slide workshop introduction

7/24/2019 Slide Workshop Introduction

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John Curran, Ph.D, P.EngCEO and Founder, Rocscience Inc.

R.M. Smith Professor Emeritus, University of Toronto

February 28-29, 2012, Perth, Australia
http://rocscience.com/home


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About Us

Established in 1996, based on 15+years research and developmentwork

20 full-time staff in Toronto, mostwith advanced engineeringdegrees

Over 6000 customers, in 110+

countries; user base includesconsulting firms of every size, andabout 200 universities


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Our Unique Offering

Competitively priced software

return on investment is rapid

Free technical support provided by engineers who developed programs

We have software from a few different companies and I just wanted to let you

know that your customer support blows all theirs out of the water.

Thank you very much! Free evaluation copy and 30-day money back

guarantee


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Our Unique Approach

Highly user-friendly programs

CAD tools

Similar interface between our 13 programs Rapid learning


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Our Software


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Examine

3D

analysis of underground

excavations

Phase2

finite element analysis

of excavations & slopes

Unwedgewedge analysis for

underground excavations

RocSupportsupport estimation using

ground reaction curves

Our Software - Excavation Design
http://www.rocscience.com/Anon/RocSupportDemo2.exehttp://www.rocscience.com/roc/software/Phase2.htmhttp://www.rocscience.com/roc/software/Examine3D.htm


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Dipsgraphical and statistical

analysis of orientation data

RocFallstatistical analysis of rockfalls

RocDatarock mass, soil & discontinuity

strength analysis

Our Software - Geotechnical Tools

Settle3D

Settle3D

3D analysis of settlements and

consolidation
http://www.rocscience.com/roc/software/Examine3D.htmhttp://www.rocscience.com/roc/software/Examine3D.htmhttp://www.rocscience.com/roc/software/Examine3D.htmhttp://www.rocscience.com/roc/software/RocFall.htmhttp://www.rocscience.com/roc/software/Dips.htm


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Our Software Philosophy

Modeling goals:

Gain insight

Explore potential trade-offs and alternatives

To achieve goals:

Allow designers to focus on engineering

Leave tedious/mundane tasks to program

Facilitate speedy modelling

Compute as fast as possible

Make it easy and fast to create models


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Part I

Introduction to slope stability


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Objectives

Goals of slope stability analysis

Basic slope stability analysis

Identifying different slope failure mechanisms

Identifying conditions under which particularmechanisms occur

Overview of slope stabilization methods


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Aims of Slope Stability Analysis

Assess equilibrium

conditions (natural slopes)

Evaluate methods for

stabilizing slope

Evaluate impact/role of

geometric and physical

parameters on stability

Discontinuity strength Height

Slope angle, etc.


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Determine impact of

seismic shock on

stability

Back analyze forprevailing conditions

at failure

Shear strength

Groundwater conditions


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Determine optimal staged

excavation or construction

sequence

Design slopes that reliably

maintain stability at

reasonable economic

costs


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Parametric Analysis

Uncertainties regarding material

properties and physical

conditions

Variability of properties from

location to location

Difficulties in measurement

Required to evaluate physical

and geometrical factorsaffecting stability


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Slope Stability Analysis

Components of analysis

Slope under consideration (geometry, geology, soil

properties, groundwater, etc.)


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Slope Stability Analysis

Components of analysis

Slope geometry

Geologic model

Groundwater Loadings on slope

Failure criterion

Failure analysis


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Failure Modes

Slope failure modes/mechanisms

Ways in which slide masses move

Identifies critical failures that should be eliminated or

minimized

Used proactively to permit early design improvements and

at less cost than is possible by reactive correction of

problems


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Failure Modes

Three major classes

Slides: Mass in contact

with parent/underlying

material moving along

discrete boundary (shear

surface)


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Failure Modes

Three major classes

Falls: Steep faces,

immediate separation of

moving mass from parent

material, intermittent

contact thereafter


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Failure Modes

Three major classes

Flows: Moving mass

disaggregates,

displacement not

concentrated on

boundary


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Failure Modes

Slides (dictated by unbalanced shear

stress along one or more surfaces)

Rotational

Translational

Compound/

Combination

Planar

Wedge Toppling


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Failure Modes

Rotational (rock and soil)

Sliding along curved surface

Common cause: erosion at base of slope


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Failure Modes


Original Surface

Failure Surface

Circular Shallow Noncircular


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Failure Modes



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Failure Modes

Translational

Slides move in contact with underlying surface

Sliding surface commonly a bedding plane,

can also be fault/fracture surface


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Failure Modes

Translational

Block slide Slab slide

Failure Surface


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Failure Modes

Translational

Block Slide Slab Slide


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Failure Modes

Aspect ratio of sliding mass

Rotational: 0.15 < D/L < 0.33

Translational: D/L


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Failure Modes

Compound

Competent stratum


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Side Relief Planes

Upper Slope Surface

Slope Face

Failure Plane

Failure Modes

Planar (rock and soil)


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Failure Modes

Planar

Movement controlled by geologic structure

Surfaces of weakness (discontinuities joints, faults, bedding

planes, etc.)

Contact between overlying weathered material and firm bedrock


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Failure Modes

Geometric conditions

necessary for planar

failure

Failure plane strikes parallel

or approximately parallel(within 20o) to slope face

Failure plane daylights into

slope face

Dip of the failure plane >friction angle of failure

plane

Upper Slope Surface

Slope Face

Slope Height

TensionCrack

Sliding Block or

Mass (Wedge)

Failure or SlidingSurface


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Failure Modes

Presence of release surfaces at

lateral boundaries of sliding

block

> >

Dip of the

slope faceDip of the

discontinuity

Angle of

friction for the

rock surface

Upper Slope Surface

Slope Face

Slope Height

Tension

Crack

Sliding Block or

Mass (Wedge)

Failure or

Sliding Surface

Failure or Sliding Surface

Release Surfaces


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Failure Modes

Forces acting on failure

block:

Weight of block, W

Normal water pressure, U

Tension crack water pressure, V

Surcharge, F

Seismic forces, S

Forces from artificial support, B

W

S

B

F

V

U


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Failure Modes

Wedge (rock)


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Failure Modes

Wedge (rock)


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Failure Modes

Wedge Geometry

1,2= Failure planes (2

intersecting joint sets)

3= Upper ground surface

4= Slope face

5= Tension crack

H1= Slope height referred

to plane 1

L= Distance of tension crackfrom crest, measured along

the trace of plane 1.

15

3

2

4

H1

L


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Failure Modes

Wedge (rock)

2 discontinuities striking obliquely across slope face

Line of intersection daylights in slope face

Dip of line of intersection > friction angle of discontinuities


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Failure Modes

General conditions for wedge failure

Plunge of line of intersection > angle of friction for rock

surface

Plunge of line of intersection < dip of slope face

Trend of line of intersection approximately parallel to dip

direction of slope face and daylights in slope face


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Failure Modes

Wedges fail if strength is exceeded

> >


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Failure Modes

Wedges cannot fail


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Failure Modes

Wedge (rock)

Active Wedge


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Failure Modes

TopplingUndercutting Discontinuities

Low-Dip Base

Plane Daylighting

in Slope Face


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Failure Modes

Toppling (blocky rock masses)

Weight vector of block resting on incline falls outside base

of block

Often occurs in undercutting beds

Goals of toppling analysis

Determine mechanism (path) and factor of safety against

toppling


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Geologic factors controlling failure modesGeologic Conditions Potential Failure Surface

Cohesionless soils

Residual or colluvial soils over shallow rock

Stiff fissured clays and marine shales within

upper, highly weathered zone

Translational with small

depth/length ratio

Sliding block

Interbedded dipping rock or soil

Faulted or slickensided material Intact stiff to hard cohesive soil

Single planar surface

Sliding blocks in rocky masses

Weathered interbedded sedimentary rocks

Clay shales and stiff fissured clays

Stratified soils

Multiple planar surfaces

Thick residual and colluvial soil layers

Soft marine clays and shales

Soil to firm cohesive

Highly altered and weathered rocks

Rotational (circular slopes with

homogeneous material, non-

circular slopes of heterogeneous

material)


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Objectives

Overview of principles of

Limit equilibrium analysis

Method of slices

Review of assumptions of different methods


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Limit Equilibrium Analysis

General approaches for analysis of slopes

Limit equilibrium

Finite element, finite difference

Back analysis

Keyblock concept (rock)

Probabilistic methods


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Attraction of limit equilibrium

Most common slope analysis method

Relatively simple formulation

Useful for evaluating sensitivity of possible failure

conditions to input parameters


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Fundamental concepts

All points along slip surface are on verge of failure

At this point in time

Driving forces (D) = Resisting forces (R)

Factor of safety(FS) = 1

D > R FS < 1

D < R FS > 1

Limiting equilibrium perfect equilibrium between forcesdriving failure and those resisting failure


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Fundamental concepts Factor of safety (factor of ignorance)

Quantitative measure of degree of stability

Accounts for uncertainty

Guards against ignorance about reliability of input parameters Lower quality site investigationhigher desired factor of safety

Higher quality site investigationlower desired factor of safety

Empirical tool to establish suitable economic bounds on design


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Uncertainties accounted for by factor of safety Uncertainty in shear strength due to soil variability, relationship

between lab strength and field strength

Uncertainty in loadings (surface loading, unit weight, porepressures, etc.)

Modelling uncertainties: including possibility critical failuremechanism SLIGHTLY different from that identified, model is notconservative


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Factor of safety DOES NOT account for possibility of grosserrors such as bad choice of failure mechanism

e.g. ignoring presence of existing shear surfaces in slope


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Fundamental concepts

Two steps for calculating factor of safety

Compute shear strength required along potential failure surface

to maintain stability

Compare required shear strength to available shear strength(which is assumed constant along failure surface)

For Mohr-Coulomb

=


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Planar failure

W

W sin

N

W cos


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Planar failure

W

W sin

N

W cos

tan

sin tan

cAFS

W

= +


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Rotational failure method of slices

Used by most computer

programs

Readily accommodates

complex slope

geometries, variable soil

and groundwater

conditions & variableexternal loads


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N slices

Zi

n Number of Slices

Number of unknowns (6n 2)

n Normal forces on base

n Shear forces on base

n Lines of action (Zi)

n-1 Interslice normal forces

n-1 Interslice shear forces

n-1 Lines of action (Zh)

1 Factor of Safety

FS?

Zh


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N slices

Zi

n Number of Slices

Total number of equations (4n)

n Moment equilibrium of slice

n Force equilibrium in X

n Force equilibrium in Y

FS?

n Mohr-Coulomb

relationship between shear

strength and normaleffective stress

M = 0

Fx= 0

Fy= 0

Zh


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N slices

Zi

Common assumption

Zi= base length of slice

i.e. normal force on slice base acts at

midpoint of base

n 2unknowns remain to make

problem determinate

These assumptions characterize

different slope stability methods

d

Zi = d/2

Zh


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Slope stability analysismethods

Ordinary (Fellenius)

Bishop simplified

Janbu simplified

Janbu corrected

Lowe-Karafiath

Corps of Engineers (I, II)

Spencer

Morgenstern-Price

General Limit Equilibrium

(GLE)


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Thrust line: connects points of application of interslice

forces


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Location of thrust line

May be assumed

May be calculated from rigorous analysis that satisfies complete

equilibrium (Spencer, Morgenstern-Price, GLE)

Simplified methods (Bishop, Janbu, Lowe-Karafiath, Army Core)

neglect location of interslice force because complete equilibrium

is not satisfied

l b l


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Slope stability analysis methods Many methods available

Methods are similar

Difference only in:

Which static equations satisfied

Which interslice forces included

Relationship between interslice

and shear normal forces

l b l


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Methods of slicesassumptions

Ordinary (Fellenius)

Assumes circular slip

surface Neglects all interslice forces

(shear and normal)

Only satisfies moment

equilibrium

One of the simplest

procedures

( tan )

sin

c l

FS

W

+

=

i i ilib i l i


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EXAMPLE:

Determine the factor

of safety for the slip

circle shown

35

c = 20k Pa

= 20

6.1 m

Li i E ilib i A l i


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Solution for slip circle Subdivide sliding sector (4)

Find area of each slice

(mid-height x breadth)

Determine weight (unitweight x area)

Find tangential and normal

force components on

sliding surface

Repeat for each slice &

sum up

12

34

N= ?T= ?

Li i E ilib i A l i


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Solution for slip circle - Ordinary

12

3

4

Tangential

T [kN]

Normal

N [kN]

Weight

W [kN]

Area

]m2[

Slice

no.

-771723.71

421631688.72

11619122411.63

1061041487.74

N= 529 T= 257

c = 20k Pa

= 20tan 529*0.364 192

7620*10.7*( ) 284180

tan 284 1921.85

257

N kN

cr kN

cr NFS

T

= =

= =

+ += = =

Li i E ilib i A l i


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Bishop (1955) simplified

Assumes interslice shear

forces = 0 (reduces # ofunknowns by (n-1))

Moment eq. about centre

and vertical force eq. for

each slice are satisfied

Overdetermined soln(horizontal force eq. not

satisfied for one slice)

( tan )

sin

c l

FS

W

+

=

Li i E ilib i A l i


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Janbu simplified



Overall horizontal force eq.

and vertical force eq. for

each slice

Overdetermined solution(moment equilibrium not

completely satisfied)

( tan )

sin

c l

FS

W

+

=

Li it E ilib i A l i


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Janbu corrected



Overdetermined solution

(moment equilibrium not

completely satisfied)

Correction factor,f0,accounts for interslice shear

force inadequacy



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Lowe and Karafiath

Assumes interslice force

inclined at angle = (groundsurface angle + slope base

angle)/2

Horizontal and vertical force

equilibrium are satisfied for

each slice Overdetermined solution

(moment equilibrium not

satisfied)



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Corps of Engineers I


inclined at angle = groundsurface angle


are eq. satisfied for each

slice

Overdetermined solution(moment equilibrium not

satisfied)



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Corps of Engineers II


inclined at angle = averageslope angle between left

and right points of failure

surface


are eq. satisfied for eachslice



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Spencer

Assumes all interslice forces

inclined at constant, butunknown, angle

Complete equilibrium

satisfied



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Morgenstern-Price

Similar to Spencers

assumes all interslice forcesinclined at constant, but

unknown, angle

Inclination assumed to vary

according to portion of

arbitrary function Satisfies complete

equilibrium



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Methods of slices

Force Equilibrium Moment

Method Horizontal Vertical Equilibrium

Ordinary No No Yes

Bishop simplified No Yes Yes

Janbu simplified Yes Yes No

Lowe-Karafiath Yes Yes No

Corps of Engineers Yes Yes No

Spencer Yes Yes YesGLE (Morgenstern-Price) Yes Yes Yes

I f C iti l S f S h


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Issue of Critical Surface Search

Example: centre of criticalfailure surface may not be

located inside grid

Probabilistic Slope Analysis


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Probabilistic Slope Analysis

Under ideal conditions FS= 1 should ensure safe design Uncertainty forces use of higher FSs

Based on past experience FS= (1.3 1.5)

Use of single FSvalue does not accurately reflect site

investigation quality Good site characterization should be lower FS

Poor site characterization should be higher FS

Often though same FS used

Slope Stabilization Methods


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Reduction of slope height Reduction/flattening of slope angle

Incorporation of benches

Application of support elements (bolts, piles, buttresses,

berms, etc.) Installation of drainage

Use of excavation techniques that minimize dynamicshocks and rock mass damage

Removal of unstable or potentially unstable materials



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Slope Stability References


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Slope Stability References


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slide workshop introduction

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