Numerical analysis of the effects of grouting on mitigating the risk of hydraulically induced failure during deep shaft excavation

• Published in: Expanding Underground - Knowledge and Passion to Make a Positive Impact on the World pp. 887 - 894
• Main Authors: Šaponjić, J., Radovanović, S., Divac, N., Divac, D.
• Format: Book Chapter
• Language: English
• Published: United Kingdom CRC Press 2023; Taylor & Francis Group
• Edition: 1
• Subjects: Permeability Coefficient; Above Ground; Groundwater Flow; BIM Model; Typical Failure Mechanisms; Grout Curtain; Geological Strength Index; Excavation Stages; Drain Element; Hydraulic Failure; Strength Reduction Method; Overburden; Aquitard Layer; Pore Pressure; Initial Groundwater Level; Partial Safety Factor; High Groundwater Table; Predominant Stability; SEM; Eurocode En; Ultimate Limit State; Excavation Base; SCL; Seepage Force
• DOI: 10.1201/9781003348030-107

Hydraulic failure presents a significant stability and safety issue for deep shaft excavation projects that deal with a high groundwater table in cohesionless soil. In urban environments, the lowering of the groundwater table for deep shaft excavations is usually not an acceptable solution to mitigate these risks due to the surface settlements it may cause, thus jeopardizing the safety of the surrounding structures. In this paper, a case study of an inlet shaft of a TBM-bored wastewater tunnel in Belgrade, Serbia was analyzed in order to show that grouting techniques can be an appropriate measure for reducing hydraulically induced instability. Since the location of the shaft is near the riverbank, the soil stratigraphy consists of granular soils with a high groundwater table. Several different variations of grout curtains around the shaft were modelled by conducting steady state flow analysis using commercial software based on the finite element method, and the obtained results were used to determine the risk of hydraulically induced failure at the bottom of the excavation. Based on the obtained results it is concluded that the application of grout curtains is a suitable solution for increasing the factor of safety for hydraulic heave problems as well as for soil failure problems. However, when encountering an aquitard layer during the earlier phases of excavation, grout curtains have no effect on lowering the pore pressures that are inducing the uplift at the bottom of the aquitard layer. Hydraulic failure presents a stability and safety issue for deep shaft excavation projects that deal with a high groundwater table in cohesionless soil. For the purpose of numerical analysis, a two-dimensional axisymmetric steady-state groundwater flow model of the inlet shaft is created using Plaxis 2D software. This chapter presents a case study of an inlet shaft of a tunnel boring machine-bored wastewater tunnel in Belgrade, and analyzes Serbia in order to show that grouting techniques can be an appropriate measure for reducing hydraulically induced instability. When the option of lowering the groundwater table is not applicable, grouting can present a rational solution to mitigate the risk of hydraulically induced failure during deep shaft excavations. When there exists a risk of uplift issues in the upper layers of excavation, grout curtains have no influence on lowering the pore pressures inducing the uplift at the bottom of the aquitard layer.