Slope stability calculations for road cutting: Tuuliharju case · Anisotropy of undrained shear strength 09/11/2017 12 SGY geotechnical seminar 2017 Ayman A. Abed HELSINKI • In

SGY geotechnical seminar 2017 HELSINKI

Ayman A. Abed, Dr. –Ing.

Postdoctoral researcher

Aalto University

E-mail: [email protected]

Slope stability calculations for

road cutting: Tuuliharju case

09/11/2017

Contents

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SGY geotechnical seminar 2017Ayman A. Abed

HELSINKI

1. Saturated soil mechanics basics, stress rotation during

cutting and stress path concept

2. On the loading path dependency of the mobilized shear

strength

3. Anisotropy of undrained shear strength

4. About drained and undrained conditions

5. Soil stability at Tuuliharju cutting

6. Final remarks

Saturated soil mechanics basics

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Terzaghi’s effective stress principle:

“All measurable effects of change of stress, such as compression,

deformation and mobilization of shearing resistance are exclusively due to

changes in effective stress”

Mohr-Coulomb failure criterion:

Failure occurs when

• Drained conditions:

• Undrained conditions:

𝜎′ = 𝜎 − 𝑝𝑤

𝜏 = 𝜏𝑓 = 𝑐′ + 𝜎𝑛′ tan𝜑′

𝜏 = 𝜏𝑓 = 𝑐𝑢

Principal stress rotation during cutting and stress

path concept

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=𝜎1′ + 𝜎2

′ + 𝜎3′

3

=1

2𝜎1′ − 𝜎2

′ 2 + 𝜎2′ − 𝜎3

′ 2 + 𝜎3′ − 𝜎1

′ 2

Initial condition


path concept

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Intermediate situation


path concept

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Final situation

Stress state along failure surface

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Direction of the major principal stress

On the loading path dependency of the mobilized

shear strength

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The failure can be triggered by adding external load at the top of the slope

(adding fill, for example)

Triaxial compression (loading path)

Strength parameters from triaxial compression are representative but non-conservative


shear strength

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Strength parameters from triaxial extension are representative but conservative

The failure can be triggered by reducing total stresses at the toe of the slope

(removal of soil by cutting, for example)

Triaxial extension (unloading path)


shear strength

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The failure can be triggered by reducing effective stresses along the slip

surface (raising of groundwater table, for example)

Triaxial extension (unloading path)

Strength parameters from triaxial extension are representative but conservative


shear strength

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• Average values for shear strength parameters from triaxial compression and triaxial

extension should be used

• The shear strength parameters from Direct Simple Shear (DSS) can be considered

as an average values and used directly

Anisotropy of undrained shear strength

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• In general CuTC > CuDSS > CuTE. For clay from Tuuliharju with Ip ≈ 35% one finds CuDSS/CuTC ≈ 0.62.

• Corrected undrained shear strength as measured by vane shear test are in good agreement with those

from direct simple shear for common clays in practice.

• Shear strength anisotropy is more pronounced in low plasticity soils.

After Larsson (1980)

Correction of vane shear test results

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To account for inaccuracy related to vane shear test in field, Bjerrum (1973)

introduced a correction factor which depends on soil plasticity.

Original chart by Bjerrum Chart as given by SGY (1999)

About drained and undrained conditions

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𝑡90 = 0.848𝐷2

𝐶𝑣

• If no information is available about 𝑇 then the materials with permeability 𝑘 > 10−6 m/s is

considered as drained for normal rate of loading whereas for 𝑘 < 10−9 m/s the material is

undrained.

Some criteria are given to judge on the prevailing drainage conditions

Proposal for critical drainage path length D

in the case of cutting

• Determine the time factor 𝑇 =𝐶𝑣𝑡

𝐷2

• Or alternatively, estimate:

T < 0.01 0.01< T <3.0 (0.4)* T > 3.0 (0.4)*

Undrained Undrained & drained Drained

• If construction time 𝑡 < 𝑡90 then undrained conditions should be considered. For example, for

Tuuliharju case 𝑡90 ≈ 10 months and the construction time is on a scale of weeks, yielding that

undrained condition is important.

Stability at Tuuliharju cutting:Notes about the calculations

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• Detailed experimental investigation program is performed by Aalto

University at the site and in the laboratory including triaxial compression

testing, one dimensional compression tests and liquid limits

determination.

• Focus is only put on the Limit Equilibrium Method in this presentation.

• Morgenstern-Price method is employed in the calculations.

• In undrained calculation, the total stress method is employed. Whereas

for drained calculations, the effective stress method is adopted with Mohr-

Coulomb failure criterion.

• Based on field measurements, the groundwater table is at an average

depth of 1.5m.

Stability at Tuuliharju cutting: Some of the measured soil properties

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Depth [m] 𝜎3′ = 25.0 [kPa] 𝜎3

′ = 50.0 [kPa] 𝜎3′ = 75.0 [kPa]

2.5 𝑐𝑢𝑇𝐶 = 30.0 𝑐𝑢𝑇𝐶 = 30.0 𝑐𝑢𝑇𝐶 = 35.0

𝜎3′ = 30.0 [kPa] 𝜎3

′ = 60.0 [kPa] 𝜎3′ = 90.0 [kPa]

4.0 𝑐𝑢𝑇𝐶 = 35.0 𝑐𝑢𝑇𝐶 = 35.5 𝑐𝑢𝑇𝐶 = 39.0

Undrained shear strength from triaxial compression tests

Stability at Tuuliharju cutting:Cutting geometry and soil profile

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SHANSEP methodStress History And Normalized Soil Engineering Properties

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This method allows to estimate a continuous undrained shear strength profile

based on some discrete measurements of undrained shear strength and over-

consolidation ratio OCR.

𝑐𝑢𝜎𝑣′ =

𝑐𝑢𝑁𝐶𝜎𝑣′ 𝑂𝐶𝑅𝑚

• The measured undrained shear strength in

triaxial compression tests are converted to direct

simple shear counterpart using a factor of 0.62

Stability at Tuuliharju cutting:Undrained shear strength as used in the calculations

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Values based on vane shear test and on SHANSEP method results are employed in the analyses

Used average valueEstimated values by different methods

Stability at Tuuliharju cutting:Safey factor for undrained conditions

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Right FS = 1.022Left FS = 1.123

Cu is estimated based on SHANSEP method

Right FS = 1.19Left FS = 1.27

Cu is estimated based on corrected vane shear results

Final remarks

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• The calculations in undrained conditions show that the cutting is not safe in

the short term. In fact, the cutting suffered failure at that location right after

construction.

• The stability calculations in drained conditions (not shown here) show that

the cut is stable for long term condition.

• The strength parameters based on Direct Simple Shear can be considered

as an average parameters and recommended for cutting stability

calculations.

• The vane shear test results are suitable for undrained calculations but after

correcting them for inaccuracy (SGY, 1999).

• The SHANSEP method provides a nice tool to build a continuous profile

for the undrained shear strength based on preconsolidation history and

effective stress profile.

Final remarks

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Acknowledgement

The results in this presentation are part of a Researcher & Development

project being funded by “Liikennevirasto” and supervised by Leena Korkiala-

Tanttu, Prof. (Aalto University), Panu Tolla, M.Sc. and Veli-Matti Uotinen,

M.Sc. (Liikennevirasto).

• The presented results are only a small part of a more comprehensive study

about cutting stability which will be soon available as a report with

recommendations for improved application of Limit Equilibrium Method.

References

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1. BJERRUM, L., 1973. Problems of soil mechanics and construction of soft clays and structurally unstable soils. Proc 8th Intl Conf SMFE Mosc. 1973 3,

111–159.

2. D’Ignazio, M., Länsivaara, T., 2016. Strength increase below an old test embankment in Finland.

3. D’Ignazio, M., Phoon, K.-K., Tan, S.A., Länsivaara, T.T., 2016. Correlations for undrained shear strength of Finnish soft clays. Can. Geotech. J. 53,

1628–1645.

4. Duncan, J.M., Wright, S.G., Brandon, T.L., 2014. Soil strength and slope stability. John Wiley & Sons.

5. JAMIOLKOWSKI, M., 1985. New development in field and laboratory testing of soils. pp. 57–153.

6. Kankare, E., 1969. Failures at Kimola floating canal in Southern Finland. pp. 609–616.

7. Karstunen, M., Krenn, H., Wheeler, S.J., Koskinen, M., Zentar, R., 2005. Effect of anisotropy and destructuration on the behavior of Murro test

embankment. Int. J. Geomech. 5, 87–97.

8. Ladd, C.C., 1971. Strength parameters and stress-strain behavior of saturated clays. Res. Rep. R71-23, Soils publication 278 280.

9. Ladd, C.C., Foott, R., 1974. New design procedure for stability of soft clays: 10F, 3T, 39R. J. GEOTECH. ENGNG. DIV. V100, N. GT7, JULY, 1974,

P763-786. Pergamon, pp. A220–A220.

10. Lambe, T.W., 1997. THE SELECTION OF SOIL STRENGTH FOR A STABILITY ANAL YSIS. Fifth Spencer JBuchanan Lect. Tex. M Univ.

11. Larsson, R., 1980. Undrained shear strength in stability calculation of embankments and foundations on soft clays. Can. Geotech. J. 17, 591–602.

doi:10.1139/t80-066

12. Lehtonen, V., 2015. Modelling undrained shear strength and pore pressure based on an effective stress soil model in Limit Equilibrium Method.

Tampereen Tek. Yliop. Julk.-Tamp. Univ. Technol. Publ. 1337.

13. SGY, 1999. Kairausopas II.

14. Terzaghi, K., Peck, R.B., Mesri, G., 1996. Soil mechanics in engineering practice. John Wiley & Sons.

15. Vermeer, P.A., Meier, C.P., 1998. Stability and deformations in deep excavations in cohesive soils.

List of Symbols

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𝑐 Cohesion 𝑤 Water content

𝑐′ Effective cohesion 𝑤𝐿 Liquid limit

𝑐𝑢 Undrained cohesion 𝑤𝑃 Plastic limit

𝑐𝑢𝐷𝑆𝑆 Undrained cohesion from Direct Simple Shear test 𝑤𝑠𝑎𝑡 Water content at full saturation

𝑐𝑢𝑁𝐶 Undrained cohesion for normally consolidate clay 𝜏 Shear stress

𝑐𝑢𝑇𝐶 Undrained cohesion from Triaxial Compression test 𝜏𝑓 Shear stress at failure

𝑐𝑢𝑇𝐸 Undrained cohesion from Triaxial Extension test 𝜎1′, 𝜎2

′ , 𝜎3′Principal stress

𝑐𝑣 Coefficient of consolidation 𝜎𝑛 Normal stress

𝐷 Drainage path length 𝜎𝑛′ Effective normal stress

𝐾0 Coefficient of earth pressure at rest 𝜎𝑝′ Consolidation pressure

𝑘 Permeability 𝜎𝑣′ Effective vertical stress

𝑚 Soil parameter OCR Over consolidation ratio

𝑝′ Effective isotropic pressure

𝑝𝑤 Pore water pressure

𝑞 Deviatoric stress

𝑡 Time

𝑡90 Time needed for 90% of consolidation to take place

𝑇 Consolidation time factor

Thank you for your attention!

Questions?

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Slope stability calculations for road cutting: Tuuliharju case · Anisotropy of undrained shear strength 09/11/2017 12 SGY geotechnical seminar 2017 Ayman A. Abed HELSINKI • In