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Landslides, a Recurrent Phenomenon

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Landslides, a Recurrent Phenomenon Landslides, a recurrent phenomenon by George D. BATHRELLOS & Hariklia D. SKILODIMOU What is a Landslide? Landslide is a general term used to describe the down-slope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such a movement (Highland & Bobrowsky, 2008). Gravity affects all objects on Earth. From the largest mountains to the smallest grains of sand, gravity pulls everything in a direction toward the center of the planet (Nagel, 2004). Mass movements are the result of shear stresses on slopes (caused by gravity, the weight of material and soil water) overcoming the inherent resistance of the materials (made up of the cohesive properties of slope particles and their internal friction) (Hart, 1986). In general, the scientists use many terms worldwide for this phenomenon, such as "landslide", "landslide events", "mass movements", "gravity movements" and "slope movements" (Terzaghi, 1950; Coates, 1977; Varnes, 1978). What does Landslides cause? Gravity The main force which is responsible for mass movement is the gravity. Gravity is the force that acts everywhere on the Earth's surface, pulling everything toward the center of the Earth. On a flat surface the force of gravity acts downward. So long as the material remains on the flat surface (Fig. 1) it will not move under the force of gravity (Nelson, 2013). Fig. 1: Influence of gravity on an object to a horizontal surface (modified from Nelson, 2013). On a slope, the force of gravity can be decomposed into two components (Fig. 2): a component acting parallel to the slope and a component acting tangential to the slope (Nelson, 2013). Fig. 2: Influence of gravity on an object on an inclined level (modified from Nelson, 2013) • The vertical component of gravity (gp) helps to hold the object in place on the slope. The parallel to the slope component of gravity (gt) causes a shear stress, which pulls the object in the down-slope direction parallel to the slope. • On a steeper slope, the shear stress or parallel to the slope component of gravity, (gt) increases, and the vertical component of gravity (gp) decreases. • The forces resisting movement down the slope are grouped under the term shear strength, which includes frictional resistance and cohesion among the particles of the object. • When the shear stress becomes greater than the combination of forces holding the object on the slope, the object should move down-slope. • Otherwise if the object consists of a collection of materials like soil, clay, sand, etc., if the shear stress becomes greater than the cohesion forces holding the particles together, the particles will separate and move or flow down-slope (Nelson, 2013). Thus, down-slope movement is favored by steeper slope angles (BUT, be careful, not the very steep slope angles more than 60o), that increase the shear stress, and anything that reduces the shear strength, such as lowering the cohesion among the particles or lowering the frictional resistance. But this is often expressed as the safety factor (Fs) the ratio of shear strength to shear stress. Fs = Shear Strength / Shear Stress Shear strength consists of the forces holding the material on the slope and could include friction, and the cohesion forces that hold the rock or soil together. If the safety factor becomes less than 1.0, slope failure is expected (Nelson, 2013). Water Water becomes an important factor in mass movement processes, as the medium of transport, for several reasons: 1. Addition of water from rainfall or snow melt adds weight to the slope. Water can permeate into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. Weight is force, and force is stress divided by area (expanse), so the stress increases and this can lead to slope instability (Nelson, 2013). 2. Water has the ability to change the angle of repose, which is the stable angle for the slope (Nelson, 2013). • Dry unconsolidated grains will form a pile with a slope angle determined by the angle of repose (Fig. 3). The angle of repose is the steepest angle at which a pile of unconsolidated grains remains stable, and is controlled by the frictional contact between the grains. In general, for dry materials the angle of repose increases with increasing grain size, but usually ranges about 30o - 45o. Fig. 3: Dry unconsolidated grains (modified from Nelson, 2013) • Slightly wet unconsolidated materials display a very high angle of repose (Fig. 4), because surface tension between the water and the solid grains tends to hold the grains in place. Fig. 4: Slightly wet unconsolidated materials (modified from Nelson, 2013) • When the material becomes saturated with water (Fig. 5), the angle of repose is reduced to small values and the material tends to flow like a fluid. This happens because the water gets between the grains and eliminates grain to grain frictional contact. Fig. 5: Slightly wet unconsolidated materials (modified from Nelson, 2013) 3. Water can dissolve the mineral cements that hold grains together. If the cement is made of calcite, gypsum, or halite, all of which are very soluble in water, water entering the soil can dissolve this cement and thus reduce the cohesion between the mineral grains (Nelson, 2013). 4. Liquefaction is occurred when loose sediment becomes oversaturated with water (Fig. 6) and individual grains loose grain to grain contact with one another as water gets between them. This can be occurred as a result of ground shaking, as in the case of earthquakes, or can be occurred when water is added as a result of heavy rainfall or melting of ice or snow. It can also be occurred gradually by slow infiltration of water into loose sediments and soils (Nelson, 2013). Fig. 6: Liquefaction (modified from Nelson, 2013) 5. Groundwater exists nearly everywhere beneath the surface of the earth. It is water that fills the pore spaces between grains in rock or soil or fills fractures in the rock. The water table is the surface that separates the saturated zone below, wherein all pore space is filled with water from the unsaturated zone above. Changes in the level of the water table is occurred due changes in rainfall. The water table tends to rise during wet seasons when more water infiltrates into the system, and falls during dry seasons when less water infiltrates (Fig. 7). Such changes in the level of the water table can have effects on the factors discussed above (Nelson, 2013). Changes in the level of the water table (modified from Nelson, 2013) Fig. 7: Other parameters Many other factors, that cause mass movements (landslides), are reported in the scientific literature (e.g. Bathrellos et al., 2009; Rozos et al., 2011). Some parameters are: • Lithology. Lithology is one of the most decisive parameters regarding the landslide manifestation. In Greece the landslides density higher percentage is attributed to Cyclothematic formations (Plio- Pleistocene fine-grained sediments and Flysch formations). • Land uses. The variation of the vegetation in an area is a parameter that seriously affects the slope failures, as slope stability is very sensitive in changes on vegetation. • Tectonic elements. Tectonic activity increases landslide events by creating steep slopes and sheared, weakened rocks. • Slope angle, • Slope aspect. The angle and the aspect of the slopes play a very important role in the manifestation of the landslides because they express the result of the combined influence of many agents. • Geometry of main discontinuities. The geometry of the main discontinuities in relation to slope geometry (aspect) is strongly related to the stability of hard soils, and soft rocks. • Anthropogenic activities (e.g. roads). The artificial and natural parts of the slopes around a road are more sensitive in landslide manifestation. • Hydrographic (drainage) network. The hydrographic axes continuously change the slopes of the rivers and can therefore be considered as one of the principal parameters in landslide manifestation. Triggering factors The usual factors which trigger landslides are intense rainfall, earthquakes and human activities (Bathrellos et al., 2009; Rozos et al., 2011; Chousianitis et al., 2016). Which are the mass movements (landslides) types? The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing (USGS, 2004). A great number of scientists have given the classification of landslides. The worldwide most used classification is that USGS (2004) and Varnes (1978) have proposed (Fig. 8). Fig. 8: Types of landslides (modified from USGS, 2004 & Varnes, 1978) According to Varnes (1978) the types of movements are: fall, topple, slide, spread and flow. Falls take place when rocks break off and material free-falls or bounds down to the base of a cliff. Topple is movement that is performed by the rotating of a unit or units around a point. In Slides cohesive blocks or material remain relatively intact, moving along a well-defined surface of sliding. Spread is lateral extension together with shear or tensile fractures. Flows are a fluid movement of loose earth material. Which are the parts of a landslide? Landslide features are presented in figure 9, with the commonly accepted terminology, according to USGS (2004) and Varnes (1978). Fig. 9: A graphic illustration of a landslide, describing its features (http://www.oas. org/dsd/publications/Unit/oea66e/p305.GIF). Recorded landslide phenomena in Hellas The affected geological formations from landslide phenomena in Hellas showed that flysch is the most critical prone geological formation while most landslide occurrences are mainly located in the western part of the country, where serious causing factors and triggering effects favor the manifestation of such phenomena on a broad scale (Koukis et al., 2005).
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