Geotechnical damage in rural areas caused by the 2015 Gorkha Nepal Earthquake

Chiaro GabrieleJSPS RESEARCH FELLOW, University of TokyoLecturer, University of Canterbury, New Zealand

Kiyota T., Pokhrel R.M., Katagiri T.Institute of Industrial Science, University of TokyoGoda K.University of Bristol, United KingdomSharma K.University of Alberta, Canada

Tokyo, 1 June 2015

Outline:1. Damage survey

2. Tectonics of Nepal

3. Geotechnical damage (rural areas)3.1 Trishuli area3.2 Melamchi area3.3 Epicentral area (Baluwa)3.4 Pokhara Valley

4. Summary

5. Recommendations3

Advanced Party of Japanese Investigation Team for the 2015 Nepal Earthquake Disaster

JGS: Japanese Geotechnical SocietyJSCE: Japanese Society of Civil Engineers

ATC3, ISSMGE: Asian Technical Committee, Int. Society of Soil Mech. & Geotech. Eng.

Figure 1. Survey team: Kiyota Takashi; Katagiri Toshihiko; Pokhrel Pradeep; Pokhrel Rama Mohan; Chiaro Gabriele; and Goda Katsuichiro (from left to right) 2

1. Damage survey • Day 1 (5/1): Kathmandu (city center)• Day 2 (5/2): Trishuli and Melamchi• Day 3 (5/3): Kathmandu (suburbs)• Day 4 (5/4): Baluwa (Epicentral area)• Day 5-6 (5-6/5): Pokhara Valley• Day 7 (5/7): Briefing Kathmandu

0 25 50 km

Pokhara

Trishuli

Melamchi

Kathmandu

Baluwa

Mw6.7

Mw7.3

Mw6.6

Mw7.8VII

VIIIVI

Figure 2. Survey routes and location of investigation sites 3

2. Tectonics of Nepal

HimalayaTibetan Plateau

MBT

MFT MCT

NEPAL

Figure 3. Schematic illustration showing the relative motion and main features associated with the type of plate boundaries in Nepal (mod. after Sapkota et al., 2013)

MCT: Main Central Thrust MBT: Main Boundary ThrustMFT: Main Frontal Thrust

5

3. Geotechnical damage in rural areas

Figure 4. Typical village in mountainous areas of central Nepal6

3.1 Trishuli areaNorthwest of

Kathmandu City

0 5 10 km

Trishuli Gerkhu

Ranipauwa

Battar

Nigala

Kathmandu City

Photo 1,2

Photo 3,4

Photo 6-14 Photo 5

Figure 5. Survey routes and location of investigation sites along Trishuli Highway

50 km southeast of the Mw7.8 (4/25)

epicenter

7

Shallow landslide on steep slope

0.01 0.1 1 100

20

40

60

80

100

Perce

ntage

finer

by w

eigth:

%

Particle size: mm

Ranipauwa sand

D50=0.21

Weathered sedimentary rocks

Figure 6. (a) shallow landslide occurred near Ranipauwa; and (b) gradation of sand sample

(b)

(a)

1

8

Damage to the Trishuli Highway

Steep slope

Figure 7. Cracks and fissures on the roadway triggered by the earthquake

Rain water and/or aftershockscan cause slope failure

2

9

Rock fall along the Trishuli Highway

Cracks on the road

Figure 8. A bus impacted by falling rocks near Nigala village

Need for protectionsystems !!!

3

4

10

Damage to the Trishuli earth dam

Hydropower plant damaged

earth dam

Figure 9. A view of the reservoir and earth fill dam as seen from Gerkhu village

Dam Length: 1150mDam Height: 12m upstream side

20m downstream sideCrest width: 4 m

5

11

crest

earth fill dam12

Figure 10. (a) a view of the reservoir as seen from the upstream side; and (b) cracking at the upstream side

(a)

(b)

6

7

12

Figure 11. (a) schematic cross-section of Trishuli dam; (b) ground settlement at the upstream side; (c) cracking on the crest; and (d) no apparent damage at the downstream side

Compacted gravelly/silty soil

reservoir

?

Upstream side

Downstream side

Crest

Need investigations to verify dam body is not damaged

(a)

(b) (c)

(d)

89

10

13

Soil liquefaction

Figure 12. Location of liquefied area at the reservoir waterfront

Figure 13. Cracking and lateral spreading of gently sloped ground

Max. distancefrom waterfront : 29m

Max. length: 52 mMax. opening: 60 cm

11

12

14

29 m

52 m

0.01 0.1 1 100

20

40

60

80

100

Perc

enta

ge fin

er b

y weig

th: %

Particle size: mm

Pote

ntilly

lique

fiable

soils

most liquefiable

soils

Pote

ntilly

lique

fiable

soils

Trishuli boiled silt

Figure 14. Liquefaction effects near reservoir waterfront: (a) boiled silt volcanoes ; (b) widespread boiled silt along cracks; and (c) gradation of boiled silt samples

(boundaries for potentially and most liquefiable soils are adopted from Tsuchida, 1970)

90% non-plastic fines (< 75 μm)

(a)

(c)(b)

Boiled silt13

1415

3.2 Melamchi areaTrishuli

Bahunipati

Melamchi

Kathmandu City

Photo 15

Photo 20

0 5 10 km

Northeast of Kathmandu City Photo 16,17

• 90 km from Mw7.8 (4/25)• 40 km from Mw6.7 (4/26)

Figure 15. Survey route and location of site investigations in Melamchi area

16

Retaining wall failure

Figure 16. Cracking on the roadway induced by retaining wall failure

Need stabilization work to avoid wall collapse

Sliding of the wall and settlement of the backfill

15

17

Ready-to-fall boulders

Figure 17. (a) shallow landslide near Melamchi town ; and (b) unstable boulders threatening houses and road

Shallow landslide and unstable rocks

16

17

(a) (b)

18

3.3 Epicentral area

Figure 18. Survey route and location of investigation sites in the epicentral area

Photo 31

Photo 18,19Photo 32

Photo 20,21Photo 22-24

Photo 26-30

19

Artificial slope

Figure 19. (a) Artificial slope survived earthquake; and (b) cracking on the slope

(a)

Expected collapse during rainy season

18

19

(b)

20

Landslides along the road to Baluwa and Barpak

Figure 20. Many shallow landslides on steep slopes near the epicentral area

20

21

Landslide No. 1

45m

Figure 21. (a) Landslide occurred near Chanaute village; and (b) unstable rocks threatening the road (looking uphill from the road)

58m

62m

21

22

68o

(a)

(b)Unstable rocks

22

71m

38m

Landslide No. 2

22

(a)

(b)

(c)

23

24

Figure 22. (a) Collapsed hill blocking the road; (b) large size boulders; and (c) limestone rock fragment

Limestone

23

107m

300m

Landslide No. 3

Figure 23. Landslide/rock avalanche occurred near Baluwa village

25

Cracks parallel to the landslide front

26

27

24

29

Figure 24. (a) temporary trail between the landslide debris and rocks; and (b) fissured rock formation threatening people

30 Unstable fissured rock

(a)

(b)

25

32Figure 26. Large-size rock fall

nearby Chanaute

30 Unstable cracked rock

(a)

31

Figure 25. Rock fall and temporary cleared road

Weathered rock

Rock block failure

Rock fall along the main road

More rock falls are anticipated during

rainy season/aftershoks

26

3.4 Pokhara ValleyPokhara Valley

0 2.5 5 km

POKHARACITY

ArmalaPhoto 31-33

Distance from epiceter:• 71 km from Mw7.8 (4/25)• 86 km from Mw6.6 (4/25)• 205 km from Mw6.7 (4/26)

Figure 27. Survey route and location of investigation sites in the Pokhara Valley27

Sinkhole in the Armala area

Figure 28. Muddy water observed after the earthquake

33

Erosion process in the subsoil

28

No new sinkholes and no change in size of existing sinkholes

Nov 2014

May 2015

34

35

Figure 29. A typical sinkhole in the Armala area

Possible sinkhole formationduring rainy season

29

4. SummaryThe post-earthquake damage survey conducted fromMay 1 to 6 can be summarized as follows:

Trishuli area:• A number of landslides and rock falls occurred and

cracking of the road were often observed

• Silt boiling and lateral spreading of a gentle slopewere the evidence of liquefaction occurrence

• Damage to an earth fill dam was observed.However, fissures appeared along the dam axis andno water leakage was reported

30

Melamchi area:A number of landslides were triggered not only by themainshock but also by a major Mw6.7 aftershock thatoccurred 40 km northeast of Melamchi

Epicentral area:Landslides and rock falls occurred in many locations.One of the large landslides blocked the road betweenBaluwa village and Barpak village making it difficult forrescue teams to quickly reach affected disaster area

Pokhara Valley:In the Armala area, while sinkhole formation has beenin progress since November 2013, no new sinkholeswere formed by the earthquake

31

The following recommendations are given by theAuthors:

• Inspection of the landslide and rock fall areas iscrucial because cracked and unstable rocks still remainon the slope. Surviving slopes may experience furtherdamage during aftershocks and/or rainy season

• Cracking at the upstream side and on the crest ofTrishuli earth fill dam suggests deterioration of thedam body, which may be accelerated to failure of thedam. Therefore, rehabilitation works are required forthe dam as early as possible after conductingdetailed investigations

5. Recommendations

32

• In the mountainous area, formation of naturallandslide dams was reported. They should besurveyed in order to evaluate possible breaching

33

Full details of survey damage are reported in

Chiaro, G., Kiyota, T., Pokhrel, R.M., Goda, K., Katagiri, T. andSharma, K. (2015). Quick report on damage survey of the 25 April2015, Gorkha Earthquake, Nepal. Soils and Foundations, in review.

Goda, K., Kiyota, T., Pokhrel, R.M., Chiaro, G., Katagiri, T., Sharna, K.and Wilkinson, S. (2015). The 2015 Gorkha Nepal Earthquake:insights from earthquake damage survey. Frontiers in BuiltEnvironment (Earthquake Engineering), in review.

34

Thank you very much for your kind attention!

AcknowledgementsMr. Pokhrel P. (Tribhuvan University, Nepal)Mr.Kato, Y. (NHK)Mr. Hirano, M. and Mr. Kaneko, J. (The Mainichi Newspapers)

35