Research On Several Crucial Problems Of Geotechnical Centrifuge Modeling Techniques

Geotechnical centrifuge modeling techniques can offer a hypergravity environment to make up the stress loss of scale model,achieving the effects of time and space compression.It is recognized as one of the most promising and potential physical simulation methods in the 21st century by the scientific community for its unique ability of ensuring the same stress-strain state between model and prototype.Geotechnical centrifuge models usually simulate the failure process and mechanism under static and dynamic loading such as landslide,dam cracking,liquefaction,foundation failure and retaining wall instability with the same materials,structures and stress-strain characteristics of prototype.Large numbers of researchers in China devoted themselves to the development of geotechnical centrifuges.In the past 20 years,more than 30geotechnical centrifuges had been built up,making China has the second largest centrifuge construction scale in the world.With the unremitting efforts of scholars,both the quantity and performance of centrifuge have amazing progress,providing a superior platform for the researches on major frontier scientific problems in geotechnical engineering and the development of interdisciplinary.Yet,with the deepening and the expansion of research scope,two sticks are gradually highlighted,summarised as response diviation between model and prototype,and large dispersion in parallel experiments.The main cause of this problem is the imbalance between development of equipment research technology and experimental simulation technology.The characteristics of force field difference and multiphase coupling lead to the complexity and uncertainty of geotechnical centrifuge simulation results.The output of high quality results depends not only on the advanced test equipment,but also on the test simulation techniques of correct,effective and complete.Thus,for improving the overall level of geotechnical centrifuge techniques and carrying out high quality results,to evolve basic simulation techniques,adaptive basic theories,design methods and new technologies are of paramount value.In this thesis,four main issues of centrifuge modeling techniques were studied.The first three could be described as stress simulation difference characteristics,sand pluviation technique and mechanism and viscosity coefficient blend standard.By means of theoretical derivation,physical testing,seismic damage investigation,numerical simulation,etc,some basic principles and practical standards of model and equipment design were established.Simultaneously,considering the facts of differences between centrifuge equipments and the lack of general evaluation test methods,the basic composition of CSIEM-40-300 large dynamic geotechnical centrifuge and the existing evaluation criteria were introduced.Based on which,the main performance evaluation method was established to provide a reference for the performance evaluation of similar equipment and the revision of relevant standards.The main achievements and innovative works are as follows:1.The differences between centrifugal and natural gravity field were analysed.Three new concepts named as general distributed stress,additional lateral stress and coupling dynamic stress were brought forward to reveal the influence of centrifugal acceleration distribution characteristics and rotation mechanism on model stress state,based on the correlation between force field characteristics and internal stress.According to which,mathematical models describing stress similitudes were established.Design criterias for effective radius of rotation,acceleration,angular velocity,model height and width are given,establishing important guidelines for equipment and model parameter design.2.A new concept and definition of stable modeling relative density is proposed.Comparative preparation tests of dry and saturated sand models were carried out by self-developed duckbill sand pluviator.Control factor influences and density dafferences between two models were analysed.Testing results and evaluation methods for model uniformity were given as well.To describe the pluviation process,mathematical models of single partical and partical flow are established according to fluid-solid coupling theory and momentum conservation law.According to which,the inner mechanism and key parameter of pluviation method are revealed,filling in the blank of the basic theory of mode preparation.3.The conflicts in actual application of domestic and foreign pore fluid viscosity blending standards were discussed.A V_s-k formula and viscous coefficient mixing standard are proposed through calibration test conducted by self-developed V_s-e-k testing device with in-situ shear wave velocity and soil samples from Wenchuan M_s8.0earthquake liquefaction site.A set of 1g and Ng scale model liquefaction numerical tests were conducted by FLAC 3D with typical liquefied and unliquefied field records of Wenchuan earthquake to verify the feasibility of numerical method.The influence and range of permeability on the increase of pore pressure ratio were dissected.A viscosity coefficient blending standard based on numerical tests is proposed.By comparing the two blending standards,the rationality of the standard and the low sensitivity of liquefaction to the permeability were proved.4.The basic indicators and functions of CSIEM-40-300 large geotechnical centrifuge are introduced.Its core composition,structure and key technologies were described as well.The main points and process of acceptance syllabus,safety control principles and performance test methods were established by way of collecting large-scale simulation test equipment standards and experiences of geotechnical centrifuge performance tests.According to acceptance syllabus,the CSIEM-40-300 large geotechnical centrifuge was checked and accepted.The applicability of proposed methods and standards were verified,providing an important reference for the formulation of relevant standards on geotechnical centrifuge performance tests and evaluation.