Economical Design Approaches for Deep Excavation and Lateral Support (ELS) in Soil with Shallow Bedrock

Deep excavation and lateral support (ELS) are integral components of urban development, particularly in the construction of multi-storey basements. The conventional ELS design employs the Mohr-Coulomb (M-C) Failure Criterion for both soil and rock parameters, even when bedrock is encountered at shallow depth. This may lead to over-conservative design by underestimating the rock mass strength and stiffness in the overall rock-structure interaction analysis. This paper explores an alternative design approach by suggesting the application of the Hoek-Brown (H-B) Failure Criterion for ELS design in the presence of shallow bedrock. While the M-C parameters are well-established for soil, the conservative nature of M-C Failure Criterion for rock parameters derived from rock joints or discontinuities defect shear strength properties can lead to avoidable costs. Additionally, the paper addresses common scenarios involving the design of temporary piled walls socketed into bedrock for deep excavation, which involve the design of groundwater seepage in rock. The paper suggests a more nuanced approach, advocating for an assessment of the likelihood of possible rock joint failures to derive sensible and cost-effective solutions. By reconsidering conventional design criteria and incorporating alternative geotechnical principles while highlighting the importance of rock testing and field observation, the proposed methodologies contribute to more efficient and economically viable deep excavation practices in rock.