Study On Soil Dynamic Characteristics, Nonlinear Seismic Response Of Complex Site And Its Methods

Recent twenty years, study on soil dynamic characteristics, nonlinear seismic response of complex site and its methods have become very active research areas, both in earthquake engineering and geotechnical earthquake engineering. This research needs deal with a great many keys and its method is not mature, although the results are much more abundant, it is obvious insufficient and inadequate for monotonous methods and impercipient physical mechanism etc. As a result, firstly, the author has expounded theoretical and practical significance of this reaearch, reviewed the current advancements at home and abroad in this field.Secondly, based on plenty of high quality strong motion observations and borehole array observations, the core problems: inversions of soil linear and nonlinear dynamic parameters, nonlinear constitutive relastions and soft soil's, numerical integration methods in time domain, effects of vertical strong motion on nonlinear seismic response of slope site, diploid nonlinear seismic response of complex site during the velocity pulse type strong motion of near fault, are analyzed. The following are main contents and innovative achievements.1. Soil dynamic 1D nonlinear constitutive relation is the key to numerical method and dynamic response of site and foundation. Firstly, the development and research advancement are summarized. Then, the important problem, which are analysis methods, the application rules of from equal amplitude cyclic load process to irregular load's, equivalent methods of test hysteretic damping, curve shapes of hysteretic constitutive etc. are analyzed, evaluated and discussed. The advantages and disadvantages of models are compared with each other. Finally, the shape of hysteretic constitutive curve can affect waveform of nonlinear strong motion in site response, which is pointed out especially.2. By analyzing nonlinear response of clay layer mantled base rock site, Los Angeles Obregon Park multi-mantled layer site, Taiwan Lotung DHB borehole array site, 5 types models, which are Pyke method, damping degradation coefficient model, Non-Masing rule model ONE and model TWO, implicit stress damping equivalent model are compared detailedly with each other. It is objective that some discrepancy between theoretical results and the strong motion observations. Each model is relatively rational and effective. Considering previous questions and the characteristics of reverse load or unload curves sometimes change more flatly at first and then acutely in soil test. First, the rules of adjustable double-parabola hysteretic constitutive curves in irregular load history are established, and function expression and implement course of A type and B type parabola are expounded. Second, by comparisons of 9 methods simulated results of soil test damping, the load and unload rules of double-parabola model accord with the test results, especially accord with the value of soil test damping. Because of the adjustment coefficient Ad can be changed, the model has more powerful capability of simulation the shape of constitutive relationship curves. Third, using the borehole array records of Taiwan Lotung DHB field test, the nonlinear seismic response of DHB site are analyzed by equivalent linear method SHAKE91, LSSRLI-1; nonlinear method in time domain - DESRA-2, Pyke model, Non-Masing rule model ONE, implicit stress damping equivalent model, damping degradation coefficient model, Non-Masing rule model TWO and double-parabola constitutive model respectively. After compared all the synthesize acceleration ground motions with the observation records, we found that the PGA of double-parabola model are almost equal to the observation value, the comparative size relations of strong motion waveform, subsequence waveform are agree well with the records. As a result, the rationality of double-parabola constitutive model and the feasibility in practical engineering application are manifested.Similarly, the idea of adjustment coefficient Ad can change the shape of hysteretic constitutive cure, which is applied in previous damping degradation coefficient model. Generalized damping degradation coefficient constitutive model is gained. The function expression and implement course are expounded too, and its rationality is also manifested by borehole array site. At last, all of constitutive models are organized into AntiPlane Nonlinear Wave Propagation program APNWP₁D.3. How to effectively and rationally describe the nonlinear response of soft soil site is one of the most important tasks in geotechnical engineering. Firstly, stiffness degradation of soft soil, dynamic stress decrease and residual strain potential in cyclic loads are discussed. Dynamic Residual Stress Potential(RSP) in polynomial type are brought forward after detailed analyzing the results of triaxial test, cyclic simple shear test and torsional test. The conceptions of relative RSP and local relative RSP for regular cyclic and irregular load respectively are presented. Simultaneously, RSP are verified by all test data. Secondly, the affinity of RSP and residual strain potential, the application rule of RSP in irregular loads are explained. At last, dynamic RSP is successfully used in seismic response of Xiamen Bailuzhou site. The results of permanent displacement, dynamic range of stress and strain, isolation vibration effect of silt layer and the difference of shear and vertical strong motion, which accord with the knowledge of residual stress characteristic and site nonlinear seismic response, and the model is proved rationality and practicability again.4. This paper emphatically analyze experiential relation model, which is abstracted from a lot of experiment data. After drastically studied two-dimensional plastic constitutive model, we put forward single hysteretic nonlinear constitutive model and double hysteretic nonlinear constitutive model of soil. The accomplished processes and steps are presented, and two numerical examples are designed for validating these methods. Through the contrast of not only the one-dimensional and two-dimensional nonlinear hysteretic constitutive curve, but also the nonlinear dynamic response, it can be seen that the two type modle are rational.5. In order to understand the soil linear and nonlinear characteristics of borehole array site not only in the small and moderate earthquake but also in the strong earthquake, verifying and improving the precision of field test and laboratory test. Firstly, the paper expounds the linear horizontal stratified site's P- SV, SH inclined incidence wave propagation method, whose Layer Transform Array is always real. The SSRA procedure is made, and its right is proved by an example of homogeneous elastic halfspace analysis. Secondly, Inversion methods, SSRA combining a global-local optimization algorithm, which is genetic algorithm (GA) - Simplex Method, are brought forward. At last, using 3 moderate and weak seismic events at Xiangtang borehole array (2#) site, soils dynamic characteristic parameters, including P velocity, damping value, frequency- dependent coefficient b, which are no dealing with in previous literatures, are calculated. S velocities of Xiangtang 2# site are on the whole greater than 1994's field test results, and are close to the 3# which is 200m far away. At the same time, perceptible soil nonlinear behavior in 60gal magnitude earthquake is detected by inversion analysis. In addition, the displacement and stress transfer function of Xiangtang 2# site are also analyzed.For the records of borehole array, we tentatively present an inversion method of hysteretic nonlinear in time domain for dynamic characteristic parameters of site soil. Then taking this method in practice by TKCH07 borehole array data in Japan Seismic monitoring Network Kik-net, the result is proved reliable, because it is almost in the test scope of sand and clay, which are accepted by international.6. Displacement and/or velocity often be inputted in wave motion explicit FEM at present, while acceleration record directly reflects original information of seismic wave. As a result, an explicit integration formula, which is based on both center difference method and Newmark -βmethod, and is reduced by equilibrium equation, is proposed. Water tower and framework structure dynamic analysis indicated that, the acceleration response of the formula is agree well with that of mode analysis associating with Duhamel integral method and Newmark -βimplicit method, when the efficiency of calculating is firstly ensured. In addition, we concisely summarize the real time processing method, which is unconventional method, based on transfer function of numeric filter in time domain, and the first step's initial value formula are recommended.7. It isn't enough in study of vertical strong motion influencing on nonlinear seismic response of slope site for its complicated strong motion input, soil nonlinearity, slope topographty condition and interface effects etc. The methods of taking half or 2/3 of shear response spectrum as vertical seismic action, recommended or prescribed compulsorily in many engineering codes at home and abroad, are used to design buildings. But how to rationally consider this effect is one of the most important tasks. Firstly, these research advancements are reviewed. The necessary of equivalent linear method of 2D wave motion explicit FEM ELPSV program with characteristic of high efficiency, real phase, actual radiation damping, is discussed. Then, strength of vertical strong motion influencing on response spectrum of slope site surface, equivalent response history and PGD and PGV in site section, seismic response of soft slope site are analyzed, and draw some primary conclusions as following: There are certain influences on shear seismic response spectrum when the period less than 0.3s, but it is very light over 0.3s. Vertical seismic response of slope site was distinctly changed by vertical strong motion. Moreover, value of vertical seismic action in code for seismic design for buildings is apt to unsafety. Simultaneously, vertical response of the right site is great than the left's. The slope top's shear response is much stronger than slope feet's for slope topography condition, while vertical seismic response is not obvious. Magnified effect in strong motion is more than in weak motion when ratio of horizontal to vertical component(av/ah) is 2/3, which agrees with the knowledge of nonlinear response. Interestingly, Interface effects make the response is lower than ambient site response. And soft site response is essentially consistent with phenomena of the hard site, except for vertical seismic response is increased previously and then decreased from site bottom to the top. At last, concise epilogue is given. All of conclusions and study results reflected quantitatively effect extent of vertical strong motion, provide beneficial basement for rational considering vertical strong motion in aseismic design of buildings and constructions in slope site.8. Using the strong earthquake observations at Ashigara Valley in Japan, the two much great single-double hysteretical nonlinear wave motion simulation program-PSVNWA₂D and SHNWA₂D are roundly checked, the author simulate not only SH nonlinear wave motion, but also P-SV nonlinear wave motion. The analysis results not only provide the theoretical strong motion of observation station, but also forecast the earthquake velocity, displacement and permanent displacement correspondingly, even, educe the single - double hysteretic nonlinear dynamic constitutive relation curves.Aiming at Suiling little slope site, we use PSVNWA₂D program to calculate the displacement and acceleration response of the surface and interface between the first layer and the second layer, when strong and weak type of Northridge and Turkey earthquake as input. The results show not only the changes of every earthquake motion and hysteretic curves, but also the nonlinear earthquake response difference between the strong motion and the weak motion in each earthquake, also difference between the two earthquake. Simultaneously, we analyze the spectrum change of transfer function, amplification coefficient and vertical earthquake motion derived from the shear response. In the research, it is sometimes found that amplification coefficient of nonlinear response at site observation station increase with the earthquake strong motion enhanced. The reason of phenomena is expatiated, and the case is proved by the strong earthquake observation and earthquake mechanism. At the same time, the paper pointed out essential difference of the transfer function of displacement, velocity and acceleration in equivalent linearization and time domain nonlinear technique. In addition, nonlinear seismic response during Loma Prieta earthquake is also analyzed.Under Kobe earthquake action, nonlinear seismic response of Quasi- Seattle- Basin site at in-plane and anti-plane cases is analyzed. The results indicated that the difference is remarkable between the nonlinear wave motion simulation of in-plane and that of anti-plane, in particular for the displacement response and permanent displacement. In the plane, the'basin focusing effect'of the nonlinear wave motion simulation is obvious, while the situation in anti-plane. is not enough. But'the basin basement edge effect'is distinct in two kinds of situations.These two effects can cause great difference of the seismic motion on the site section. The characteristics and the rules of the nonlinear seismic response are not only beneficial to the forecast and the estimate of seismic motion of important engineering, but also helpful to nonlinear aseismic design of the important construction structure.9. Inputting the near fault unidirectional and bidirectional velocity big pulse, complex site response is calculated by equivalent linearization method, the result is examined by itself. Rational condition and limitation of the traditional equivalent linearization method in this project are pointed out. In view of shortcomings, dynamic diploid nonlinear anlaysis explicit method in time domain for 1D site seismic response, which taking both physical nonlinear and geometrical nonlinear into account, is put forward. Then this method is taken to the APNWP₁D procedure, and establishing independent switch to control it. We decompose the input motion of near fault velocity big pulse into two kind of independent mechanisms - the elasitc big deformation mechanism and the diploid nonlinearity (elast-plastic big deformation) mechanism. At the same time, the parted and unitive double-mechanisms are proposed. For the two-dimensional complex ground, in view of the near fault seismic motion decomposition strategy, which are used to unidirectional and bidirectional velocity big pulse seismic motion analysis, the method of dynamical plastic theory combined big deformation mechanism is proposed. It has established bigger two-dimensional diploid nonlinear dynamical procedure software DNDYN₂D. Under TCU068 unidirectional pulse and Northridge NWPCR bidirectional pulse action, The diploid nonlinear dynamic response of complex site at Institute of Agricultural Sciences on the Hualin road was analysed. The result indicated that, when seismic motion was unidirectional velocity pulse, the parted diploid- mechanism was more reasonable than unitive diploid-mechanism. However the unitive diploid-mechanism is more suitable for the analysis of bidirectional velocity pulse action. The result also quantitatively present the permanent displacement, acceleration time history and peak value of site at Institute of Agricultural Sciences on Hualin road.This method may use for strong nonlinear seismic response of near fault stratified site, even on condition of seismic strong motion with several metres permanent displacement (PGA almost greater than 400gal) at bedrock as input. At the same time, more satisfactory answer in diploid nonlinear seismic response of 2D site for engineering also can be obtained.In the end, existing problems are pointed out and the research problems, which need to be done in future, are suggested.