Study On Seismic Risk Assessment Of Embankment In Plain Areas

The paper, focusing on the phenomena of frequent occurrence of earthquake and severe damage to highways in China, develops the seismic risk assessment on highways in plain areas. The paper aims to have a comprehensive understanding of risk degree of highway subjected to seismic hazard and vulnerable highway sections susceptible to earthquake in order to reduce the loss of seismic induced hazard on highway and achieve the objective of harmonious and friendly relationship among the driver, vehicle, road and natural environment.Based on the summary and conclusion of research achievement of foreign and domestic seismic risk assessment for highways, an example of expressway from Xi’an to Baoji is presented. The paper evaluates seismic geology and its activity in the studied region and near-field region and develops the research on the seismic risk, embankment vulnerability to seismic damage and risk assessment in plain areas in term of risk assessment theories and steps. The conclusions have been listed as follows:1) Through field investigation and data collection of Xi-Bao expressway, the paper analyzes the general profile of Xi-Bao expressway and its terrain, topography, meteorology, hydrology conditions as well as seismic regionalization in areas along the highway alignment, which are divided into studying region and near-field region. The seismic active tectonic and the historical seismic profiles are analyzed and the seismic geological condition and seismicity are also evaluated studying region and near-field region.2) Based on the analysis of influencing factors on seismic hazards, the seismic peak ground acceleration(PGA) is selected as the risk assessment index. Probabilistic seismic risk assessment models including seismic zone division, mechanism model of seismic source and prediction model of seismic occurrence in the studying area are built, and seismic parameters in the mechanism model of seismic source are also determined, i.e. value a and b, upper limit of earthquake magnitude Mu, lower limit of earthquake magnitude M0, seismic annual average occurrence rate and seismic space distribution function and others. On the analysis of the attenuation relationship of PGA, seismic risk probability calculation method and the influence of earthquake ground motions, the paper conducts the seismic risk analysis and computation in Xi-Bao expressway and plots the seismic risk regionalization map. From the results: Xi-Bao expressway crosses the land with the basic seismic intensity Ⅶ and Ⅷ, and evaluation results is a little bit higher than the fourth generation of the seismic regionalization map, which is consistent with current seismic active state of Wei-He fault basin.3) Theoretical seismic hazard vulnerability assessment method is applied to evaluate the embankment vulnerability of highway section K1074+520. By the analysis of the cause, characteristic and influencing factors of seismic damage, the severity of seismic impact on embankment are classified into the five degree: non-damage, slight damage, medium damage, and severe damage and total destroy. The maximum horizontal displacement rate εmax and the maximum settlement rate ζmax on the top of embankment are taken as probabilistic seismic demand analysis. After the determination of the mechanics parameters of embankment filling soil, soil foundation and retaining wall, relying on the FLAC3 D software, the three-dimensional finite difference models on embankment without retaining structures and that with retaining structures are built. a series of samples(IDA,εmax)and(IDA,ζmax)are obtained by the input of selected fifteen earthquake records acting as incremental dynamic analysis IDA offered by PEER, and statistical relation between IDA and εmax, ζmax respectively are obtained using regression method. Probabilistic seismic analysis is conducted assuming the relation conforms to logarithmic normal distribution. it is formulated that the probability of different seismic damage degree inflicted on embankment with retaining wall and that without embankment on the condition of PGA, and seismic hazard vulnerability curves are also been plotted.4) The paper also analyses the definition of risk and the risk countermeasures. The risk of embankment seismic damage is defined as the probability of occurrence of different seismic damage grade, and mathematical expressions for the risk of embankment seismic damage are provided. In the research of site seismic intensity probability distribution and the transforming relationship between the seismic intensity and PGA, site GPA probability model is built. The risk of seismic damage on embankment without retaining structures and that with retaining structures are calculated using Monte Carlo method. According to the evaluation results of seismic risk assessment of Xi-Bao expressway, seismic risk curves in the studying region are developed by regression method and formulations of seismic damage risk on embankment are deduced using seismic risk curves. it is formulated that the probability of different seismic damage grade occurrence on embankment with retaining wall and that without embankment for a period of time in the future. The paper provides contents and application range of the risk of embankment seismic damage calculated by Monte Carlo method and risk curves. The risk of embankment seismic damage is categorized into three degree, i.e., high degree, medium degree and low degree and its acceptability is also be determined. The countermeasures for seismic damage risk on embankment are provided from the four aspects of planning control, contingency plan establishment, reserve of supply and equipment as well as engineering protection.