Geotechnical Assessment, Excavation Method Selection, and Design Framework for an Integrated Underground Tunnel System for Traffic Decongestion and Flood Mitigation in Greater Accra, Ghana

Greater Accra, the seat of Ghana's government and commerce, faces two self-reinforcing infrastructure crises. Chronic road congestion and annual catastrophic flooding together drain an estimated USD 600–900 million from the national economy every year. Surface-level responses such as bus rapid transit, lane expansions, and open-channel dredging over the years have repeatedly failed to close the structural gap because neither crisis can be resolved without addressing the subsurface. This paper proposes and geotechnically assesses an integrated underground transport and drainage tunnel network for the Greater Accra Metropolitan Area (GAMA), comprising nine route segments totalling 178.72 km of single-bore alignment (357.44 km of twin-bore excavation) at an estimated total cost of USD 27.6 billion. The underground network is threaded through three geologically distinct terrains: the competent Cape Coast Granitoid in the north-western inland sector (UCS 60–120 MPa; Q = 7.2–9.5); the fractured and potentially swelling Birimian shale in the central corridor (UCS 15–40 MPa; Q = 0.8–1.5); and the soft, saturated Holocene coastal alluvium and Accraian shale in the south and east. Each terrain requires a different excavation approach, likely an Earth Pressure Balance TBM, a hard-rock TBM, the New Austrian Tunnelling Method, or a hybrid of these to match domain-specific support categories and integrated drainage provisions. A co-designed 3 m × 2 m drainage gallery captures groundwater and stormwater overflow below the invert of flood-critical tunnel sections, supplementing the Odaw Basin discharge network and reducing flood frequency at the highest-risk hotspots by an estimated 58–72%. The nine-segment route network connects and forms a complete closed loop that links the north-western ring and eastern arc. The study explicitly documents ten critical assumptions and ten acknowledged weaknesses and concludes by specifying a three-phase geotechnical investigation programme needed to carry the project from AACE Class 5 pre-feasibility to feasibility. This work directly addresses Sustainable Development Goals 6, 9, 11, 13, and 15.