Discussion On Seismic Bearing Capacity Of Gravity Retaining Wall's Foundation

Gravity retaining wall is widely used in railway and highway engineering.Accoring to the result of investigation,in the western area of China where the high intensity earthquake occurred frequently,various kinds of damage was caused by earthquake to the gravity retaining wall,such as sliding,overturning,wall cracking,foundation instability and so on.The large deformation or instability of foundation caused by the loss of bearing capacity during the earthquake often preceded the damage of wall body,turning to be the weakness of earthquake resistant behaviour of the retaining wall.To study the changes of the bearing capacity of the retaining wall foundation under the effect of earthquake and provide reference for seismic design,the upper-bound theorem of limit analysis is used to solve the ultimate bearing capacity of retaining wall's foundation under seismic load.The change mechanism and influence factors of ultimate bearing capacity are analyzed,and the method of determining the the allowable bearing capacity of the retaining wall foundation under seismic condition is discussed.The main research contents and results of this thesis are as follows:(1)Based on pseudo-static approach and upper-bound theorem of limit analysis,the method of calculating the ultimate bearing capacity of retaining wall foundation under seismic load is proposed.Through a typical example,the variation of the ultimate bearing capacity with seismic acceleration and its mechanism as well as the influence of strength parameters of foundation soil on it are analyzed.The results show that the ultimate bearing capacity decreases approximately linearly as peak acceleration increases when seismic fortification intensity isn't more than 9 degree.The attenuation rate of ultimate bearing capacity is negatively correlated with cohesion of foundation soil,but not significantly affected by internal friction angle.Main cause of the decrease in ultimate bearing capacity is that earthquake aggravates the inclination and eccentricity of base load causing reduction of failure zone of foundation,while the seismic inertia force of foundation soil has no significant effect on it.(2)Considering that the pseudo-static approach doesn't take into account Frequency spectrum,phase and other vibration elements,a method of calculating the ultimate bearing capacity based on pseudo-dynamic approach is proposed.The difference between the calculation results by the two approach is analyzed.The results show that the pseudo-dynamic solution of ultimate bearing capacity is obviously higher than the pseudo-static solution,which is duo to the vibration phase difference of soil layer in different depth in the process of bedrock seismic wave propagating to the surface.The shorter the seismic wavelength is,the more significant the difference between the calculation results by the two approach is.The seismic acceleration at ground surface is the decisive factor of the ultimate bearing capacity,which always reaches the amplitude when the foundation is subjected to ultimate failure.Since the wavelength of recorded earthquake is much greater than the depth of the shallow foundation failure area,the difference between the calculation results by the two approach is not so significant,actually.The pseudo-static approach is better for solving the ultimate bearing capacity cause it is more convenient than the pseudo-dynamic approach.(3)The provisions on seismic bearing capacity of foundation from seismic design codes of various domestic industries is explained,and the principle to adjust the safety factor and the strength of foundation soil under seismic condition is summarized.Combined with the calculation results of the ultimate bearing capacity,a correction method with suggested correction factor related to both seismic fortification intensity and soil property were proposed for seismic allowable bearing capacity of gravity retaining wall's foundation.The results show that the allowable bearing capacity in seismic condition is less than that in natural condition when seismic fortification intensity is more than 7 degree,the correction factor for 8 and 9 degree are approximately 0.9 and 0.7 respectively.Finally,some opinions and suggestions on the shortcomings of provisions on seismic bearing capacity of foundation in seismic design codes is proposed.