COMPARATIVE EVALUATION OF DIFFERENTIAL SETTLEMENT AND LOAD-TRANSFER MECHANISMS BETWEEN RAFT AND DEEP PILE FOUNDATIONS OF CONSTRUCTION PROJECTS IN SOFT SOILS: APPLICATION TO THE BIIC (R+5) BUILDING IN COTONOU

Urban development along the Beninese coastline involves medium-rise projects in areas with sandy formations of low surface compaction and a shallow water table, necessitating a complex foundation system. This study presents a comparative evaluation of geotechnical performance, execution constraints, and economic impacts between a general raft foundation and pile foundations for the BIIC building (R+5) of the Financial City of Cotonou. The methodology is based on a G3-type geotechnical characterization derived from pressuremeter and core drilling data up to 25 m, followed by dimensioning according to the NF P 94-261 and NF P 94-262 standards, respectively for shallow and deep foundations. The design profile reveals a shallow layer of loose sand (equivalent net limit pressure Pl* = 0.39 MPa) beneath a stabilized water table at a depth of 1.99 m, overlying a very dense horizon (Pl* = 3.47 MPa) at a depth of 3.50 m. The results indicate that the general foundation slab transmits a stress of 232.80 kPa and has an allowable ultimate limit state (ULS) bearing capacity of 1006.48 kPa, but induces a maximum settlement of 4.81 cm. Conversely, piles with a diameter of 0.80 m, anchored at 8.50 m, limit the settlement to 0.80 cm and provide a bearing capacity of 2546.23 kN. Although both options meet regulatory bearing capacity requirements, the decision analysis shows that the optimal choice depends on site constraints. The general foundation slab requires significant management of groundwater inflows (groundwater drawdown). Before soil improvement, the pile solution, although more expensive in terms of equipment, offers superior structural safety and eliminates the hydraulic risks associated with excavation.