A comparison of the design approach recommended in the South African national standard for a reinforced soil wall with a finite element design to identify an opportunity to optimise the design.
Naidoo, Rannel Sashnee.
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Reinforced soil structures are used extensively for retaining wall applications in South Africa, with the design methodology recommended within the civil engineering fraternity as specified in the design code SANS 8006-1:2018, which was previously published as the SANS 207:2011. Given the improvement in the geosynthetic market, the design methods recommended within the design codes may be over-conservative. Although there have been revisions to the codes over the years, there has been limited improvements to the recommended design methods in the code. The design methods were developed at a time when there were limitations in computing capabilities and much of the designs were undertaken by simplified analytical ‘hand’ calculation. Numerical tools available today together with improved reinforcement quality provides an opportunity to optimize reinforced soil wall designs. Using an existing reinforced soil wall constructed at the N2 – Umgeni Road Interchange as a case study; the design code was used to design the reinforced soil wall. This design was verified using a finite element analysis using Rocscience’s Phase 2 software. The finite element analysis confirmed the adequacy of the reinforced soil wall developed using the design code. The displacements determined in the analysis concludes that the acceptability of the wall would be dictated by serviceability limit states rather than ultimate limit state. It is however concluded that the design code may indeed be over-conservative in light of the stresses, vertical displacement and factor of safety achieved in the FEA. This conservatism is attributed to the several partial and reduction factors applied in the determination of the reinforcement stresses and design strength. A parametric study was undertaken to assess the effects of altering the reinforcement configuration to identify opportunities to optimise the analytical design of the case study wall. The results confirm that various combinations of increasing the vertical spacing, or reducing the length, coverage ratio and tensile strength of the reinforcement could be acceptable, however is dependent on the deformation and strain tolerances of the structure, hence resulting in possible cost and construction time savings while still maintaining the integrity of the reinforced soil wall.