slope stability -mdh - u of s engineering stability... · w / h = embankment slope (slope angle the...

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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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