Fugro Engineering Services (FES) was appointed by Bouygues to carry out a major geotechnical/CPT survey of soils around the causeway for foundation design and earthquake risk assessment.

The work had to be carried out adjacent to the existing road in areas of mangrove swamp using the temporary access roads.

A total of 88 CPTs were required at 79 locations, both on land and over water, with pore pressure dissipation tests to determine the settlement characteristics of the soils at 44 locations. Electric seismic cone tests were completed at 14 locations to determine the shear wave velocity for seismic risk analysis, and in-situ shear vane tests at three locations on the soft clay soils.

There are no CPT rigs in Jamaica so FES shipped a standard 20 ton capacity unit from the UK and mounted it onto a ballasted truck hired locally.

The detachable CPT unit was transferred to a spudleg pontoon barge for seabed measurements along the causeway. Additional over water CPTs were carried out from a heavy pontoon close to the existing Hunt's Bay Bridge to assist in the design of new foundations for the new bay bridge crossing. Measurements were taken to depths of 30m.

Special laboratory tests were carried out on the soft clays at Fugro's UK laboratories to determine the settlement characteristics of the embankments.

The results led to the decision to install thousands of vertical wick drains to rapidly construct the embankments on the soft soils.

The contract also required the installation of instrumentation to monitor the soft ground during embankment construction, including vibrating wire piezometers for groundwater pressures, magnetic extensometers for settlement and inclinometers for horizontal soil movements.

The data collected from a trial embankment area allowed Bouygues to design a construction process to build a stable embankment on the very soft organic soils.

MJ Information No: 21506