GEL 105 -- Lecture Notes Feb. 16-18

GEL 105 -- Lecture Notes: Mass Wasting -- Feb. 16-18, 2010 Slide 1 - The only thing that landslides cause are tsunamis Unstable slope underwater creates mass displacement of water (only in oceans) Slide 2 - Natural hazards are increasing because population has increased Regolith: loose particles such as soils and sediments (includes vegetation associated with soils) Snow avalanches, mudslides, etc are all landslides Creep - mass wasting; not usually seen but still causes massive structural damage Slide 3 - Notice how many deaths are caused by landslides caused by other natural hazards, especially earthquakes, floods, and volcanic eruptions Slide 4 - Slopes are most stable at the angle of repose or at less than the angle of repose Geology - what is slope made out of? Angle of repose: maximum angle material can have and still be stable; depends on material being stacked; when angle exceeds this, mass wasting occurs Slide 6 - Cohesion = friction + compression External supports: man-made structures Gp = perpendicular Slide 7 - When gravitational pull is bad Slide 8 - Vector: arrow showing direction and magnitude ----------> vs. -----> : 1st vector is greater because it's longer (greater magnitude) g = gravity (direct pull towards the center of the earth gp = gravity to hold you in place perpendicular to slope; adds to shear strength gt = transverse or tangent; parallel to slope; opposes shear strength (adds to shear stress) Slide 9 - Slopes with underlaying rock parallel to slope are significantly less stable than slopes with underlying rock perpendicular to slope Sedimentary rocks are formed in layers Strength of rocks is determined by the angle of the slope in relation to the angle of repose and the dipping of the rock layers (parallel v. perpendicular) Slide 10 - Water reduces friction and helps create smooth movement of sediment Moving water creates erosion of rock layers which weakens layers above the erosion (undercutting) Slide 11 - To maintain angle of repose, the base of the material will widen Water creates surface tension when not completely saturating the material Over-saturation leads to less friction, smaller angle of repose Slide 12 - Direct relationship between water pressure and and the amount of water above the point Slide 13 - Vegetation that creates weakened slopes are rare, usually found in CA High topography (changes in elevation) + fire = mudflows/landslides after rainfall Slide 15 - Liquefaction doesn't occur in solid rock Slide 16 - When sand is found above and below rock layers, it weakens the rocks in between those layers Slide 18 - Magma moving underground will heat the rocks above it, causing expansion and subsequent earthquakes Lahar: mudflow associated specifically with volcanic eruptions Slide 19 - Mount St. Helens is the only time that a landslide has triggered a volcano Slide 20 - Sand grains are so small that you can't see or feel them as individual grains When disturbed by human processes, quick clay can be activated Slide 21 - Always happens in water, which allows for creation of random orientation Only requires pressure to rearrange particles Slide 23 - Problems with this picture that will cause mass wasting: Dipping is same direction as slope; water added ground from pool and septic systems; noching: space between house and rock layer is greater than the angle of repose Slide 24 - Overloading: placing too much weight on top of rock face that is unstable Overloading + Rock dipping + Waves cutting through cliff = unstable Slide 25 - Rockies have a rain shadow that creates less water erosion Appalachians - rock type + rock layering + lots of weathering = less stable western side Rockies more consolidated rocks (newer rock) Appalachians more eroded, lots of soil and vegetation, chemical weathering from mountaintop removal mining Slide 27 - Type of movement creates different categories/classes of landslides Rapid = landslide Slow = long-range effects, heavy damage sustained over long period of time Slide 28 - Creeps are SLOW Mudflows and debris flows are faster, fall into a range of speeds Flows look like a "pouring" of debris/mud down a slope Slide 29 - Retaining walls - flexing outward and broken Bent trees Leaning man-made structures (poles, tombstones) ** Be able to identify all of these as signs of creep ** Slide 30 - Occurs on ANY degree of slope Water underground freezes and expands, then thaws and contracts All slopes creep except those that are entirely SOLID rock (bedrock) Slide 31 - Be able to notice same signs of creep (trees, structural damage) Bent trees occur when creep pulls sediment and vegetation down a hillside but trees are still trying to grow towards the sun Slide 33 - Very small materials moved downhill by water; typically faster than debris flows Slide 34 - Can contain larger boulders and poorly sorted rocks/sedimentary materials Less water = slower, very viscous Viscosity: resistance to flow Tend to caues more damage than mudflows because of large materials present Slide 35 - Fastest; only resistance is friction of material with air as rocks enter free fall Usually caused by frost WEDGING Slide 36 - Water penetrates cracks in rocks to wedge rocks up Talus: rock material fallen from rock face during a rock fall; degree of slope is angle of repose Most common in areas with seasonal climate change Slide 37 - Range of motion dependent on amount of water present 2 main types of slides: slump and rock slide Main difference: slump is curved and rock slide occurs on planar surfaces Slide 38 - Scarp: area left on rock face when land slumps down Slump block: entire section of land that slumps down Most materials stay intact throughout the slide Slide 39 - Notice how exposed scarp is curved Slide 40 - Typically faster than slump Scarps are not curved Caused by water undercutting Rock layers are parallel to slope Entire sections of rock detach from the slope and slide down Slide 42 - Rates of movement are measured relative to cm/sec Debris avalanches include snow avalanches Slide 43 - Slope reduction: reducing the angle of the slope (usually through terracing) Retention structures include rock bolts Slide 45 - Terracing (making steps) More stable, but no slope is completely stable Slide 47 - Netting used only for rock falls Netting captures rocks but doesn't keep rock falls from occuring Slide 48 - Rock bolts and netting are usually used together Slide 50 - Creep still present even in reversed slopes (where rock is perpendicular to slope direction) Slide 51 - Tunnels prevent rocks from falling on cars in sloped areas prone to rock falls, but doesn't stop mass wasting from occurring Samantha Schleich GEL 105 -- Lecture Notes Mass Wasting