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 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 surface2. 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

SinkholeFormation

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 BlvdThe 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 movementMass movement resulting from expansive soil.