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Chapter 5 - Mass Wasting (Landslides).docx

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Rob Milne

Chapter 5: Mass Wasting The Frank Slide: • April 29 1903 – tragic landslide in Frank Alberta. • Atleast 76 people died • People evacuated but soon returned to mining until 1911 when the government forced people to abandon for fear that the mountain was not stable. • Example of a “rock avalanche” – landslide involving sudden failure of a large mass of rock that rapidly fragments and travels as a streaming mass at high speeds. Contributing Factors • Cause – internal or external factor that, over time, reduces the stability of a slope and brings it to the point of failure. • Trigger – an event that sets off the landslide – final straw 1. Geology – water infiltrates and slowly dissolves the carbonates 2. Glaciations – Glacial erosion steepened the overall slope of the mountain, reducing its strength. 3. Mining – mining of coal at the base of the mountain may have reduced the stability of the limestones higher on the mountain. 4. Weather – could have been the final straw. The region experienced heavy snowfalls and unusually warm temperatures in April. Water put pressure on rock. Lessons Learned 1. Large landslides cannot be prevented – difficult to monitor as well 2. Geology is important – underlying cause of most landslides. The type of rock, the dip direction, the presence of faults and the degree of fracturing of rock. 3. Human activity can trigger landslides – coal mining in this case. Or irrigation or landscaping. 5.1 Introduction to Landslides Mass wasting and landslide – downslope movements of rock or sediment as a result of gravity. It can be slow or fast and can involve small or large volumes of sediment or rock. Types of Landslides There are four variables that underpin most landslide classifications 1. Mechanism of the movement (fall, topple, slide, flow, or complex movement) 2. Type of material (rock, consolidated sediment, or organic soil) 3. Amount of water present 4. Rate of movement • Fall – bounding of rock or blocks of sediment from the face of the cliff • Slide – downslope movement of a coherent block of rock or sediment along a discrete failure plane. o Slump – failure plane is curved upwards • Flow – slow to rapid downslope movement of sediment in which particles move semi- independently of one another, commonly with the aid of water o Debris flows – mixtures of mud, debris, and water. o Creep – very slow downslope movement of rock and soil.  Sackung – huge slow moving landslides.  Topple – slow creep like movement in which a rock mass pivots about a point • 1965 Hope Slide - January 1965 – a huge mass of rock slid away from Johnston Peak (BC). Raced down the slop and reached the valley floor killing 4 motorists • Rock Avalanches – reach high speeds, travel long distances, override large obstacles, deadly. o 1970 Peru – earthquake lead to an avalanche which destroyed the city of Yungay and claimed 8000 lives. • Subaqueous (underwater) landslides – complex events which occur when a slump or slide on the submerged slope of a delta of at the edge of the continental shelf can change into a debris flow or a turbidity current that travels great distances from the point of failure. The mass can rapidly transform into a turbid flow or mud, sand, and water an then travel hundreds of kms along the seafloor. 5.1 Case Study – Estimating the Velocity of Landslides from Their Run­up and Superelevation • Superelevation refers to the tendency of some flows to rise higher on the outside of a bend in their paths than on the inside. • Pandemonium Creek Landslide – travelled 9km from a steep rock face to the Knot Lakes. • Velocity of the debris as it ran up the valley was v^2 = 2gh where v = velocity, h = max vertical runup g = gravitational acceleration. Forces on Slopes • Slope stability can be determined by: o Driving Forces – move rock or sediment down a slope and resisting forces – oppose such movements. Resisting force is the shear strength of the slop material • Factor of safety (FS) – how slope stability is evaluated. It is the ratio of the resisting forces to the driving forces – if the factor of safety is less than or equal to 1, the resisting forces exceed the driving forces and the slope is considered stable. If the FS is equal to 1, the driving forces equal the resisting and a slope failure can be expected. • driving and resisting forces can be determined by: o type of material, slope and topography, climate, vegetation, water, time Role of Material Type • Material composing a slope can affect both the type and frequency of landslides. Mineral composition, degree of cementation or consolidation, and the presence of planes of weakness. • Rotational and Translational – rotational have curved slip surfaces whereas translational slides have planar slip surfaces. • Common type of translational slide is a debris avalanche – shallow slide of sediment or soil over bedrock. o The failure plane is generally either at the base of the organic soil or in colluvium, a mixture of weathered rock and other debris below the soil. The Role of Slope and Topography • Slope stability is strongly influenced by slope and topography, more specifically by slope steepness and topographic relief. • Topographic relief refers to the height of a hill or mountain above the land below. Areas of high relief are generally prone to landslides. The Role of Climate • Climate is the characteristic weather typical of a place or region over years or decades. o Climate influences the amount and timing of water that infiltrates or erodes a hillslope and the type and abundance of hillslope vegetation. The Role of Vegetation 1) Vegetation provides a protective cover that reduces the impact of falling rain. 2) Plant roots add strength and cohesion to slope materials 3) Vegetation adds weight to the slope, which could increase the likelihood that slope will fail. The Role of Water 1) Many landslides happen during rainstorms when slope materials become saturated 2) Other landslides develop months or even years following deep infiltration of water into a slope 3) Erosion of the toe of the slope by a stream reduces the mass of resisting material and decreases the slopes stability. • Water also contributes to the liquefaction of fine granular sediments. When disturbed, water saturated silts and sands can lose their strength and flow as a liquid. • Spontaneous liquefaction of Leda Clay in the St. Lawrence Lowland in Southern Quebec and Ontario triggered large landslides that destroyed many homes and killed more than 70 people in the 20 century. • Freezing of water in fractures in rock can destabilize slopes and trigger rock falls. • Water is also implicated in shallow landslides – thaw flow slides – also called permafrost. This happens when the water starts defrosting from the winter. The Role of Time • Slope failure may occur without an obvious trigger when the resisting forces finally fall below the driving forces. 5.2 Geographic Regions at Risk from Landslides • Mountainous areas • Western Cordillera of BC, Yukon, Alberta, and the Appalachian provinces of Quebec and New Brunswick. • In the US: mountainous areas of the west coast, the rocky mountains, Alaska range, appalachain mountains. • 3 factors that increase incidence of landslides: o Urbanization and development will increasingly expand into landslide prone areas o Tree cutting
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