Complex geology slope stability analysis by shear strength ...home.agh.edu.pl/~cala/prezentacje/MSSR.pdf · Complex geology slope stability analysis by shear strength reduction

Complex geology slope stability analysis by shear

strength reduction

Marek Cala, Jerzy Flisiak Department of Geomechanics, Civil Engineering & GeotechnicsAGH University of Science & Technology(former Univ. Of Mining & Metallurgy)

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.

==

trialtrial

trialtrial FS

tgarctgFScc ϕϕ

20Department of Geomechanics, Civil Engineering & Geotechnics

Shear strength reduction technique (SSR)

• It’s well known fact that for simple slopes FS obtained from SSR is usually 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).

25 m

25 m

hg

45o

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


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

1.4

1.5

1.6

1.7

1.8

1.9

2

FSWeak layer 1 m thick

FLACFelleniusBishopJanbu

SSR versus LEMWeak layer 1m thick

Hard soil c=75 kPa, φ=30o

Soft soil c=25 kPa, φ=10o

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

FSWeak layer 5 m thick

FLACFelleniusBishopJanbu

SSR versus LEMWeak layer 5m thick

Hard soil c=75 kPa, φ=30o

Soft soil c=25 kPa, φ=10o

25 m

25 m

hg

45o


SSR versus LEM

Bishop FS = 1.731

FLAC FS = 1.54

20 m

1

m

16 Department of Geomechanics, Civil Engineering & Geotechnics

SSR versus LEMbenched slope case

15 m

10

m

10 m

20.918 m15 m15 m

45o

40o

γφ=20 kN/m = 20

c = 10 kPa

3

o


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


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.

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 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


Modified shear strength reduction technique Velocity vectors Displacement vectors

Plasticity indicators Shear strain rateFS2 = 1.24


MSSR versus LEMbenched slope case

FS =0.901

BishopFS=0.921

FS =1.002

BishopFS=1.008

FS =1.243

BishopFS=1.228



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



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


Department of Geomechanics, Civil Engineering & Geotechnics

Shear strength reduction techniqueLarge, complex geology slope case

8

Modified shear strength reduction technique Displacement vectors

Shear strain rateFS2 = 0.87


Modified shear strength reduction technique Plasticity indicators






Modified shear strength reduction technique Displacement vectors

Velocity vectorsFS5 = 1.29





MSSR versus LEMlarge, complex geology slope case

FS =0.671

FS =0.872

FS =1.023 FS =1.174

BishopFS=1.351

FS =1.295

BishopFS=1.255


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 SSRtechnique 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 !!!


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Complex geology slope stability analysis by shear strength ...home.agh.edu.pl/~cala/prezentacje/MSSR.pdf · Complex geology slope stability analysis by shear strength reduction