A study of mechanisms controlling the slope stability in South Gippsland, Victoria

Slope instability has proven to have significant impacts on safety and infrastructure. A wide variety of slope stabilisation methods have been purposed and utilised. In the study area of South Gippsland, located in the Victoria state in south-east Australia, the slope instability is one of the most common geotechnical issues where most slopes are traversed by the rural road network over an undulating landscape. To mitigate the negative impacts from the phenomenon of slope instability, two well-developed slope stabilisation methods, the geogrid-reinforced retaining wall with gabion basket wall facing and the laterally-loaded pile, are integrated as one long-term retaining structure utilised in the study area. These two methods are connected by a steel rail that is welded at the top of the protruding I-beam post embedded in the concrete pile to form a buttress to the lowest row of the gabion basket wall facing. This distinctive local practice has the advantages of providing a long-term support to one specific slope site where repeated failures happen; hence, it can reduce the ongoing engineering cost to the local government authority significantly. Moreover, the ease of the handling and transportation of gabion basket which can be developed from the local materials makes this distinctive integrated slope-stabilisation method more suitable for the rural area. For the gabion-faced geogrid-reinforced retaining wall and the pile retaining structure involved in the integrated method, the effect of the reinforcement properties and the dimension variables on the performance of the stabilised slope has been investigated in detail from previous studies. The threshold value of the critical design parameters, such as the embedded length of the geogrid of retaining walls and the embedded length of the pile, has also been determined. However, as two independent methods integrated in one intact structure, the change in the properties of one separate structure not only influences the behaviour of the slope but affects the performance of the other structure. In addition, the existence of one structure already alters the behaviour of the other structure. This coupled effect between two methods makes the understanding of reinforcement mechanisms of this distinctive structure less straightforward. The hydraulic factors with respect to the rainfall condition and the groundwater condition are disadvantageous to the stability of the slope in the study area based on the previous investigations. In Australia’s wettest 24-month period between 2010 and 2012, over 100 slope failures occurred in the study area and most of them have been treated appropriately by the integrated method. The predicted annual mean rainfall precipitation in the study area shows an increasing trend and the tendency of the extreme rainfall events with short duration and high intensity also increases, therefore, the performance and the response of the integrated method stabilised slopes under aforementioned climate conditions needs a comprehensive investigation. This thesis will enhance the insight for the integrated slope-stabilisation method from the aforementioned aspects. The Finite Element Method is implemented to capture the behaviour of the slope and the reinforcement under various conditions. With the assistance of the Strength Reduction Technique, the stability of the slope has also been evaluated. A series of the typical published numerical examples are reproduced throughout the thesis for the purpose of the validation of the adopted method for the simulation under specific conditions. Among others, the deflections of the wall-facing, the critical slip surface, and the behaviour of the geogrid and the embedded pile are considered and studied. When the simulation conducted with the consideration of the fluid-solid coupling, the hydraulic characteristics of the unsaturated soil, the changes in pore-water pressure, saturation degree, and the matric suction are also considered. In addition, the slope configurations with different design parameters adopted not only for the comparison purpose but also for the consideration of the variety in the slope site conditions. The numerical study indicates that under the effect of the gravity, the stability of the gabion-faced geogrid-reinforced retaining wall stabilised slope can be improved significantly from the additional contribution of the embedded pile. More shear strength of the soil is also mobilised by the installation of the pile as indicated by the redistribution of the critical slip surface. When the uniform surcharge load imposes on the surface of the backfill of the embankment, the integrated method stabilised slope still yields satisfactory performance, good bearing capacity of the integrated method thus can be demonstrated. While the negative impact from the hydraulic factors with respect to the groundwater table conditions and the rainfall infiltration on the stability of the natural slope has been investigated comprehensively, the role of the road embankment infrastructure and the slope stabilisation work under the negative impact of the hydraulic factors has received less attention in the literature. This thesis will remedy the gap in this research field to a certain extent. The steady seepage analysis has been conducted to study the effect of various groundwater table conditions on the behaviour of the slope and the reinforcement. The numerical results indicate that with the growth of the groundwater table level, the negative impacts impose on the stability of the slope is increasing, whereas the associated drainage system of the retaining wall based on the Australian Standards can mitigate these negative impacts effectively by reserving the strength of the geogrid and the embedded pile, and the matric suction distributed in the reinforced soil zone. Moreover, the finite element analysis of transient water flow through the variably saturated soil of the integrated method stabilised slope under various rainfall conditions has been conducted. It is shown that the rainfall intensity imposes a significant influence on the stability, road edge deflections, and the matric suction distribution of the reinforced slope. The failure mechanism of the slope also exists different modes subjected to various rainfall intensities. Three typical rainfall patterns derived from the Australian climate condition influence the stability and the matric suction distribution of the integrated method stabilised slope to various degrees, and the most critical rainfall pattern has also been identified.