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Fall 2015 No.27
Unearthing the Cleverness of Soil
Prof Charles W W Ng has made studying the complexities of soil his life work, creating fresh hope for managing landslides and slope stability through geotechnical, geo-environmental and bio-engineering, and even finding similarities with human behavior
A secondment to a geotechnical engineering team when working at a leading international engineering consultancy firm in the UK in the late 1980s provided the earth-shattering realization for then young graduate structural engineer Charles W W Ng as to his future career path.
Delving deep
“The team was looking into a technical design involving excavation – four stories deep – and next to the River Thames,” recalled Prof Ng, who joined the School of Engineering in 1995 and is now Chair Professor of Civil and Environmental Engineering. “When I saw that even a major consultancy was finding this challenging, I knew it must be an area worth researching.”
It was a surprise move, particularly for Prof Ng, who had previously envisaged doing an advanced structural engineering at his master’s study but switched to reading a PhD focused on geotechnical engineering. As a master’s student, he had rated the geotechnical area among the most difficult subject for anyone to do well in. But do well he certainly has, becoming a world authority on unsaturated soil mechanics, slope stability and sustainability, and winning multiple awards east and west for his research, most recently a State Scientific and Technological Progress Award, Second Class, one of the State Council’s highest honors.
Research goldmine
“From day one, I have had a curious mind,” he said. “This accidental assignment sparked my interest and I discovered a goldmine for research there.”
Prof Ng undertook his PhD at the University of Bristol, UK, combining his knowledge of structures and his deepening understanding of soil mechanics to undertake novel research on soil-structure interaction and multi-propped excavation. He then became as a postdoctoral research associate at the University of Cambridge, a world leader in soil mechanics and centrifuge modeling, before being recruited by HKUST to take the field forward in Asia and beyond.
His specialty on his return to Hong Kong became unsaturated soil mechanics, realizing this complex area was still waiting to be explored more widely, locally and globally, and starting one of the first postgraduate courses on the subject in Asia in the late 1990s. Application-wise, an interest in slopes was a natural corollary of living in Hong Kong, with its hilly terrain and buildings perched at all levels. Prof Ng later moved from studying loose-fill slopes (Hong Kong has over 6,000) stabilizing with soil nails to green slopes using plants as stabilizers.
Centrifuge model test setup for a 45-degree vegetated slope
Integrated bioengineered (ecologically balanced) live cover for natural slopes
His respect for the natural materials he works with continues to grow: “I have found soils to be extremely clever. Why? If you really know them, you know they have a memory, known technically as the over consolidation ratio. Soil properties are different from man-made materials. Just like humans, they are stress dependent. If you push them lightly, they respond differently from if you push them hard. They are also path dependent, which means they vary depending on their direction of natural geological or applied loading path, similar to humans.”
While plants and their roots have been used previously for stability, Prof Ng argues they have not been effectively utilized. “When you also know unsaturated soil mechanics, different reasons for why and how to use plants emerge. The roots are actually far more important for creating suction in the ground. There are two side effects as plants suck water from the soil: the soil will get drier and gain strength; and permeability will be reduced, making it harder for water to get in. Both are beneficial to slope stability,” he said. Such findings and their geotechnical and environmental implications have been published in major international journals, including Géotechnique and Canadian Geotechnical Journal.
Root causes
The topicality and impact of Prof Ng’s studies have led to an unprecedented array of multi-million dollar research grants, including three major funding awards from the Research Grants Council in Hong Kong (one Theme-based Research Scheme and two Collaborative Research Fund grants), participation in a Mainland China 973 project, and funding from the National Natural Science Foundation of China. Research projects have ranged from investigating root-soil-water interactions to develop bioengineered live cover systems to preventive methods to deal with environmental hazards from municipal solid waste landfills.
In the large-scale theme-based project, which began in January 2016, he heads an international team looking into debris flow mechanisms and sustainable mitigation of risks in Hong Kong. More extreme weather heralds the potential for disastrous numbers of landslides. 
Debris flows on June 7, 2008 blocked the North Lantau Expressway for 16 hours
Photo credit: Head of Geotechnical Engineering Office and Director of Civil Engineering and Development, HKSAR Government (from plate 8 of GEO report no. 273)
Landmark geotechnical centrifuge facility
Prof Ng served as Director of the University’s pioneering and world-leading Geotechnical Centrifuge Facility from establishment in 2001 until early last year. When it came on stream, the HKUST centrifuge was equipped with the first 2D shaking table globally and the world’s most advanced four-axis robotic manipulator. It has gone on to assist the University’s researchers, other academics, government departments and industry to undertake tests on soil behavior and properties that could not have been carried out in the region otherwise.
One memorable occasion, among many, involved simulating conditions in the field to show why a newly constructed but unoccupied 13-story apartment building collapsed in Shanghai in 2009. The test was successfully carried out with live feed from the HKUST centrifuge to the senior manager concerned in Shanghai, watching via the Internet. “Nerve-wracking but important” is how Prof Ng recalled that day.
Comparison of model and prototype
Married to soil
If soil is like a human being in behavior, the reverse can also be true. Prof Ng, who jokingly refers to himself as “married to soil”, keeps a punishing professional schedule that has seen him publish over 200 SCI international journal articles and some 200 conference papers, and author two reference books. He has also been invited to deliver about 50 keynotes, state-of-the-art reports and special lectures over six continents. He is currently associate editor of the Canadian Geotechnical Journal, a leading publication in the field, and chair of the Awards Committee of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). He also chaired the organizing committee for GeGe2015, the first international conference covering both geo-energy and geo-environment and held in December 2015 at HKUST.
In addition to these activities, he is Associate Vice-President (Research and Graduate Studies) for HKUST overall, bringing his own successful experience of research funding and project leadership to strategic planning in University-wide research development and postgraduate education. With his own graduate students, Prof Ng seeks to guide them to achieve what they want to be and to see issues from an all-round perspective. It is an approach that appears to have served his students well. He has graduated 27 PhD and 35 MPhil students, with eight students being admitted to Cambridge for further studies, and others taking up academic positions overseas and in Hong Kong.