Bridge Foundations at Large Water Depths
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The Norwegian Public Roads Administration (NPRA) plans for a significant upgrade of the E-39 highway route along the west coast of Norway between Kristiansand and Trondheim, where the ferry connections will be replaced by fixed links. This study concerns two topics that were identified in the initial phase of the study. The first topic concerns the progressive failure of cyclic loaded rock anchors, and the second topic concerns soil-structure interaction (SSI) in terms of macro modeling. The two topics are investigated both experimentally and numerically. The topic of cyclic loaded rock anchors emerged from the need to anchor large tension forces to the ground. The bridge design determines the forces transferred to the foundations. For example, anchoring the suspension bridge main cables may constitute a cable pull of more than 1 GN (e.g., the Akashi Kaikyo bridge, Japan, Furuya et al. ). Main cables may be anchored to solid rock using rock anchors fixed to an anchor plate inside the rock (e.g., Hardangerbrua, NPRA ). Floating bridges or submerged floating tube bridges (SFTB) may require the anchoring of mooring lines in permanent tension, where cyclic loads (in one-way tension) may appear to various degrees, caused by environmental conditions and traffic loads. Rock anchors may be used to transfer the tension loads to the ground. The cyclic load action in combination with long-term creep may nonetheless alter the rock anchor capacity. The study investigates the failure mechanism occurring at the rock anchor - grout interface, which is one out of four possible failure modes of a rock anchor. Laboratory tests are performed on 47 samples of rock anchors grouted into pre-drilled holes in a concrete block simulating the rock. A model predicting the rock anchor capacity as a function of the cyclic load history and current state (slip) is suggested. The model is calibrated for the test conditions performed in the laboratory. Back calculations of laboratory tests are performed to validate the model. The model is also implemented in the finite element (FE) code ANSYS through a user-defined model applied in interface elements along the anchor body. The laboratory work and the model are described in Journal of Rock Mechanics and Geotechnical Engineering (in print). The second topic relates to SSI and macro modelling. The geotechnical design of a structure typically considers bearing capacity and settlements/displacements. These evaluations are commonly performed using bearing capacity formulations. There are, however, a variety of factors required to adjust for the load inclination, the embedment and the shape. The evaluations may alternatively be performed using FE. The representation of both the structure and the elements in a comprehensive FE code may be time consuming. A macro model, which is a simplified methodology to enhance the SSI process, is investigated. The macro model represents the SSI in one single element, which in turn can be applied in a node to the structural model representing the non-linear foundation behaviour. This methodology was pioneered within the offshore energy industry, e.g., for jack-up foundations (e.g., Houlsby and Cassidy , Bienen and Cassidy ). This methodology applies the plasticity theory to model the nonlinear foundation behaviour and includes a yield surface, an elastic domain, a non-associated flow rule and a hardening law. A macro model for a shallow foundation resting on a granular material is developed and validated by prototype testing in a 4 m × 4 m × 3 m sand bin at NTNU. The model can be used for bridge foundations, anchors or other foundations resting on granular materials. The model and experimental work are presented in the Journal Computers and Structures.