Model Tests And Calculation Analysis On Vertical Dynamic Responses Of Piles In Inclined Strata

For the merits of large capacity,low settlement and good feasibility to cross complicated geological condition,pile foundations are widely used for the supporting foundations of bridges and embankments—with their mechanical performances greatly dependent on pile-soil interaction.When subjected to the traffic dynamic loads,In the inclined strata represented by the slope topography and the ground with inclined bedrock,the stress pattern of surrounding soil is not axisymmetric anymore;different piles under the same cap often have different frictional lengths and thus behave differently in the load-transfer mechanism and dynamic responses aspects;boundary inclination would change the wave propagation paths,and sometimes causes reflected waves.However,most of the existing research focus on the piles embedded in horizontal soil sites,and few literatures report dynamic characteristic of the piles in inclined strata.This thesis adopts combined methods of model tests,numerical simulations and theoretical analysis to comprehensively study the dynamic response characteristics,the load-transfer and deformation mobilization mechanism,wave propagation paths,pile-soil-pile interaction mechanism and the calculation method for the dynamic responses of piles embedded in inclined strata and subjected vertical dynamic loads.Highlight is put on the strata inclination effects on dynamic behaviors,and the corresponding reasons.Moreover,we also propose a simplified method for calculating the dynamic responses of pile foundations in inclined stratum conditions.The main results are as follows:(1)Groups of model tests were carried out to study the dynamic behaviors of single piles under cyclic dynamic loads,embedded in horizontal strata,slope ground and the soil sites with the inclined bedrock,respectively.We reveal the effects of the strata inclination on the dynamic response mechanism of single piles under the combined vertical loads of various mean loads,loading frequencies and dynamic amplitudes from the perspectives of vibration displacement at pile head,dynamic strain along pile shaft and the soil stress at pile tip.The test results show that,whereas the inclination of soil site exerts relatively limited influences on the pile shaft responses,it can obviously change the dynamic response of the soil field—leading to direction-dependent surrounding soil responses.To be specific,the inclined bedrock condition and slope topography will cause different response patterns with each other: under the former situation,the displacement of the soil on the upper side of the bedrock is greater than the displacement on the lower side where locates at the same depth,and the influence of the bedrock inclination becomes larger as the depth of the soil increases;The slope ground effects are mainly within a certain depth range from the ground surface and weakens with increasing depth.The directional difference in displacement response relies on the radial distance from the soil to the pile axis—that is,the soil displacement on the side of the slope toe is more significant when the radial distance is small while the soil displacement on the side of the slope crest will exceed the one on the slope toe if the radial distance exceeds a certain range.(2)The model tests on the dynamic characteristics of group piles in the inclined strata were carried out.The test results find that the different positions of the pile cap exhibit different vibration under the strata inclination condition.The cap partition at downslope direction vibrates more intensively and the displacement at downslope shows“amplification effects”,which shows an asymmetrical pattern.Then,we study the causes of that differential vibration phenomenon on the pile cap in the inclined stratum and reveal the load transfer mechanism through the finite element algorithm.The results show that the differential vibration of the cap is caused by the differential vibration of the piles under the cap.The group piles in the inclined stratum are mutually different in the length of the non-embedded segment and the frictional segment of piles,and in the soil layer thickness from pile tip to the bedrock,which results in significant differences in load transfer and displacement mobilization of the different pile shafts under the same cap.Furthermore,we analyzed the depth profiles of the dynamic displacement and axial force of the piles with different shaft lengths and different stiffness ratios between the pile and soil,from the perspectives of sloping ground and inclined bedrock sites,respectively.(3)Numerical simulation was carried out to study the wave propagation path and pile-soil-pile dynamic interaction mechanism on slope ground.The finite element method(FE)was used to calculate the peak displacements and arrival times at different depths of the soil site around the vibrating pile,and the two physical quantities were converted into amplitudes and phases commonly used in vibration problems.The comparison with existing rigorous solution has proved the accuracy of FE method.After that,we carried out the research on the wave propagation path in the slope deposit.The results show that the wave propagation behaves obvious directionality,and the wave attenuates faster in the upslope than in the downslope.Generally,horizontal paths dominate the downward waves,and the soil vibration attenuation in downslope is close to the situation in horizontal strata,while the propagation paths of partial upward waves are deflected and no longer in the horizontal direction.Furthermore,we employ the research on the dynamic interaction between the active and passive piles on the slope ground.The results show that the wave diffraction caused by the rigidity of passive pile cannot be ignored,and the vibration of the passive pile on the slope ground is still mainly controlled by the contact area between the pile and soil.(4)Dynamic response characteristics,the load transfer law and the interaction mechanism of the group piles under the inclined bedrock condition and subjected to the dynamic load of traffic is carried out through the numerical algorithm based on finite element method.The results show that,various socket lengths cause the phenomenon of differential vibration on the piles cap in sloping bedrock field,and the responses of the cap on the lower side of the inclined plane are greater than that on the upper side of the inclined plane.In addition,under the condition of inclined bedrock,the outspreading vibration waves influence the passive piles located to the upper side and the lower side,to different extent.The displacement of the passive pile on the upper side along inclination is close to that in the horizontal bedrock filed;by contrast,the displacement of the passive pile on the inclined upper side is slightly smaller than that of the horizontal bedrock,which could be contributed to the partial energy absorption by the bedrock with larger impedance when the vibration propagates towards the upper side.(5)The calculation method for the influence of the bedrock boundary on the dynamic response of a single pile and the calculation method of the dynamic response of capped piles in sloping ground are carried out,respectively.The results show that the vertical impedance of the single pile fluctuates around the impedance curve of the one in the horizontally infinite stratum.The amplitude and frequency of the fluctuation are closely related to the distance from bedrock boundary to the pile axis.The closer the pile foundation is to the bedrock,the smaller the frequency,but the greater the amplitude of the fluctuation.In addition,based on slope-pile-soil-pile dynamic interaction mechanism in(3),we establish a calculation model for the dynamic interaction of two piles in sloping ground.Then we deduced the pile-pile interaction factor and the vertical dynamic impedance of capped piles considering the influence of the ground inclination.The results show that the slope effects are manifested in three aspects: one is that the pile-pile interaction factors are different in the upslope and downslope directions,and the upslope direction is slightly greater than the downslope direction;the other is the interaction factors for different slope angles are also different.The steeper the slope is,the more intensive the slope effects become.Third,the slope angle has a significant effect on the peak value of the dynamic impedance curve.And the larger the distance between piles is,the more significant the slope effects are.When the distance between piles is 10 d,the real fraction of the dynamic impedance is reduced by about 30% compared with the one in horizontal soil sites,and the imaginary fraction is reduced by about 50%.