Transitioning from High-Cost Deep Foundation to Gentle Structures”: A Pilot Geotechnical Approach for Marshlands of Kibugabuga- Shinga – Busoro Road, the Road Infrastructure located in the East African Rift Valley/ Rwanda

In Rwanda, road infrastructure projects in marshland areas are frequently characterized by two extremes: prohibitively expensive deep-foundation designs or premature structural failure of bridges and culverts. This study investigates the Kibugabuga-Shinga-Gasoro project as a pilot model to resolve this dilemma. Through a multi-layered geotechnical analysis involving deep borehole logging (30m) and seismic characterization, we identify a critical soft-soil bottleneck between 4m and 7m depth (SPT N-values ≤ 2). Furthermore, we integrate the seismic risks associated with Rwanda’s location in the Western Branch of the East African Rift Valley, proposing a Peak Ground Acceleration (PGA) of 1.6 m2 for structural design. The paper analyzes the interplay between varied swampy materials,including peat, compressible clays, and sand deposits,and calculates a scouring depth of 1.68m as a primary factor in foundation instability. It is argued that the traditional reliance on deep piling to reach bedrock (often ≥ 30m deep in this project) creates an unsustainable financial burden for the project. Instead, the proposed implementation of Gentle Structures an engineering paradigm shift toward ground improvement, flexible culverts, and lightweight fills. Our findings suggest that this model can reduce foundation costs by up to 35% while significantly improving or maintain the life cycle of structures in saturated terrains, serving as a strategic blueprint for future national road rehabilitation programs .