Group members : Wesley Teo, Tan Yi Ting, Chiang Wen Jun, Jonathan Chua
Hello! Welcome to our blog.
Posted on this blog are the various mitigation strategies used to prevent the hazards of mass movements with regard to slopes.
Please take some time to look through them! =)
Sheet Cover
In this picture, the hazards of mass movement are mitigated with the covering of the slope using a large sheet waterproof material, often made from canvas or plastic. This helps to prevent landslides, flows and heaves. In this case, the slope may be exceptionally vulnerable to slope failure and may therefore require the placement of such a material over it.
The removal of this mechanism of mass movement is essential to hazard mitigation because water adds weight, lubricates slope materials and increases pore water pressure.
Water and sand or soil grains are drawn together by capillary attraction. This attraction is a result of surface tension where exposed surface of liquids contract to the smallest possible area. If too much water comes in contact with a slope and its materials, the soil becomes saturated and may turn into a slurry that can flow down the slope.
Another important aspect of the mechanism of water in the study of this particular slope is that of the reduction of shear strength. Water can reduce shear strength in 6 ways. However, for this slope only a few are relevant. Firstly, it can reduce the natural cohesiveness as water will push the soil clumps or sand grains further apart and reduce friction. It can also add weight by filling the pores of sedimentary rocks which are highly porous. Lastly, it can also dissolve materials and break down the materials that bind the rock together.
The sheet can also prevent rainsplash and raindrop impact, which has the capacity to erode soil, sand and fine regolith, leading to the mass movement process of soil creep.
With the sheet cover in place, water from precipitation will fall on the sheet and slide down to the bottom of the slope where there is a drain that will channel the water away. This allows for the slope to remain relatively dry, especially during times of heavy thunderstorms or extended periods of rain.
This form of mitigation is also essential because much of the roads and highways had to be constructed through an area with exceptionally hilly and undulated terrain. As such, most of the slopes still exist and may have also become weakened during the course of the construction because of the removal of materials that may have been supporting the slopes.
Done By : Wesley Teo, 2AD2
Vegetation
This picture is taken in genting highlands.The picture shows how terracing and vegetation can help prevent mass movement. The terracing allows the steep slope to be more stable by lowering the gradient to produce more stable angles. The vegetation on the terraces provides anchoring effects, and prevents the soil from being unconsolidated by binding together soil particles and holding soil to bedrock with their roots, this helps to prevent the initial stages of soil erosion.
Vegetation also absorbs water, decreasing the water saturation limit of the float material. However, vegetation can also destabilize slopes. Vegetation exposed to the wind transmits dynamic forces into the slope it also increases infiltration and thus may increase pore pressure.
Done By : Tan Yi Ting, 2AD2
Terracing
Terracing regrades the slope to produce more stable angles. This prevents particles from sliding downslope under the influence of gravity. There are also drains at the foot of every terrace to collect any loose particles that may still slide down the slope. This reduces the rate of soil erosion very significantly, because without this measure, soil particles would be able to slide down the slope very rapidly, which might lead to loss of life and property if the landslide occurs near populated areas.
The process of terracing is done in 5 steps as follows:
1. Drainage
This increases the shear strength of the materials by reducing the pore-water pressure.
2. Terracing and drainage
Regrading of the slope to produce more stable angles.
3. Loading the toe and retaining walls
Material deposited at the slope foot(toe) reduces the shear stress. Retaining walls are used to stabilise the upper-slope. In this case a steel-mesh curtain is used.
4. Stabilisation by retaining wall and anchoring
The toe is stabilised by a retaining wall. The upper slope has rock anchors and mesh curtain. Drains improve water movement and shotcrete is used to reduce infiltration into the hillside.
5. Toe stabilisation and hazard-resistant design
The toe is stabilised by gablons and earth fill. The railway line is protected by a hazard-resistant design structure.
Done By : Chiang Wen Jun, 2AD2
Slope Drainage
In this picture, the hazards of mass movement are mitigated with drainage lines within the cliff face, field drains, gravel trenches and by intercepting overland flow. Shallow surface drains are used to intercept the overland flow while vertical drainage is employed to remove the water from the cliff face as well as the body of the cliff.
Some of the mass movement processes operating on cohesive materials occur over very long time spans. One of the most widespread of these processes is soil creep. Soil creep involves the movement of slope sediments in a series of numerous cyclical. The cyclical effects of temperature fluctuations, variations in moisture, and gravity on inclined soil sediments often cause this process.
Soil creep can only occur on a slope. It occurs when some mechanism causes the surface soil layer to expand and contract. Mechanisms that can cause this expansion and contraction include cyclical changes in soil temperature and moisture. Expansion moves soil particles upward and perpendicular to the angle of the slope. When contraction occurs the soil particles move downward under the influence of gravity. As depicted in the animation, the angle of this movement differs from the angle of the movement due to expansion. The net result is a slow downslope movement of the slope's materials. Thus, the drainage prevents this from occurring by removing water from the cliff face.
The drainage is especially useful in clay cliffs which are susceptible to slumping and rotational slides which are triggered by high water content in the clay. The method deals with the main factor of cliff-face failures – the action of water – and therefore it is one of the most effective way of strengthening the cliff.
However if successful, changes in the cliff hydrology can have impacts on the ecology and land uses of the cliff top. Successful drainage schemes may also result in subsidence of cliff-top land as the cliff dries out. Implementation of a comprehensive drainage network sometimes requires a considerable level of technical know-how and expertise.
Done By : Jonathan Chua, 2AD2
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