Site Assessment Report The Regional Office, Phuentsholing reported the road damage at ch.12.00 km on Ganglakha – Dungna GC Road by the massive landslide that was triggered by heavy rainfall (Ref: - letter no. RO/DoR/PLing/2014-2015/W-28/294 dated 19th September, 2014). Regional office reported that restoration of this road stretch is not possible due to this massive landslide and proposed to re-align the road as an alternative option. In this regard, RO requested geotechnical engineer to be deputed at the earliest to check the feasibility of two options proposed by them for realignment. The road was also scheduled to be blacktopped by RO this financial year. Therefore, in pursuance the undersigned made a site visit to assess the proposed realignment options. The undersigned also assessed the landslide to check whether the damaged road stretch could be restored. Following officials from RO also visited the site; 1. Mr.Dorji Wangdi, Chief Engineer, RO, Phuentsholing. 2. Mr.P.L.Puri, Executive Engineer, SD Tala, RO, P/ling. 3. Mr.Tshering Dendup, Assistant Engineer, SD Tala, RO, P/ling. Assessment:• Team observed that about 1 km of road length have been affected adversely from ch.12.00 onwards due to mass movement of hill at Taguna. The road sunk about 100 m from its original level at the main slide area as of 27th September, 2014 with numerous fresh tension cracks observed at road approaching towards slide area. It mainly indicates the very active large scale landslide. The head scrap at crown of the slide starts right from the ridge of the hill and extended towards the settlement areas of Kungkha village. The nearest structure was 7 m from the edge of the slide. • The area was covered with thick vegetation and soil condition was highly saturated. It was difficult to ascertain the outcropped bedrock due to thick overburden all over. However, highly weathered and crushed boulders of quartzite and phyllite were seen at the slide area. It was also observed that prominent water was flowing out from the slide at numerous locations which signifies high groundwater table in the area. One of the causes of this massive landslide could be the significant amount of groundwater that facilitated movement during incessant rainfall due to excessive pore water pressure in the soil. • Along with groundwater condition, geological intrinsic cause was also ruled in. Looking at the land morphology in the vicinity along with observation of Gneiss rock type assimilating with Quartzite and Phyllite rocks indicates the sudden change in the geological formation in the area. Most massive landslides with clear slide surface also occur in the upper soil layer of extremely weathered or crushed rockbed beneath. This could also be one of the factors that caused sudden downward movement. Main Central Thrust (MCT) could possibly traverse or pass through this particular area. • The village tshogpa Mr.Mon Bahadur Rai of Kungkha village said that the first subsidence occurred in year 2000 during heavy torrential rainfall that lasted for about a week. Thereafter the area started to slide every year during heavy rainfall. It was also been reported that Regional 1|Page Site Assessment Report Office every year fills up the crack developed on the road in this area. This indicates the creep or area in mass moving slowly over the years. • After thorough site assessment in view of the magnitude, large-scaled coverage, present dilapidated road conditions and future possibility of further movement, it is beyond one’s imagination to restore the road with the appropriate mitigation measures. The assessment team found that the road cannot be restored at all at the same place neither can cut the hill to make the road trafficable, therefore, recommended to by-pass the road or realigned from other suitable areas. 1. Taguna Landslide area shown on Google map 2|Page Site Assessment Report 2. Panoramic view of Taguna Landslide at ch.12.00 Km 3|Page Site Assessment Report Existing road Original alignment About 100 m Subsided road 3. About 1 Km of road stretch has been subsided about 100 m from the original alignment at ch.12.00 Km 4|Page Site Assessment Report 4. Condition of road as of 27th September, 2014 ( Tension cracks ) 5. Highly fractured Quartzite bedrock outcropped near slide area 5|Page Site Assessment Report Re-alignment Options:The tentative re-alignment routes have already been identified by RO, Phuentsholing, the undersigned just visited these routes to ascertain the feasibility of the options. These options are; 1. Option A: - In between ch.9.00 to 10.00 km a take - off may be taken to by-pass this chronic landslide area. Viewing the landform of the area, gentle slope and saddle above the existing road, the option may be favorable to by-pass the slide. However, it is to be ensured that hairpin zigs should be avoided as much as possible in short interval as this may aggravate instability of the area as the terrain condition is not much different from Taguna slide in terms of geological formation, soil condition which is moist and saturated. In this area, vegetation is dense; it may be due to high water table and the sub-soil is saturated throughout the year. Though at present, it is certain that there is no threat to the area by the slide in the vicinity. It is recommended that detailed survey is carried out to ascertain the exact road length from the take-off point to by-pass slide area. The preliminary survey showed around 6 km fresh cut is needed to join to other side of the hill. 6. Area for re-alignment identified by RO,Pling ( 1st Option ) 6|Page Site Assessment Report 2. Option B: - The road should follow the existing farm road of Chongyakha village which 15 Kms from the take-off point located at ch.7.00 Km (also known as Sanu Zero locally) on Ganglakha – Dungna GC Road. After reaching Chongyakha village, fresh cut of around 2 km should be carried out to join the existing road at Dophulakha which is 4 Km far from Kungkha village towards Dungna. The tentative road length will be 17 Km to join the existing road and to reach Kungkha village, road users have to come back 4 km from this point, thus making 21 km in total road length from Sanu zero to reach Kungkha village which was only 12 Km earlier. However, the exact road length will be known only after carrying out detailed survey. Also present Chongyakha farm road needs major maintenance beyond ch.7.00 Km from Sanu zero and the geometric correction in some areas. The two major unstable areas have also been observed on this farm road. Chongyakha village farm road 7. The take-off point for Option B 7|Page Site Assessment Report Tentative alignments of Option A & Option B Conclusion 1. It is extremely remote to re-store the existing road stretch at ch.12.00 km after assessing the site. This large scale landslide is still active and may slide further dragging whole of the hillock in the vicinity in nearer future. Whole of the hill is geologically very unstable. Therefore, it is strongly recommended to re-align or by-pass this stretch for the road connectivity. 2. Both the options mentioned above are feasible for re-alignment but RO P/ling should take into account the shortest route possible to minimize the cost involvement in construction and subsequent maintenance cost. The exact road length will be ascertained only after carrying out detailed survey of the road. 3. Other alternative could be the connectivity from Lokchina side. RO P/ling should explore the possibility of linking Ganglakha – Dungna GC road to Lokchina in future which has connectivity to Phuentsholing via Amochu. 4. This road connects the five gewogs (i.e. Dungna, Getena, Lokchina, Metedkha and Phuentsholing) of Chhukha Dzongkhag. The connectivity to these gewogs are completely cutoff, therefore there is a dire need to restore the road immediately. 8|Page Site Assessment Report Submitted by:- (Dilip Thapa) Executive Engineer Geotechnical Section, Design Division 9|Page