The Zanzibar urban roads upgrade required the delivery of the island’s first double-carriage flyovers at Mwanakwerekwe (Flyover 1) and Adani/Amani (Flyover 2) to support growing transport demand and urban development. In both locations, the critical geotechnical challenge was not the bridge substructure itself, but the need to retain high approach embankments within a constrained urban footprint. The MSE wall heights varied from near zero at the start of the approaches to approximately 9 m at the abutments, while the bridge abutments themselves were piled, leaving the retained approach fills to be supported by the earth-retaining system.

A conventional retaining solution would have increased footprint demand, reduced constructability flexibility and imposed a less efficient solution for long, variable-height urban approaches. The project also had to address:

• high live traffic surcharge of 20 kN/m² on the reinforced soil block for both flyovers;

• seismic design demand using 0.1g PGA;

• barrier impact loading, with 60 kN horizontal impact considered in the design;

• groundwater encountered at shallow depth, approximately 2 m below ground at Flyover 1 and 1 m below ground at Flyover 2;

• differential settlement risk associated with variable founding conditions, including coarse deposits and stiff fine-grained strata.

The engineering problem was therefore to provide a retaining system that was:

• structurally stable under internal, external and overall failure modes,

• tolerant of long-term settlements,

• constructible in staged urban works,

• compatible with shallow approach embankments and bridge interface geometry,

• and durable for long-term highway service.