Unsaturated soil slopes are often encountered in most of the Civil
Engineering infrastructure such as retaining walls, dams, road
embankments and mining pits. Rapid population and economic growth in
hilly urban areas such as Hong Kong and Northern Taiwan and the
recurrent rainfall-induced landslide problem have greatly increased the
attention given to slope stability problems. Yet, unsaturated soil theory is
not commonly used in geotechnical practice. Direct measurement of
unsaturated soil parameters is time consuming and expensive, there is also
a resistance to change established design methods and a lack of practical
examples incorporating unsaturated soil theory in routine geotechnical
practice. While it is acknowledged that numerous studies have been
conducted on unsaturated slope stability, most of these studies have
considered individual cases with a limited range of parameters.
This study conducted a series of numerical parametric studies on
unsaturated silty slopes based on regional slope and hydrological
parameters considering no infiltration conditions, 1D long-term infiltration and 2D transient infiltration to narrow down on parameters of engineering
importance and identify the parameter range where suction strength
significantly affects ???. Given the design factor of safety of slopes under
normal conditions in Taiwan is 1.5, slope cases of engineering interest
were considered to have ??? between 1.0 and 1.5 after considering the
effects of rainfall infiltration.
Firstly, a numerical study on unsaturated silty slopes under no infiltration
conditions was conducted. The numerical model was first validated using
two cases, one was a hypothetical case, the other a small scale landslide
case. In the latter, the use of pedo-transfer functions was demonstrated as
an alternative to obtaining unsaturated soil parameters. For the parametric
study, the water table was identified as the main controlling parameter of
the effects of suction strength on slope stability. By comparing the ???
between cases with suction and without suction, it was observed that the
suction effects were most significant for slopes with ? ≤ 30 m and water
table below the mid-slope range, ??⁄? ≤ 0.5, with a ??? % difference
≥ 5 %.
Secondly, the effects of 1D steady rainfall infiltration on unsaturated silty
slopes were examined to give insight on what slope conditions the matric
suction can still exist in the long-term condition. Practical procedures were
proposed on the slope conditions where the suction strength significantly
influenced the slope long-term stability. The borderline silt/clay slopes
possessed significant apparent cohesion for a broad variety of parameters
including infiltration rate, ?⁄?? = 0 - 0.5, water table, ??⁄? ≤ 0.3, slope
height, ? = 10 - 30 m. Borderline sand/silt slopes possessed significant
apparent cohesion over a narrower parameter range, ?⁄?? = 0.3 - 0.6, water
table, ??⁄? ≤ 0.0, slope height, ? = 10 m.
Thirdly, a review of common geotechnical software on numerical analyses
of unsaturated slope case studies under two-dimensional transient rainfall
infiltration effects was carried out. The aim was to highlight the software
capability to analyze typical rainfall-related slope problems and provide
useful information to the users on important factors that may affect the
software output. Distinct coupling approaches, failure modes and failure
search criteria caused significant differences in the initial stages of rainfall
infiltration. At critical conditions, most software programs yielded similar
output having ??? differences ≤ 20 % . The ??
largely determined the
failure time and hydrological response of unsaturated slopes to rainfall
infiltration.
The final section consisted of a parametric study considering the effects of
two-dimensional transient rainfall infiltration on unsaturated silty slopes
based on regional slope and hydrological parameters. Generally, the main
rainfall-induced failure mechanisms were observed to be wetting front
advancement, loss in suction and a rise in the water table, which reconfirms
previous findings in literature. This study went a step further to establish
hydraulic parameter boundaries and link the slope parameter selection to
the expected failure mechanism. Based on this study's findings, practical
suggestions are given for slope design, monitoring and disaster mitigation
in geotechnical practice.

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