in foundation pit excavation.The interactions between a concave terrain and the stress waves caused by the cylindrical charge are investigated by means of field tests and theoretical analysis based on a boundary integral equation method.The effects of the width and depth of the concave terrain and its distance to the blast source on its vibration reduction efficiency are examined.An evaluation model for the vibration reduction efficiency of concave terrain is developed through regression analysis.The research results demonstrate that the P wave is the dominant component in causing the ground vibration.Due to the existence of the concave terrain,the P wave is partially reflected in front of the concave terrain and radiates behind the concave terrain.Hence,the ground vibrations on the base of and behind the concave terrain are much smaller than those without the concave terrain.Parametric analyses show that the vibration reduction efficiency of concave terrain is little affected by its width,but is related to its distance to the blast source,its depth,and the depth of the blast hole.(2)The boundary integral equation method is employed to analyze the influence of a tunnel on the ground vibration induced by the cylindrical charge for both cases with and without a concave terrain.Meanwhile,the effect of a concave terrain on the dynamic response of a tunnel is examined.The effects of the pre-cracks with different stiffnesses and depths,which are considered a substitute for deep concave terrains,on the dynamic response of a tunnel are also evaluated.The research results demonstrate that for the case without a concave terrain,the tunnel will reduce the horizontal ground vibrations behind it,but will amplify the vertical ground vibrations in its environs.The maximum amplification coefficient of vertical vibration appears at a farther distance to the blast source with the tunnel depth increasing.When there exists a concave terrain,the effect of the tunnel on the ground vibrations behind the concave terrain depends on the position of the concave terrain.Additionally,the concave terrain will reduce the maximum dynamic hoop tensile stress at the tunnel wall.The maximum dynamic hoop tensile stress becomes smaller with the depth of the concave terrain increasing and the distance of the concave terrain to the blast source decreasing.The pre-crack with a smaller stiffness and a larger depth can effectively reduce highfrequency components of stress waves.The increase in the pre-crack depth results in the stress wave diffraction distance increasing,thus improving the reduction efficiency of the pre-crack for the maximum dynamic hoop tensile stress at the tunnel wall.The reduction effect of the pre-crack on the maximum dynamic hoop tensile stress at the tunnel wall becomes weaker with the increase in the distance of the tunnel to the blast source.(3)The large-scale valley topography has a significant amplification effect on the ground vibrations caused by earthquakes.The boundary integral equation method is developed to investigate the dynamic response of the valley topography under the combined action of selfweight and seismic waves.In this research,seismic waves refer specifically to stress waves induced by earthquakes.The influence mechanism of incident wave frequency,slope height,and slope inclination on ground motions is discussed.Numerous numerical calculations are performed to determine the ground motion amplification factor for the slopes composed of different lithologies.Meanwhile,the mechanism of tensile failure at the upper surface of the slope during the earthquake is explored.The stress development inside the slopes with different heights and inclinations under P or SV waves with different frequencies is discussed.The research results demonstrate that the incident wave frequency determines the location where the incident wave superposes with the reflected wave,thus,it controls the changing trend of ground motion with the elevation.The slope height does not affect the amplitude of the scattered Rayleigh wave but changes the time difference between the scattered Rayleigh wave and the incident wave,which results in the amplitude of the scattered body wave varying.The slope inclination affects not only the time difference between the incident wave and the scattered waves but also the amplitude of the scattered body and Rayleigh waves.The influences of slope height and slope inclination on the acceleration response spectrum ratio at the upper surface of a slope are closely related to lithology.An analysis of the stress development process inside a slope reveals that the diffracted Rayleigh wave travelling along the upper surface is the major source of tensile stress.The self-weight causes compressive stresses at the upper surface of the slope.SV waves with the same frequency and amplitude are more likely to cause tensile failure at the upper surface of a steep slope than P waves.High-frequency SV waves cause stronger tensile stresses at the upper surface than on the lower part of a steep slope,while low-frequency SV waves prioritize producing tensile failure on the lower part of a slope.A gentle slope is more likely to suffer tensile failure on its lower part.(4)Due to the limitation of terrain,tunnels are unavoidably constructed near the slope surface in the valley area.The boundary integral equation method is developed to investigate the interaction between the tunnel and the slope under the combined action of in-situ stress and seismic waves.Then,the effects of two anti-seismic measures,such as installing an isolation layer around the tunnel and strengthening the surrounding rock,on the dynamic response of the slope and the tunnel are analyzed.The research results demonstrate that the tunnel has a significant amplification effect on the ground motion above 10 Hz.The slope can amplify not only the static stresses induced in the lining by in-situ stress but also the dynamic stresses caused by seismic waves.Stronger dynamic stresses in the lining are produced by the seismic wave with a lower frequency.The stresses induced in the lining by P waves below 2.4 Hz are weaker than those induced by SV waves with the same amplitude and frequency.The influence of the slope on the stress state of the lining cannot be significantly weakened by increasing the distance between the tunnel and the slope surface.The isolation layer can improve the amplification effect of the tunnel on high-frequency ground motion.After the isolation layer is installed,the tensile and compressive stresses in the circular lining are reduced,but the tensile stresses at the arch foot of the horseshoe-shaped tunnel are intensified.If the surrounding rock around the tunnel is strengthened,the amplification effect of the tunnel on the ground motion will be weakened.Meanwhile,due to the strengthening surrounding rock,the tensile stresses induced in the lining by in-situ stress are intensified,while the dynamic stresses caused by seismic waves are reduced.(5)A comparison is made between the dynamic response of the tunnel-slope system to blast loads and earthquakes.Although the peak particle velocities caused by seismic waves in the tunnel lining are smaller than those induced by blasting waves,seismic waves can cause stronger tensile stresses.It is because there are obvious differences in the type,frequency spectrum,and duration of the dominant stress wave caused by earthquakes and blasting.Additionally,the slope has an amplification effect on the ground motions induced by the seismic waves,resulting in the ground motions increasing with the increase of elevation.However,the ground vibrations induced by blasting waves decrease sharply with the increase in elevation.The amplification effect of the slope on the ground vibrations induced by blasting waves can be ignored.