This project involved the construction of the new Essau Farafenni highway. At Kerewan the road was to cross the River Gambia. The geology at this location comprised extensive soft marine clayey silts, and to construct both the bridge and the embankments on bearing piles would have been prohibitively expensive. Harrison Geotechnical was engaged to: design and undertake an investigation of the soils, analyse the findings and design an economic solution to the problem.

Some boreholes already had been drilled but owing to the sensitive nature of the soils the information obtained was not accurate or reliable enough. An additional investigation was carried out using a Cone Pressuremeter. This instrument is a sophisticated combination of CPT and pressuremeter which in this case was the only practicable way to provide extremely accurate measurements of geotechnical parameters. The analysis indicated that the possible failure mechanisms for the proposed embankments arose from catastrophic shear failure and excessive long-term settlement.

The solution was to accelerate the dissipation of excess pore pressures developed during the construction of the embankment, thereby increasing the factor of safety against shear failure and accelerating the consolidation of the embankment. This was achieved by installing vertical drains linked to a geotextile wrapped drainage blanket on which the embankment was constructed coupled with monitoring the pore pressures during construction.