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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 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? Engineering Properties of Soils: Worked Example of Soil Shear Strength USGS Experimental Debris-Flow Flume Show flume video Vaiont Worked Example – Estimating debris flow speed from the scar and deposit 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 Carbonates and Landforms in Fla. Sinkholes Sinkholes • May be natural or man made • Water table – High • Dissolution • Vertical flow • Pressure change – Low • Decreases support SINKHOLE DAMAGE Sinkholes OrlandoWinter Park, FL Sinkholes West central Florida sinkholes Sinkhole Formation 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 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 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 Sinkhole Map Sinkholes • Detection Methods – Drilling – Ground penetrating radar – Resistivity Sinkholes 12/22/04: Deltona's Howell Blvd The Informed Volusian Sinkholes Caves • Common in karstic terranes • Mostly form by the dissolution of limestone • Mechanisms – Acidic meteoric and groundwater (most caves) • CaCO3 + H2CO3Ca+2 + 2 HCO3-1 – Biological Activity (more recent idea) • H2S + 2O2 + Biological activity H2SO4 • H2SO4 + CaCO3 + 2H2O CaSO4•2H2O + HCO3-1 Crystals in Cave Gypsum Crystals: Selenite (variety) Size: 20 X 50 feet Other Subsidence In Florida • Oxidation of Histosols Perception of the Landslide Hazard • Negligible • Residents are not easily swayed by technical information Cause of Portuguese Bend Landslide Portuguese Bend Oft reported mass movement Mass movement resulting from expansive soil.