Research On Dynamic Response Characteristics And Seismic Fortification Of Representative Earth Fissure Sites In Fenwei Basin

As a typical slow-varying geological hazard with wide distribution,strong disaster-causing ability and high difficulty in prevention and control,earth fissures have caused serious damage to urban roads,bridges,underground pipelines and buildings,which is also a huge obstacle to the development of urbanization.Existing researches on earth fissures mainly focus on regularities of distribution,activities characteristics,formation mechanisms and disaster–causing mechanisms.However,research on the dynamic response of earth fissure sites has not reached consensus.Due to the lack of the support of ground motion data at earth fissure site,the current specifications‘Specification for site investigation and engineering design of Xi`an ground fissure’（DBJ61–6–2021）and‘Code for seismic design of buildings’（GB50011–2010）temporarily ignore the dynamic response of a site with earth fissure.The tectonic activities are strong and the seismic activities are frequent in the study area,the widely distributed earth fissures expose the structures in the site to a huge risk of earthquake damage.Therefore,the dynamic response of earth fissure sites and the seismic fortification standards of earth fissure site requires to be solved immediately.Relying on the National Natural Science Foundation of China‘Contrastive research on dynamic site effect of ground fissures in Fenwei Basin’,this paper takes the representative earth fissures in Fenwei basin as the research object to carry out systematic analysis.According to previous investigation,exploration and formation mechanism researches on earth fissure,18representative earth fissures in Fenwei basin were selected to carry out systematic microtremor testing.24 survey lines and 450 were measuring points were laid in Dationg basin,Taiyuan basin,Linfen basin,Yuncheng basin and Weihe basin.A total of nearly 100 hours of microtremor data were recorded.Firstly,the Fast Fourier Transform analysis,HVSR（Horizontal to Vertical spectral ratio）single–point spectral ratio analysis,Acceleration response spectrum and Arias intensity were employed to reveal the characteristics of dynamic response of earth fissure site.Secondly,taking the peak value of original site that far from the fissure,the amplification factors of site amplification effect were calculated.Taking the whole area and local site as the objects,through the fitting analysis for multi-region and multi-type of earth fissure site,the attenuation laws of amplification effect were summarized.The empirical formulas of amplification effect were established and the influence range were divided.Thirdly,through the numerical analysis of the ground motion response on representative earth fissure sites,the changing laws of ground peak acceleration were obtained.The influence factors of the amplification effect of the site were discussed.The empirical formulas of ground motion in earth fissure sites were fitted.The basic seismic fortification intensity and fortification range of the earth fissure sites were proposed.Finally,the earth fissure site models with multi-angle crossing tunnels,multi–distance adjacent tunnels and multi–distance adjacent stations were established.After numerical calculation,the basis characteristics of the response of underground structure including internal force,displacement,deformation,soil pressure and soil-structure slip were revealed.The variation law of amplification effect on structures were summarized.The basic seismic acceleration and fortification range of the underground structure in earth fissure site were also proposed.The conclusions of the paper are as follows:（1）The characteristics of the microtremor including frequency bandwidth,spectrum pattern and predominant frequency at various locations in the earth fissure site are basically the same,which are hardly affected by the development of earth fissure in the site.The peak of Fourier spectrum peak,H/V spectrum ratio peak,acceleration response spectrum peak and Arias intensity increase significantly near the fissure.The amplification effect attenuates regularly with the distance from the earth fissure.These peaks tend to stable after 25 m,which can be regard as the farthest influence range of the amplification effect.The hanging wall effect can be found at the site with normal-slip earth fissures.（2）Based on the amplification factor of the dynamic response of the earth fissure site and its attenuation process,the total influence range can be divided into three risk zones,the safe zone beyond the farthest influence range,the low-risk zone where the amplification factor gradually increases to 1.5,and the high-risk zone where the amplification factor is higher than2.The farthest influence ranges of the amplification effect of the microtremor response on the representative earth fissure sites in the five basins are basically the same,which are all around 21–24 m.（3）The dynamic response of the site with earth fissure derived from the fault at basin margin（siteⅠ）is the strongest and the influence range is farther.The high–risk range is about11.1 m on hanging wall and about 6.0 m on footwall.The farthest influence range is about 24.2m on hanging wall and about 23.0 m on footwall.The dynamic response the site with earth fissures controlled by buried faults in the basin is slightly weaker than that of the siteⅠ.The high–risk range is about 7.3 m on hanging wall and about 5.7 m on footwall.The safe range is about 23.3 m on hanging wall and about 21.0 m on footwall,which are basically consistent with siteⅠ.As to the siteⅢwith small–scale tensile earth fissures,the dynamic response are the weakest.The amplification effect on this site will not exceed 1.5 times and the influence ranges are about 23 m on both side of earth fissure.（4）A significant seismic amplification effect can be found on the internal forces,displacements,deformations,earth pressures and soil-structure slips of the tunnel through the earth fissure,the tunnel section near the earth fissure and the underground station structure near the earth fissure.The smaller the horizontal angle between the earth fissure and the traversing structure,and the closer the horizontal distance between the structure and the fissure,the stronger the amplification effect of the dynamic response.The structural response amplification effect only increases with the intensity of the input seismic,which is same as the site surface response characteristics.（5）According to the basic characteristics of the site amplification effect and its change law,the basic design seismic acceleration,intensity of protection and design range of three types of typical ground fracture sites were summarized:for the Class I earth fissure sites,out of 24 m on hanging wall and 23 m on footwall,the standard value of A₀ can be used for the basic design seismic acceleration.1.5A₀ is suggested between 11–24 m on hanging wall and 6–23 m on footwall.2A₀is suggested between 6–11 m on hanging wall and 3–6 m on footwall.3A₀is suggested within 6 m on hanging wall and 3 m on footwall.The fortified range is slightly smaller than that of Class I sites in Class II sites.The basic design seismic acceleration in Class III sites is suggested to be increased to 1.5A₀ within 23 m.The basic design seismic acceleration of underground structures and their design ranges were also given in combination with the basic characteristics of seismic response in representative earth fissure sites.The farthest fortified range of structures near the fissure is about 10 m wider than the surface.The tunnels crossing through the earth fissures are fortified according to their structure lengths.The fortified range on hanging wall is farther and increases as the intersection angle decreases.