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 cm

60 cm

60 cm

20 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

Drying

Wetting

k drying

k wetting

ksat drying

ksat wetting

Volumetric water content

Mat

ric s

uctio

n (k

Pa)

Con

duct

ivity

k (

cm/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 (

kPa)

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 8.15

BC1TP1, 100-105 cm

2.14 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)