To efficiently use of underground space for meeting increasing infrastructural needs while minimizing environmental impacts, a significant number of tunnels and ever deeper basements are being excavated in densely populated cities like London, Shanghai and New York. Practising structural and geotechnical engineers are facing tougher and tougher design challenges. In this keynote lecture, the effects of stress relief due to deep excavation and twin tunnelling on the capacity and deformation of friction piles are discussed. In the first part of the lecture, centrifuge model tests carried out to investigate the capacity and deformation of a friction pile with and without considering vertical stress relief due to a 20-m deep basement excavation are reported. For comparison purposes, in-flight pile load tests were conducted both at the ground surface and at the formation level after an in-flight simulation of the excavation. A non-dilatant pile-soil interface was adopted for simulating piles installed in normally consolidated clays and loose sands, whereas a dilatant pile-soil interface was used for modelling piles constructed in overconsolidated clays and dense sands. In addition, discrete element modelling (DEM) of these two pile-soil interfaces was carried out to reveal the governing mechanisms and the changes in pile shaft resistance. A simple calculation method allowing for the effects of vertical stress relief on pile capacity is then proposed. In the second part of the lecture, a series of three-dimensional centrifuge model tests and numerical back-analyses are reported to study the influence of twin tunnelling on an adjacent friction pile and pile group. Moreover, the effects of the construction sequence of twin tunnelling on an existing pile are explained. Design implications of these two types of stress relief (i.e., vertical and horizontal) on piled foundations are highlighted.
Professor Charles W. W. Ng is currently the CLP Holdings Professor of Sustainability, Chair Professor in the Department of Civil and Environmental Engineering and Associate Vice-President for Research and Development at the Hong Kong University of Science and Technology (HKUST). He is the President of the International Society for Soil Mechanics and Geotechnical Engineering (2017-2021).
Professor Ng was elected an Overseas Fellow of Churchill College, Cambridge University, in 2005 and also a Changjiang Scholar (Chair Professor of Geotechnical Engineering) by the Ministry of Education in China in 2010. He is Fellow of the Institution of Civil Engineers, the American Society of Civil Engineers, the Hong Kong Institution of Engineers and the Hong Kong Academy of Engineering Sciences. Currently, he is an Associate Editor of the Canadian Geotechnical Journal.
Professor Ng has published some 330 SCI journal articles and 250 conference papers and delivered more than 50 keynotes and state-of-the-art reports across six continents. He is the main author of three reference books: (i) A Short Course in Soil-structure Engineering of Deep Foundations, Excavations and Tunnels by Thomas Telford, and (ii) Advanced Unsaturated Soil Mechanics and Engineering and (iii) Plant-Soil slope Interaction by CRC: Taylor & Francis.