El Salvador, Jan. 2001

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

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 Needs to be 1.25 or greater Calculations show that about 50 m 3 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 Orlando- Winter Park, FL

Sinkholes West central Florida sinkholes

Sinkhole Formation

Sinkholes Types of sinkholes –Solution or subsidence sinkholeSolution 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 sinkholesCover-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 sinkholesCover 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) CaCO 3 + H 2 CO 3  Ca HCO 3 -1 –Biological Activity (more recent idea) H 2 S + 2O 2 + Biological activity  H 2 SO 4 H 2 SO 4 + CaCO 3 + 2H 2 O  CaSO 4 2H 2 O + HCO 3 -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.