Implementing the 5A Control Framework: Case Studies Demonstrating Economic and Safety Benefits in Deep Excavation

Abstract

The design of excavation and lateral support (ELS) systems has traditionally been constrained by the conservative 3A empirical rule, often resulting in over-conservative and heavy strut systems. The Geotechnical Engineering Office (GEO) has initiated a paradigm shift with its Technical Guidance Note No. 54 (TGN 54) and Publication No. 1/2023, advocating for an enhanced 5A Approach. This new methodology introduces a five-tier performance-based control framework (Alert, Alarm, Action Levels 1-3), linking rational and often relaxed deformation limits (e.g., 0.5%Hₑ for Action Level 3) to explicit monitoring and response protocols. This paper presents two detailed case studies demonstrating the successful application of the codified 5A framework. Our findings indicate that designing to the 5A deformation limit, in accordance with the TGN 54 control mechanism, resulted in a 15% reduction in the weight of steel struts and a decrease of 30% to 50% in preload force compared to conventional 3A designs. A key outcome was a notable enhancement in construction safety: lighter struts facilitated reduced crane lift weights, simplified handling, and smaller preload forces, thereby lowering the risk of damaging existing underground utilities and accelerating installation processes. Additionally, material cost savings of approximately 15% were achieved. This paper serves as a blueprint for implementing this advanced methodology in urban excavation projects.