This master's thesis deals with the design and analysis of the excavation support in Brdo near Ljubljana using the Plaxis 2D programme, which applies the finite element method. This analysis is particularly complex due to the very unfavourable geotechnical and hydrogeological conditions at the site, which has two aquifers: the upper aquifer and the lower aquifer of Ljubljana, separated by a soft clay layer with low permeability. The primary method of excavation support investigated was the use of steel sheet piles anchored with geotechnical anchors. This method is cost-effective and the sheet piles can be reused several times. In view of the difficult hydrogeological conditions and the flexibility of the sheet pile wall construction, the excavation must be carried out in several phases. After each phase, the sheet piles are anchored with pre-stressed anchors to further reduce possible movement.
An alternative solution is to use a reinforced concrete diaphragm wall. Whilst this method is more difficult and costly to implement, it allows the use of a single-sided formwork and provides an extremely effective barrier against water ingress into the pit. Due to the greater stiffness of the diaphragm wall, one less row of anchors is required compared to sheet piling, which reduces the number of excavation phases required and enables the work to be completed more quickly. We modelled the diaphragm wall in two different ways and compared the results. In the first modelling method, plate elements were used (similar to sheet piles), where the friction between the soil and the protective structure was not taken into account. Alternatively, we modelled the diaphragm wall with 2D surface elements, taking into account the actual thickness of the structure and thus more accurately accounting for the effect of soil-structure interface friction. Although there are differences between the calculation results for both variants, we found that for a membrane thickness of 60 cm, friction along the soil-structure interface has no significant influence on the extreme values of the internal forces and displacements. The verifications for the ultimate limit state and the design of the structural elements were carried out in accordance with the Eurocode standards.
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