Complex geology slope stability analysis by shear...

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Complex geology slope stability analysis by shear

strength reduction

Marek Cala, Jerzy FlisiakDepartment of Geomechanics, Civil Engineering & GeotechnicsAGH University of Science & Technology

Slope stability Shear strength reduction technique (SSR)

� The stability of slopes may be estimated using 2D limit equilibrium methods (LEM) or numerical methods.

� Due to the rapid development of computing efficiency, several numerical methods are gaining increasing popularity in slope stability engineering.

� The factor of safety (FS) of a soil slope is defined as the number by which the original shear strength parameters must be divided in order to bring the slope to the point of failure.

20Department of Geomechanics, Civil Engineering & Geotechnics

==

trialtrial

trialtrial FS

tgarctg

FS

cc

ϕϕ

2

• It’s well known fact that for simple slopes FS obtained from SSR isusually the same as FS obtained from LEM (Griffiths & Lane, 1999; Cala & Flisiak, 2001).

• However, for complex geology slopes considerable differences between FS values from LEM and SSR may be expected (Cala& Flisiak, 2001).


Shear strength reduction technique (SSR)

25 m

25 m

hg

45o

Several analyses for the slope with weak stratumwere performed to study the differences between LEM and SSR.

Weak layer 1m thick


SSR versus LEM

0 10 20 30 40Distance of weak layer from slope crest

1.4

1.5

1.6

1.7

1.8

1.9

2

FS

Weak layer 1 m thickFLACFelleniusBishopJanbu

25 m

25 m

hg

45o

Hard soil c=75 kPa, φφφφ=30o

Soft soil c=25 kPa, φφφφ=10o

3

Weak layer 5m thick


SSR versus LEM

25 m

25 m

hg

45o

0 10 20 30 40 50Distance of weak layer from slope crest

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

FS

Weak layer 5 m thickFLACFelleniusBishopJanbu

Hard soil c=75 kPa, φφφφ=30o

Soft soil c=25 kPa, φφφφ=10o

16

Bishop FS = 1.731

FLAC FS = 1.54

20 m

1

m

Department of Geomechanics, Civil Engineering & Geotechnics

SSR versus LEM

4

15

15 m

10

m

10 m

20.918 m15 m15 m

45o

40o

γφ=20 kN/m = 20

c = 10 kPa

3

o


SSR versus LEMbenched slope case

14

FLAC/SLOPE (Version 4.00)

LEGEND

16-Jul-02 18:17

Factor of Safety 0.90

Shear Strain Rate Contours 5.00E-07 1.00E-06 1.50E-06 2.00E-06 2.50E-06 3.00E-06

Contour interval= 5.00E-07(zero contour omitted)Boundary plot

0 2E 1

-1.500

-0.500

0.500

1.500

2.500

3.500

4.500

5.500

(*10^1)

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000(*10^1)

JOB TITLE : bench

Itasca Consulting Group, Inc. Minneapolis, Minnesota USA


SSR versus LEMbenched slope case

5

13 Department of Geomechanics, Civil Engineering & Geotechnics

Modified shear strength reduction technique 1. Apply classic SSR technique to calculate FS1 (FLAC/Slope).

2. Export *.dat file to FLAC. Calculate the initial, stable situation by increasing c and φ. φ. φ. φ.

3. Find the representative number of steps (Nr) which characterises the response time of the system. Use 1.1Nr for further calculations.

4. Calculate situation for FS1(check out for communication between FLAC and FLAC/Slope and elimination of any mistakes).

5. Reduce c and φ φ φ φ to find further FSi ( prepare *.dat file manually or using Excel; each time start from the initial, stable *.sav file).


Modified shear strength reduction technique

Velocity vectors Displacement vectors

Plasticity indicators Shear strain rateFS2 = 1.00

6



Velocity vectors Displacement vectors

Plasticity indicators Shear strain rateFS2 = 1.24

10

FS =0.901

BishopFS=0.921

FS =1.002

BishopFS=1.008

FS =1.243

BishopFS=1.228


MSSR versus LEMbenched slope case

7

9


LEGEND

30-Jun-02 8:24


User-defined Groupsbedrockzwietrzelina_wetily_podweglowekontakt_spag_wI_wetI_poklad_weglaily_miedzyweg_dolnekontakt_spag_wII_wetII_poklad_weglastary_zwal_wew

Boundary plot

0 2E 2

-2.500

-1.500

-0.500

0.500

1.500

2.500

3.500

4.500

(*10^2)

0.500 1.500 2.500 3.500 4.500 5.500 6.500 7.500(*10^2)

JOB TITLE : 5_layer_wet



Shear strength reduction techniqueLarge, complex geology slope case

8


LEGEND

30-Jun-02 8:24


Shear Strain Rate Contours 5.00E-07 1.00E-06 1.50E-06 2.00E-06 2.50E-06 3.00E-06 3.50E-06

Contour interval= 5.00E-07(zero contour omitted)Boundary plot

0 2E 2

-2.500

-1.500

-0.500

0.500

1.500

2.500

3.500

4.500

(*10^2)

0.500 1.500 2.500 3.500 4.500 5.500 6.500 7.500(*10^2)

JOB TITLE : 5_layer_wet



Shear strength reduction techniqueLarge, complex geology slope case

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Displacement vectors

Shear strain rateFS2 = 0.87



Plasticity indicators


9







Displacement vectors

Velocity vectorsFS5 = 1.29

10





2

FS =0.671

FS =0.872

FS =1.023 FS =1.174

BishopFS=1.351

FS =1.295

BishopFS=1.255


MSSR versus LEMlarge, complex geology slope case

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Conclusions • For a simple, homogeneous slope, FS calculated with SSR are

usually the same as FS obtained from LEM. • In the case of a simple geometry slope consisting of two

geological units, FS calculated with SSR may be considerably different than FS from LEM.

• In the case of complex geometry and geology slopes SSR technique is much more “sensitive” than LEM.

• Another step forward is the modified shear strength reduction technique – MSSR.

• Application of SSR/MSSR with FLAC may be recommended for the large-scale slopes of complex geometry.

• Such a powerful tool as MSSR with FLAC gives the opportunity for the complete stability analysis for any slope.

• Limitations: visibility, interpretation. • Verification !!!


Complex geology slope stability analysis by shear...

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