Presentation on theme: "MASS MOVEMENTS. IMPORTANT CONCEPT: ROLE OF GRAVITY Gravity causes the downward and outward movement of landslides and the collapse of subsiding ground."— Presentation transcript:
IMPORTANT CONCEPT: ROLE OF GRAVITY Gravity causes the downward and outward movement of landslides and the collapse of subsiding ground. Eventually it will flatten all slopes. The force of gravity is the mass of a body x the sine of the slope. If the initial resistance to motion is removed, the body will move. Earthquake and heavy rain could give initial energy.
IMPORTANT CONCEPT: CREEP Movement down slope of soil and uppermost bedrock Most commonly seen by its effects on telegraph poles, fences and trees. The soil zone slips in ultra slow movement as particles shift in response to gravity. Block diagram showing the effects of creep.
IMPORTANT CONCEPT: CREEP How creep works Surface materials expand perpendicular to slope (1 to 2) as a result of freezing, wetting or heat of sun. Upon thawing,drying or cooling they contract (2 to 3) under pull of gravity vertically. The surface materials do not go back to original position. Thus have a slow movement down slope i.e. creep
IMPORTANT CONCEPT: LANDSLIDE Fast moving mass-movement Causes most fatalities. Landslide is a mass whose center has moved downwards and outwards. Has a tear-away zone upslope where material has pulled away and a pile-up zone where material had accumulated.
IMPORTANT CONCEPT: LANDSLIDE Major topographic features Features include crown, head scarp, basal surface of rupture, transverse cracks, transverse ridges and radial cracks. All created in the downward and outward movement
IMPORTANT CONCEPT: EXTERNAL PROCESSES CAUSING FAILURE On arcuate failure surfaces have balance between the driving mass and the resisting mass. Changing either can create a landslide Processes include: 1) steepen slope, 2) remove support from bottom of slope, and 3) add mass high up on slope.
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE FAILURE Clay forms during chemical weathering due to acidic fluids such as water, CO 2 charged water and organic acids decomposing minerals created at high pressures and temperatures. Clay has totally different internal structure. Clay minerals are built like books and have many unfilled atomic positions in the crystal structure. Typically, clay can have their strength dramatically reduced by adding water which also causes expansion.
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE FAILURE Fine grained rock flour left behind during the retreat of the glaciers and deposited in a nearby sea. The clay and silt particles are loosely packed and held together as a rock by sea salts. When the sea retreats, the sediments are uplifted and the glue removed by fresh water. Anything can cause the house of cards to collapse Quick Clay: Ontario, Canada 1993
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE FAILURE (The five roles of water) 1)Sediments have high porosities. When these void spaces are filled with water the sediment is much heavier and the driving mass increased. 2)Water is easily absorbed and attached externally to clay minerals with a major decrease in strength. 3)Water flowing through rocks can dissolve the minerals that bind the rocks together. The removal of the cement makes the rock easier to move or a slope easier to collapse. 4)Water can physically erode loose material creating caverns. 5)Pressure builds up in water trapped in the pores of sediments being buried deeper and deeper. Sediments can compress but water does not compress. Get abnormally high pore-water pressures which “jacks up” the sediment and makes it very easy to move.
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE FAILURE (The role of flowing water) Schematic cross section of ground water flowing through poorly consolidated rock. The water will carry sediments to the stream creating a series of caverns that seriously weaken a hill.
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE FAILURE (Slope stability) Addressed by use of Coulomb/Terazaghi equation where s = c + (p - h w ) tan ø Where s = resistance due to shear, c = the cohesion of the sediment layer p = load of sediment and water above a slide surface h w = weight of water above the potential surface. F = internal angle of friction. Strength comes from cohesion + the weight of the sediment. Weakness from the pore water pressure and the internaL angle of friction. Clays have high cohesion but a very low failure angle. Sands have poor cohesion. Granites have very high failure angle.
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE FAILURE (Quick sand) Example of the Coulomb-Terazaghi equation. The pore water pressure h w equals the weight of the sands p. Leaves cohesionless sand with no shear stress. With no shear stress you will sink into the sand when you walk on it.
IMPORTANT CONCEPT: ADVERSE STRUCTURES 1)Ancient slip surfaces are weaknesses that tend to be reused over time. These surfaces are especially slippery when wet. 2) The orientation of the sediment layers can create strong or weak conditions. Sediment layers dipping into the hill are very stable, dipping in the same direction but shallower than the slope have daylight bedding. Potentially dangerous condition. 3) Rocks have inherent weakness that set-up slope failure. Lack of cement, clay layers, soft rocks, splitting joints, faulting surfaces. TRIGGERS Basic causes bring slopes close to failure: Rain, earthquakes or humans
MASS MOVEMENTS CLASSIFICATION (Speed of movement and water flow) On left have mass movement speed versus moisture content. On right have rates of travel for mass movements
MASS MOVEMENTS CLASSIFICATION (Falls, Flows, Slides and Subsides) Falls and subsides involve vertical drops. Slides and flows involve downward and outward motion. Sliding involves a coherent mass. Flowing involves the moving mass behaving like a viscous fluid.
Most common mass wasting types Avalanche Slump Creep Flow Rockslide
Slumps Movement of coherent block of material along a curved surface. More likely to occur when slope is undercut from below, material is saturated. Common on river banks where stream erosion oversteepens banks. Debris flows, earth flows, mud flows Flows are movements in which the material deforms chaotically as it moves. Debris flows -- contains large boulders, gravels Earth flows -- sandy material Mudflow -- mud with considerable water
Deposits of Mass Wasting Mass wasting produces sediment deposits. These deposits commonly contain a wide range of sizes of particles (they are "unsorted"). The deposits do not show layering (they are "unstratified")
How do we prevent landslides and mass wasting? This is NOT possible. A better question is “how do we lessen the effects” of mass wasting? 1. Remove weight from slope 2. Engineering controls 3. Vegetation and Geofabric TM 4. Cables and anchoring systems 5. Tunnels built over highways thus reducing weight