Geotechnical Assessment and Mitigation Strategies for Construction of the Nembe–Brass Road in the Niger Delta, Nigeria

Abstract

The proposed Nembe–Brass Road is a strategically important infrastructure project with significant implications for the socio-economic development, connectivity, and resource potential of the Niger Delta and Nigeria at large. Despite its inclusion in successive development plans, the road has remained unimplemented for over six decades, partly reflecting the formidable geotechnical and environmental challenges associated with its alignment. A critical gap in the development of the road has been the lack of adequate site-specific geotechnical characterization and an integrated assessment of how the highly compressible and weak subsoil conditions can be safely and economically managed. This study addresses this gap through a geotechnical assessment of the subsoil along the approximately 48-km proposed alignment, with particular emphasis on identifying the engineering constraints and evaluating feasible ground improvement and foundation solutions. The investigation reveals that the alignment is predominantly underlain by very soft, weak and highly compressible clay, with natural moisture contents ranging from 50–200%, undrained shear strengths of approximately 2–14 kPa, and bulk unit weights between 11 and 17.6 kN/m³. The soils are largely underconsolidated, while the observed liquidity indices indicate highly sensitive and deformable soil conditions, with plasticity characteristics suggesting varying depositional origins. Bearing pressures are critically low along more than 70% of the alignment, while estimated settlements may reach approximately 5 m under a stress increment of 100 kPa in some sections. These conditions constitute major risks to embankment stability, excessive and differential settlement, long-term serviceability, and construction sustainability. The study further establishes that conventional mitigation measures involving direct excavation and replacement or extensive placement of fill are impractical across large sections of the alignment because of the extreme softness, high water content, low density, and limited bearing capacity of the subsoil, as well as their potential to cause significant ecological and socio-economic disturbances within the sensitive Niger Delta environment. In response to these identified constraints, a zoned and hybrid ground treatment strategy is proposed. Piled foundations are recommended primarily between approximately 20 and 30 km chainage, particularly across sections characterized by thick clay sequences and river crossings, where piles can provide long-term stability while minimizing extensive disturbance to the underlying eco-hydrological system. Although relatively expensive, piling offers a robust solution where conventional ground treatment may be inadequate. In other sections, a combination of preloading and Prefabricated Vertical Drains (PVDs) is proposed to accelerate consolidation, increase soil strength, reduce post-construction settlement, shorten construction time, and minimize environmental impacts. The study therefore demonstrates that an integrated combination of targeted piling and ground improvement, selected according to the prevailing geotechnical conditions along each section of the alignment, provides a more technically viable, economically rational, and environmentally responsible strategy for overcoming the critical subsoil constraints to the development of the Nembe–Brass Road.