How do slopes become unstable

Soil with coarse textures and low levels of organic carbon, consequently, generated greater lightness values than in soils with finer textures and higher contents of organic matter. The analysis used in this study confirms the multivariate relationships and the outcomes pointing to this direction. Finally, in relation to soil organic carbon content effects on lightness, this research had discovered the same relationship found by Konen et al.

This could be caused by the joint migration of the clay and Iron formed in the slope soils. The higher duration of the annual dry period at altitudes of less than m, higher temperatures with the consequent rising in dehydration of the Iron forms, could be the factors of this greater reddening [ 13 ]. This result is also consistent with the finding through the Munsell Soil Colour Chart that indicates the hues for overall samples were YR Yellow-Red which were influenced by high concentration of Iron oxides in studied area.

Iron oxides can reflect the surrounding of the environments in which they are formed and are considered as colouring agents for most of slope soil samples. Curi [ 12 ] stated that in the soil systems, hematite, goethite and probably maghemite which are classified under Iron oxides are the main pigmenting agents in influencing soil colour.

The hues that indicate stable slope are between 2. From the findings it can be concluded that the yellowish colour for most of the unstable slope soil samples are caused by a yellow to brown iron oxide mineral called goethite.

Generally, these soils have lower iron contents extracted by the sulphuric acid digestion than the other. That occurs either because the parent material had a low iron content or because iron was removed from the soil by percolating water.

Due to the yellower colour, it is relatively easy to distinguish the horizons for instance the red colour dominance for most of the stable slope soil samples are due to hematite and a dark red due to iron oxide. The content of iron oxides extracted by Non-ferric Red Oxisols are quite variable in texture, which ranges from medium to very clayey.

The parent material for these soils is very variable and ranges from sandstones to pelitic rocks, with the major requirement being relatively high iron content. In addition, with respect to the overall soil samples, the conditions of slope are essential in influencing the chroma values in such a way that the stable slopes has a slightly high chromatic value than the unstable slopes.

As was stated before, this is interrelated to the larger amount of Iron formed in the stable slopes which might be caused by modification and illuviation of slope structure. This means that the colour of the texture becomes more uniform as the contents of these minerals increases, which may be attributable to the processes of reduction and enlargement.

Regarding to the relationship between soil properties, the study had identified that soil texture, total organic carbon TOC , Iron oxide and Aluminium concentration were strongly interrelated with soil colour variables at the studied areas.

It is also recommended that in order to explain and detect the colour of slope soils, the function and availability of lightness-darkness as an analytical factor should be highlighted, together with the amount of Iron oxides. The correlative relationships between chromaticity variables and soil erosion suggest that all these properties may potentially be used as an indicator of slope failure.

These findings should be used to trigger further investigation of the reasons or sources for the failure of the slope soil and an assessment made of the potential risks to humans or the environment if the failure continues. Through this study it showed that the weakening of the slope soil properties occurred mostly due to erosion effect towards the existing soil properties.

Consequently, serious attention should be emphasised on each slope along the highways particularly the unstable slopes in order to reduce harmful effects. Most of the landslides occurred during the rainy days when the soil is relatively wet. It would require special preventing strategies such as slope levelling, terracing and practicing in planting suitable vegetation in slope areas.

Vegetation and slope stability are interrelated by the ability of the plant life growing on slopes to both promote and hinder the stability of the slope.

The relationship is a complex combination of the type of soil, the rainfall regime, the plant species present, the slope aspect, and the steepness of the slope. Any study of soil properties should take serious attention towards any vegetation above the slope area as this factor is crucial in influencing the loss of several nutrients. Planting vegetation will increase the organic carbon in soil thus the ions of organic carbon will bind with ion in clay and hydrogen in soil.

These reactions will strengthen the soil structure. Knowledge of the underlying slope stability as a function of the soil type, its age, horizon development, compaction, and other impacts are the major underlying aspect of understanding how vegetation can alter the stability of the slope.

Our study did take note of vegetation, but for future studies, a more thorough study with regard to the vegetation of the areas in conjunction with certain soil properties would be interesting to be highlighted.

The research findings showed that unstable slope was more likely to occur if there is no plant life growing on the top of soil. The less vegetation growing in the soil the more likely that erosion will happened.

Vegetation can protect the soil from the impact of the rain and slows down the infiltration process. Plants with deeper roots are better at holding the soil together and protect it from erosion. But the mass of rock continued up the walls of the valley and buried them. The avalanche killed people. Changes in Hydrologic Characteristics - heavy rains can saturate regolith reducing grain to grain contact and reducing the angle of repose, thus triggering a mass-wasting event. Heavy rains can also saturate rock and increase its weight.

Changes in the groundwater system can increase or decrease fluid pressure in rock and also trigger mass-wasting events. The valley runs along the bottom of a geologic structure called a syncline, wherein rocks have been folded downward and dip into the valley from both sides see cross section below. The rocks are mostly limestones, but some are intricately interbedded with sands and clays. These sand and clay layers form bedding planes that parallel the syncline structure, dipping steeply into the valley from both sides.

Fracture systems in the rocks run parallel to the bedding planes and perpendicular to bedding planes. The latter fractures had formed as a result of glacial erosion which had relieved pressure on the rocks that had formed deeper in the Earth.

Some of the limestone units have caverns that have been dissolved in the rock due to chemical weathering by groundwater. Furthermore, the dam site was built near an old fault system.

During August and September, , heavy rains drenched the area adding weight to the rocks above the dam. On October 9, at P. The slide mass was 1. As the slide moved into the reservoir it displaced the water, forcing it meters above the dam and into the village of Casso on the northern side of the valley.

Subsequent waves swept up to meters above the dam. Although the dam did not fail, the water rushing over the dam swept into the villages of Longorone and T. Vaiont, killing 2, people. Waves also swept up the reservoir where they first bounced off the northern shore, then back toward the Pineda Peninsula, and then back up the valley slamming into San Martino and killing another people.

The debris slide had moved along the clay layers that parallel the bedding planes in the northern wall of the valley.

A combination of factors was responsible for the slide. First filling of the reservoir had increased fluid pressure in the pore spaces and fractures of the rock. Second, the heavy rains had also increased fluid pressure and also increased the weight of the rock above the slide surface. After the slide event, parts of the reservoir were filled up to m above the former water level, and even though the dam did not fail, it became totally useless.

This event is often referred to as the world's worst dam disaster. In this area the rocks have been folded into a synclinal structure with rock layers dipping gently toward the Pacific Ocean. Rocks near the surface consist of volcanic ash that has been altered by chemical weathering to an expanding type clay called bentonite.

Below these altered ash layers are shales that are interbedded with other thin volcanic ash layers that have been similarly altered to bentonite clay. The area had the appearance of an earth flow, with a very hummocky topography with many enclosed basins filled with lakes.

Prior to the s the area had been used for farming. In the s demand for ocean views led to the development of the area as an upscale suburb. But, no sewer system was available, so wastes were put into the ground via septic tanks. In the area began moving down slope toward the ocean. Rates of movement were fastest several months after the end of the winter rainy season and slowest during the summer dry season.

In the next three years the earthflow moved as much as 20 meters, but in the processes the expensive homes built on the flow became uninhabitable. Movement was caused by a combination of wave erosion along the coast removing some the mass resisting flow, added water due to the disposal of wastes, watering of lawns, and rainfall causing the bentonite clays to expand and weaken, and by the added weight of development on top of the flow.

A landslide, therefore, is a process that changes a slope from a less stable to a more stable state. No subsequent movement will occur until changes take place which, once again, affect the balance of opposing forces see Figure 2. In general the resisting strength of material decreases as the clay content rises. Clay slopes, therefore, are particularly prone to landsliding. Slides also often occur on slopes developed in a combination of impermeable fissured clays overlain by massive, well-jointed cap rocks of limestone or sandstone.

The ultimate cause of all landsliding is the downward pull of gravity. The stress imposed by gravity is resisted by the strength of the material. A stable slope is one where the resisting forces are greater than the destabilising stresses and, therefore, can be considered to have a margin of stability. By contrast, a slope at the point of failure has no margin of stability, for the resisting and destabilising forces are approximately equal.

The quantitative comparison of these opposing forces gives rise to a ratio known as the "factor of safety" F :. The Factor of Safety of a slope at the point of failure is 1.

On slopes of similar materials, progressively higher values represent more and more stable situations with greater margins of stability. In other words, the higher the value the greater the ability of the slope to accommodate change before failure occurs.

These changes are usually divided, for the sake of convenience, into internal and external groups. External changes increase the stress placed on slope-forming materials, while internal changes reduce or weaken their resistance to movement. The majority of landslides are, therefore, the product of changing circumstances or alterations to the status quo High-Point Rendel.

The shear strength of a material depends upon both the nature of the material itself and the presence of water in fissures and pores. A slope is only as strong as its weakest horizon, often a clay. Clays such as the Gault Clay which contributes to landslip problems in several British Study Areas are known as brittle materials because once they have been subject to more than the maximum stress they can withstand and have failed, further displacements are possible at lower levels of stress.

In other words the shear strength of the clay declines from a peak value to a lower residual value. Water content has a major influence on reducing shear strength, not because of lubrication, as is often stated, but due to the fact that water in the ground exerts its own pressure which serves to reduce the amount of particle to particle contact. Within saturated horizons the pore-water, therefore, bears part of the load by exerting an upthrust or buoyancy effect known as pore-water pressure.

Although soil or rock particles can resist both normal and tangential shearing forces, fluids can support compression forces but cannot resist shearing forces. Therefore, friction or resistance to movement depends on the difference between the applied normal stress and the pore-water pressure. This difference, or the part of the normal stress which is effective in generating shear resistance, is known as the effective stress.

Reactivated failures in which movement occurs along pre-existing shear surfaces where the materials are at residual strength. The importance of this distinction is that once a slide has occurred it can be made to move under conditions that the slope, prior to failure, could have resisted. In other words reactivations can be triggered much more readily than first time failures. As slope movements are the result of changes which upset the balance between resistance and destabilisation, the stability of a slope is often described in terms of its ability to withstand potential changes see Figure 2.

Stable ; when the margin of stability is sufficiently high to withstand all transient forces in the short to medium term excluding excessive alteration by human activity;. Marginally stable ; where the balance of force is such that the slope will fail at some time in the future in response to transient forces attaining a certain level of activity; and.

This perspective makes it possible to recognise that the work of destabilising influences can be apportioned between two categories of factors on the basis of their role in promoting slope failure. Preparatory factors which work to make the slope increasingly susceptible to failure without actually initiating it ie cause the slope to move from a stable state to a marginally stable state , eventually resulting in a relatively low Factor of Safety;.

Triggering factors which actually initiate movement, ie shift the slope from a marginally stable state to an actively unstable state. When considering the actual cause of landsliding this relative simplicity gives way to complexity as there is a great diversity of cause or factors. In broad terms, however, they can be sub-divided into internal causes which leads to a reduction in shear strength and external causes which leads to an increase in shear stress Figure 2.

It is not uncommon for ground movements to be caused by anthropogenic influences. In this way, the overloading of a slope by buildings and embankments, excavations without protection mechanisms on a slope during construction works, the raising of the level of groundwater, dynamiting, the inappropriate use of primary material or unsuitable allocation of land can all increase the ground movement hazard.

Anthropogenic effects can also contribute to long term destabilisation of the slope, in relation to other activities like deforestation, insufficient forest management, over-grazing, intensive exploitation and denuding of the land. Many classifications of ground movements have been proposed based on criteria such as the mechanism of the movements, the composition of the materials, the speed of the processes or the mechanisms of release.

The fall process starts with the disaggregation of rocky or loose material on a steep slope along the length of a surface on which only a few detached movements have developed. The flow process for example earth flow results from the continuous movement of a superficial ground area quickly leaving the detachment zone, in a compact way to start with, but not generally keeping this compact character.

The distribution of speeds within the moving masse is similar to that of a viscous flow. These principal types of movements, including those that often produce progressive transition mechanisms, can result in different forms of failure see Figure 2. In the case of falls the displaced material, which detaches itself from the bedrock dependant on the discontinuity surfaces dip, schistosity, fissures or fractures , travels most of its distance in the air.

The phenomena can be classified in three categories: stone and rock falls, falls in the strict sense and collapses. In general, these can be subdivided into three areas: detachment zone; transition zone and accumulation zone.

This process is the main feature of the continual degradation of a rocky cliff, determined by geological conditions, exposure and weathering. It is only possible to estimate the volume of rocky material that is potentially at risk of falling through detailed studies of the rock.

When describing stone or block movements it is useful to distinguish between the rebound and rotation phases. Forests play a very important role in that the kinetic energy of most blocks is greatly reduced by their impact against the trees. During a fall a large volume of rock, breaking up quite intensely, detaches itself from the bedrock in a block and falls. In exceptional cases, considerably larger volumes can fall. In practice, detailed studies of the bedrock, involving a deep analysis of the spatial orientation of discontinuity surfaces, are required in order to estimate the volume of rock presenting a potential risk of falling.

See Plate 2t. The initial mechanism can be explained, for example, by the development of an inclined slide surface. The trigger for collapse is determined by the topography and also by the interaction between the components of the collapsed mass and their intense fragmentation. Because of the great volume involved, collapses can permanently change the landscape.

These enormous collapsed masses often form natural barriers in mountain valleys, obstructing watercourses and creating a dam; if there is a catastrophic breach of the barrage, there is a possibility of flooding for the regions downstream.

In the case of a topple there is a forward rotational element in the detaching rock mass. The rock mass is usually found to be leaning forward in it's original position and on failure rolls forward breaking up as it travels downslope.

The shearing of layers occurs when the tips of inclined or even sub-vertical rock layers topple down slope under the effect of gravity.

Internal variations in the composition and structure of rocks can significantly affect their strength. Schist, for example, may have layers that are rich in sheet silicates mica or chlorite and these will tend to be weaker than other layers. Some minerals tend to be more susceptible to weathering than others, and the weathered products are commonly quite weak e.

The side of Johnson Peak that failed in Hope Slide is made up of chlorite schist metamorphosed sea-floor basalt that has feldspar-bearing sills within it they are evident within the inset area of Figure The foliation and the sills are parallel to the steep slope.

The schist is relatively weak to begin with, and the feldspar in the sills, which has been altered to clay, makes it even weaker. Unconsolidated sediments are generally weaker than sedimentary rocks because they are not cemented and, in most cases, have not been significantly compressed by overlying materials.

This binding property of sediment is sometimes referred to as cohesion. Sand and silt tend to be particularly weak, clay is generally a little stronger, and sand mixed with clay can be stronger still. The deposits that make up the cliffs at Point Grey in Vancouver include sand, silt, and clay overlain by sand. As shown in Figure Glacial till — typically a mixture of clay, silt, sand, gravel, and larger clasts — forms and is compressed beneath tens to thousands of metres of glacial ice so it can be as strong as some sedimentary rock Figure Apart from the type of material on a slope, the amount of water that the material contains is the most important factor controlling its strength.

This is especially true for unconsolidated materials, like those shown in Figure Granular sediments, like the sand at Point Grey, have lots of spaces between the grains. Those spaces may be completely dry filled only with air ; or moist often meaning that some spaces are water filled, some grains have a film of water around them, and small amounts of water are present where grains are touching each other ; or completely saturated Figure Unconsolidated sediments tend to be strongest when they are moist because the small amounts of water at the grain boundaries hold the grains together with surface tension.

Dry sediments are held together only by the friction between grains, and if they are well sorted or well rounded, or both, that cohesion is weak. Saturated sediments tend to be the weakest of all because the large amount of water actually pushes the grains apart, reducing the mount friction between grains.

This is especially true if the water is under pressure.