Relationship Between Construction Costs and Reliability of Quay Walls

Abstract

Structures have to meet a particular level of safety. Therefore, in the European design codes (Eurocodes), three reliability classes (RC) are introduced based on the potential consequence of failure of the structure. For each of the RC a maximum allowable probability of failure is introduced, corresponding to a reliability index. In recent research by Roubos et al. (2018), it is suggested that the marginal costs of safety investments for quay walls is quite low. Therefore, it is questionable whether the current reliability classes and the corresponding set of partial factors, as defined in the Eurocodes and CUR 211, are functional for quay walls. This gave rise to the present study. This thesis investigates the relationship between the construction costs and the reliability index for two quay walls located in the Port of Rotterdam; 1) a double anchored combi-wall and 2) a combi-wall with a relieving platform.
In this study, more insight is acquired into the relationship between the construction costs and the reliability index of quay walls. Firstly, the two quay walls are designed semi-probabilistic in RC1, RC2 and RC3, using D-Sheet Piling for the double anchored combi-wall and using Plaxis 2D for the combi-wall with a relieving platform. Thereafter, the construction costs of these designs are calculated and compared. Besides that, the influence of the partial safety factors, which are defined in the Eurocodes and distinguish the reliability classes, on the construction costs is quantified. The same was done for the influence of three of the critical failure mechanisms; ‘passive resistance inadequate’, ‘sheet pile profile fails’ and ‘tension member anchorage fails’. For these failure mechanisms the reliability indices are estimated using the reliability analyses module of D-Sheet Piling, which is based on a probabilistic level II analysis, the First Order Reliability Method (FORM).
It appeared that the marginal costs of safety investments for both quay walls is relatively low, even significantly lower than suggested by Roubos et al. (2018). It followed that the differentiation in construction costs between the reliability classes is considerably less than the differentiation in construction costs between quay walls in practice. Therefore, it seems that the current reliability classes and the corresponding set of partial safety factors, as defined in the Eurocodes and CUR 211, are non-functional for quay walls. Besides that, it can be concluded that when designing a quay wall, the determination of the angle of internal friction of the soil strongly influences the construction costs, followed by the surface- and crane loads. The influence of the cohesion of the soil and the bollard load on the construction costs is very small. Furthermore, the influence of the failure mechanisms ‘passive resistance inadequate’ and ‘tension member anchorage fails’ on the construction costs of the double anchored combi-wall is relatively low. Therefore, it is suggested that the reliability index of the quay wall can be increased in a economically attractive manner by increasing the length of the tubular piles of the combi-wall or the steel sectional area of the anchor rod. Due to these influences, it can be economically beneficial to increase the target reliability index of the failure mechanism ‘passive resistance inadequate’ and decrease the target reliability index of ‘sheet pile profile fails’.