Side-Anchored Floating Bridge at Bjørnafjorden - Design Analysis for Plate Anchors in Clay and Cyclic Bearing Capacity Evaluation.

Abstract

This thesis discusses the anchoring system for a planned side-anchored floating bridge crossing the Bjørnafjord. The anchoring system consists of 32 anchors divided into eight groups, four on each side of the bridge alignment. Due to the risk of submarine landslides hitting one or several anchor groups by eroding the sediment basin plate anchors installed in the seabed is considered instead of the now planned suction, mixed and gravity anchors. The sediments on the seabed basin consists of soft clay with strength increasing linearly with depth. Plate anchors has been considered for locations having a sediment thickness greater than 16 m. A design code published by DNVGL is used for the anchor design. The landslide scenarios evaluated are slides eroding 5 m, 10 m and 15 m of the sediments at the anchor locations. It has been determined that smaller plate anchors penetrated deep will have a lower risk of getting influenced by a landslide than bigger anchors at a shallower depth. How small the plate anchors can be depends on how thick the sediment layer is. The anchor areas are ranging from 25 m$^{2}$ to 64 m$^{2}$ penetrated at respectively deep and shallow depth.

A cyclic bearing capacity analysis considering the cyclic shear strength for the soil due to its impact on the anchor design has been computed. A cyclic time series accounting for wave loads have been scaled up to represent both dynamic wind and wave loads. The Bjørnafjord clay has not got any cyclic laboratory tests completed therefore cyclic laboratory tests for the clay at Gjøa are computed. The cyclic amplitude from the scaled up times series was too low to have an impact on the design of the plate anchors.