Based on triaxial shear tests of structural cube loess samples with different water contents

residual strength effect and relationships between structural indexes and strength parameters are studied

based on which the uniform strength formula for plane strain is corrected. Variations of Mohr circle of soil stress behind the wall under the seismic conditions are analysed. By using the method of level layer element

calculating seismic passive earth pressure of loess filler considering intermediate principal stress

structure and residual strength effects is established. The results indicate that cohesions and friction angles calculated by unified strength theory increase with unified strength parameter b. The structure of loess has a huge influence on cohesion while the effect of friction angles is not obvious. The total seismic passive earth pressure coefficient increases with the increase of b value and structural index

while it decreases with the increase of horizontal seismic coefficient. The residual strength makes a more contribution to seismic passive earth pressure coefficient for large structure than for small structure.