THESIS
2015
v leaves, vi-xvi, 128 pages : illustrations ; 30 cm
Abstract
The use of vegetation to stabilise slopes from shallow failure has been
increasingly attractive from the environmentally friendly perspective. Previous
research mainly focused on mechanical effect (i.e., root reinforcement) of vegetation
on slope stability, while the hydrological effect (i.e., water uptake (or transpiration)
induced negative pore-water pressure (PWP)) is less studied.
The objective of this research is to investigate effects of vegetation on slope
stability. To achieve this, a novel technique capable of modelling both mechanical and
hydrological effects of vegetation in geotechnical centrifuge was developed. The
technique involves using a branch cutting to model the plant root to which vacuum is
delivered to generate a pressure gradient. This enables water to be...[
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The use of vegetation to stabilise slopes from shallow failure has been
increasingly attractive from the environmentally friendly perspective. Previous
research mainly focused on mechanical effect (i.e., root reinforcement) of vegetation
on slope stability, while the hydrological effect (i.e., water uptake (or transpiration)
induced negative pore-water pressure (PWP)) is less studied.
The objective of this research is to investigate effects of vegetation on slope
stability. To achieve this, a novel technique capable of modelling both mechanical and
hydrological effects of vegetation in geotechnical centrifuge was developed. The
technique involves using a branch cutting to model the plant root to which vacuum is
delivered to generate a pressure gradient. This enables water to be extracted from the
soil into the branch cutting and thus a negative PWP to be induced in the surrounding
soil. This technique was implemented in two series of centrifuge tests (rainfall test
and rising g-level test) to investigate: (i) retention characteristics of the induced
negative PWP subjected to the infiltration of rainfall with different durations; (ii)
effect of antecedent water uptake duration on wetting response of the induced
negative PWP; and (iii) failure characteristics of vegetated slopes. Transient seepage
analyses were carried out to further interpret results obtained from centrifuge tests.
It is found that negative PWP can be induced by the newly developed technique
and the PWP distribution is reasonably consistent with field measurements of
vegetated slopes. After a one-hour 108 mm/hr rainfall (20-year return period), at least
-5 kPa PWP can be uniformly maintained within 2.5 times root depth. Given that
rainfall duration, there exists a threshold rainfall intensity (135 mm/hr), higher than
which would not help in further destroying the negative PWP as the soil has reached
its infiltration capacity. Once the rainfall duration further extends, however, the
negative PWP is destroyed significantly as more water is infiltrated in. On the other
hand, regardless of antecedent drying durations, the effect of water uptake can reach
2.5 times root depth. While upon rainfall with an extended duration (i.e., 2.5 hrs), long
pre-drying duration does not contribute to maintaining the negative PWP, which
implies that hydrological effect of vegetation vanishes. Stability analysis of the model
slopes shown that compared with the mechanical contribution of vegetation to factor
of safety, hydrological effect (once negative PWP maintained) is more pronounced.
Moreover, due to the presence of negative PWP, critical slip surface goes deeper.
Unlike the conventionally assumed pullout failure in literature in assessing the
stability of vegetated slopes, failure was observed in the rising g-level test while the
roots almost remain intact. The failure initiated at slope toe, and slip surfaces were
generally shallow and parallel to slope surface. Due to the shallow slip surface depth
(30-45% of root depth), the confining stresses around roots in the sliding zone are low
and do not provide enough frictional force to pull the embedded roots out.
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