| The water diversion pipeline project from the Hanjiang to the Weihe river is one of the large-scale water conservancy projects in China and one of the most important water conservancy hubs in the Yellow River basin.It aims to alleviate the shortage of water resources in the Yellow River basin and promote the development of local agricultural irrigation,urban water supply and industrial water use.The area involved in the project has frequent seismic activity and thick loess cover along the line,and the length and span of the water transmisson pipeline are large.Due to the difference of soil properties along the pipeline and the buried depth of the pipeline,under the action of seismic load,the loess foundation under the water transmisson pipeline is prone to uneven seismic subsidence,which poses a serious threat to the safe operation of the project.Therefore,the typical undisturbed loess of the north main line of the second phase of the the water diversion pipeline project from the Hanjiang to the Weihe river is selected as the research object.Based on field investigation,indoor conventional tests and dynamic triaxial tests of soil,the backbone curves,dynamic elastic modulus and damping ratio of undisturbed loess under different confining pressure conditions are studied.By comparing and analyzing the seismic subsidence characteristics of undisturbed loess along the water transmisson pipeline,the formula of seismic subsidence coefficient of undisturbed loess is established to predict the seismic subsidence of undisturbed loess foundation of water transmisson pipeline,and the reasonable seismic subsidence risk assessment is carried out according to the current norms and criteria related to seismic subsidence.This study can provide a theoretical basis for the seismic design and seismic subsidence evaluation of the undisturbed loess foundation of the water diversion pipeline project from the Hanjiang to the Weihe river.The specific contents and results are as follows:（1）Under the same confining pressure,with the increase of dynamic strain,the area surrounded by the hysteresis curve of undisturbed loess increases,and the slope of the long axis of the hysteresis curve decreases gradually.With the increase of dynamic stress level,the center offset of the hysteresis curve of undisturbed loess in Liangjia Village and Chenliang Village is more significant,showing a strong strain accumulation.The backbone curve of undisturbed loess is approximately hyperbolic,and its dynamic stress increases nonlinearly with the increase of dynamic strain.The influence of confining pressure on the backbone curve is more significant.The greater the confining pressure,the greater the dynamic stress corresponding to the dynamic strain of the undisturbed loess.（2）The dynamic elastic modulus of undisturbed loess along the water transmisson pipeline shows an inverse S-shaped attenuation trend with the development of dynamic strain.Under the same confining pressure,the initial dynamic elastic modulus of undisturbed loess in each site is quite different.In general,the initial dynamic elastic modulus of undisturbed loess in Sunjiabao Village is the largest,and that in Chenliang Village is the smallest.The damping ratio of undisturbed loess along the water transmisson pipeline generally shows an S-shaped growth trend with the development of dynamic strain.However,when the dynamic strain is larger,the damping ratio decreases with the increase of dynamic strain.Under the same dynamic strain level,the damping ratio of undisturbed loess along the water transmisson pipeline is relatively discrete with the change of confining pressure,and the damping ratio is approximately concentrated in the range of 0.10-0.32.（3）The seismic subsidence coefficient of undisturbed loess along the water transmisson pipeline increases with the increase of dynamic stress,and shows obvious nonlinear characteristics.With the increase of confining pressure,the seismic subsidence coefficient of undisturbed loess in the same site gradually decreases.In the early stage of dynamic load,the seismic subsidence coefficient of undisturbed loess in each site is relatively close.With the increase of dynamic stress,the seismic subsidence coefficient of undisturbed loess has a certain degree of difference.Among the loess sites along the pipeline,the undisturbed loess in Chenliang Village has the strongest seismic subsidence,and the undisturbed loess in Sunjiabao Village is the weakest.（4）Through regression analysis of the influence of confining pressure and dynamic stress on the seismic subsidence coefficient of undisturbed loess,the formula of seismic subsidence coefficient of undisturbed loess is established.It is verified that in the initial seismic subsidence deformation stage,the predicted value of the seismic subsidence coefficient will have a certain deviation from the measured value.However,as the dynamic stress gradually increases,the predicted curve of the seismic subsidence coefficient is in good agreement with the measured value.The absolute error is in the range of 1×10-4-5×10-3,and the relative error is mainly in the range of 1.15%-19.73%.The formula of seismic subsidence coefficient of undisturbed loess can better describe the seismic subsidence deformation law of undisturbed loess foundation of the water diversion pipeline project from the Hanjiang to the Weihe river.（5）Under low confining pressure,the difference of seismic subsidence critical dynamic stress of undisturbed loess in each site is small,but with the increase of confining pressure,the difference gradually increases.By analyzing the variation laws of seismic subsidence coefficient and seismic subsidence characteristic dynamic stress,the seismic subsidence sensitivity coefficient characterizing the seismic subsidence sensitivity of undisturbed loess is proposed.The seismic subsidence sensitivity coefficient can describe the growth rate of the slope of the undisturbed loess seismic subsidence curve in the range of critical dynamic stress and ultimate dynamic stress,reflecting the steep increase of seismic subsidence deformation.When the dynamic stress is the same,the seismic subsidence sensitivity coefficient of undisturbed loess gradually decreases with the increase of confining pressure.（6）According to the layered summation method and the formula of seismic subsidence coefficient,the amount of seismic subsidence of the loess sites,the amount and difference of seismic subsidence of the water transmission pipeline foundation are calculated respectively.Except that Sunjiabao Village belongs to Grade I seismic subsidence loess site,the seismic subsidence loess site grade of other sites is Grade II.From northeast to southwest,the amount of seismic subsidence of the undisturbed loess foundation along the water transmission pipeline is 3.96,9.68,7.75,6.84,11.55,and 7.73 cm,respectively,and the difference of seismic subsidence is 5.72,1.93,0.91,4.71,and 3.82 cm,respectively.Due to the fact that the water transmission pipeline is made of steel pipes,which can withstand large stress and is not easy to deformation.Therefore,the seismic subsidence difference between the undisturbed loess foundation of the water transmission pipeline has little effect on the water transmission pipeline. |
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