slope stability assessment - g&p geotechnics sdn. · pdf file ·...

Slope Stability Assessment

By Ir. Liew Shaw Shong

G&P Geotechnics Sdn Bhd

One-Day Short Course on Slope Engineering, Promenade Hotel at Kota Kinabalu on 30 June 2010


Contents

• Overview

• Chap. 1: Failure Mechanisms (Contributory & Triggering)

• Chap. 2: Uncertainties & Assumptions

• Chap. 3: Slope Mass Properties

• Chap. 4: Methodology & Analysis

• Chap. 5: Factor of Safety

• Chap. 6: Hazard & Risk Assessments

2

Failure Mechanisms

Contributory & Triggering factors

Analytical Approaches :

• Deterministic approach

• Probabilistic approach

Natural slopes & Engineered slopes

Hazard & Risk

Overview on Slope Stability Assessment

3One-Day Short Course on Slope Engineering, Promenade Hotel at Kota Kinabalu on 30 June 2010

Contents

• Overview








Chap 1 Failure Mechanisms

• Landslide: Any perceptible down slope movement of rock or soil

• Necessary Condition for Slope Instability

– Gravitational force (Disturbing Force)

• Sufficient Conditions for Slope Instability

– Reduction of Slope Strength (Resistance) – Water!

– Additional disturbing forces (surcharge, groundwater drawdown)

– Change of slope geometry (steepening)

• FOS ≥ 1 (Stable) or FOS < 1 (Unstable)

FOS =


Types of Landslides

(Nelson 2004)

Types of Movement

Falls

Topples

Slides

Rotational

Translational

Lateral Spreads

Flows (Slow Creep .vs. High Mobility)

Complex (Combination above)


Classification of Mass WastingT

ype

of M

ovem

ent

Flo

wS

lide

Fal

lClassification

Creep

Earth Flow

Mudflow

Avalanche

Rotational Slide

Rock Slide

Debris Fall

Rockfall

MaterialDebris

Debris

Saturated Debris

Debris or Rock

Debris

Bedrock

Debris

Bedrock

VelocityImperceptibly Slow

Slope and Material Dependent <5 km/hr

Very Fast 100 km/hr

Slow-mod. (short)

Fast

Fast

Fast

Natural Factors for Landslides

• Geological Structures

• Weak or sensitive materials • Weathered materials• Sheared, jointed, or fissured materials• Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth)

• Contrast in permeability and/or stiffness of materials (Perched water regime)


Natural Factors for Landslides

• Morphological Features

• Tectonic or volcanic uplift• Glacial/erosion rebound• Fluvial, wave, or glacial erosion of slope toe or lateral margins

• Subterranean erosion (solution, piping) • Deposition loading slope or its crest • Vegetation removal (by fire, drought)• Thawing Freeze-and-thaw weathering• Shrink-and-swell weathering


Summary of Landslide Factors

Triggering Mechanisms• Intense Rain-Fall

• Water-Level Change

• Ground Water Flow

• Human activity

• Rapid Snowmelt

• Volcanic Eruption (Mt. St. Helen, USA)

• Earthquake Shaking (921 Chi-Chi Earthquake, Taiwan)


Mt. St. Helen (USA)


Earthquake Induced Landslide


Shape of Slip Surface

Structurally Controlled 3D Blocks

•Planar

•Rigid Wedge Block

•Toppling

2D Plane Strain Failure

•Circular Failure

•Wedge Failure

Channelised Flow


Distressed area

Channelised Flow – Technically challenging

Buildings

Modeling Debris Runout Modeling Debris Runout & Mobility& Mobility

Hydrological & Hydrogeological Cycles

(Nelson 2004) 15One-Day Short Course on Slope Engineering, Promenade Hotel at Kota Kinabalu on 30 June 2010

Hydrological & Hydrogeological Cycles

(Nelson 2004)

Retention

Precipitation

Transp

iratio

n

Evaporation

Evaporation

Underground Seepage

Surface Runoff

Infiltration/Percolation

Saturated

Unsaturated


Effect of Water on Soils

Phreatic Surface

Vadose Zone

Saturation Zone

Degree of Saturation

Porewater Pressure

-u 0 +u

0 100%

Physics in Vadose Zone


`Hygroscopic

Zone


Physics in Capillary Action


u : -ve


Dry sand grains will form a pile. The slope angle is determined by the angle of repose (i.e., the steepest angle at which a pile of unconsolidated grains remains stable - controlled by the frictional contact between the grains. It usually lies between about 30 and 37 degrees.

(Nelson 2004)

Dry sand

Angle of repose

Grain-to-grain frictional contact



Slightly wet soil materials exhibit a very high angle of repose because surface tension between the water and the grains tends to hold the grains in place.

Wet sand

Angle of repose

Surface tension thin film



When the material becomes saturated, the strength may reduce significantly and it may tend to flow… water (between the grains) eliminates grain to grain frictional contact.

Angle of repose

Fully saturated sand

Water surrounds the grain and prevent grain-to-grain contact



Toe erosion undermining the slope mass


Tunneling/Piping Phenomena in the Vadose Zone (Jones, 1981)



Effective Stress and Strength

Before Rainfall After Rainfall

σe = σT – P σe = σT – (P+∆P) σT : Total Stress

P : Fluid pressure of groundwater (or soil water)

∆P : Positive pore pressure increment (suction reduction or

positive hydrostatic pressure)

σe : Effective stress (stress supported by the soil skeleton)

Note: fluid pressure is negative (less than atmospheric) if unsaturated and becomes positive when saturated

P+∆P

σT

P

σT

Slope Stability

Steps to perform Slope Stability Analysis

• understand the development & shape of slopes• understand the external effects (likes seismic loading, surcharge, saturation) on slope and embankments

• determine the short-term & long-term stability conditions & mode of operational conditions

• understand failure mechanisms & choose right stability analytical model

• evaluate the possibility of failure of natural or engineering slopes


Contents

• Overview








Chap 2 Uncertainties & Assumptions

• Idealisation & Simplification of Geological/Hydro-geological/Geotechnical models

– Subsurface conditions (Inadequate investigation)

– Relict structures

– Groundwater regime (Rapid Drawdown/Artesian/Perched Water Table)

– Geomorphological conditions

• Accuracy of analytical model

– Postulation of possible Failure Mechanisms

– Upper bound solution

– Equilibrium of Slide forces & moment


Model Idealisation & Simplification

FOS = 1.07

Bedrock

Residual Soils


Road Cutting

Model Idealisation & Simplification

FOS = 1.42

Bedrock

Residual Soils


Road Cutting

Rock Mass Structures


Groundwater Regime



• Idealisation & Simplification of Geological/Hydro-geological/Geotechnical models

– Subsurface conditions (Inadequate investigation)

– Relict structures

– Groundwater regime (Rapid Drawdown/Artesian/Perched Water Table)

– Geomorphological conditions

• Accuracy of analytical model

– Postulation of possible Failure Mechanisms

– Upper bound solution

– Equilibrium of Inter-slide Forces & Moment


Cutting Earthworks

Soil Nails That Have Been Installed

Finite Element Analyses

Dev. of Plastic Points In FEM (After Cutting of 12 Upper Berms)


Limit Equilibrium Stability Analyses

Global Stability FOS=1.01

Full Installation of Soil Nail (Except Berm 1)



• Slope Geometry & Loading conditions

- Assessment of probable loading scenarios

- Future possible changes of site conditions

• Mass Properties

– Stress conditions (Stress path/Change of principal stress direction)

– Reliability of mass properties of slope

– Strength anisotropy

– Mobilized strength, peak/residual strength


Variation of Site Conditions


Raising Water

Inundation

Recess of Water (Drawdown condition)

Phreatic Water Table & Steady State Seepage



• Slope Geometry & Loading conditions

- Assessment of probable loading scenarios

- Future possible changes of site conditions

• Mass Properties

– Stress conditions (Stress path/Change of principal stress direction)

– Reliability of mass properties of slope

– Strength anisotropy

– Mobilized strength, peak/critical/residual strengths


Contents

• Overview








Chap 3 Slope Mass Properties

Sources of Deriving of Strength Parameters

• Laboratory tests (Tri-axial strength tests, shear box test)

• Field tests (Vane shear tests, piezocones)

• Observation of Rock Mass Structure & Joint Surface Condition (Homogenous & Isotropic)

• Back-analysed operational mobilised strength parameters

Failure Criteria

• Mohr-Coulomb criterion (homogeneous soil strength)

– τ = c’ + σ’n tanφ’ (Saturated soils)– τ = C’ + σ’n tanφ’ (Unsaturated soils)

• Generalised Hoek-Brown criterion (rock mass strength, 2002)

– σ’1 = σ’3 + σci (mbσ’3/σci+ s)a

where mb, s & a are function of GSI & D (disturbance factor)


Geological Strength Index

• Estimate Jointed Rock Mass Peak Strength & Deformation Parameters

• Block size/Discontinuity Spacing < 25% of Dimension of Excavation (otherwise governed by rock structure)

• GSI depends on

• Rock Structure (Interlocking condition)

• Block Surface (Roughness condition)


APPLICATION


DISCONTINUITY TESTS


Unconfined Confined

DISCONTINUITY TEST


σ3 σ1

ROCK SLOPE


ROCK MASS SHEAR STRENGTH


ROCK SLOPE STABILITY ANALYSIS

Limit Equilibrium Analysis


ROCK SLOPE AND DISCONTINUITY

Depend on discontinuities surface friction 49One-Day Short Course on Slope Engineering, Promenade Hotel at Kota Kinabalu on 30 June 2010

SLIP SURFACE SHAPES


FAILURE ENVELOPE

The Mohr-Coulomb criterion expressed the relationship between the shear stress and normal stress at failure along a shear surface.

( ) ( ) βσσσσσ 2cos2

1

2

13131 −++=n

'tan' φστ nc +=


PROBLEMS WITH MOHR-COULOMBMohr-Coulomb criterion is applicable for discontinuities and discontinuous rock masses,

hence remains one of the most commonly applied failure criterion. But several key

limitations apply to rock slope stability analyses.

Non linear failure

envelopesScale effect


m and s are derived from

empirical charts that are

related to rock mass quality

DESIGN PARAMETERS


STABILITY ANALYSIS DESIGN PROCESS


Contents

• Overview








Chap 4 Methodology & Analysis

Analytical Models

• Limit Equilibrium Method

– Kinematic stability of rigid body (Markland Analys)

– Slide methods

– Predefined slip surface for FOS computation

– Either force equilibrium or moment equilibrium or both

• Finite Element/Difference Method

– Deformation

– Failure mechanisms

– Strain & stress compatible

Analysis Types

• Drained analysis (Effective stress)

• Undrained analyses (Total stress)


Chap 4 Methodology & AnalysisLimit Equilibrium

Method

Static Equilibrium Inclination & Relationship of Inter-slice Forces

Force Moment

Ordinary/Swedish – 1936 - � Ignore H & V

Janbu (Simplified) – 1954 � - Ignore V, Consider H

Janbu (Generalised) � - Consider both V & H (Line of Thrust & Moment Equilibrium of Slides)

Bishop (Simplified) – 1955 - �� Ignore V, Consider H

Morgestern-Price - 1965 �� Consider both V & H (Variable)

Spencer – 1967 �� Consider both V & H (Constant)

Lowe-Karafiath � - Consider both V & H

Corps of Engineers � - Consider both V & H

Sarma – 1973 � � V = C + H tan φ57One-Day Short Course on Slope Engineering, Promenade Hotel at Kota Kinabalu on 30 June 2010


Equilibrium

∑F=0

∑M=0

VH


Generalised Limit Equilibrium Frameworks (GLE)

– Fredlund (1977 & 1981)

V=H λ f(x)

H : Interslice normal force

V : Interslice shear force

INFINITE SLOPE FAILURE

resista

nceβ

Sliding surface

h

b

Vertical

slice

W



FORCE EQUILIBRIUM

– THE SLICE

W

W P

W N

cl + W Ntan φφφφ'

WN = Wsinβ

l = length of sliding

surface

'tan' φσ ncs +=

Terzaghi Equation as based of

Mohr-Coulomb Failure Envelope




Conditions :

• Groundwater Regime (Rapid Drawdown/Artesian/Perched Water Table)

• Submerged slope

• Tension cracks

• Surface loadings/surcharge

• Global/local stability


Contents

• Overview








Chap 5 Factor of Safety

• Definition of FOS:

– Resistance/Disturbance (Fr/Fd, Mr/Md, H/Hc, τavai./ τmob.)

– Strength Reduction Factor

• Determination of FOS:

– Condition of Application (Long/short terms)

– Reliability of soil parameters & analytical model

– Consequences


Chap 5 Factor of SafetyReferences FOS Requirements

BS6031 1.3~1.4 for first time slide

1.2 for slide with pre-existing slip surface

JKR Road Works 1.2 for unreinforced slope & embankment on soft ground

1.5 for reinforced slope

Hong Kong Geoguide

1.0~1.4 for new slopes depending on risk categories

1.0~1.2 for existing slope depending on risk categories

NAVFAC DM7.1 1.5 for permanent loading condition

1.15 to 1.2 for transient load

Britain National Coal Board 1970

1.5/1.35 (peak/residual strength used) for risky slope

1.25/1.15 (peak/residual strength used) for non-risky slope

Canada, Mines Branch 1972

1.5/1.3 (peak/residual strength used) for risky slope

1.3/1.2 (peak/residual strength used) for non-risky slope


Table 2. Partial factor recommended in EN1997-1 Annex A - Updated in 2007Design Approach 1 Design Approach 2

Design Approach 3

Combination 1 Combination 2 Combination 2 - piles & anchors Combination 1 Slopes

A1 M1 R1 A2 M2 R2 A2 M1 or M2 R4 A1 M1 R2 A1 M=R2 A1 A2 M2 R3

ActionsPermanent

Unfav 1.35 1.00 1.00 1.35 1.35 1.35 1.00

Fav 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Variable Unfav 1.50 1.30 1.30 1.50 1.50 1.50 1.30

Soil

tan φφφφ' 1.00 1.25 1.00 1.25 1.00

Struct

Actions

Geotech

Actions

1.25

Effective cohesion1.00 1.25 1.00 1.25 1.00 1.25

Undrained strength 1.00 1.40 1.00 1.40 1.00 1.40

Unconfined stregnth 1.00 1.40 1.00 1.40 1.00 1.40

Weight density 1.00 1.00 1.00 1.00 1.00 1.00

Spread

footings

Bearing 1.00 1.00 1.40 1.00

Sliding 1.00 1.00 1.10 1.00

Driven piles

Base 1.00 1.30 1.10 1.00

Shaft (compression) 1.00 1.30 1.10 1.00

Total / combined 1.00 1.30 1.10 1.00

Shaft in tension 1.25 1.60 1.15 1.10

Bored piles

Base 1.25 1.60 1.10 1.00

Shaft (compression) 1.00 1.30 1.10 1.00

Total / combined 1.15 1.50 1.10 1.00

Shaft in tension 1,25 1.60 1.15 1.10

AnchorsTemporary 1.10 1.10 1.10 1.00

Permanent 1.10 1.10 1.10 1.00

Retaining

walls

Bearing capacity 1.00 1.00 1.40 1.00

Sliding resistance 1.00 1.00 1.10 1.00

Earth resistance 1.00 1.00 1.40 1.00

Slope Earth resistance 1.00 1.00 1.10 1.00

Chap 5 Factor of Safety


Contents

• Overview








Chap 6 Hazard & Risk Assessments

Risk Mitigation over :

� Natural Terrain Slopes

� Engineered Slopes

• Unreinforced Slopes• Reinforced Slopes


� Experience from man-made slopes not necessary applicable to natural terrain

� Low frequency high magnitude event� Visual Impact

∴ New technical approach:� React-to-known hazard� Mitigation works� Risk Assessment� New analytical tools, e.g. debris mobility

evaluation, landslide susceptibility map, etc.

Natural terrain landslide risk (Hong Kong Experience)


Many landslides

in a heavy rainstorm

Many landslidesMany landslides

in a heavy rainstormin a heavy rainstorm

Small failure can be serious for HighrisesSmall failure can be Small failure can be serious for Highrisesserious for Highrises

Increasing risk

due to developing closer

to natural hillside

Increasing risk Increasing risk

due to developing closerdue to developing closer

to natural hillsideto natural hillside

Low-frequency

large-magnitude event

LowLow--frequencyfrequency

largelarge--magnitude eventmagnitude event


Natural Terrain Landslides Natural Terrain Landslides Natural Terrain Landslides

Natural Terrain FailureNatural Terrain Failure

Propensity ~ 1 / 2km2 /yr

Fatality (PLL) ≤≤≤≤ 5 / yr

Propensity ~ 1 / 2kmPropensity ~ 1 / 2km2 2 /yr/yr

Fatality (PLL) Fatality (PLL) ≤≤≤≤≤≤≤≤ 5 / yr5 / yr

ManMan--made Slope Failuremade Slope Failure

Potential Loss of LifePotential Loss of LifePotential Loss of Life

Propensity ~ 1 / km2 /yr

Fatality (PLL) ≤≤≤≤ 10 / yr

Propensity ~ 1 / kmPropensity ~ 1 / km2 2 /yr/yr

Fatality (PLL) Fatality (PLL) ≤≤≤≤≤≤≤≤ 10 / yr10 / yr

60% Steep 60% Steep

Natural HillsideNatural Hillside


UNACCEPTABLE

ALARP

INTE

NS

E

SC

RU

TIN

Y

RE

GIO

N

10- 3

10- 4

10- 5

10- 6

10- 7

10- 8

F/yr

Fatality ≥ N

1 10 100 1,000

UnacceptableUnacceptableUnacceptableUnacceptable

ALARPALARPALARPALARP

Intense

Intense

Intense

Intense

Scrutiny

Scrutiny

Scrutiny

Scrutiny

Region

Region

Region

Region

Personal Individual Risk (IR) Criteria

Existing Facility:

New Facility:

IR with 10-5 per yr

IR with 10-4 per yr

Interim Risk Standards Interim Risk Standards Interim Risk Standards Interim Risk Standards

(GEO Report No. 75)(GEO Report No. 75)(GEO Report No. 75)(GEO Report No. 75)

Technical Challenge – Natural terrain landslide risk

Societal Risk Criteria : FSocietal Risk Criteria : F--N CurveN Curve


Broadly Broadly Broadly Broadly AcceptableAcceptableAcceptableAcceptable

One-Day Short Course on Slope Engineering,

Promenade Hotel at Kota

73

Risk Management Framework

Risk Reduction Trend (Hong Kong Experience)

GEOestablished

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Postulated risk trend

Landsliprisk

Checknew works

Upgradeold slopes

Rehousesquatters

Publiceducation


Thank you

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