Case studies on settlement induced by tunnel excavation works in Hong Kong : causes, effects and resolution

Abstract

Tunnel excavation in densely populated cities like Hong Kong often induces ground settlement, potentially compromising the structural integrity of adjacent buildings and utilities. This dissertation consolidates project data of two of the recent tunnel projects in Hong Kong, namely Kai Tak West (KTW) of the Central Kowloon Route (CKR) (Case Study 1) and To Kwa Wan (TKW) Station of the Shatin Central Link (SCL) (Case Study 2) which employed cut-and-cover methods. Ground deformation control mechanisms, including geotechnical instrumentation for real-time monitoring of ground deformation, implementation of 3A or 5A Levels and the associated pre-defined actions, are examined in both case studies. The case studies also analyze the causative factors of settlement, induced effects to the nearby sensitive receivers and the resolution of settlement-related issues, offering insights into future tunnelling projects. Both case studies identify groundwater drawdown during bulk excavation and dewatering as the primary cause of ground settlement. Case Study 1 revealed significant settlement in the vicinity of tunnel excavation, primarily caused by prolonged and deformation in thick, compressible clay/silt layer due to pore water dissipation, resulting in unexpected settlement patterns and magnitudes exceeding the anticipated levels, with greater settlement magnitudes encountered at increasing distances from the excavation area. The Case Study also revealed an empirical correlation between ground deformation and construction sequencing, including the installation of vertical structural elements in the ELS system, dewatering and bulk excavation, strutting, construction of permanent structures and backfilling. Additional factors, such as the implemented ground control mechanisms and remedial measures, can also determine the settlement patterns. Settlement-induced effects, including minor disruption to the serviceability of the nearby Kowloon City Ferry Pier (KCFP), underscore the importance of effective groundwater control and the timely implementation of adequate mitigation and remedial measures, especially in excavation sites with adverse geological and hydrological conditions, as key resolutions to the undue ground deformation. In Case Study 2, repeated exceedances of triggering levels in the vicinity of TKW station indicate ineffective stipulation of AAA triggering levels, emphasizing the need for realistic AAA benchmarks, strict adherence to predefined response actions when triggering values are exceeded, and other robust administrative measures, including effective on-site and document auditing and a clear communication framework among contractors, consultants, government authorities and other relevant stakeholders, to address settlement issues effectively.