slope stability -mdh - u of s engineering stability... · w / h = embankment slope (slope angle the...
TRANSCRIPT
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LANDSLIDESLANDSLIDES
1.1. TopographyTopography
2.2. StratigraphyStratigraphy
3.3. Material PropertiesMaterial Properties
4.4. GroundwaterGroundwater
5.5. Slide MechanismSlide Mechanism
ELEMENTS OF SLOPE STABILITY ANALYSIS
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1. Material Properties ( φ and c’)2. Internal Stress (σ)3. Pore Pressure Conditions
Resistance to Sliding is a function of:
FRICTIONAL MODEL
FS =Resistance to sliding
Mobilizing Forces
Mobilizing Forces a function of:
1. Elevation Difference (Height)2. Slope Angle3. Weight of Material(s)
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SOILPARTICLES
PORESPACE
σ
External ForcesSOIL COMPOSITION
σ
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T
T
SOILPARTICLES
External Forces
Pore Space
RT
Friction
SOIL COMPOSITION
σ σ σ
σ
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T= R = σ tan φ the block slides
Points where T = R(for different values of σ)
φ
R = Frictional Resistance(Resistance to sliding)
T = Mobilizing Force(Gravity ??)
T
When T > R the block slides
FrictionalModel
Sliding ≡ Failure
σ = Normal Force
Tan φ = coef. of friction
SLIDING BLOCK EXPERIMENT(Dry Soil)
T1T2T3
σ1 σ2 σ3R
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σ
Tβ
Sliding takes placeWhen slope angle β = Φσtan β = σtan φ
Gravel Φ = 35o
Till Φ = 25o - 30o
Silt Φ = 25o
Clay Φ = 7o to 25o
i.e.. Natural angle of a dry slope is the friction angle Φ
TILTING PLATFORM EXPERIMENT(Dry Soil)
R
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T
T
External Pressure
SOILPARTICLES u
Hydraulic Pressure in Pore water
uPore Space Filled with
Water
SOIL COMPOSITION(Saturated soil)
σσ
Net Contact Force (σ-u)Positive – Pore Water Pressure
Positive pore water pressure reduced friction!
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σ’ = (σ - u) = effective stress
T Ru
u
– u
T
TWhen T > R = (σ ) tan φ the block slides
Frictional Model with Water
σ
σ
σ
SLIDING BLOCK EXPERIMENT(Saturated Soil)
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β
Slope angle β is less than φbecause of hydraulic pressure
T u
TILTING BLOCK EXPERIMENT(Saturated Soil)
σ
Frictional Model with Water
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SERM 11
T = σ-(-u) tan ΦT
T
SOILPARTICLES
-u
External Force
-u
Suction in the pore water
Pore space filled with water
When T = R failure
Stress Effective in Mobilizing Friction = σ – (-u)
σ
σ
T R
-u
σ
σ
SLIDING BLOCK EXPERIMENT(Unsaturated Soil)
Frictional Model with Water
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Slope angle β may be greater than Φbecause of negative hydraulic pressure (-u)
β
T-u
TILING BLOCK EXPERIMENT(Unsaturated Soil)
σ
Frictional Model with Water
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COHESION
φ
T1T2T3
σ1 σ2 σ3c
T = R = c + (σ - u) tan φ - the block slides
Independent of normal stress (s)Theoretical bases controversial
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T
σ
φφR
Residual State
Strain
Stress
COHESION AND RESIDUAL STRENGTH
T = R = c + (σ - u) tan φ - the block slides
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LANDSLIDE TOPOGRAPHY(Landslides in clay)
Upper Scarp
Toe
Mud Wave
TensionCracks
Slip SurfaceSurface of weakness
Layer of clayHigh pore pressures
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H
Foundation
Dyke/Embankment
w
h
W
Piezom. Surface (Pressure Heads)H = Slope HeightW = Embankment Widthw / h = Embankment Slope(Slope angle the most important)
GEOMETRIC FACTORS AND SLOPE / FOUNDATION STABILITY
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Style A – Shallow Circular Slip
Embankment
Till
LANDSLIDE STYLES
Common for uniform clays or till in embankment and foundations
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Style B - Deep Seated Circular Slip
Embankment
Soft Clay
LANDSLIDE STYLES
Common for soft clay foundations supporting high dykes
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Style C – Deep Seated Composite Slip
Embankment
Soft Clay or Till
LANDSLIDE STYLES
TillSoft ClayHighly plastic
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Tension crack
Bulge
Tension crack
Seepage
Alignment stake
Embankment
SHALLOW SLIP – INITIAL STAGE
Foundation
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1.2 m woodstakes
Tensioncrack
Penmarkings
SHALLOW SLIP – INITIAL STAGE
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Dyke SlidingMass
Stratified Silt
Conventional Stability Model
HYDRAULIC PRESSURE IN CRACKS
Clay
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Dyke
Stability Model with Tension Crack
Fluid
HYDRAULIC PRESSURE IN CRACKS
Clay
Stratified SiltSlidingMass
HydrostaticForce from
Fluid in Crack
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Deep-seated slip
Shallow slip
Soft Clay
SLIPS ON EMBANKMENT SLOPES
Soft Clay
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2:1 SlopeRegina Clay
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Pond
Ditch
Actual slopeangle
Apparentlyflatter
slope angle
Pond
Ditch
Actual slopeangle
ApparentlySteeper
slope angle
DITCH ADJACENT TO DYKE
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Transcona Grain Elevator, near Winnipeg, ManitobaTranscona Grain Elevator, near Winnipeg, Manitoba
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Built in 1913Started filling with grain September 1913
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October 19, 191327° tilt toward the west
RAPID DYKE CONSTRUCTION
Example
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CHEMICALLY INDUCED CHANGES IN SOIL BEHAVIORExample Seepage under water retention dyke (Alex Man,
Jim Graham, Marolo Alfaro, Tee Boon Goh, 2004)
• Instability of dykes at a freshwater reservoir in Southern Manitoba have been occurring on an irregular basis along the length of the dikes
• It is unclear why some sections have become unstable while others have remained stable
• None of the instabilities at has resulted in an uncontrolled release of water
The Problem
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Water
Clay CoreRip-Rap
Upper Foundation96% - 99% ClayLower Foundation ~72% Clay
8 m
3 to 4 m
TYPICAL DYKE SECTION
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0
50
100
150
200
250
300
0 2 4 6 8 10 12 14 16Strain (%)
q (k
Pa)
200 kPa
200 kPa 2"
400 kPa
400 kPa 2"
500 kPa
Background Stable
CIŪ TRIAXIAL TEST RESULTS
Unstable
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CHEMISTRY vs. DEPTH
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New CentreLine
FILL
ExistingRoadway
FILL N
416
417
418
401
402
403
404
405
413
414
415
406
407
408
409
410
411
412
Pore Pressure Monitoring – Highway # 17 Example
South of Onion Lake, Sask.
RAPID DYKE/EMBANKMENT CONSTRUCTIONExample
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0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
Calculated factor of safety
B C
oeffi
cien
t
H = 20 m
H = 15 m
H = 10 m H = 7 m H = 5 m
Piezo Reading
(m)
Total Head u
(m)
∆ u (m)
Fill Elevation
(m)
Fill Height h
(m)∆ h
B Coefficient
5.29 -1.11 0.00 562.34 0.90 0.005.50 -0.90 0.21 562.34 0.90 0.005.64 -0.76 0.35 562.62 1.18 0.28 0.716.13 -0.27 0.84 563.27 1.83 0.93 0.506.06 -0.34 0.77 563.26 1.82 0.92 0.446.27 -0.13 0.98 563.26 1.82 0.92 0.55
0.00
Time
B c
oeffi
cien
t
560.0561.0562.0563.0564.0565.0
Elev
atio
n (m
)
0.0
1.0
0.5
RAPID DYKE/EMBANKMENT CONSTRUCTION
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DYKE REPAIR INITIATED FAILURE
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DYKE REPAIR INITIATED FAILURE
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Berm constructionBerm
FillDrain
STABILIZATION
Toe Drain constructionSeepage line no drain
Seepage line with drain
Drain
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Cutoff wallSTABILIZATION
Cutoff Wall
Trench drain constructionSeepage line no drain
Seepage line with drain
Drain
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Flatten slopeSTABILIZATION
Reduced downstreamslope
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Key Elements of StabilityKey Elements of Stability
1.1. Material StrengthMaterial Strength2.2. Hydraulic PressureHydraulic Pressure
Groundwater and/or Brine/ SeismicGroundwater and/or Brine/ Seismic3.3. Gravity (Slope Angles)Gravity (Slope Angles)
MANAGEMENT OF STABILITY
Key Field ObservationsKey Field Observations
•• CracksCracks•• Seepage on Dyke SlopeSeepage on Dyke Slope•• Alignment ChangesAlignment Changes
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