Nonlinear Effects under Seismic Impacts
Identification of Nonlinear Soil Behavior in Strong Ground Motion
Tech Area / Field
- ENV-SEM/Seismic Monitoring/Environment
- OBS-NAT/Natural Resources and Earth Sciences/Other Basic Sciences
3 Approved without Funding
Institute of Physics of the Earth, Russia, Moscow
- Geo-Research Institute, Japan, Osaka\nKyoto University / Disaster Prevention Research Institute, Japan, Kyoto
Краткое описание проектаThe purpose of the project is study and quantitative description, in terms of higher-order transfer functions, of the nonlinear elastic behavior of the main classes of soils, for predicting the soil response in strong ground motion.
For a long time, nonlinear elastic response of subsurface soils in strong ground motion was one of the most urgent problems in seismology, inducing controversies and discussions. The behavior of various soils under a dynamic loading was studied in laboratory and field experiments, by penetration, dynamic sounding, pile testing, seismoacoustic and vibration methods, etc. However, soil behavior in situ in strong ground motion is not explored enough, can not be satisfactorily described by existing models and often is not properly understood. This is partially due to the fact that in strong ground motion soils do not only change parameters of propagating seismic waves, but also change their properties. Phase transitions can occur, when soils are liquefied or saturated with gas bubbles. As known, such multi-phase media possess strong elastic nonlinearity. These processes occurring in soils in strong ground motion are important for understanding the soil behavior, and the experimental data can be obtained from observations of the soil behavior in situ.
Strong motion records provided last years by seismic vertical arrays represent valuable experimental data, which allow estimation of the soil behavior at different depths, from the surface down to the location of the deepest device (Pavlenko and Irikura, 2001a). The method for processing vertical array records, proposed in this project, and the following nonlinear identification of the soil layers allows extracting maximum information on the soil behavior from the records. Accumulation of experimental data on strong ground motion in various soil conditions could give us the answers to virtually all the questions, which are interesting for geotechnical engineers (Archuleta et al., 2001): Where in the soil column does the response become linear? At what level of strain (stress) does the nonlinear response occur? How is the nonlinear response manifested in the observed ground motion? What properties of the soil that can be measured in situ are most important in affecting nonlinearity? If a soil column experiences nonlinear response, does it return to a state with the same linear (or nearly linear) response? At what time period? etc..
Realization of the proposed project will fill the gap in our knowledge of the urgent problem of the nonlinear behavior of soils in strong ground motion, nonlinear elastic properties of sediments, and nonlinear wave effects in seismic fields. The project will result in the analysis and generalization of experimental data on the behavior of the main classes of soils in strong ground motion in situ, in order to determine the typical features of nonlinear transformations of seismic signals in soils, to construct physical models of the soil response in strong ground motion, to explain the character of the nonlinear behavior of the main classes of soils, including soil liquefaction, to determine the types and quantitative characteristics of nonlinearity of the main classes of soils. The possibility of predicting the response of soil columns in strong ground motion will be estimated, and recommendations for seismic microzonation will be given for accounting for soil nonlinearity in strong motion. Knowledge of the types and quantitative characteristics of elastic nonlinearity of sedimentary subsurface soils allows a correct formulation and solution of problems of propagation of finite-amplitude seismic waves in the Earth's crust (such as, waves from earthquakes and artificial seismic sources, storm microseisms, etc.), a correct estimation of possible nonlinear wave effects: generation of higher harmonics and subharmonics of the main frequencies of propagating waves, changes in spectra of propagating signals, non-elastic absorption, degradation of elastic moduli, recurrence phenomena, seismic solitary waves, and others.
Investigation of nonlinear elastic properties of soils is a traditional topic in the Institute of Physics of the Earth of Russian Academy of Sciences. Scientific publications on this topic prepared by russian scientists appear since the late 1960's. In 1960-90's, multiple field experiments were carried out, aimed at studying the behavior of soils in situ, which gave origin to a number of Ph.D. and Doctoral dissertations. Last years, powerful computers appear, and nonlinear identification of the main classes of soils, based on calculations of multidimensional higher-order transfer functions of subsurface soils (computer programs are elaborated in the Institute of Physics of the Earth by the project leader Pavlenko O.V.) became possible. During the project realization, numerical simulation of the propagation of seismic signals of various shapes and intensities in nonlinear-elastic and elastic-plastic media will be widely applied, based on computer programs developed by Pavlenko O.V. The programs use calculation algorithms elaborated by Joyner and Chen (1975).
The collaborators of the project, Japanese scientific teams under the leaderships of Professor K. Irikura and Professor Y. Iwasaki, are engaged in the experimental and theoretical study of strong ground motion, such as, source mechanisms, path effects of seismic wave attenuation and scattering, and site effects of earthquakes. They have a unique experience in theoretical and experimental seismic research in Japan and experimental data on strong ground motion, which includes earthquake records of past years, as well as records of recent earthquakes provided by the borehole network Kik-Net. This can now be used within the framework of the proposed project.
Most of the Russian participants of the project (67% of 6 = 4 persons) have been involved previously in military technologies, such as, methods for detection and discrimination of underground nuclear explosions and rocket launches. These studies were considered to become an additional tool for the remote control of ban treaty and strategic missile launchings. Many of them are not well-known scientists, because they were considered to be classified in the Former Soviet Union. So the proposed project will provide the job (data processing, development of analytical and computer models, etc.) for highly qualified scientists and engineers, previously involved in military research.
We are going to perform the following:
- to estimate in situ stress-strain relations for the main classes of soils, such as, reclaimed fill, loess, sandy soils, clays, loams, gravels, etc., of various density and saturation with water, using all available strong motion records, provided by seismic vertical arrays, and to compare them with similar curves obtained in laboratory experiments;
- to summarize and generalize the obtained data and find regularities in the behavior of various classes of soils in strong ground motion, to establish the connection between the behavior of a soil in strong ground motion and its composition and saturation with water;
- by means of numerical simulation, to investigate degradation of the shear modulus and increase in the nonelastic absorption with increasing the intensity of propagating seismic signals in various classes of soils, to compare the results with available experimental data;
- by means of nonlinear system identification technique, to determine the types and quantitative characteristics of elastic nonlinearity of the main classes of soils, to classify soils by manifestations of their elastic nonlinearity;
- to distinguish linear and nonlinear (which are due to quadratic, cubic, and higher-order nonlinearities) components in the available records of strong ground motion;
- to distinguish the classes of soils with characteristic odd types of elastic nonlinearity, to investigate the connection between degradation of the shear modulus in such soils and the odd-order nonlinear components in their response;
- to distinguish the classes of soils with characteristic even types of elastic nonlinearity, to investigate the connection between the nonelastic absorption in such soils and the even-order nonlinear components in their response;
- to study the mechanisms and regularities of generation of subharmonics of the main frequencies of propagating signals, to determine factors influencing subharmonic generation;
- to study transformations of spectra of seismic signals, propagating in soils, and determine the medium properties, influencing these spectral transformations;
- to evaluate the possibility of estimating the response of the main classes of soils in strong ground motion by their response in weak motion;