Physical Modeling Of Seismic Response Of Highfill Slope By Hypergravity Shaking Table

With the implementation of the Western Development Strategy,hillfill slope construction has developed rapidly.Earthquake,as one of the main natural disasters,has brought tremendous effects and hazards to geotechnical engineering.Hillfill slope projects are usually constructed with higher height,complex geological profile(inclined "bedrock-weak sublayer-rockfill" slope)and complicated geological condition(high earthquake-intensity region).The dynamic response and hazard formation of highfill slope are the major concerns in seismic design and engineering practice.Few specifications and codes listed detailed guideline for how to design highfill slopes by considering dynamic response characteristics.It brings many challenges for highfill slope construction.Hypergravity shaking table modelling is an important method to reveal the seismic response and hazard formation of highfill slope.However,restricted by current facility capacity,it is difficult to reproduce the in-situ stress field for highfill slope.The development of physical modeling for highfill slope requires sophisticated use of the capability of facilities by utilizing hypergravity generalized scaling law in an improved way.It brings great theoretical and practical significance for investigating the dynamic response and hazard formation of highfill slope.In this paper,firstly,the shaking control precedure of hypergravity shaking table based on frequency response function is proposed,and well shaking control of hypergravity shaking table is the premise for physical modelling.Secondly,the hypergravity generalized scaling law is derived,which provides theoretical basis for full-section modelling dynamic response of high fill slope with strain scaling factor equal to unity,when the in-situ stress field could not be reproduced.The applicability of the hypergravity generalized scaling law is verified by modelling the dynamic response of flat deposit and slope site.Thirdly,the dynamic response and hazard formation of highfill slope at Ganzi Airport is revealed by utilizing the hypergravity generalized scaling law.The main research works and conclusions are as follows:1.According to the configuration of ZJU-400 hypergravity shaking table,the physical model and computation model of the bucket-shaker-container system are established with consideration of the servo-hydraulic subsystem nonlinearity.The effects of amplitude,model mass,stiffness and damping ratio on frequency response function are conducted based on the computation model,which include shaking amplitude,model payload,model stiffness and model damping ratio.Then a suite of centrifuge model tests were conducted to observe the frequency response function.It is found that the shaking amplitude has the dominant influence on FRF.NWhen the output ratio is more than 20%,the amplitude of frequency response function decreases with the shaking amplitude.The FRF cluster model of ZJU-400 hypergravity shaking table is established and a simple feed-forward control procedure is proposed to tune the command motion,which facilitates the possibility of well controlled shaking with only one excitation.Validation model tests show that the proposed procedure secures a satisfactory accuracy of the waveform replication for both sine waves and major component of earthquake motion.The proposed shaking control procedure is valuable for only needing one shake without model turbulence.2.The application scope and limitation of traditional scaling law(both stress and strain scaling factor equal to unity)and generalized scaling law(both stress and strain scaling factor unequal to unity)are summarized.Based on Hardin Equation and the modulus degradation characteristic of soil material,the hypergravity generalized scaling law is derived with stress scaling factor unequal to unity and strain scaling factor equal to unity.When the in-situ stress field could not be reproduced for the limit of facility capacity,the hypergravity generalized scaling law comprehensively considers the stress dependeny of soil materials and scale down the base input motion to model the prototype shear strain in model test.This study provides more physical meaning in hypergravity modelling for dynamic response of large-scale geotechnical engineerings.3.Based on the precise shaking control of hypergravity shaking table,the applicability of the hypergravity generalized scaling law is verified when modeling the dynamic response of the flat deposit by using the approach of "modeling of models.When applying the hypergravity generalized scaling law without considering the stress dependence of modulus attenuation,the derived prototype shear strain under lower effective stress is bigger than that under higher effective stress.When considering the stress dependence of modulus attenuation,the derived prototype shear strains under different effective stresses show a good consistency under different effective stresses.Under a higher effective stress,the modulus attenuation is closer to the prototype,then the stress dependence of modulus attenuation makes smaller effect.4.The dynamic response of high fill slope is modelled under different centrifuge accelerations by using the "modelling of models" technique.The prototype shear strains show good consistency under different effective stresses.The feasibility of the hypergravity generalized scaling law is verified to reveal the dynamic response of two-dimensional slope.Because of the discrepancy of stiffness and strength along the soil profile,the relatively soft silty clay layer between the overlying slope and the underlying bedrock enhances the de-amplification of peak acceleration and the amplification of displacement considerably.High fill slope with bedrock-weak sublayer-rockfill structure is characterized by multiple vibration modes,and multiple amplification peaks appear in acceleration response spectrum(ARS)amplification factor.Because of the filling height,the highfill slope has lower fundamental frequency.5.The physical modeling technique was used to reveal the seismic response of the highfill slope at Ganzi Airport under the engineering designed conditions(backfill at the foot of slope,low water level).At the surface,the displacement became the biggest,and asymmetric dilatation was observed at the surface under strong earthquake.The relationship between rockfill material density and cumulative strength was established to predict the density and settlement after a certain earthquake.Also as the relationship between the shear modulus of rockfill material at small strain Gmax and cumulative strength was established to predict the stiffness of rockfill materials after earthquake.Seismic response was further analyzed under a series of adverse conditions,such as rising water level,rainfall and removing backfill materials.With the water level rising,the filtering effect of the underlying bedrock was enhanced,and the acceleration and displacement amplification decreased.Rainfall increased the density and stiffness of the rockfill materials,then the acceleration amplification increased and the residual displacement decreased.When removing backfill materials,the highfill slope moved easily at the free boundary.The acceleration and transient horizontal displacement increased and transient vertical displacement decreased.While obvious residual deformation was not observed,and further monitoring should be reinforced.In this research,the shaking control technique of hypergravity shaking table is proposed based on the frequency response function through numerical simulation and experimental verification,which provides the basis for the hypergravity shaking table tests.Hypergravity generalized scaling law was developed through formula derivation and test verification,which offers a method to model the seismic response for highfill slope.This research has guiding significance for the seismic design of large-scale geotechnical seismic engineering.