1. El Salvador, Jan. 2001

2. 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

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 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?

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 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

16. 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

17. 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

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) CaCO 3 + H 2 CO 3  Ca +2 + 2 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

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.