Hydro-Mechanical Properties and Stability Analysis of
Four Landslide-Prone Hillslopes in Western North Carolina
30 October 2013
York W. Lewis1, Alexandra Wayllace1, Jonathan W. Godt2, Richard M. Wooten3, and Ning Lu1
1Colorado School of Mines2U.S. Geological Survey3North Carolina Geological Survey
Objectives• Test Hydrological and Geotechnical properties
• Calculate Factor of Safety
• Determine pore-water pressures needed to cause slope failure.
Photos courtesy of NC Geological Survey
Site Locations
• Slope angles average 31 degrees.
Poplar CoveMooney Gap
Bent Creek
Maps courtesy of NC Geological Survey
Typical Soil Horizon Thicknesses
O
A
B
C
30 cm60 cm
60 cm20 cm
Classification: Silty Sands (SM), Silty Clays (SC)
Laboratory Tests• Index Properties for soil classification
• Direct Shear (unsaturated & saturated) for shear strength parameters
• Transient Release and Imbibition Method (TRIM) for soil-water retention, hydraulic conductivity, and suction stress functions
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.451E-1
1E+0
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1Bent Creek 1, 100-105cm Depth
DryingWettingk drying
k wettingksat dryingksat wetting
Volumetric water content
Mat
ric s
uctio
n (k
Pa)
Con
duct
ivity
k (c
m/s
)
Drying Wettingn=1.97 n=1.67a=0.14 kPa-1 a=0.16 kPa-1
qr=0.20 qr=0.20qs=0.42 qs=0.35ksat=5.8E-5 cm/s ksat=1.2E-6 cm/s
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.51E-1
1E+0
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6
1E-1
1E+0
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6Bent Creek 1, 100-105cm Depth
Suction drying
Suction wetting
Suction stress drying
Suction stress wetting
Volumetric water content
Mat
ric s
uctio
n (k
Pa)
Suc
tion
stre
ss (k
Pa)
Hydraulic Conductivity Function & Soil Water Retention Curve Suction Stress Characteristic Curve
& Soil Water Retention Curve
Test Results
Infinite-Slope Stability Analysis
ɸ’ = Effective friction angle (degrees)β = Slope angle (degrees)c’ = Effective cohesion (kPa)ss= Suction stress (kPa) g = Unit Weight (kN/m3)z = Vertical depth (m)
(Lu and Godt, WRR, 2008)
Sample In situ Factor of Safety
Pore Water Pressure (kPa) for FOS = 1
MG1TP1, 115-125cm
2.11 6.7
MG4Tp1, 70-80 cm
2.34 7.7
PC2TP1, 50-65 cm 2.72 8.15BC1TP1, 100-105
cm2.14 13.5
Conclusions• Stability calculations indicate modestly stable slopes under field conditions.
• Results show that rainfall events need only increase pore water pressures by 6.7 kPa to 13.5 kPa in order to cause landslides in the region.
• Use of improved hydrologic/geotechnical testing methods can quantitatively assess regional landslides.
Acknowledgement: NASA (Grant # NNX12AO19G)