Offshore soil investigation: Straits of Messina: Solutions to unique problems

An offshore soil investigation was carried out in 1992 under difficult environmental conditions in the Straits of Messina, in order to obtain geotechnical design data for a proposed buoyant tunnel. A number of unique situations must be anticipated and overcome during the planning and execution of soil investigations in such areas. The Straits of Messina are particularly infamous for their treacherous currents and pose additional problems such as considerable sea-bed slopes and water depth variations across the Straits. The Messina channel is over 350 meters deep and in only 5 kilometers wide at the proposed crossing. The Messina investigation was further complicated by the soil conditions, very dense sands and gravels, and the need to determine dynamic soil properties. These difficulties were resolved by innovative procedures. Currents and shallow water problems were overcome by careful scheduling of operations to utilize periods of slack water, use of a dynamic positioning vessel with optimal station holding capabilities, and the use of helical spirals on the drill string. The sea bed frame was specially modified with a hydraulic levelling system to accommodate the sloping sides of the strait. A seismic cone was used to measure in-situ shear wave velocity, and a high capacity small diameter cone was needed to penetrate the dense granular deposits.