Research On Failure Mechanism And Aseismic Measures For Underground Engineering Under High Intensity Earthquake

China has the most complex geological structure in the west region, which is unique in the world. Since Quaternary Period, tectonic movement and earthquake activity are frequent, active faults develop. In such complicated geological conditions of tibetan plateau, the tunnel may have to be constructed beside active fault and in high seismic intensity area.Therefore, research on tunnel failure mechanism in high seismic intensity area and the proposed aseismic measures accordingly will greatly benefit the project location selecting, design and construction of tunnel engineering.Research work of this paper starts from study of tunnel seismic damage mechanism in high seismic intensity area. Firstly, dynamic mechanical properties of rock and a new vibration absorbing material-foamed concrete are studied in this paper,then, reasonable selection method for seismic wave and a new earthqukae input method in aseismic analysis for underground engineering are both considered, futhermore,on the basis of the above two work,characteristic of tunnel seismic response in high seismic intensity area and stability evaluation are carried out deeply, and the corresponding aseismic methods are summarizd and analyzed,meanwhile, remote earthquake monitoring of the tunnel is also carried out. The main achievements are conducted as follows:1. Study on method of earthqukae input in aseismic analysis for underground engineering based on infinite element dynamic artificial boundaryA crucial link in underground engineering aseismic analysis is appropriate earthquake input method, but now a considerable number of people apply the earthquake input method used in surface buildings, which is not appropriate for underground buildings. Therefore, a new earthquake input method based on infinite element dynamic artificial boundary is proposed by this paper, in this new method, radiation damping of the strata and reflection, scattering of the seismic wave in the strata are both considered, and equivalent earthquake load formulae in different boundary surfaces for seismic waves vertically incident from the bottom of computational domain are deduced, using the wave field separation method. Meanwhile, a numerical example is conducted to verify the validity and accuracy of the method.2. Development of a new vibration absorbing material-foamed concrete and research on its dynamic mechanical propertyA new vibration absorbing material-foamed concrete has been developed, which has light weight, low strength and good ductility, and the formula of which is selected through orthogonal test, then, dynamic triaxial test is carried out on foamed concrete,analysing the deformation and strength characteristics within a seismic strain rate range,the results show that compressive strength of this material increse with increasing of strain rate;the material has no dilatancy phenomenon because of its porosity;the axial deformation of foamed concrete is obvious under dynamic compressive loading, however, it still keeps integrity after failure,which shows that foamed concrete has good ductility and can dissipate considerable seismic energy.3. Research on seismic response characteristics of tunnelSeismic response characteristics of the tunnel is studied in this paper, the results show that the displacements of cavern increase with the increasing earthquake amplitude and duration time; the cavern displacement induced by low-frequency earthquake is larger than high-frequency earthquake, which means that the cavern failure is more easily induced by low-frequency earthquake; for shear wave, the displacement is the smallest when wave incidence is horizontal,and the biggest when wave incidence is vertical; cavern displacement will decrease with the increasing buried depth, however, this trend is just fit for general geological condition, actually, seismic damage can also appear when buried depth is deep under the cross influences of surrounding rock quality,high in-situ stress and fault.4. Research about safety influences of near-field seismic faults on tunnelEarthquake initiated by seismic fault of the engineering near field area is different from general far-field earthquake, which has long period and large velocity pulse. Due to the lack of seismic monitoring records, firstly, a near-fault pulse-type seismic record is generated synthetically using equivalent velocity pulse model combined with high-frequency earthquake compponents and real near-field seismic parameters, and then this artificial wave is used to simulate the quake induced by strike-slip fault movement near the tunnel portal using kinematic seismic source model, which can be applied to evaluated the safety of the tunnel. Besides, a preliminary study is also carried out about safety influences of earthquake-induced secondary fault movement on tunnel. Meanwhile, the safety influence of tunnel slope failure on the tunnel portal is evaluated as well.5. Research on tunnel aseismic methods in high seismic intensity area.Firstly, two general ideas about tunnel aseismic design are concluded,which are property change of rock surrounding of a certain range and property change of tunnel structure itself, and various anti-seismic and seismic-relieving measures of the tunnel now are all summarized according to the two ideas, then a series of studies are carried out on tunnel aseismic design methods, which inlude researches on sectional form optimization, shock absorption effects of seismic isolation layer, shock-reduction effects of surrounding rock strengthening and anti-breaking structural design.Among the research on seismic isolation layer, the shock absorption effect of foam concrete is verified through numerical experiment, using crushable-foam rate-dependent constitutive model. The result shows that foamed concrete has good shock absorption effects as seismic isolation layer of tunnel lining, meanwhile, an economic and areasonable thickness of the isolation layer is proposed according to the specific conditions of practical engineering. Besides, two ideas on earthquake reduction of isolation layer and traditional earthquake resistant of stiffness increment are compared specially. At last, summarizing the above work, some applicable suggestions are proposed on tunnel aseismic design.6. Seismic research and earthquake monitoring for Galongla TunnelUsing the above research results synthetically, research on Galongla Tunnel seismic response is carried out, evaluating the influences of both near-fault pulse-type earthquake and far-field earthquake on inlet region containing non-causative fault, then, anti-seismic and seismic-relieving measures suitable for Galongla Tunnel are proposed. Meanwhile, remote automatic earthquake monitoring of the tunnel is carried out. By burying accelerograph in inlet region of the tunnel, three-direction acceleration records can be obtained, which provide the first hand data about research on real tunnel seismic response.