DESIGN OF COST-EFFECTIVE RETAINING WALL AT THE JUNCTION OF APPROACH BRIDGE OVER A RAMP-Paper

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The paper describes the design and construction of a cost-effective retaining wall for a new phosphate-ore scalping station in a remote West African mining complex. The project required a 12.7 m-high ramp adjoining an approach bridge. Conventional reinforced-concrete and precast-block gravity walls were considered expensive, while a reinforced-earth wall was initially preferred but could not be tendered because suitable contractors were unavailable locally.<br /><br />The team therefore developed a stepped-wall solution comprising several smaller reinforced-concrete retaining walls. Initial geotechnical assessments showed that the available borrow soil was inadequate: its allowable bearing capacity was 160 kN/m², below the required 200 kN/m², and the system’s calculated global stability factor was 1.14, below the target of 1.5.<br /><br />To make the design feasible, four backfill mixtures using locally available gravel, sand, laterite, stone dust and fines were laboratory-tested. Mix 2 was selected for economy and adequate bearing capacity. The design also specified a minimum 2 m foundation embedment and replacement of loose soil beneath the lowest wall with compacted sand or crushed stone.<br /><br />With the improved fill and foundation treatment, the stepped-wall system achieved a static global factor of safety of 1.57 and a seismic factor of 1.30. Construction was completed, and the plant was commissioned in December 2021.<br /><br />The case demonstrates that stepped retaining walls can provide a practical, economical alternative to taller conventional walls when reinforced-earth construction is not locally viable. It also highlights the value of testing and improving locally available materials to meet geotechnical design requirements.

Keywords

retaining wall

stepped-wall solution

geotechnical design

phosphate-ore scalping station

West African mining

reinforced concrete

backfill mixture

global stability factor

bearing capacity

local materials