Tokyo, 1 June 2015 Geotechnical damage in rural areas caused by the 2015 Gorkha Nepal Earthquake Chiaro Gabriele JSPS RESEARCH FELLOW, University of Tokyo Lecturer, University of Canterbury, New Zealand Kiyota T., Pokhrel R.M., Katagiri T. Institute of Industrial Science, University of Tokyo Goda K. University of Bristol, United Kingdom Sharma K. University of Alberta, Canada
Outline: 1. Damage survey 2. Tectonics of Nepal 3. Geotechnical damage (rural areas) 3.1 Trishuli area 3.2 Melamchi area 3.3 Epicentral area (Baluwa) 3.4 Pokhara Valley 4. Summary 5. Recommendations 3
Advanced Party of Japanese Investigation Team for the 2015 Nepal Earthquake Disaster JGS: Japanese Geotechnical Society JSCE: 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 Pokhara Baluwa Mw6.6 VII Mw7.8 VI VIII Trishuli Melamchi Mw7.3 Kathmandu Mw6.7 0 25 50 km Figure 2. Survey routes and location of investigation sites 3
2. Tectonics of Nepal MFT MBT MCT Himalaya MCT: Main Central Thrust MBT: Main Boundary Thrust MFT: Main Frontal Thrust Tibetan Plateau 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) 5
3. Geotechnical damage in rural areas Figure 4. Typical village in mountainous areas of central Nepal 6
3.1 Trishuli area Northwest of Kathmandu City Trishuli Gerkhu Photo 6-14 Photo 5 50 km southeast of the Mw7.8 (4/25) epicenter Battar Photo 3,4 Nigala Ranipauwa Photo 1,2 0 5 10 km Kathmandu City Figure 5. Survey routes and location of investigation sites along Trishuli Highway 7
Shallow landslide on steep slope (a) 1 Percentage finer by weigth: % 100 80 60 40 20 Weathered sedimentary rocks 0 0.01 0.1 1 10 Particle size: mm (b) D 50 =0.21 Ranipauwa sand Figure 6. (a) shallow landslide occurred near Ranipauwa; and (b) gradation of sand sample 8
Damage to the Trishuli Highway Steep slope Rain water and/or aftershocks can cause slope failure 2 Figure 7. Cracks and fissures on the roadway triggered by the earthquake 9
Rock fall along the Trishuli Highway 4 3 Need for protection Cracks on systems!!! the road Figure 8. A bus impacted by falling rocks near Nigala village 10
Damage to the Trishuli earth dam Hydropower plant damaged earth dam Dam Length: Dam Height: Crest width: 1150m 12m upstream side 20m downstream side 4 m 5 Figure 9. A view of the reservoir and earth fill dam as seen from Gerkhu village 11
(b) crest 7 2 1 earth fill dam 6 (a) Figure 10. (a) a view of the reservoir as seen from the upstream side; and (b) cracking at the upstream side 12
(b) (c) 8 Upstream side 9 Crest Need investigations to verify dam body is not damaged (d) (a) reservoir Compacted gravelly/silty soil? 10 Downstream side 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 13
Soil liquefaction 11 Figure 12. Location of liquefied area at the reservoir waterfront Max. distance from waterfront : 29m Max. length: 52 m Max. opening: 60 cm 12 Figure 13. Cracking and lateral spreading of gently sloped ground 14
(a) 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) 29 m 13 (b) Percentage finer by weigth: % 100 80 60 40 20 Potentilly liquefiable soils Boiled silt 52 m 90% non-plastic fines (< 75 μm) most liquefiable soils Potentilly liquefiable soils (c) Trishuli boiled silt 14 0 0.01 0.1 1 10 Particle size: mm 15
3.2 Melamchi area Trishuli Northeast of Kathmandu City Photo 16,17 Melamchi Photo 20 Photo 15 Bahunipati Kathmandu City 90 km from Mw7.8 (4/25) 40 km from Mw6.7 (4/26) 0 5 10 km Figure 15. Survey route and location of site investigations in Melamchi area 16
Retaining wall failure Need stabilization work to avoid wall collapse Sliding of the wall and settlement of the backfill 15 Figure 16. Cracking on the roadway induced by retaining wall failure 17
Shallow landslide and unstable rocks (a) (b) Ready-to-fall boulders 17 16 Figure 17. (a) shallow landslide near Melamchi town ; and (b) unstable boulders threatening houses and road 18
3.3 Epicentral area Photo 26-30 Photo 32 Photo 18,19 Photo 31 Photo 22-24 Photo 20,21 Figure 18. Survey route and location of investigation sites in the epicentral area 19
Artificial slope (b) Expected collapse during rainy season (a) 19 18 Figure 19. (a) Artificial slope survived earthquake; and (b) cracking on the slope 20
Landslides along the road to Baluwa and Barpak 20 Figure 20. Many shallow landslides on steep slopes near the epicentral area 21
Landslide No. 1 (a) (b) Unstable rocks 45m 22 68 o 58m 62m 21 Figure 21. (a) Landslide occurred near Chanaute village; and (b) unstable rocks threatening the road (looking uphill from the road) 22
Landslide No. 2 (a) (b) 23 (c) 71m 22 38m 24 Limestone Figure 22. (a) Collapsed hill blocking the road; (b) large size boulders; and (c) limestone rock fragment 23
Landslide No. 3 27 26 Cracks parallel to the landslide front 107m 300m 25 Figure 23. Landslide/rock avalanche occurred near Baluwa village 24
(b) Figure 24. (a) temporary trail between the landslide debris and rocks; and (b) fissured rock formation threatening people (a) 30 Unstable fissured rock 29 25
Rock fall along the main road 30 Unstable cracked rock (a) Weathered rock Figure 25. Rock fall and temporary cleared road More rock falls are anticipated during rainy season/aftershoks 31 Rock block failure Figure 26. Large-size rock fall nearby Chanaute 32 26
3.4 Pokhara Valley Pokhara Valley Photo 31-33 Armala Distance from epiceter: 71 km from Mw7.8 (4/25) 86 km from Mw6.6 (4/25) 205 km from Mw6.7 (4/26) 0 2.5 5 km POKHARA CITY Figure 27. Survey route and location of investigation sites in the Pokhara Valley 27
Sinkhole in the Armala area Erosion process in the subsoil 33 Figure 28. Muddy water observed after the earthquake 28
No new sinkholes and no change in size of existing sinkholes Nov 2014 34 Possible sinkhole formation during rainy season 35 May 2015 Figure 29. A typical sinkhole in the Armala area 29
4. Summary The post-earthquake damage survey conducted from May 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 slope were the evidence of liquefaction occurrence Damage to an earth fill dam was observed. However, fissures appeared along the dam axis and no water leakage was reported 30
Melamchi area: A number of landslides were triggered not only by the mainshock but also by a major M w 6.7 aftershock that occurred 40 km northeast of Melamchi Epicentral area: Landslides and rock falls occurred in many locations. One of the large landslides blocked the road between Baluwa village and Barpak village making it difficult for rescue teams to quickly reach affected disaster area Pokhara Valley: In the Armala area, while sinkhole formation has been in progress since November 2013, no new sinkholes were formed by the earthquake 31
5. Recommendations The following recommendations are given by the Authors: Inspection of the landslide and rock fall areas is crucial because cracked and unstable rocks still remain on the slope. Surviving slopes may experience further damage during aftershocks and/or rainy season Cracking at the upstream side and on the crest of Trishuli earth fill dam suggests deterioration of the dam body, which may be accelerated to failure of the dam. Therefore, rehabilitation works are required for the dam as early as possible after conducting detailed investigations 32
In the mountainous area, formation of natural landslide dams was reported. They should be surveyed 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. and Sharma, K. (2015). Quick report on damage survey of the 25 April 2015, 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 Built Environment (Earthquake Engineering), in review. 34
Thank you very much for your kind attention! Acknowledgements 35