The Observational Method for building pits in soft-soil conditions

A study on measurement-processing and feasibility of the Observational method Ab Initio approach.

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In the Observational Method Ab Initio approach a flexible design and construction plan is established to allow anticipation to observational feedback during the construction phase. This way, the structural design can be optimized to the in-situ conditions which is beneficial from both safety and economic point of view. In the application of building pits in the soft-soil conditions of the Netherlands the method has the additional value to verify SLS criteria and timely detect unforeseen events. So far limited building pits have been executed via this design strategy. The main reasons for this are the lack of a design procedure and the problematic quantification of safety of the flexible retaining wall design. A strategy for the execution of the Observational Method Ab Initio approach to retaining wall design is described by the CIRIA guideline C760. In this study, the suitability of this 5 step-strategy has been investigated by means of a benchmark. This investigation indicated that, although the CIRIA guideline C760 contains a valuable design strategy, only a qualitative description of safety is provided. Therefore, this study introduces a methodology for real-time measurement-processing with the use of a Bayesian update. The Bayesian update combines the information of the predictive computer model with the information obtained from measurement sets during construction. By describing this information via probability density functions different uncertainties in both the design and construction phase can be weighted in the outcome of the Bayesian update. Consequently, the retaining wall behavior can be re-assessed throughout construction. This methodology is applied to measurement sets gathered at the construction of two different building pits in the Netherlands. Both case studies showed that with the Bayesian update and consequential calibration new parametric distributions can be found. Those parametric distributions describe the retaining wall behavior from which safety definitions in term of a reliability index can be derived. The performance of this methodology for measurement-processing depends on the accuracy of the calculation model and the measurement interpretation. Especially in the case of unexpectedly high and/or fast progressing retaining wall displacements, measurement interpretation is necessary to select the best strategy to redirect the structure. Although this measurement interpretation is a challenge, it is believed that the Observational Method Ab Initio approach complemented with the Bayesian update is a promising design strategy. Its application to the construction of building pits definitely has economic potential and would be favorable for risk management.