Assessment of Scouring Effect of Msingi Masonry Arch Bridge in Mkalama, Singida, Tanzania

This study presents an integrated geotechnical and hydraulic assessment of the Msingi Masonry Arch Bridge in Mkalama District, Singida, Tanzania, to evaluate scour vulnerability, subsurface strength, and structural load capacity. Field investigations included Dynamic Probing Light (DPL) testing, core sampling, and particle size analysis at six test pits (DS1–DS6), alongside laboratory tests adhering to BS 1377:1990 standards. Results revealed significant spatial variability in soil gradation and compaction, with deeper layers demonstrating high bearing capacities (up to 1555.8 kN/m²), while surface strata exhibited loose conditions and higher susceptibility to erosion—particularly in zones with elevated fines content. Hydraulic modelling, using site-specific parameters such as hydraulic radius (1.88 m), channel slope (0.0082), and Manning’s coefficient (0.017), predicted a scour depth of 2.6 m, compared to the observed 2.0 m. Structural analysis using the MEXE method yielded an allowable axle load of 28.05 tonnes, translating to a foundation pressure of 98.6 kN/m², which is within safe soil capacity limits. Despite current structural stability, the narrow scour margin and near-threshold loading conditions indicate elevated long-term vulnerability. The study recommends immediate installation of scour countermeasures, selective foundation deepening in weak zones, and routine monitoring to enhance the resilience and longevity of the bridge.