On the temporal evolution of earth pressures in deep excavations in soft clay

Doktorsavhandling, 2024

Urbanisation and sustainable development of cities drives the need to increasingly utilise underground space. Consequently, there is more demand for deeper and larger excavations in urban areas, pushing the limits of current engineering experience. The vast majority of the reported observations of earth pressures in deep excavations, however, are on lateral earth pressures only and cover the construction stage. Reports on the performance in the serviceability stage are scarce, especially for underground structures in soft clays. In particular, there is a lack of investigations on the evolution of earth pressures below permanent structures at the base of deep excavations. Additionally, quantifying the magnitude and evolution of earth pressures due to delayed heave restrained by structural elements, remains challenging. This thesis investigates the temporal evolution of earth pressures acting on underground structures in deep excavations in soft clay, by means of field monitoring and numerical analyses. The ultimate goal is to generalise the results and develop non-dimensional design charts that quantifies the magnitude and evolution of earth pressures beneath the base of deep excavations and underground structures. The research consists of three parts i) benchmarking of a soil model (Creep-SClay1S) against the observed response of two well-documented excavations, ii) field monitoring of the hydro-mechanical response of soil elements below the base of an excavation and underground structure, and iii) a parametric study, using the Finite Element Method, designed and evaluated using dimensionless parameter groups to generalise the results. The first part demonstrates that the Creep-SClay1S model can be used to compute the magnitude and evolution of earth pressures acting on underground structures in soft clays with sufficient accuracy. Subsequently, the field monitoring of the hydro-mechanical response with clustered instruments enabled unique observations of the evolution of effective stresses and the stress ratio K=σ’h/σ’v at soil element level. The parametric study quantifies the impact of the normalised time between the end of excavation and the completion of the restraining structure at the base of the excavation on the emerging magnitude of the effective heave pressures for several scenarios. The results compare well to the field monitoring data and physical model tests. The work presented in this thesis reveals the mechanisms that control the development and evolution of earth pressures in deep excavations. The combination of field monitoring, dimensional analysis and the numerical modelling of the system response have been integrated into design charts. The results can readily be used as a tool for industry to assess the magnitude of effective heave pressure and complement detailed project-specific analyses.