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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
After two weeks of testing, suctions recorded at all depths in
bare box B increase by nearly the same amount of 6 kPa,
suggesting that there was again unit gradient downward flow.
The downward water flow during the monitoring period in box
B was expected because surface evaporation was greatly
minimised by covering the soil surface using the plastic sheet
(see Figure 1). On the contrary, uniform suction distributions
are not observed in both vegetated boxes G and T. It can be seen
that there are significant increases in soil suctions at all depths
in these two boxes, particularly within plant root zones in
shallower depths. This is not surprising because hydraulic
gradient near soil-atmosphere interface is much higher than that
in deeper depths. Measured suctions at all depths in box G are
higher than those in Box B. When grass is present, the peak
induced suction due to grass root-water uptake (i.e., 50 kPa) is
found to be six times higher than that in bare box B (i.e., 8 kPa)
at 30 mm depth. Although measurements at 140 and 210 mm
depths were made outside the root zone of grass, suctions at
these depths are also influenced by the root-water uptake
happened in shallow depths. As identified by Ng et al. (2012),
vertical influence zone of induced suction in soil vegetated with
grass could be as deep as four times of grass root depth.
For box T, suctions recorded at all depths are higher than
those in the bare box B. When the tree is present, the peak
suction induced at 30 mm depth (i.e., 67 kPa) is found to be
almost eight times higher than that in bare box B (i.e., 8 kPa).
This is attributed to tree transpiration by intercepting 50 % of
incoming radiant energy (see Figure 3) and also soil evaporation
by receiving remaining energy fallen on soil surface. It is
important to recognize that the observed distribution of suction
in box T is consistent with that of RAI shown in Figure 2. The
amount of suction induced is found to increase with an increase
in RAI proportionally. For a tree having a higher RAI, this
means that a larger surface area of tree roots is available for
root-water uptake to induce higher suctions.
When compared to box G, it can be seen that suctions
recorded at all depths in box T are about 34 % higher (see
Figure 4). This is likely because the root diameter and root
depth of the tree are larger and deeper than those of grass,
respectively. As compared to short, fine grass roots, the
characteristics of tree roots are more favorable for root-water
uptake. At deeper depths of 140 and 210 mm, the measured
suctions in box T are expected to be higher than those in box G.
This is because suction measurements at these two depths were
made within the tree root zone in box T, but those in box G
were outside the root zone of grass.
6 SUMMARY AND CONCLUSIONS
In this study, a series of laboratory tests were carried out to
explore the effects of two different plant types (grass and tree)
on induced suction and its distribution in compacted silty sand
in an atmospheric controlled plant room. Measured induced
suction in vegetated soils were compared and correlated with
energy distribution and plant characteristics.
After two weeks of testing, peak suctions (i.e., 50 kPa and
67 kPa) induced in silty sand vegetated with grass and tree are
found to be at least six and eight times higher than that (i.e., 8
kPa) in bare soil, respectively. The additional suction induced in
vegetated soils is attributed to plant transpiration through energy
interception by plant leaves. For the tree having a LAI of 4.6,
the maximum amount of energy interception by tree leaves is up
to 50 % of the incoming energy supplied in the atmospheric-
controlled room. It is important to recognize that the measured
vertical distribution of suction in vegetated soil depends on RAI.
The magnitude of suction induced is found to be directly
proportional to RAI. When RAI is higher, higher surface area of
tree roots is available for root-water uptake to take place and
higher suction is hence induced.
When comparing soil vegetated with grass and tree, peak
suction induced by tree is 34 % higher. This is because root
diameter and root depth of the tree are larger and deeper than
those of grass (which has finer and shorter roots), respectively.
As expected, these characteristics of tree roots are more
favorable than those of grass for root-water uptake.
7 ACKNOWLEDGEMENTS
The authors would like to acknowledge research grant
(2012CB719805) from the National Basic Research Program
(973 Program) (No. 2012CB719800) provided by the Ministry
of Science and Technology of the People's Republic of China,
research grant (HKUST9/CRF/09) provided by the Research
Grants Council (RGC) of the Hong Kong Special
Administrative Region, and also financial support from Hong
Kong PhD Fellowship Scheme (HKPFS) for the third author of
this paper. Experimental work carried out by final-year project
students, Messrs Kow Hong Yee, Ho Chu Pan and Wang Zi
Jian are acknowledged.
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