Slopes - Balance between: •FORCE (EXOGENETIC) •RESISTANCE (ENDOGENETIC) STRESS > STRENGTH = MASS MOVEMENT FORCES Slope Failure is due to: 1.Increase in Stress (i.e. the forces attempting to pull a mass downslope) 2.Reduction in Internal Resistance (Shear Strength) Factors contributing to a High Shear Stress 1. Removal of lateral support Fluvial or glacial erosion Wave action Faulting Weathering: frost action > weakening rocks Mass Movement events Man-made quarries and pits 2. Overloading Weight or rain, snow, talus (scree), vegetation 3. Transitory Stresses Earthquakes, movement of trees in wind Factors contributing to a High Shear Stress 4. Removal of underlying support Undercutting by rivers, waves Weathering – eg sub surface solution Mining 5. Lateral Pressure Freezing of water Swelling of Clay (Hydration) Dilitation WEAK RESISTANCE Factors contributing to a Low Shear Strength 1. Compostition and texture Weak materials - eg Sedimentary layers Loosely packed materials 2. Weathering Effects Disintegration of granular rocks Hydration of Clay minerals Dissolution of cementing materials in rock / soil 3. Changes in porewater pressure Saturated pores > push grains apart > decreases friction due to reduction in cohesion Factors contributing to a Low Shear Strength 4. Changes in Structure Spontaneous liquefaction – eg in Clays 5. Vegetation Removal of trees Removing tree roots OCR 7.3.11 “Slope Processes” Mass Movement Types 1. Heave (Wetting and Drying / Freezing and Thawing) 2. Flow (Material does not retain its form) 1. Soil Creep 2. Solifluction •Increasing Speed 3. Earthflow / Debris Flow •Increased Water Content 4. Mudflow 3. Slides 1. Landslide 2. Rotational Slump 3. Debris / Rock Avalanche 4. Rockfall 1. Sheetwash •When the volume of water is too high for the infiltration capacity of the soil - the water flows over the soil surface. Sheetwash is a very fast ‘Flow’ event •Case Study Example > Lynmouth Flood, 1953 2. Rills and Gullies •Small river Sheetwash. channels resulting from •Small ephemeral channels (seasonal temporaray - do not flow all year round) / •Caused by a significant increase in precipitation - I.e. an extreme rainfall event •As the water table rises - the water flows on the surface of the soil - as the regolith has become saturated 3. Heave •A slow form of mass movement •Occurs due to periods of: •Wetting and Drying - eg ‘swelling of clay minerals’ •Freezing and Thawing •Hence favours Cold, Wet conditions - eg Tundra Environments - eg White Mountains, Alaska •A significant form of mass movement in environments where flows and slides are not common Terracettes (the small terraces) on the grassy side of the Frenchman River valley in Grasslands National Park near Val Marie, Saskatchewan. Often due to Creep 4. Mudflow •A type of Flow - involving a significant amount of water •Very rapid - flowing at over 1km/hr •Similar to Earthflows - except they are thinner in consistency - a higher water content •Where are they likely to occur? •In mountainous areas after heavy rain •In periglacial areas at the time of maximum summer thaw •Jokulhlaups – eg Southern Iceland, 1995 •On slopes of erupting volcanoes 5. Debris Flow / Earthflow •A type of “Flow” •Occurs on slopes 5-15 Degrees •Rates between 1mm/day > 1m/ sec. •Produces Flow Tracks •Thicker in consistency than mudflows > but having a lower water content •Common in humid areas - where deep regolith The Aberfan Disaster 21 October 1966 The Slumgullion earth flow in the San Juan Mountains of south-western Colorado. This landscape in Franciscan rocks of the California Coast ranges (USA) is primarily eroded by earthflows 6. Gravity Falls •Extremely fast form of mass movement •Weathering Limited Slope The speed at which the slope develops over time is only ‘limited by the rate at which that slope weathers - as the mechanism of transport (eg via gravity) is always available to transport (or erode) weathered material - eg a Free-Face •Lines of weakness occur along the joints in the rocks •Case Study Example : Wast Water Screes, Lake District Rockfall - showing material (Talus) that fell from the cliffs above 7. Rotational Slumps / Rotational Slip •A type of Slide •Rotational movement along a curved / arcuate slip plane •Often found in areas of mixed geology •Eg a permeable chalk layer overlying an impermeable clay layer. •The permeable chalk layer can become saturated, heavier and the stress on the slope outweighs the strain > leading to a slide •The build up of water at the base of the chalk layer also acts as a lubricant, reducing the friction > promoting a Slide •Eg Vaiont Dam, Langarone Italy 1966 Police Point landslide occurred in May, 1967 in the valley of battle Creek in the Cypress Hills of south-eastern Alberta. The upper part of the landslide consist of a series of rotated slump blocks. Rotational failure and earthflow in shaly rocks in the California Coast ranges (USA) 8. Solifluction •A Type of Flow> material therefore does not retain its form •Common in Cold Climates - eg White Mountains,Alaska •Can occur on slopes with shallow gradients •Regolith - becomes saturated - eg due to: •High precipitation •Seasonal melting of snow •Often forms Solifluction Lobes - eg Cheddar Gorge •NB: Where this process occurs in Periglacial Areas - I.e. due to the Seasonal melting of permafrost layer over a frozen subsoil = Gelifluction 9. Landslide / Debris Slide / Landslip / Rockslide •A Type of “Slide” •Where the entire mass of material moves along a Slip Plane therefore tends to keep its shape (as opposed to a flow) •Often in places where there is active undercutting at the foot of slopes •Endogenetic factors important: •Eg Amount and degree of jointing in the rock •Strength of the Rock •Angle of slope •Water lubricates these joints / planes > reducing the strength of the slope •Speeds vary between 1mm/day > 10m / sec Debris avalanche tracks eroded into mountainous slopes in the Alaska Range in Alaska ending in debris cones. The original steep relief was created by glacial scour. Frost weathering weakens the bedrock, triggering the debris avalanches which scour the avalanche chutes. A rockslide in the Front Ranges of the Rocky Mountains in south-western Alberta.