Deficiencies of the vertical slice method in assessment of the seismic earth pressures

Conventionally, when the method of slice is adopted to assess the earth pressures behind the retaining wall considering a curvilinear failure surface, the mobilized soil mass usually is discretized into vertical slices. The point of application of the resultant earth thrust predicted using this technique (implemented in a recently published analytical approach, the log-spiral-Rankine (LSR) method) is reported to be insensitive to the magnitude of the horizontal seismic force. This result conflicts with those observed/suggested in several experimental tests and design guidelines, which state that the point of application will raise considerably toward the midpoint of wall as the horizontal seismic force increases. This study revealed the errors introduced by the vertical slice method and proposes to use the inclined slices (parallel to the effective gravitational field) as the remedy. The LSR method is thus re-formulated and also extended to the more general scenario in which a non-vertical wall supports an inclined frictional-cohesive backfill under seismic condition. The proposed inclined slice method is found to be more conservative than the conventional vertical slice method. The results obtained from the two methods of slices are compared and verified with those yielded by a few existing advanced analytical/numerical approaches.