Geotechnical Challenges for the Numerical Prediction of the Settlement Behaviour of Foundations in Rosenheim’s Seeton

The Rosenheimer Basin in the alpine region near the German-Austrian border is a deep ancient glacial lake, which has been filled with fine-grained sediments over the course of the past 10.000 to 100.000 years. Due to the rapid economic growth in the region over the last few decades, the high population density and the increasing utilisation of infrastructure in the region a significant demand for development exists, particularly for structures with large loads. The design and construction of such structures represents a significant challenge for engineers due to the sensitive soft, fine-grained lacustrine sediments. This article focusses on the conception and design of a cable-stayed bridge pylon in Rosenheim. The foundation consists of bored piles connected to a pile cap, with additional displacement piles and vertical drains for soil improvement. As the stiffness and strength of the sensitive lacustrine sediments are strongly reduced due to the disturbances caused by pile installation, displacement piles in combination with vertical drains around the piles are prescribed in order to re-consolidate the soil (“soil healing”) and increase the shaft friction after the pile installation. In order to take into account the effect of the soil disturbance and the healing effect of the soil improvements on the foundation behaviour realistically, high quality pile loading tests were carried out. Based on the results of laboratory and field tests, and the simulation of the single pile loading tests using a visco-hypoplasticity constitutive model, a 3D Finite-Element Model was developed and calibrated to predict the time-dependent behaviour of the mixed foundation. The numerical prediction shows that the serviceability requirements of the foundation can be fulfilled and the scheduled underpinning of the superstructure to compensate the foundation settlements will likely not be required prior to 50 years of operation. An extensive monitoring program will be implemented during the construction to validate and, if necessary, to adjust the numerical model to realistically predict the long-term deformation behaviour of the bridge foundations.