The paper is the first to establish that the classical quasi-static analytical calculation of the retaining wall for seismic load indirectly reflects the wave nature of the impact of the sandy backfill soil on the rear surface of a retaining wall. The evidence provided includes the comparative results of the designed gravity retaining wall made from reinforced concrete and having a front cantilever that consider and disregard soil friction under the calculated operational static and seismic loads. The calculations were carried out for given dimensions of the retaining wall and characteristics of sandy backfill soils on weak clay foundation soils. It is emphasized that the calculation results are quite consistent with the research data on the behavior of sandy soils in the plate base under dynamic wave loads. These results were used to obtain the dependence necessary for determining the angle of soil friction against the rear surface of the wall under seismic load of varying intensities. It seems quite convincing that the calculation of the retaining stack for a seismic load of 9 points can be carried out without taking into account soil friction, since in this case, under seismic action, soil slippage along the rear surface of the retaining wall is possible. With a seismic load of 7 and 8 (or less) points, the angle of friction of the soil against the rear surface of the retaining wall should be determined from the obtained dependence to determine the angle of friction of the soil under a seismic load of varying intensity given in this article.