Enhancing Pore Water Pressure Monitoring by Fully Grouted Piezometers: A Case Study of Crossrail and Prospects of Application in Hong Kong

This paper presents a comprehensive case study on pore water pressure measurements using fully grouted piezometers for London Crossrail project, focusing on the insights gained and exploring their potential applications in Hong Kong. Multi-level vibrating wire piezometers, strategically placed in single boreholes fully backfilled with carefully designed cement-bentonite grout, were deployed in a 'greenfield' research site in Hyde Park, London, anticipating the passage of two earth pressure balanced tunnel boring machines (EPBMs) constructing twin-bore Crossrail tunnels through London Clay. The performance of these fully-grouted piezometers are discussed, while presenting the pore water pressure measurements in low-permeability ground before, during and after Crossrail tunnel construction. It is evident from the steady-state measurements that there exists an under-drained steady-state pore water pressure profile within the low-permeability ground separating the upper and lower aquifers, also being influenced by a nearby London Underground tunnel. During the EPBM passage, these fully-grouted piezometers were capable of reliably measuring the rapid response of pore water pressure. The advantages and practical challenges of employing fully-grouted piezometers for pore water pressure monitoring are explored, accompanied by their potential applications in Hong Kong, particularly reclamation and deep excavations in low-permeability ground. This paper shares perspectives on enhancing the reliability and benefits of pore water pressure measurements for civil engineering projects in Hong Kong.