Alignment and Design of a 73-Km Long Coastal Road in the South-Central Segment of the Niger Delta, Nigeria

A 73-km East-West Coastal Highway that traverses six major rivers within the Mangrove and Coastal Hydro-meteorological Zones of the Niger Delta is to be built. The total number of river crossings along the five intervening sections of this road is 36.The Niger Delta Sub-region lies at the southern-most portion of Nigeria. Geotechnical investigations along the road profiles showed between 10-18 meters of thick Organic Clays (OH) underlain by 2.50-4.50m thick Silty-clays (OL) along the first three Sections (A,B,&C) of the road. These have saturated densities (γsat) of 10-15.40 kN/m2; PI ~15.00-35.00%; cohesion (c) ≤24.50-68.50kPa, low strength (qult ≤ 12.00 kPa) and relatively high settlement values of δult ~ 0.056m-0.072m. Poorly-graded sands (SP) and well-graded sands with high bearing capacity values (482 – 4,250kPa) lie beneath these at depths of 20m and 30m, respectively. Most of the road alignments were submerged, with few points lying 0.30m above water level during the time of the investigations (December – March). Sections D &E of the road have relatively thinner soft layers (2.00 – 2.50m thick) underlain by sands (SP and SW) with relatively high bearing values of 582-4,250kPa. The large thicknesses of compressible layers underlying most portions of the road alignment require special pavement construction techniques such as: (i) Excavation of 2.50m of the soft layer materials; (ii) Emplacement of vertical pre-fabricated Geo-drains; (iii) Emplacement of woven geotextiles atop the pre-fabricated Geo-drains, (iv) Emplacement of about 4.50m high sand-dump on top of the woven geotextiles, (v) Allow for settlement of the underlying soft layer corresponding to t50, in this case ~1.14 years. Settlement computations obtained prior to- and after pre-loading phases were 0.0608m and 0.670m, respectively. Geosynthetic reinforcements were to be used in the pavement construction of the highway in order to attain a four-fold pavement structure consisting of: (a) Bound layers made up of (i) Overlay, (ii) Surface layer and (iii) Binder layer course; (b) Either bound or Unbound made up of (i) Base; (c) Unbound layers made up of (i) Sub-base, (ii) capping and (iii) Protection layer; (d) Sub-grades made up of (i) Stabilized sub-grades and (ii) Sub-grade proper. For most portions of the remaining Sections D and E, where the thin upper soft layers are less than 1.25m these are to be scraped off before emplacement of the Bound layer directly on top of Sub-grades. This paper describes the geotechnical characteristics of the sub-soils along the entire 73-km of the road alignment and the pavement design considerations adopted