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Ganglakha-Dungna GC Road Geotechnical Assessment Report

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.
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
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.
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
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2. Panoramic view of Taguna Landslide at ch.12.00 Km
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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
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4. Condition of road as of 27th September, 2014 ( Tension cracks )
5. Highly fractured Quartzite bedrock outcropped
near slide area
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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 )
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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
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Tentative alignments of Option A & Option B
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.
Site Assessment Report
Submitted by:-
(Dilip Thapa)
Executive Engineer
Geotechnical Section, Design Division