Geotechnical Investigations for Design of Foundations for Erosion and Flood Control Structures at Unwana Beach, Afikpo, Ebonyi State, South-Eastern Nigeria

Erosion occasioned by annual floods along the Cross River at Unwana Beach, Afikpo, situated on latitude 8° 00’ 00’’ North of the Equator and longitude 5° 40’ 15’’ East of the Greenwich Meridian, has threatened the location of a Water Pumping and Treatment Station nearby. Geotechnical investigations carried out using the Shell-and-Auger rig along the shoreline of the Unwana Beach indicated that the subsurface consists of between 3.00 to 8.00 meters of Yellowish brown silty clay layer (CL) underlain by about 3.00 meters of Dark, clayey sands (SC). These are further underlain by between 0.00 – 4.00 meters of Reddish gray, mottled silty clay (CL) and between 1.00 – 1.50 meters of Black stiff silty clays (MH). Underlying all these is a Black fissile Shale layer that extends beyond the limiting 20.00-meter depth of boring prescribed by the clients. Standard Penetration Tests (SPT) indicate that the upper-most Yellowish brown silty Clay layer has N-values of between 15 and 45, while the underlying Dark clayey sand layer has average N-values of 20. The Reddish brown, mottled silty clay layer (CL) beneath has average N-values of 22, while the Black fissile shale layer has N-values more than 50 ( that is, refusal ). The computed allowable bearing capacities (based on N-values and Terzaghi’s classical soil mechanics approach) for the subsurface materials at the project site indicate that the upper Yellowish brown silty clays (CL) have q(allowable) of 236.06 and 139.51 kPa respectively; Dark clayey sands (SC) have 188.84 and 195.67 kPa respectively and the Reddish gray mottled silty clays (CL) have 173.11 and 201.60 kPa respectively. Bathymetric surveys carried out perpendicular to the shoreline at five sections indicated that the maximum depth to the river bed at the proposed site for a Landing Jetty, at the date of investigations ( 11-22-2002), was 6.20 meters. Steel sheet piles were recommended and used as foundation systems for the shore protection works with the length of sheet piles equal to H + Df + h, where H = depth to the bottom of the river bed at low-low water, Df = depth of embedment of pile into the bearing medium and h = height of sheet pile above the river bank cliff (free-board) at the time of investigations ( 18th – 28th November, 2002). Wales of steel-type were used as reinforcement for the emplaced sheet piles, with their vertical separations approximately equal to 1.50 meters. Steel tie-backs were used to restrain the emplaced sheet piles from undergoing flexural and / or buckling failures, with tie-back vertical separations equal to 1.50 meters and tie-back horizontal separations approximately equal to 2.00 meters. Additionally, anti-corrosion protection for the tie-backs was asphaltic materials and concrete encasements