A comparison of analytical and numerical determination of hydraulic bottom-heave - A case study of lime-cement column implementation in underpass road 2970 in Hasslerör, Sweden

Abstract

The determination of hydraulic bottom-heave is commonly derived by simplified analytical methodologies, which yields an overview but is unable to capture the complex behaviour as possible in numerical analyses. The use of numerical analyses is ever-increasing, but the accuracy of the methodology is highly sensitive. The thesis aims to establish the differences between the analytical and numerical modelling methodologies in determination of hydraulic bottom-heave. Furthermore, lime-cement columns are studied as a possible mitigation measure against slope stability and hydraulic bottom-heave failure. The analytical analyses are based on the Swedish design guidelines, total safety method and the direct method. The numerical analyses are divided into four scenarios which encompass the Mohr-Coulomb, Soft Soil, Hardening Soil and NGI-ADP constitutive model. The analyses are based on a case study of an underpass road in Hasslerör, Sweden. Additionally, key input parameters are established in a sensitivity analysis. The analytical and numerical analyses predict that mitigation measures are required to ensure sufficient stability of the excavation. The predicted deformations and factor of safety depend on the selection of constitutive models. The sensitivity analysis implies that the selection of groundwater conditions, strength parameters and calculation type are of most importance to the results. Conclusively, the thesis implies that the analytical analyses should serve as a first order estimate to establish whether numerical modelling is necessary. Furthermore, the sensitive nature of the numerical modelling clarifies the importance of representative input data and expertise of the geotechnical engineer. The numerical analyses conclude that lime-cement columns have a stabilizing impact with regards to both slope stability and hydraulic bottom-heave failure. However, the influence of the lime-cement columns with regards to hydraulic bottom-heave cannot be considered in the current Swedish design guidelines. The thesis recommends further research and development to verify the stabilizing effects of lime-cement columns with regards to hydraulic bottom-heave.