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El Salvador, Jan. 2001

El Salvador, Jan. 2001. How can Factor of Safety (FS) Change?. Safety Factor(FS) FS =1.25 ; minimum for slope FS = 10; minimum for a structure Change of FS Time Erosion Water. FS = RF/DF. Finish Factor of Safety Example from last lecture. FS = SLT/ W sine 

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El Salvador, Jan. 2001

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  1. El Salvador, Jan. 2001

  2. How can Factor of Safety (FS) Change? • Safety Factor(FS) • FS =1.25; minimum for slope • FS = 10; minimum for a structure • Change of FS • Time • Erosion • Water FS = RF/DF

  3. Finish Factor of Safety Example from last lecture FS = SLT/ W sine  S = shear strength of clay L = length of the slope T = unit thickness W sine  = driving force Using numbers in text, the SF is only 1.125. Needs to be 1.25 or greater Calculations show that about 50 m3 must be removed for FS = 1.25 Note Assumptions: 1. Planar Slip surface 2. Not considering effect of fluid pressure – what is it?

  4. Engineering Properties of Soils: Worked Example of Soil Shear Strength

  5. USGS Experimental Debris-Flow Flume Show flume video

  6. Vaiont

  7. Worked Example – Estimating debris flow speed from the scar and deposit

  8. Subsidence • Causes • Withdrawal of fluids (gas, oil, water) • Collapse of surface soil and near-surface rocks • Karst topography (Kars Mts., Slovenia) • Result from dissolution • Limestone or dolomite • Gypsum and salt

  9. Carbonates and Landforms in Fla. - Sinkholes

  10. Sinkholes • May be natural or man made • Water table • High • Dissolution • Vertical flow • Pressure change • Low • Decreases support

  11. SINKHOLE DAMAGE

  12. Sinkholes Orlando- Winter Park, FL

  13. Sinkholes West central Florida sinkholes

  14. Sinkhole Formation

  15. Sinkholes • Types of sinkholes • Solution or subsidence sinkhole • Occur where limestone is exposed at or near the surface • Covered by soil and insolubles as the depression forms • Usually not large • Bowl shaped: steepness determined by rate of subsidence

  16. Sinkholes • Types of sinkholes • Cover-subsidence sinkholes • Cover is mainly non-cohesive, permeable sand • Develop by gradual subsidence • Generally only a few feet in diameter and depth • Generally remain small, cavities are filled as fast as they develop

  17. Sinkholes • Types of sinkholes • Cover collapse sinkholes • Clayey, cohesive overburdens can bridge LS cavities • Changes in water table level (pumping effect) weakens the bridging overburden, resulting in collapse • Lack of support on falling water table • Upward pressure in the case of a rising water table

  18. Sinkhole Map

  19. Sinkholes • Detection Methods • Drilling • Ground penetrating radar • Resistivity

  20. Sinkholes 12/22/04: Deltona's Howell Blvd The Informed Volusian

  21. Sinkholes

  22. Caves • Common in karstic terranes • Mostly form by the dissolution of limestone • Mechanisms • Acidic meteoric and groundwater (most caves) • CaCO3 + H2CO3Ca+2 + 2 HCO3-1 • Biological Activity (more recent idea) • H2S + 2O2 + Biological activity H2SO4 • H2SO4 + CaCO3 + 2H2O CaSO4•2H2O + HCO3-1

  23. Crystals in Cave Gypsum Crystals: Selenite (variety) Size: 20 X 50 feet

  24. Other Subsidence In Florida Oxidation of Histosols

  25. Perception of the Landslide Hazard • Negligible • Residents are not easily swayed by technical information

  26. Cause of Portuguese Bend Landslide

  27. Portuguese Bend Oft reported mass movement Mass movement resulting from expansive soil.

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