Probabilistic system identification and reliability updating for hydraulic structures - Application to sheet pile walls

Abstract

Given its geographical location and history, water defense is of utmost importance for the Netherlands. Structural Health Monitoring (SHM) offers a promising approach for system identification of hydraulic structures in this water defense system. The aim of SHM is to set sensors on structures and use the monitored responses to identify structural parameters of interest. However, many pertaining questions are unanswered concerning realistic hydraulic structures and monitoring systems: What type of sensors (e.g. strain gauge, SAAF, etc.) can and should be used in the monitoring system? How many sensors are needed, where and when to install these sensors? What is the influence of construction stages of structures on system identification? Considering the evaluation of a structure, what is the influence of system identification as well as construction stages on reliability (failure probability) of structures? Considering practical implementation: which computational algorithm is suitable and feasible? How to construct a proper surrogate model of the mechanical model to reduce computational time?
To answer these questions, a single anchored sheet pile wall is studied using a probabilistic approach. The sheet pile wall is modeled using the finite element (FE) method, synthetic data are used and Bayesian approach is adopted to cope with measurement uncertainty and model uncertainty. The information conveyed by sensors is quantified by the Kullback–Leibler (KL) divergence between prior and posterior distributions. Moreover, the correlation in model uncertainty of various structural responses is quantified by comparing a full-scale experiment from the literature and a corresponding calibrated 3D finite element model.
The results show that:
• A combination of different sensor types (in our case they are SAAF and strain gauge) should be used in the monitoring system (e.g. the combination of four different types of sensors outperforms the strain sensors on the sheet pile wall by conveying 40% more information with respect to the former); 
• Even limited number of sensors can convey sufficient information. In our case, 3 sensors placed at proper locations can convey 90% information carried by 6, 8 and 9 sensors considering different responses. They should be installed as early as possible;
• The failure probability computed using posterior from system identification largely decreases compared with that computed using prior (the ratio of prior and posterior failure probabilities can go up to 1510 in our case); 
• Delay of the start of monitoring during the construction stages decreases the information conveyed by sensors in system identification (the conveyed information can decrease by 50% in our case) and increases the computed failure probability in reliability analysis: the ratio of prior and posterior failure probabilities can be as large as 3010 ); 
• MultiNest performs well in Bayesian inference in high dimensional problems; 
• Gaussian process regression (GPR) with anisotropic radial basis function (RBF) kernel and white kernel as well as an adaptive infilling criterion is capable of constructing an accurate surrogate model even when it goes to high dimensionality. The error of surrogate model prediction can be explicitly explained.
To my knowledge the work presented in this thesis is the first application of combined system identification and reliability assessment for hydraulic structures, and the first detailed analysis of the effect of sensor installation time on system identification and structural reliability of hydraulic structures.
The findings imply that probabilistic system identification is a promising approach to substantially reduce our uncertainty in modelling hydraulic structures and in turns to increase their calculated safety. The approach has the potential to extend the working life of aging hydraulics structures and save costly strengthening and replacement. The analysis framework can also be applied to other structures in civil engineering.