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J.H. van Dalen
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1
The Observational Method for building pits in soft-soil conditions
A study on measurement-processing and feasibility of the Observational method Ab Initio approach.
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.
...
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.
Master thesis
(2018)
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Jay Srigopal, Kenneth Gavin, Jan van Dalen, Cor van der Veen, Richard de Nijs
Müller Verpress piles are driven displacement piles which inject grout at the enlarged tip of the pile into the surrounding soil. Performed full-scale tests were not able to be loaded the pile upon failure, as the pile can resist forces which are significantly larger than its design capacity. This type of pile is known for its large tensile capacity and it is used often to bear major loads in large structures, but the ultimate true capacity has not been identified yet. A better understanding in the behaviour of a MV pile can be crucial in an optimal design of such structures, as construction costs and obtained safety level can be reduced. From provided full-scale tests the uncertainties about the shape of the circumference of the injected grout body and the acting soil pressures have been reduced. Further a different relation is found between the mobilized shaft friction and the cone resistance. As the soil becomes stiffer and holds a larger cone resistance, more shaft friction is mobilized.A two-dimensional numerical model of a cross section of the pile is generated to obtain the actual shape of the circumference and the maximum shaft friction after installation. The measured grout discharges and mobilized shaft friction from pile tests were in the same range as the results obtained from this model, validating that the results of the model are within reason. All available full-scale tests were extrapolated by application of the model and generated similar capacities as the method by measured shaft friction, setting a minimum boundary for α_t at 1.4%. A second numerical model of a complete full-scale tested pile is successfully generated, including the adaptations of obtained parameters from the other model. The adaptations consist of a new circumference, different relation for EA and larger values for the maximum shaft friction. The model showed results which are in the same range as the analytical derivations regarding the course of forces and mobilized shaft friction. The modelled results do however differ slightly in value. This difference in results can be explained by the absence of detailed soil investigation. The lack of detailed geotechnical parameters would increase the accuracy of the model, as the model is very sensitive to variation in the geotechnical parameters. The produced adaptations of multiple parameters have proven however to be a better indication of the total capacity than the current norms. Therefore the norm can be adapted into a more progressive code based on the produced relations of this thesis. The adaptations in EA relation, circumference and mobilized shaft friction comprehend the behaviour of the Muller Verpress pile with a better understanding. Changing these parameters are preferred over the adaptation of the α_t factor, as the theoretical background remains in the design rule.
...
Müller Verpress piles are driven displacement piles which inject grout at the enlarged tip of the pile into the surrounding soil. Performed full-scale tests were not able to be loaded the pile upon failure, as the pile can resist forces which are significantly larger than its design capacity. This type of pile is known for its large tensile capacity and it is used often to bear major loads in large structures, but the ultimate true capacity has not been identified yet. A better understanding in the behaviour of a MV pile can be crucial in an optimal design of such structures, as construction costs and obtained safety level can be reduced. From provided full-scale tests the uncertainties about the shape of the circumference of the injected grout body and the acting soil pressures have been reduced. Further a different relation is found between the mobilized shaft friction and the cone resistance. As the soil becomes stiffer and holds a larger cone resistance, more shaft friction is mobilized.A two-dimensional numerical model of a cross section of the pile is generated to obtain the actual shape of the circumference and the maximum shaft friction after installation. The measured grout discharges and mobilized shaft friction from pile tests were in the same range as the results obtained from this model, validating that the results of the model are within reason. All available full-scale tests were extrapolated by application of the model and generated similar capacities as the method by measured shaft friction, setting a minimum boundary for α_t at 1.4%. A second numerical model of a complete full-scale tested pile is successfully generated, including the adaptations of obtained parameters from the other model. The adaptations consist of a new circumference, different relation for EA and larger values for the maximum shaft friction. The model showed results which are in the same range as the analytical derivations regarding the course of forces and mobilized shaft friction. The modelled results do however differ slightly in value. This difference in results can be explained by the absence of detailed soil investigation. The lack of detailed geotechnical parameters would increase the accuracy of the model, as the model is very sensitive to variation in the geotechnical parameters. The produced adaptations of multiple parameters have proven however to be a better indication of the total capacity than the current norms. Therefore the norm can be adapted into a more progressive code based on the produced relations of this thesis. The adaptations in EA relation, circumference and mobilized shaft friction comprehend the behaviour of the Muller Verpress pile with a better understanding. Changing these parameters are preferred over the adaptation of the α_t factor, as the theoretical background remains in the design rule.