A Preliminary Study On The Law Of Earthquake Ground Response In The Seasonal Frozen Regions

In1986, the earthquake swarms of Dedu occurred in China. The Deduearthquake swarms are comparatively rare which occur in the seasons of bothwinter and summer. The phenomena of the seismic damage show that thestudy of frozen ground seismic response is very significant. The shocks ofseasonal frozen ground in China are widely distributed, and many earthquakesoccurred in the seasonal frozen ground areas. As a kind of special soil type,the permafrost (frozen soil) doesn’t just have the typical non-lineal soil andtemperature, but the change of the confining pressure will cause the uniquedynamic performance of the soil. While the current Seismic Design Code(GB050110-2010) set the classification of site and the anti-seismic designbased on the conventional mud, neglecting the impact to the permafrost.Nowadays, the research achievements of the impact of the site frozen soilin the seasonal frozen region to the seismic ground motion, accelerationresponse spectrum and engineering structure have yield few results. Most theconclusions are qualitative understanding, and the main conclusion holds thatthe existence of the permafrost will lead to the motion-damping seismicreduction effect on the seismic ground motion of the soil to some extent whichdoesn’t accord with the actual earthquake disaster phenomena. Meanwhile,the shear wave velocity test of the site permafrost is basic blank; thelaboratory test results are not sufficient; the research achievements of therelationship of the amount of ice, the permafrost temperature and the shearwave velocity of permafrost haven’t been published openly; there isn’treliable published achievement applied to the dynamic range of the shearmodulus ratio and damping ratio in the permafrost actual shear strain; theexamples are mostly based on the self-developed calculation procedure, sothe reliability of the method remains to need a further discussion.The thesis applying the soil seismic response linearization calculationprocedure (LSSRLI-1) which is commonly used in the earthquake engineering;basing on the national norms, the engineering sites of the seasonal frozenregions are classified; according to different conditions, the calculation andthe analysis to the seasonal frozen ground are carried out; studying thedifference of the vibration response to the surface land and the surfaceacceleration response spectrum; summarizing the law of influence from thedifferent parameters on the seismic response of the seasonal frozen ground;putting forwards to the preliminary proposal on the existing norms of thefrozen ground in the seismic design.The main tasks and research results of this thesis described as follow:1、Applying the different calculation methods to the various classificationto the ground (the site is classified into four main categories and eightsubcategories), calculate the vibration response to the surface land including eight kinds of site types, five different depth of frozen soil, two differentfrozen degree and two hundred kinds of working conditions. In the contrary ofthe calculating results of the different frozen depth, it reveals the differentlaws with the change of surface acceleration and response spectrum in variouscategories of site.2、Study the permafrost nonlinear effects by changing the dynamic shearmodulus ratio and the control point value on the maximum shear strain curve;calculate the six kinds of dynamic shear modulus ratio and two kinds ofdifferent frozen depth of eight site types in the96working conditions underthe control points change of±2%、±4%，±6%, and compare the surfaceacceleration response spectrum in site.3、The results show that the most significant effect of the site types in thepermafrost layer of the seismic response characteristics lies between the Ⅰ,Ⅱsite. We need to consider the impact of the permafrost layer when the shearwave velocity is larger (Vs>200m/s) as well as the site is thinner (h<10m),which is less significant in other workings to the entire seismic response ofthe permafrost.4、The impact of the thickness of permafrost layer and the hardness(shear wave velocity) of frozen soil on the surface acceleration responsespectrum is obvious, but less on the dynamic shear modulus ratio. The shearwave velocity will be the focused study research in future.5、The high-frequency amplification of the frozen ground in the differentearthquake intensity is still exists, but the earthquake peak acceleration isn’tnecessarily higher in winter than in summer.6、The frozen soil shear strain is small. The maximum of frozen soilshear strain in the worst site which is affected by the seasonal frozen soil isabove the10-4to10-6magnitude. The permafrost dynamic shear modulusand damping ratio can’t be measured by the strain within the dynamic tri-axialtest, and need more accurate instrument for testing.7、 Based on the current site classification measures of the roomtemperature soil state, it’s dangerous to research the seasonal frozen site undercertain condition, which needs to add the frozen working conditions toachieve the goal of considering the most unfavorable effect on the earthquake.