1167
Soil suction induced by grass and tree in an atmospheric-controlled plant room
Succion du sol induite par l'herbe et l'arbre dans une chambre atmosphérique contrôlée
Ng C.W.W., Leung A.K., Garg A., Woon K.X.
The Hong Kong University of Science and Technology, Hong Kong
Chu L.M.
School of Life Sciences, The Chinese University of Hong Kong
Hau B.C.H.
School
of Biological Science, The University of Hong Kong
ABSTRACT: Vegetation is commonly found on soil slopes and landfill covers worldwide. Although mechanical contribution of roots
has been considered in some stability calculations, any effects of suction induced by root-water uptake on the reduction of water
permeability and the increase of shear strength are generally ignored. Due to a lack of experimental study, relationships among
induced soil suction, atmospheric conditions (such as radiant energy) and plant characteristics (such as leaf area index (LAI) and root
area index (RAI)) are not well understood. In order to improve our fundamental understanding of soil-water-plant-atmospheric
interaction, this study measures and compare induced suction and its distribution by grass (
Cynadon dactylon
) and tree (
Schefflera
heptaphylla
) planted in completely decomposed granite (silty sand) in an atmospheric-controlled plant room. By monitoring the
responses of induced suction in each vegetated soil for two weeks, the effects of the two different plants on suction distributions are
highlighted and discussed. Observed suction induced is interpreted in conjunction with measured interception of radiant energy by
plant leaves through LAI and also measured plant root characteristics through RAI.
RÉSUMÉ : La végétation est généralement présente sur les pentes du sol et les couvertures d'enfouissement partout dans le monde.
Bien que la contribution mécanique des racines ait été prise en compte dans certains calculs de stabilité, les effets de la succion
induite par la racine sur la réduction de la perméabilité à l'eau et sur l'augmentation de la résistance au cisaillement sont généralement
ignorés. En raison d'un manque d'études expérimentales, les relations entre la succion du sol induite, les conditions atmosphériques
(tels que l'énergie rayonnante) et les caractéristiques de la plante (comme l'indice de surface foliaire (ISF) et l'indice de surface de
racine (ISR)) ne sont pas bien compris. Afin d'améliorer notre compréhension fondamentale de l'interaction sol-eau-plante-
atmosphère, cette étude mesure et compare la succion du sol et sa distribution par l'herbe (
Cynadon dactylon
) et l'arbre (
Schefflera
heptaphylla
) plantés dans un sol en granite complètement décomposé (sable limoneux)) dans une chambre atmosphérique contrôlée.
En surveillant les réponses de la succion induite dans chaque sol pendant deux semaines, les effets des deux plantes différentes sur les
distributions de succion du sol sont mis en évidence et discutés. La succion du sol observée est interprétée avec l'interception de
l'énergie rayonnante mesurée par ISF et aussi les caractéristiques des racines mesurées par ISR.
KEYWORDS: vegetation, suction, unsaturated soil, leaf area index, root area index.
1 INTRODUCTION
Many soil slopes and landfill covers worldwide are covered
with vegetation. While there are engineering needs to search for
sustainable and environmentally friendly slope stabilisation
methods, shallow slopes in particular, vegetation has been
considered to be an alternative, through mechanical and
hydrological processes (Greenwood et al. 2004). Mechanical
root reinforcement has been researched to be beneficial to slope
stability (Greenwood et al. 2004). On the contrary, research on
the effects of suction due to root-water uptake on the reduction
of water permeability and the increase in shear strength is
relatively scarce. Due to the lack of rigorous and systematic
research, effects due to plant-induced soil suction are generally
ignored when analyzing transient seepage and stability of a
vegetated soil slope.
As far as plant-induced suction is concerned, some studies
reported in literature mainly focused on responses of soil
moisture/suction due to the presence of grass species (Ng et al.
2012). Investigation on the effects of other plant species such as
tree on root-water uptake and induced changes of soil
moisture/suction are rather limited.
It has been well-recognized that the amount of transpiration
and root-water uptake of any plant species strongly depend on
characteristics of plant leaf and root and atmospheric parameters
such as radiant energy. Correlations between intercepted radiant
energy and leaf area index (LAI) have been commonly used to
interpret plant-induced changes of soil moisture (Monsi and
Saeki 1953). LAI is a dimensionless index defining the ratio of
total one-sided green leaf area to projected area of a plant on
soil surface. For a plant having a higher LAI, a larger leaf area
is available to intercept radiant energy for transpiration. While
root-water uptake always takes place below ground, this
common approach may be less direct because LAI is an index
reflecting characteristics of plant leaf above ground. Further
investigation on relationship between induced suction and other
more relevant plant characteristics such as root area is needed.
To improve our understanding on the interaction among soil,
water, plant and atmosphere, a test program was conducted to
measure the magnitude and distribution of induced suction in
vegetated soil. Two plant species, namely
Schefflera
heptaphylla
(tree) and
Cynadon dactylon
(grass), which are
commonly found in Hong Kong and other parts of Asia
including Vietnam, India and Malaysia, were selected for
investigation and comparison. Three test boxes with and
without vegetation were purpose-built and they were monitored
for two weeks under identical, controlled and constant
atmospheric conditions.
