R E S E A R C H @ H K U S T
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The new system used HKUST’s world-
class geotechnical centrifuge to simulate
both the mechanical and hydrological
(suction) effects of different types of root
architecture and hence their impact on
induced soil suction and slope stability.
The research group was the first to
identify possible types of slope failure
involving different types of root
architecture. They also derived new
analytical solutions to calculate plant-
induced soil suction by different root
architecture to predict the stability
of vegetated soil slopes. With this
understanding, the HKUST team could
advise on the type of vegetation to plant
to increase slope stability in what they
called “green slope engineering”. In
addition, the researchers identified plant
characteristics of geotechnical value.
Through measuring the leaf area index,
for example, engineers could estimate
induced suction in the ground and hence
analyze slope stability. The research was
supported by two multidisciplinary
Collaborative Research Fund projects in
Hong Kong and a National Basic Research
Program(973 project) inMainlandChina.
Mitigating Debris Flows with
Multiple Flexible Barriers
In addition to green slope engineering
initiatives, Prof Ng has now set out to
add to global understanding of natural
terrain landslides and mitigation
measures through a cutting-edge
investigation into the interaction
between debris flows and multiple
flexible barriers. Such barriers are
especially useful solutions for densely-
populated, hilly cities, for example
Hong Kong, and for protecting inter-city
infrastructure, such as roads and railways,
across mountainous regions, for example
Belt and Road initiative countries.
In 2016, Prof Ng brought together a
multidisciplinary international research
team to launch the five-year project,
funded under the Theme-based
theories of unsaturated soil mechanics,
these trend-setting frameworks enable
researchers and engineers to simulate
interaction among cyclic mechanical,
hydraulic (water content and suction),
and thermal (temperature) behavior of
unsaturated soils in different states and
stress-, path-, and strain-dependency.
To verify this theoretical work,
Prof Ng’s research team has developed
and patented novel experimental
apparatuses, including a suction-
controlled stress-controllable pressure
plate extractor, and a temperature-
controllable cyclic triaxial apparatus
equipped with a novel double-cell total
volume measuring system, known
as the “HKUST Inner Cell”. This has
been licensed to a UK company and
nearly 200 universities and research
institutions worldwide have adopted
the plate extractor and the HKUST
Inner Cell, the latter of which is
now among the standard measuring
methods globally.
The world’s largest man-made 160-meter long
flume in Kunming, Mainland China. This facility
has a channel width of six meters and will be able
to simulate debris flow volumes of up to 500 m
3
.
Nature’s Engineers
Based on such in-depth knowledge of the
properties and behavior of unsaturated
soils, Prof Ng and his team have gone on
to make a series of innovative research
contributions to slope stability and
sustainability. One such area is soil-
plant-atmospheric
interactions
and
bioengineered live cover systems. The
researchers discovered that not only
do plant roots provide mechanical
reinforcement,butmoreimportantly,they
also induce soil suction via transpiration
(hydrological effects), increasing soil
shear strength and reducing water
infiltration in slopes and landfill covers.
“Plants are intelligent natural ‘engineers’,”
Prof Ng noted. “They can cost-effectively
stabilize man-made structures and
natural slopes, and contribute to the
development of environmentally friendly
and sustainable cities.”
Through this and related discoveries,
the team then developed and patented an
innovative artificial model root system.
Eco-friendly
Three-layer
Landfill Cover
Hong Kong’s landfills are filling up
faster than expected. The amount of
waste in the city’s overflowing landfills
has increased by 80% in the past
30 years, with a large percentage
coming from the construction sector.
Based on advanced unsaturated soil
mechanics, Prof Ng’s research team
has invented and patented a new
environmentally friendly three-layer
landfill cover system to protect the
environment from gas emissions from
the landfill body and to minimize water
infiltration into the waste after closure
of a landfill. This type of landfill cover
system has since been extended to use
construction waste as cover materials.
No artificial materials, such as geo-
membranes, are needed to minimize
rainfall infiltration and gas emissions from
the landfill. The elimination of artificial,
non-environmentally friendly geo-
membranes can also prevent interface
failure of a traditional landfill cover system.
Unlike traditional soil cover systems,
the HKUST system is suitable for all
weather conditions. It is
self-regenerative, durable, and
virtually maintenance free.
Landfill Cover Composition
Trial at Shenzhen, Xiaping landfill site
A schematic of the novel three-layer landfill
cover system using recycled crushed concrete
(above) and the field trial using recycled
waste at the Xiaping landfill site in Shenzhen,
China (left). Recycled crushed concrete
is both practical and cost-effective,
widening potential interest in both
developing and developed countries.
Flat ground
(planting density)
Sloping ground
(bare and vegetated covers)
Flat ground
(biochar effects)
Total area
~600 m
2
25 m
12 m
15 m
20 m
Coarse recycled concrete layer
Fine recycled concrete layer
Low permeability soil layer
Solid waste
