Progressive Landslide Analysis in Canadian Glacial Silty Clay in Churchill River

Progressive Landslide Analysis in Canadian Glacial Silty Clay in Churchill River

S. Bernander ¹ , R. Dury ¹ , J. Laue ¹ , S. Knutsson ¹ , L. Elfgren ¹

1

Luleå University of Technology, Luleå, Sweden

Robin DURY

Luleå Tekniska Universitet

Email: robdur-6@ltu.student.se Phone: + 33 6 80 38 87 19

Contact

Bernander, S. 2011. Progressive landslides in long natural slopes. Formation, potential extension and configuration of finished slides in strain-softening soils. Doctoral thesis, Lulea University of Technology, ISBN 978-91-7439- 283-8.

Bernander, S. (2016). Comments on the Engineering Report by Nalcor/SNC-Lavalin of December 2015 prepared for Grand Riverkeeper, Labrador, Inc.

Churchill, 31 pp.

Bernander, S., Kullingsjö, A., Gylland, A. S., Bengtsson, P. E., Knutsson, S., Pusch, R., Olofsson, J., & Elfgren, L. (2016). Downhill progressive landslides in long natural slopes: triggering agents and landslide phases modelled with a finite difference method. Canadian Geotechnical Journal, Vol. 53, No. 10, pp. 1565-1582, dx.doi.org/10.1139/cgj-2015-0651

Dury, Robin (2017). Progressive Landslide Analysis. MSc Thesis, Luleå University of Technology, Luleå, Sweden. To be published at http://ltu.diva- portal.org/

References

• At Muskrat Falls in Newfoundland, Fig. 1-2, a large hydro power plant is built.

• A land ridge composed of loose layers, the North Spur, will be used as a natural dam.

• Three different failure surfaces have been studied for different soil properties, Fig. 3-4.

Failure analyses have been performed for three cases:

• Case 1: A traditional Limit Equilibrium Method (LEM) for a horizontal failure surface

• Case 2: A Progressive Failure Analysis (PFA) with a Finite Difference Method (FDM) for an inclined failure surface (4%) in the upper clay layer, Bernander et al. (2011, 2016), Dury (2017)

• Case 3: As Case 2 for a curved failure surface in the lower clay layer

Fig. 4. Section A-A of the North Spur at Muskrat Falls. In red, the failure surfaces corresponding to the three cases studied. The blue triangle represents the additional pressure on the cut-off wall due to the rising water.

0 50 100 150 200 250 300 350 400 450 500 550 600

50

25

0

-25

Sand

Silty clay

Lower clay

Cut-off wall

(m)

• The horizontal failure surface in Case 1 is safe (both with LEM and PFA)

• The inclined and curved failure surfaces in Cases 2 and 3 are unsafe for many material properties according to the Progressive Failure Analysis (PFA).

• For a pressure on the cut-off wall by rising water of N _w = 0,5ρ _w H ² = 0,5∙10∙(39-17) ² = 2420 kN/m, the safety factor for the inclined surface in Case 2 for a clay with c = 60 kPa and c/c _R = 0,2 kPa will only be F = N _resistance /N _applied = 1007/2420 = 0,4. This is very unsafe.

• Fig.5 illustrates a case taking also the earth pressure on the cut-off wall into consideration. This gives safety factors <1 for clays weaker than c = 100 kPa with c _R /c≤ 0,2 to c = 60 kPa with c _R /c≤ 0,45 kPa.

• Fig.6 illustrates the inclined failure surface in Case 3 in a similar way as in Fig. 5.

Fig. 5 and 6. Safety factors as a function of the sensitivity ratio for different shear strength for case 2 and 3

• Tests ought to be carried out to check the real properties of the soil (stress – deformation relationships as in Fig. 1).

• One way could be to drive groups of piles into the soil and to check the settlements for possible liquefaction in the areas around the assumed failure planes.

• If the tests indicate materials that give unstable results, the North Spur ridge ought to be stabilized, Bernander (2016), Dury (2017).

Fig. 1. Satellite view of the North Spur at Muskrat Falls Fig. 2. Construction site at Muskrat Falls http://muskratfalls.nalcorenergy.com/

0 10 20 30 40 50 60 70 80 90 100

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Sh ear str ess 𝜏 (kP a)

Deformation 𝛿 (m)

Shear stress 𝜏 as a function of the deformation 𝛿 for two different cases of study

Fig. 3. Softening behavior of materials for two different cases of study

Failure surface (1)

Failure surface (3) +39

+17

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

0,2 0,25 0,3 0,35 0,4

Sa fe ty fact or F

Ratio c

/c Safety factor 𝐅

=

𝐍_{𝐚𝐩𝐩𝐥𝐢𝐞𝐝}

as a function of the sensitivity ratio

for different shear strengths from 50 to 100 kPa (Case 2)

c=100 kPa c=80 kPa c=70 kPa c=60 kPa

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

0,2 0,25 0,3 0,35 0,4 0,45 0,5

Sa fe ty fact or F

Ratio c

/c Safety factor 𝐅

=

𝐚𝐩𝐩𝐥𝐢𝐞𝐝

as a function of the sensitivity ratio

for different shear strengths from 60 to 110 kPa (Case 3)

c=110 kPa c=90 kPa c=70 kPa c=60 kPa

Shear stress along failure surface (2) for c=60 kPa and c

/c=0,2 when submitted to the critical load 𝐍

= 𝟏𝟎𝟎𝟕 𝐤𝐍/𝐦

Fig. 7. Shear stress along the failure plane for for c=60 kPa and c

/c=0,2

0 10 20 30 40 50 60 70

0 100 200 300 400 500

Sh ear str ess τ ( kP a)

Position along the failure plane (m)

Total shear stress τ

In−situ stress τ₀

Sand Silty clay Sand

c=60 kPa, c

/c=0,2 c=90 kPa

Results

Method Introduction

Discussion

c=90 kPa, c

/c=0,2

c

=12 kPa c

=15 kPa c=60 kPa

Failure surface (2) +6

2420 kN/m