Investigation of lateral earth pressure acting on rigid retaining wall with cohesive backfill under dynamic conditions

Within the scope of this research, four models of rigid retaining walls, specifically gravity walls with 8 m height and horizontal surface, were designed using clay soil as backfill with cohesion = 20 kN/m2, unit weight = 17 kN/m3 and friction angle = 25o. The foundation soil was sand with cohesion = 0 kN/m2, unit weight = 18 kN/m3, and friction angle = 30o, assuming the presence of an effective and suitable drainage system behind the wall to prevent additional lateral water pressure. The lateral earth pressures for these walls were calculated in both static and dynamic conditions. Based on these pressures, the dimensions of the walls were designed. For the static condition, Rankine's method for cohesive soil was used. Regarding the dynamic condition, Nakajima et al. (2023) method, a newly published approach, was employed. This method incorporates soil cohesion into the calculation of seismic active earth pressures, and it has been combined with the modified Mononobe-Okabe method, which can be applied in cases of high seismic loading. For the four models of retaining walls, the first two were designed under dynamic conditions using the horizontal seismic coefficient kh defined by the Turkish Building Earthquake Regulation (TBDY 2018). In the first model, the surcharge load on the backfill soil was included, whereas it was not included in the second model. The remaining two models were designed using the horizontal seismic coefficient kh from American Association of State Highway and Transportation Officials (AASHTO), including the surcharge load on the soil behind the wall in the third model and omitting it in the fourth. This was done to assess the impact of the surcharge load on the wall's dimensions and lateral displacements, as well as to examine the differences in design based on whether the seismic coefficients were taken from the Turkish or AASHTO standards. Additionally, stability analyses were performed for all four designs in both static and dynamic states, and these were compared with safety factors related to overturning, sliding, and bearing capacity as specified by the Turkish standard for the first two designs and the AASHTO standard for the last two, the equation used for these checks were obtained from Das and Sivakugan (2018). Furthermore, Plaxis 2D software version 2024 was utilized to calculate both static and dynamic displacements, which were then evaluated against the permissible limits according to the design standards followed. The results confirmed the unsuitability of this soil for use behind retaining walls, where the dimensions of the designs were relatively large given that the lateral water pressure xxiv was not considered based on the assumption of a suitable drainage system is available. Also, comparisons between the Turkish standards and AASHTO showed a difference concerning horizontal seismic coefficient kh, where kh resulting from the Turkish standard was 40% larger, affecting the dynamic lateral pressure, which was greater in the first two designs comparing with the last two. Consequently, the dimensions (base width) of the first two designs were approximately 20% larger than the last two designs. Therefore, it can be said that the Turkish standard is conservative in calculating kh. The research also highlighted the importance of including dynamic calculations in the design of these walls due to the significant impact of seismic forces on the retaining walls and on the lateral active earth pressures. Furthermore, the displacement results from the Plaxis 2D software indicated the necessity of improving the soil used if construction is required or additional load is to be added on it.