| Many cities is located in the riverside river, the sea coast, subway construction pass through soft soil or sand layer inevitably in the future. For example, liquefaction strata were encountered in the past in China Nanjing, Shenzhen and other cities subway, and the shield tunnelof the Taipei Rapid Transit system. Sand produce liquefaction phenomenon inevitably under the action of the earthquake, Past earthquake damage show that underground structures in liquefaction field damaged easily under earthquake. Especially in the strong earthquake, Soil liquefaction may have great damaging effects on the underground structures. Structures in liquefaction field faced with floating by foundation liquefaction, uneven subsidencing after the earthquake, and tunnel lining damaging due to ground liquefaction additional forces. Hanshin Earthquake show that clearly subway and other underground structures could lead to serious earthquake damage in the position that may occur large deformation and displacement. Underground structures such as subway stations are an important part of lifeline engineering, its seismic problems has become a important part of study for city earthquake engineering and disaster prevention and mitigation. The seismic analysis of underground structures liquefaction field is a problem to be solved. The maininvestigation work focuses on the following.(1) The research on typical liquefaction characteristics of free fieldThe numerical simulation experiments was completed for the liquefaction in free field with FLAC3D. Experimental results show the typical characteristics of soil liquefaction:excess static pore-water pressure increased and effective stress decreased. Confirmed the isolation effect of liquefaction:the sand soil can not transfer shear, the displacement, velocity and acceleration amplitude of sand in liquefaction reduced significantly, shear stress decreased. The excitation frequency impacted the liquefaction characteristics, when the input frequency closed to the natural frequency of the site, the degree of liquefaction is high, and displacement and deformation is large. The degree of liquefaction at the bottom of field is higher with low-frequency input. The dynamic response frequency characteristics of the liquefaction field was 'Low frequency amplifing, high frequency filtering". The degree of liquefaction is lower with smaller acceleration input, the degree of liquefaction is higher with larger acceleration input. Sand relative density have a greater impact for the free field liquefaction, the degree of liquefaction is higher for low density soil, the degree of liquefaction is lower for high density soil.(2) The numerical experiments for structural stiffness impacet on liquefaction fieldSet a partition wall in the free field, Constructed a non-free field。The series numerical simulation test was completed for the liquefaction in non free field with different stiffness structures using FLAC3D. Studied the PL characteristic of the non-free field with different stiffness structures according to the reaction from structure to earth soil, and revealed the liquefaction distribution features in non-free PL field. Experiment showed that the non-free PL field distribution approaches non-uniform in the level, the pore-water pressure distribution shifted, the contour of pore pressure presents the fluctuation shape, nearby the structure forms the low pressure region, forms the high pressure region several meters far to the structure, contrast with the free PL field, the ppr in low pressure region reduces obviously, approximately40%, the ppr in high pressure region is higher than the free field. The structure suppressed the vicinity soil to liquefy, the structure strengthened the liquefaction in distant place soil. The structure rigidity is bigger the inhibition range to be bigger, the invigoration effect to the distant place liquefaction to be also bigger. The structure rigidity is smaller the inhibition range to be smaller, the invigoration effect to the distant place liquefaction to be also smaller. The double partition wall numerical experimentation confirmed the structure anti-liquefaction mechanism again in this document. Researching results of effects on free field liquefaction distribution characteristics by structural stiffness will provide experimental basis for the dynamic response analysis of non-free liquefaction field with underground structures.(3) Numerical experiment and analysis of anti-liquefaction measure with gravel drainage layer around underground structureThe gravel drainage layer was set around underground structure, The numerical liquefaction experiment was completed separately in free field, the non free field and the gravel drainage layer non-free field using FLAC3D. Researched the liquefaction distributed characteristic of the gravel drainage layer non free field. The anti-liquefaction effect of the gravel drainage layer was investigated, and analysed dynamic characteristic change of underground structures after imposition a gravel drainage layer in liquefaction field. Calculations indicate that, the gravel drainage layer around underground structure wasn't liquefied, the excess static pore pressure obviously reduced in distant liquefaction field, the structure didn't float, horizontal drift reduced. The overall quality and rigidity of the structure with gravel drainage layer increased, the stability of the structure increased.(4) The development for unified twin shear damage concrete constitutive modelTo meet the needs for simulation analysis dynamic response characteristics and earthquake damage mechanism of the metro underground structures using FLAC3D, based on Yu MaoHong unified twin shear theory and Mazars damage theory, the numerical scheme of elastoplastic unified consfimfive model in FLAC3D Was studied. Studied. And the numerical format of the elastoplastic constitutive model based on the unified strength theory was derived. In the FLAC3D environment developed twin shear damage constitutive model TwinShear_Damage_Model considering concrete material strength, stiffness parameters corresponding reduction by damage. The corresponding UDM interface codes were programmed.(5) The seismic response numerical experiments and analysis of Dakai subway station in liquefaction fieldDakai subway station was constructed using a opening excavation method, Site soil is sandy and gravel soil, Hanshin earthquake characteristics is gravel liquefaction, Dakai subway station site liquefaction can not be excluded from the Kobe earthquake, this paper keep the station on the liquefaction field for seismic dynamic response analysis, completed seismic response numerical experiments of Dakai subway station in liquefaction field, Explore the impact of liquefaction field to Dakai subway station structure internal force, take Dakai subway station structure as study object, Using FALC3D software completed numerical simulation experiments, Finn and elastic constitutive model were used to site soil, elastic constitutive model and mohr constitutive model were used to Dakai subway station structures, the dynamic response numerical experiments of Dakai subway station were completed in liquefaction field and elastic field, experiments revealed the internal forces characteristics of subway station in the liquefaction field, Compared the dynamic response differences of the subway station structure in liquefaction field and elastic field. At the same time, compared the dynamic response differences of the subway station structure by level and vertical together ground motion and only level ground motion, Studied the impact of subway station internal force with vertical ground motion, Compared the difference of structure internal force between two kind constitutive model (elastic constitutive model, Mohr constitutive model). Completed the structure dynamic response experiments on different contact stiffness between soil and structure, experiments showed that the contact stiffness of the contact surface have a great influence on the acceleration response of contact surface.
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