Improving identification of a significant soil layer from CPTU combined with acoustic data

The Norwegian Public Roads Administrations (NPRA) plans to build world’s longest floating bridge with 5 km length to cross the 570 m deep fjord Bjørnafjorden. The bridge is planned with mooring systems connecting it to anchors on the seabed. The evaluation of soil condition on the seabed is crucial both for design of the anchoring systems and for the evaluation of possible slides that may affect the foundations. Detailed seabed mapping revealed variable seabed conditions and identified up to 45 historic submarine slides. Sub-bottom profiling data was used to interpret past slide events and their slip planes. The sub-bottom profiling revealed a soil layer that is distinctly visible from acoustic data. This soil layer coincides with the slip plane identified from a number of the historic slides. Identification of this layer is considered important to make more realistic slope stability evaluations. The question is whether it is possible to identify this layer with only geotechnical data, particularly CPTu data. Current approaches to establish soil layering as recommended by NS-EN ISO standard and different CPTU based soil behaviour classification charts are investigated. This work shows that these evaluation approaches do not correspond with the results from acoustic data in a consistent and reliable way. Approaches based on CPTu alone might overlook this important layer. However, by revisiting CPTu measurements in relation to the acoustic data, this work established trends in CPTU data and proposed approaches that could identify this slip plane. This approach is more consistent and ensures that the information from geotechnics and geology complement each other in identifying significant layers. This work illustrates that combining acoustic and CPTu data improves geotechnical interpretation and understanding of critical ground conditions. This approach is considered valuable also for other fjord sediments.