appendix iii: geological investigation along road … · geological& subsoil investigation...

GEOLOGICAL& SUBSOIL INVESTIGATION REPORT: SASEC ROAD CONNECTIVITY PROJECT

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APPENDIX III: GEOLOGICAL INVESTIGATION ALONG ROAD CORRIDOR & SUB-SOIL INVESTIGATION OF MAJOR BRIDGE SITES AND LANDSLIDES

Pritulal Sharma Geotechnical Engineer KYINGKHOR CONSULTANCY SERVICES, THIMPHU Client: SASEC ROAD CONNECTIVITY PROJECT DEPARTMENT OF ROADS, THIMPHU


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CONTENTS Contents ................................................................................................................................................................ .................. 2

EXECUTIVE SUMMARY ................................................................................................................................................ 4

INTRODUCTION ................................................................................................................................................................. 5

SECTION A: GEOLOGICAL INVESTIGATION ALONG ROAD CORRIDOR ...................................... 7

I WORKING METHODOLOGY ................................................................................................................... 7

II GEOLOGY .......................................................................................................................................................... 7

III SUMMARY OF FINDINGS.................................................................................................................. 12

IV CONCLUSION ............................................................................................................................................... 13

SECTION B ......................................................................................................................................................................... 15

(GEOLOGICAL STUDY and SUB-SOIL INVESTIGATION of MAJOR BRIDGE SITES and LANDSLIDES) ................................................................................................................................................................... 15

I METHOD OF ASSESSMENT ................................................................................................................ 16

II GEOLOGY OF BRIDGE SITES ............................................................................................................ 16

III SEISMIC REFRACTION SURVEY ...................................................................................................... 21

IV SEISMIC SURVEY AT MAJOR BRIDGE LOCATIONS ON NGANGLAM-DEWATHANG ROAD .......................................................................................................................................... 24

V SHUGURI SLIDE .......................................................................................................................................... 42

VI GEOLOGICAL STUDY AT PASAKHA ROAD & BHAWANIJHORA SLIDE ................... 44

1 GENERAL ................................................................................................................................................................ .. 53

The factor of safety: a deterministic approach .......................................................... 54

Slope failure in Circular mode: .......................................................................................... 55

Rotational failure: method of slices ................................................................................. 55

2. STABILITY ASSESSMENT (SHUGURI SLIDE)..................................................................................... 56

HAZARD DUE TO LANDSLIDE ....................................................................................... 60

REMEDIAL MEASURES ...................................................................................................... 60

RECOMMENDATION AND CONCLUSION ............................................................... 61

3 STABILITY ASSESSMENT (BHAWANIJHORA SLIDE) .................................................................... 61

HAZARD DUE TO LANDSLIDE ....................................................................................... 64

RECOMMENDATION AND CONCLUSION ............................................................... 65

VII CONCLUSION ......................................................................................................................................... 65

ANNEXURES ............................................................................................................................. 66

GOOGLE EARTH IMAGES: GENERAL & GEOLOGICAL FEATURES ................................... 66

LOCATION MAP OF NGANGLAM-DEWATHANG ROAD ............................................................... 71

Geological Map of Bhutan (Reproduced from Bhargava ed. 1995) .............................................. 71

Geological Formations through Road Corridor ....................................................................................... 72


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Geologic Map Of Bhutan Showing Main Thrusts ................................................................................... 72

LABORATORY TEST RESULTS ................................................................................................................... 74

REFERENCES ................................................................................................................................................................ .. 76


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EXECUTIVE SUMMARY The objective of the study is geological and geotechnical investigation of road alignment and bridge sites of Nganglam-Dewathang road. Surface geological mapping of the complete road corridor and subsoil investigation by seismic refraction technique was carried out. Soil and rock sampling and laboratory analysis of the samples from unstable areas of road corridor was conducted. Sieve Analysis, Bulk Density and Direct Shear test was conducted on soil samples from some unstable areas. Point Load and Water Absorption Test was also carried out for the rock samples collected from major bridge sites. As per the Bhutan Geological Maps, the major landslides/unstable areas observed are found mostly near the MBT which have been avoided from the road alignment during detailed design. Except for Shuguri slide at Km 6, the geotechnical investigation of road alignment of Nganglam-Dewathang road is located on stable formations.

Surface geological study and seismic refraction on the bridge sites, and laboratory tests of samples indicate stable geological formations on Kirung Ri, Menchudrang Ri and Tshokhi Ri on Nganglam-Dewathang road. Seismic study and geological observations carried out at Shuguri slide zone indicate loose formation (highly weathered rocks). Seismic traverses have also been made across the Bhawanijhora slide debris (along Pasakha road crossing), along foothill of this slide above masonry wall and above (and parallel to) second gabion wall (Pasakha side). Compact colluviums and alluvium gravels and boulders possibly with weathered rocks underlie below these loose materials. Unlike in Dewathang-Nganglam section the surface geological features like the exposure of bedrock is absent in the widespread dry bed of Bhawanijhora which is covered for more than 5m depth with the slide debris. The recommended remedial measures especially for major unstable areas (Shuguri slide) on Nganglam-Dewathang section and Bhawanijhora slide at Pasakha road should be followed during construction.


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INTRODUCTION BACKGROUND SASEC Road Connectivity Project is a project funded by the Asian Development Bank (ADB), the detailed project designs and procurement assistance of which was entrusted to M/s Kyingkhor Consultancy Services, Thimphu. Under the SASEC Road Connectivity Project, Nganglam-Dewathang highway, a part of the second southern east-west national highway and Pasakha Access road will be constructed/improved. The Department of Roads (DOR) under the Ministry of Works and Human Settlements is the executing agency for this Project.

SCOPE AND OBJECTIVES OF THE CONSULTING SERVICES

Detailed geological investigation is an important part of study for detailed design of this project During the implementation of the civil works, a total of 74.5 km of National Highway connecting Nganglam to Dewathang under Pemagatshel and Samdrupjongkhar Dzongkhags will be constructed and about 1km of Pasakha access road connecting Pasakha Industrial area from the Indian border at Khokla will also be improved/upgraded. The detailed TOR of the design consultants are in Appendix I of the Main Report on Detailed Design of this project.

The detailed geological investigation of the road corridor including the major landslides and bridge sites for the proposed Nganglam-Dewathang National Highway and a section of Pasakha road passing through Bhawanijhora slide debris has been carried out to investigate the geological and geotechnical conditions in the project area. Such study is important for design and evaluation of slope stability assessment, its stabilization and erosion control measures; slope drainage; stability and ground condition for retaining walls and major engineering structures. The study is also equally important in selecting the alignment to avoid major unstable sites wherever possible. Further the study also involves geological study in selecting the bridge sites.

The report consists of two sections namely; Section A (Geological Investigation along the Road Corridor) and Section B (Geological Study and Sub-Soil Investigation of Major Bridge Sites & Landslides).

Surface Geological Investigation along Nganglam-Dewathang road corridor is described in Section A. Section B describes on the surface geology as well as the sub-soil investigation by seismic refraction technique on major landslides and bridge sites. Section B also consists of geological and subsoil study at Bhawanijhora debris slide and its impact on Pasakha access road connecting Pasakha Industrial area from the Indian border at Khokla.

GENERAL APPROACH OF THE CONSULTANT

After complete mobilization of the team and the required resources, the Task schedule and the Consultants’ schedule was discussed and finalized, and the assignments planned and executed in accordance with the findings and recommendations of the feasibility study carried out by IMC Worldwide, UK in association with Messers Gyaltshen Consultancy and Himalayan Engineering and Management Consultants. Site visits and consultations with the relevant agencies especially DGM and concerned stakeholders have been conducted as necessary to ensure that adequate and reliable data/information is gathered for the project design.


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COLLECTION OF DOCUMENTS, LITERATURE REVIEW AND DESK STUDY

All the relevant documents like feasibility study reports, detail design reports, alignment maps and drawings, documents on climate change adaptation measures has been collected and referred/reviewed. The proposed road alignment was reviewed prior to actual start of the field survey works.

MEETINGS & CONSULTATIONS

Meetings and consultations were held in Thimphu with PCU/DOR, for relevant project information, and necessary participation and support. After the road alignments were studied the team moved to the field for further consultations and field work. Meetings and consultations were also held with the Gewog and local communities for information, cooperation and support.


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SECTION A: GEOLOGICAL INVESTIGATION ALONG ROAD CORRIDOR

I WORKING METHODOLOGY a) Geological Mapping

Geological mapping was carried out along the project road corridor gathering information on the major unstable areas of the project road section. Brunton Compass, Clinometers, Measuring Tapes, Geological Hammer, Digital cameras and handheld GPS were used in geological mapping. It comprised the following studies:

• Collection of information on geological, geographical, and geotechnical conditions; • Information on slopes and land use pattern of the area, identification of rocks types, its

angle of dip and direction; • Drainage condition, surface water/ground water conditions based on ground

observations, and seepage condition; • Identification of material types (colluviums, residual, alluvial soils etc); • Identification and record of existing slope instability features (major landslides,

rockslides, gully erosion, debris slides, areas of major bank scouring/toe-erosion and sinking or subsidence areas;

b) Geotechnical Study:

Geotechnical study of landslide and unstable areas was carried out including assessment of soil parameters. Visual identification of overlying soil type such as colluviums, alluviums and residual soil was made in the field. The assessment of soil was carried out in the field by visual identification of overlying soil. The in-situ rock strength was measured by geological hammer.

II GEOLOGY 1 General Geology Prior to starting the geological and geotechnical assessment of the project areas, a desk study

of the geology of Bhutan was conducted. Table presented below on The General Stratigraphic

Order of the Bhutan Himalayas and Geological Map of Bhutan Himalayas indicates various

formations and rock types, major faults and thrusts, and folds etc.

Table: General stratigraphic order of the Bhutan Himalayas


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Tethyan succession Conglomerate, shale, siltstone, sandstone, limestone, volcanic

--------Unconformity--------

Thimphu Group Gneiss, migmatite, amphibolites and high grade meta-sediments

--------Thimphu Thrust (MCT)---------

Jaishidanda Formation

Biotite-garnet+saturolite schist with tectonic slivers of granite gneiss

------------Jaishidanda Thrust-------------

Shumar Formation Quartzite, phyllite and chlorite-mica schist, Barsong limestone, a few tectonic slivers of granite gneiss in the lower part

-------------Shumar Thrust------------

Deo Ri Formation Diamictite (A lithified, conglomeratic, siliciclastic rock which is unsorted, with sand and/or coarser particles dispersed through a mud matrix. The term is commonly used today in preference to ‘tillite’, which has clear genetic connotations), phyllite, quartzite

----------Unconformity? – locally tectonised

Buxa Group Gritty feldspathic quartzite, limestone, dolomite, quartzite conglomerate, slate, quartzite, phyllite, tectonic scales of Permian Setikhola Formation

http://www.encyclopedia.com/doc/1O13-silicification.html�

http://www.encyclopedia.com/doc/1O13-sand.html�

http://www.encyclopedia.com/doc/1O13-matrix.html�


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------------Buxa Thrust-----------

Damuda Subgroup Sandstone, siltstone, shale, coal beds

----------MBT-----------

Siwalik Group Sandstone, siltstone, shale, clay, conglomerate

---------Fault (MFT)--------

Quaternary succession

Sand, conglomerate, silt, clay


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2. Geology through the Road Corridors Four geological formations occupy the proposed Nganglam-Dewathang road corridor. The geological formations are indicated in the Geological Map. The Table below describes the types of rock found in these geological formations.

Group Formation Main Rock Types Remark

Siwaliks I-III Sandstones, siltstones, shale, clay, conglomerate and local lignite

Buxa Group Manas Dolomite, limestone, Phyllite, Quartzite, and some local conglomerate

Diuri Formation Phyllite, siltstone, diamicitite and

quartzite

Setikhola Formation Feldspathic sandstone, siltstone, shale,

carbonaceous shale, coal

The geological maps and Google Earth Images of the study area are appended in the Annexure for reference. Geological mapping and geotechnical studies done along the road corridor are described section wise as follows:

Nganglam (0 Km)-Kirungri Bridge

The take off (Km 0) of proposed Nganglam-Dewathang highway starts from Nganglam town through the right bank slope of Kirungri following the existing Nganglam-Chokhorling farm

Cut slope failure


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road. This existing Farm Road has gradient that fits to the highway ruling gradient of 5%-7% which will require grade improvement in most of the locations for the 15 km stretch of the farm road. The geological formation falls under I-III Formation of Siwalik group with sandstone, siltstone, clay and silt. Approximately after 50m of the take off bed rock of fractured sandstone (dipping 90°/54°) is exposed for a stretch of 150m along the road on both the hill and valley side. Cut slope failure has occurred in the monsoon of 2013 at the upper part of the hillside after 0.05Km along this stretch (refer photos above). Maintenance of proper batter slope is recommended in this stretch with slope angle not exceeding 45°. Bio-engineering and maintenance of back slope is also recommended in the section between 0.46Km and 0.50Km. Drangnala Sing Borang village (1 Km) is stable site with gently sloping terrain. Hill side between 1.35Km and 1.45Km is the water-shed area. It is a drinking water source for the nearby settlement. Surface drains with water draining into a collection tank should be provided in this stretch. Between 1.8Km and 2.1Km fractured sandstone rock is present. Some cut slope failure is observed due to high batter slope in the existing road. Bio-engineering and maintenance of back slope angle at 1:1 (V: H) with 5m height is recommended. Unstable site with dark grey carbonaceous shale is present at 2.31Km to 2.33Km requiring bio-engineering and catch drains at the hill side as the area is marshy. Between Km 2.52Km and Km 2.58 the slopes are unstable due to seepage and presence of dark grey carbonaceous shale where proper drain management is required. Kirungri Bridge to Km 15.5 The alignment crosses Kirungri immediately after the Dizama settlement. The alignment continues to follow the farm road even after crossing Kirungri, over which a bailey bridge has already been built. The farm road is followed up through Shuguri village till 15Km before reaching the Yargewong village/settlement. This stretch is stable except at certain stretches where black soils are observed especially before reaching Shuguri settlement (refer Shuguri slide (Km 6) in the Google Earth image) Deposits of shallow-seated colluvial debris of phyllite (dark grey gravelly silt) are also observed between Km 11.6 to Km 11.9 just before Lungkhangma (the Nye stone) at Km 13. Yargewong Km 15.5 to Chokhorling Km 26.5 The road alignment deviates from the farm road at Km 15, and passes above the Yargewong settlement through stable formation. The central line pegging (Ghat tracing) for detailed design has been marked after the proposed highway diverts from the existing farm road at 15Km. The road alignment passes intermittently through rocky terrain (hard, strong quartzite). This stable mostly rocky stretch (Km 17 to Km 17.5 and Km 19 to Km 22) will also be potential source of construction materials. The alignment runs above the water source of both Yargewong and Chokhorling near Km17 and Km 27 respectively. Chokhorling to Rigi saddle The formation between Chokhorling till the bridge crossing at Tshokhiri is mostly gravelly soil with phyllite and quartzite as rock types. After Tshokhiri the rocks are mostly quartzite and phyllite and other rocks of Diuri formation.


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Few landslides are present between the Chokhorling mobile tower and Rigi saddle. However the road alignment has avoided such unstable areas. Hence the alignment passes mostly through the stable areas. The main drinking water source at Khalatsho lies below the road alignment. The shifting of bridge crossing further downstream at Tshokhiri (Km42) has avoided most of the rocky stretches. Rigi saddle to Rishore: This stretch too passes through stable area. No major instabilities are present except the Rigi saddle near the Rigi saddle. The Rigi slide however which lies towards south of the saddle does not affect the road as the alignment corridor avoids this unstable slope. The presence of strong bed rock of hard phyllite on both the slopes at the Rigi saddle indicates stable that the saddle is a stable area. The Diuri bridge site has been shifted further downstream (Km 57) avoiding steep rocky stretch and shortening the road length. Rishore to Dewathang (Terminal Point) The road corridor is stable all the way till it reaches Dewathang. Minor cut slope failures are expected in some watershed areas between Rishore and Chenary in Dewathang side. Besides water management, protection walls and bio-engineering are suggested in such zones.

III SUMMARY OF FINDINGS Based on the field observations and gauging, the findings are summarized as follows:

• The Nganglam-Dewathang road corridor passes through four Geological Formations namely, Siwalik, Manas, Diuri and Setikhola Formation

• The road alignment takes off at Nganglam town following the Chokhorling farm road through gentle to moderately sloping hill.

• Between Km 0.46 to Km 0.5, loose gravelly silt of sandstone and phyllite is present at the hill side. Gabion wall and Bio-engineering and maintenance of back slope angle is recommended in this section.

• Hill side between 1.35Km and 1.45Km is the water-shed area. It is a drinking water source for the nearby settlement. Surface drains with water draining into a collection tank should be provided in this stretch.

• Between 1.8Km and 2.1Km fractured sandstone rock is present. Some cut slope failure is observed due to high batter slope in the existing road. Bio-engineering and maintenance of back slope angle at 1:1 (V: H) with 5m height is recommended.

• Unstable site with dark grey carbonaceous shale is present at 2.31Km to 2.33Km requiring bio-engineering and catch drains at the hill side as the area is marshy. Between Km 2.52Km and Km 2.58 the slopes are unstable due to seepage and presence of dark grey carbonaceous shale where proper drain management is required.

• The terrain is mostly stable till Kirungri. • Mostly gently sloping (15° to 30°) terrain between Kirungri to Shuguri with colluvial soil

cover with some shallow seated failures. • Shuguri slide (Km 6) is an area of concern as it is an unstable area due to presence of

highly weathered phyllite and black soil and improper water management. The Shuguri is presented separately in Section B of the Report on its impact and control measures.

• The road corridor passes through flat to very gentle terrain between Shuguri to Lungkhangma before descending to Yargewong village

• The road alignment passes above the community’s main drinking water source at Chokhorling and Khalatsho. These stretches are observed to be stable.

• Rocky and steep terrains are present between Yargewong and Gazawong, (Km 17 to 17.5 and Km19-22).


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• The major Rigi slide has been avoided by realigning the road through the stable Rigi saddle (which has strong bedrock of phyllite exposed on surface).

• Between Rishore and road alignment terminal point at Dewathang, the terrain is gentle to moderately gentle slopes with rock exposure in some locations.

• Most of the unstable areas and rocky stretches through which the Nganglam-Dewathang road was aligned during feasibility have been avoided by realigning in such stretches (like Rigi slide).

• Bridge locations for Menchudrang Ri, Tshokhiri and Diuri has been shifted to avoid wider bridge span and stronger foundation. Shifting of these bridge locations have not only shortened road length but also avoided rocky stretches especially in case of Tshokhiri and Diuri.

• The road alignment terminates at the S/J-Trashigang highway in a stable area. • Geological investigation indicates that the unstable areas observed are located close

to the MBT line (refer Geological Maps in Annexure).

IV CONCLUSION

• Alignment runs mainly through stable formation except at some areas where it intersects the MBT along the road corridors.

• The major Rigi slide has been avoided at the Nganglam-Dewathang road by realigning through the stable Rigi saddle (which has strong bedrock of phyllite exposed on surface).

• The major bridge sites and landslides including the Pasakha road section has been studied in detail and the findings with recommendation are presented in Section B of this report.

• The Shuguri slide in Nganglam-Dewathang road and the Bhawanijhora slide at Pasakha section are the two major unstable sites. These unstable sites too are explained in Section B of this report.

• The major bridge sites namely Kirungri, Menchudrang Ri, Tshokhiri and Diuri on Nganglam-Dewathang section have been located at reasonably stable locations where there are presence of bed rocks.

• For the black clay soil (found in some locations between Nganglam and Yargewong along Nganglam-Dewathang road corridor) at least one meter of clay soil shall be removed and replaced by suitable improved sub-grade material.

• The road alignment passes above the community water sources at Chokhorling and Khalatsho. These stretches are observed to be stable.

• Water management, protection walls and bio-engineering are suggested in some watershed areas between Rishore and Chenary.

• Most of the unstable areas and rocky stretches through which the Nganglam-Dewathang road was aligned during feasibility study have been avoided by realigning in such stretches (like Rigi slide). Other smaller slides between Rigi saddle and Diuri Bridge have also been avoided.


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• Bridge locations Tshokhiri and Diuri have been shifted to avoid wider bridge span and stronger foundation. Shifting of these bridge locations have not only shortened road length but also avoided rocky stretches of the road.

• The road alignment terminates at the S/J-Trashigang highway in a stable area.


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SECTION B

(GEOLOGICAL STUDY AND SUB-SOIL INVESTIGATION OF MAJOR BRIDGE SITES AND LANDSLIDES)


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I METHOD OF ASSESSMENT

The method of assessments was based on the following:

1. Undertaking site visits to the proposed bridge locations and major landslides to familiarize with the existing site conditions;

2. Advise the surveyors to incorporate geotechnical requirement in the topo- survey of the bridge sites.

3. Carrying out studies such as the site condition, surface and subsurface investigations, using seismograph; Locations of test points were decided in consultation with the Team Leader/Bridge Engineer.

WORKING METHODOLOGY

The following methodology was adopted for study of the bridge sites:

1. Sub-soil investigation was carried out by Seismograph ES 3000 available with the Department of Roads, Thimphu, to determine on the type and depth of materials. The location of bridge site was decided in consultation with the Project Team officials.

2. ES 3000 (12-channel) Seismograph was used to conduct the seismic refraction survey and Seisimager/SW program was used for data analysis and result interpretation.

3. Surface geology of the sites was studied using Brunton Compass, Clinometers, Measuring Tapes, and Geological Hammer, Digital cameras and handheld GPS.

It comprised of the following studies:

• Collection of Information on slopes and land use pattern of the area, identification of rocks types, its dip amount and direction and information on discontinuity, fissures, faults,

• Identification of material types; soil/rock classification, geotechnical strength of soil/rock materials and the configuration of soil/rock strata,

• Identification and recording areas of major bank scouring/toe-erosion

II GEOLOGY OF BRIDGE SITES I Kirungri Bridge

Right Bank


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The surface geology of Kirungri Bridge site is as follows:

Left Bank: The bed rock (strong, dark grey quartzite) is exposed at the edge of the river close to the bridge abutment. Bed rock dipping almost vertical are also seen inside river.

Right Bank: This bank has presence of strong, dark grey quartzite dipping at N26°E at 46° angle. The terrain slopes at 145°/65° at the upstream while the downstream terrain is very gentle. A soil slip that had occurred above the hard and stable rocky formation at the upstream is stabilizing.

Left Bank


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II Menchudrang Ri Bridge Site: Left Bank:

The bed rock consisting of hard phyllite with some quartzite intercalation dips at 330°/55°. It is slightly weak to strong, slightly weathered to fresh, light to dark grey in color. Thin layer of colluviums (boulders) with soil cover overlie the bed rock which is exposed for a height of about 5m from river bed.


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Right Bank

The right bank of the bridge site has flat top surface which is about 10m high from the river bed. The bed rocks are exposed along the slope which is overlain by thick colluviums boulders

and thin soil cover. The bed rocks are similar as that present in the left bank. The hard phyllite with some quartzite intercalation dipping at 330°/45°.

III Tshokhi Ri Bridge Site:

The geo co-ordinates taken (by hand held GPS) at bridge axis at river crossing is 26.87247° N and 91.37826° E. Photo shows bed rock exposed at left bank

Left Bank: It is flat to very gentle sloping terrain with rock exposure at the river site as shown in the photos above. This flatter terrain has thin alluvial covers (boulders and gravels of quartzite) and bed rock of strong quartzite. The terrain slopes towards west.

The rock type which is exposed towards river side is quartzite rock, strong to very strong, fresh, light grey to dark in color with exposed height of around 3m from river bed. Rock dips at 360°/54° and has continuity of more than 30m. Spacing of the exposed rock varies between 5cm to 20cm.

Left Bank

Right Bank

Right Bank


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Right Bank: This bank has flat top surface with steeper slope (about 8m high) between river bed and flatter top. The rock is strong, slightly weathered quartzite similar as that existing in the left bank. Rock dips at 360°/45°-55°, with spacing of 10-25cm.

IV Diuri Bridge Site:

It is located at the geographical coordinates (recorded by a handheld GPS) of 26° 52'38.8" N and 91° 26'23.8"E. The left bank slope lies in Dewathang side and right bank at Nganglam side of the proposed road.

The surface geology of Diuri Bridge site is as shown in the photos below:

Left Bank: The bed rock consisting of strong, dark grey quartzite and diamicite is exposed at the edge of the river at the proposed

Left Bank (Dewathang side)

Right Bank (Nganglam side)

Diuri Bridge Point


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location of the bridge abutment. Bed rock dipping at 45° towards north has dominant discontinuity along dip with joint spacing more than 20cm, indicating that the formation is stable. The height of the rock from river bed is about 4m.

Right Bank: This bank has presence of strong, dark grey quartzite and diamicite dipping N60°E at 15°. The terrain is very gentle sloping. The proposed location of the bridge abutment is at the rock exposure as seen in the adjoining photo. Only one major discontinuity set along the dip is observed with joint spacing more than 10cm. The exposed height of the exposed rock is 1.5 to 2m. Though the location of abutment is in the flood plain the exposed bed rock at nearby

upstream and downstream and across the river suggests the depth continuity of the bed rock for at least 3m to 4m at the proposed site. The rock diamicite is a strong cemented matrix of hard and strong quartzite with other local rocks.

III SEISMIC REFRACTION SURVEY The subsurface can generally be represented by different layers such as residual soil on the top, weathered layer, colluviums or alluvium in the middle, and the bedrock. These different layers have different wave velocities. Seismic waves travel at a different speed in different materials.

The compressional velocities of sound in some common materials are as follows:

- unsaturated weathered surface material (120 – 200 m/s) - unsaturated sand and gravel or alluvium (360 – 480 m/s) - saturated sand and gravel or alluvium (1200 – 1800 m/s) - metamorphic rock (3000 – 7000 m/s) (Haeni, 1988).

Nonetheless, the velocity of the seismic waves can significantly decrease if the formation has fractures and joints.

Generally, the velocity increases with depth, where the top soil has the lowest velocity and bedrock has the highest velocity. It may be noted that refraction seismic technique may not be effective in areas where a high velocity layer is underlain by a low velocity layer.

Refraction seismic method involves placing several receivers (geophones) in line with a seismic source on the ground surface. After triggering the seismic source, the signal that reaches the geophones is recorded for a specified duration for that particular shot record. The


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inter-geophone spacing was chosen depending on the available profile length, and the seismic energy was generated by striking a 7.5-kg hammer on a metal plate.

The first arrival time at the geophones contain information about the velocity and the depth of the underlying layers; the arrival time versus receiver offset plot is called the time-distance plot.

For this project, the refraction seismic data were recorded using Engineering Seismograph which can be quickly carried into remote areas to determine sub-surface profile at bridge, areas of instability, and for general determination of bedrock levels along the route. The seismic data were acquired using the 12-channel ES 3000 (12-channel) Seismograph provided by Department of Roads, Thimphu. A 7.5 kg hammer was used as source of seismic energy with 12 geophones spaced at 2m interval for a shot record. The seismic energy was generated by striking the hammer on a metal plate. For each traverse 5 shots were recorded at -4m, 0m, 11m, 22m and 26m locations along the traverse.

Traverses along and across the bridge axis were conducted on each bank of the bridge sites.

Data Processing: These field survey data have been analyzed using Seisimager/SW program (Pickwin and Plotrefa) available with Geotechnical Section, DoR, Thimphu. The recorded seismic data were uploaded to computer and seismic data processing was performed using SeisImager. The first arrival times were picked using the Pickwin module and tomographic inversion of the travel time data were carried out in Plotrefa module to generate the velocity model. The depths to the top of the underlying layers are calculated from the point on the travel time vs. offset plot, where the slope (1/velocity) changes. Result Interpretation: The material type has been determined by comparing the result of p-wave velocity from tomographic inversion and P-wave velocity characteristic of various materials indicated in Tables below together with the surface geological findings of the site. Corresponding Safe Bearing Pressure of the material at 5m to 10m depth for shallow bridge foundation can be determined from the Standard Safe Bearing Pressure Chart provided below.

Material Type Velocity Remarks

Top soil/Colluviums/uncompacted alluvium

300- 1000 m/s

Fractured/weathered rock/ highly compacted alluvium with high boulder content.

1000-2000 m/s

Bedrock > 2000 m/s

The velocity over phyllite in Nganglam, through correlation with borehole data, was found to be about 2000 m/s (refer Tshering DGM (2007)

Table showing ranges of velocity during interpretation of seismic velocity model for different


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materials – Source DGM

Table: Characteristic P-wave velocities range. (Modified after Anon, 1995).

Material P-wave velocity material

P-wave velocity

m/s m/s

Air 300 weathered sedimentary rock

300 – 3000

ry sand 400 - 900 metamorphic rock 1000 – 6000

Clay 300 – 1800 Un-weathered basalt 1000 –

4300

Weathered igneous and metamorphic rock

450 - 3700 limestone 500 –

6700

Note that material descriptions are crude and does not account for variations in, for example, water content, number of discontinuities, or whether discontinuities are open, filled, or closed, etc. These factors influence the velocity values far more than most of the material constituents.

Slope Stability and Environmental Analysis, DGM-ITC Training Course 2004

Safe Bearing Pressure (SBP)

Upper table: some typical values for SBP on different rock types; lower table: SBP for jointed rock based on rock strength and fracture spacing (or RQD)


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IV SEISMIC SURVEY AT MAJOR BRIDGE LOCATIONS ON NGANGLAM-DEWATHANG ROAD

Considering the joint spacing of 6cm and average UCS of 25MPa for moderately strong rock and referring the following Table for Safe Bearing Pressure for quartzite/gneiss the SBP of at least 0.3MPa can be considered for bed rock at the bridge site. The seismic results and surface geological observation for most of the traverses in the proposed major bridge locations on Nganglam-Dewathang road indicates presence of rock within 3m to 10m depth.

The seismic studies conducted at the three bridge sites are as follows:

The seismic traverses are designated as T1, T2 etc. and the Left Bank and Right Bank of the bridge sites as LB and RB respectively. The colors of tomography display P-wave velocity through various materials along the traverses. The depth and location of various material types along the traverse can be estimated from the Tomographic inversion images of the traverses. The materials have been categorized based on the p-wave velocities indicated in the above tables and the surface geological studies carried out at site.

Kirungri Bridge Site:

Seismic results for Kirungri bridge site at its left bank indicate bedrock (quartzite) at depth of 6m from the river bed below the abutment. Strength of the rock increases as we move deeper.

The

geological study indicates colluviums surface at hill side of right bank and alluvial deposit at the flood plain on both left bank and downstream of right bank. There is also the presence of outcrop of quartzite in the river below the abutment.


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Considering the result of seismic analysis and the geological assessment it is concluded that strong bed rock of quartzite is likely below 6m from the surface at river bed level below the existing abutment.

At right bank too traverses have been made along and across bridge axis respectively. The seismic result indicates the presence of weak rocks and/or colluviums of quartzite boulders and gravels below the existing abutment till riverbed level. Correlating the seismic result and surface geology, presence of strong quartzite bed rock is indicated starting beneath the existing abutment and upstream. Outcrops observed here at riverbed level indicate its continuity for more than 5m as indicated in the seismic result.

Tomographic Inversion on Kirungri Ri bridge sites are presented below:


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Traverse Kirungri LB T1 (Traverse is along bridge axis and -4m located at the existing abutment side and 26m is at Shuguri side):

Kirungri LB T2 (Traverse across bridge axis with -4m at the upstream side, 11m below the existing abutment and 26m location at downstream side):

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Elev

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( m)

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

( m)Di st ance

(m/s)

334541748954116113681574178119882195240126082815302232283435364238484054

Scal e = 1 / 25


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(m/s)

330431531631732832933103311331234133414351535163617361836193720372137

Scal e = 1 / 25


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Kirungri RB T2 (Traverse across bridge axis with -4m at the upstream side of abutment, and 26m location at downstream side at the old road):

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Elev

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( m)

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

( m)Di st ance

(m/s)

30045060075090010501200135015001650180019502100225024002550270028502999

Scal e = 1 / 25


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Traverse RB T1 (Kirungri): (Traverse is along bridge axis and -4m located at the existing abutment side and 26m along road at Dizama side):

Result Interpretation on Kirungri bridge site:

Left Bank: Considering the result of seismic analysis and the geological assessment it is concluded that strong bed rock of quartzite is likely below 6m from the surface at river bed level below the existing abutment.

Right Bank: Correlating the seismic result and surface geology, presence of strong quartzite bed rock starting beneath the existing abutment and upstream is indicated. Outcrops observed

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Elevation

(m)

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 (m) Distance

(m/s)

300 450 600 750 900 1050 1200 1350 1500 1650 1800 1950 2100 2250 2400 2550 2700 2850 2999

Scale = 1 / 250


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here at riverbed level indicate its continuity for than 5m upstream as indicated in the seismic result.

The surface geology and seismic study indicates favorable bridge foundation on both the banks of Kirungri Ri at the proposed crossing point. Down depth continuity of bed rock for both banks is more than 5m.

Menchudrang Ri

Seismic results for Menchudrang Ri bridge site at its left bank indicate bedrock (strong phyllite) at the river bed below the proposed abutment. Weak and fairly weathered phyllite is also indicated 3m-4m behind the proposed abutment (-3m location of the Seismic Traverse lies over the proposed abutment). Strength of the rock increases as we move deeper below the traversed surface

The geological study indicates colluviums and soil cover surface at hill side of left bank and alluvial deposit at the flood plain at the downstream. The presence of outcrop of quartzite in the river immediately below the abutment for about 5m stretch downstream indicate that the site is stable and suitable for abutment construction at the proposed location

Considering the result of seismic analysis and the geological assessment it is concluded that the presence of strong bed rock of phyllite likely below 5-6m from the surface at river bed level.

At right bank too traverses have been made along and across bridge axis respectively. The seismic result indicates the presence of weak rocks and/or colluviums of quartzite boulders and gravels and or weathered rocks below the proposed abutment for a depth of 5m. Presence of slightly weak to moderately strong phyllite is indicated after 7m-8m depth from the proposed location of abutment. Correlating the seismic result and surface geology, presence

Left Bank Abutment

Right Bank Abutment

Menchudrang Ri


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of phyllite bed rock is indicated at 7m to 8m underneath of the proposed abutment. The Continuity and strength of the rock increase as we move down depth below the abutment.

Tomographic Inversion on Menchudrang Ri bridge sites are presented below;

Menchudrang Ri LB T1 (Traverse across bridge axis with -4m at the upstream side, -3m at the proposed abutment and 26m location at downstream side):

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( m)

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

( m)Di st ance

(m/s)

3165457731001123014581687191521442372260128293058328635153743397242004428

Scal e = 1 / 25


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Traverse RB T1 (Menchudrang Ri): (Traverse is along bridge axis and 0m located at the proposed abutment side and 26m along road at Khalatsho side):

Menchudrang RiRB T2 (Traverse across bridge axis with -4m at the upstream side of abutment, and 26m location at downstream side and 11m crossing the 11m point of above Traverse RB T1):

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Elev

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( m)

-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

( m)Di st ance

(m/s)

30045060075090010501200135015001650180019502100225024002550270028502999

Scal e = 1 / 25


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Result Interpretation on Menchudrang Ri bridge crossing:

Left Bank: Considering the result of seismic analysis and the geological assessment it is concluded that the presence of strong bed rock of phyllite likely below 5-6m from the surface at river bed level.

Right Bank: Correlating the seismic result and surface geology, presence of phyllite bed rock is indicated at 7m to 8m underneath of the proposed abutment.

Down depth continuity of bed rock for both banks is more than 5m. The surface geology and seismic study indicates favorable bridge foundation on both the banks of Menchudrang Ri at the proposed crossing point.

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( m)Di st ance

(m/s)

3184375566757949131032115112701390150916281747186619852104222323422460

Scal e = 1 / 25


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Tshokhi Ri Bridge Site:

Seismic refraction for Tshokhi Ri bridge site at its left bank indicate bedrock (strong quartzite) below the proposed abutment. Strength of the rock increases as we move deeper. Possible formations: Depth of top soil and alluviums: 1m to 1.5m and hard formation (quartzite rock) below it for >6m. Bed rock observed is strong quartzite.

The geological study indicates alluviums and soil cover surface at the flood plain that occupies the left bank. The presence of outcrop of

quartzite at the riverside below the abutment indicates that the site is stable and suitable for bridge construction.

At right bank too traverses have been made along and across bridge axis respectively. The seismic result indicates the presence of soil cover and/or colluviums of quartzite boulders and gravels and or weathered rocks below the proposed abutment for a depth of 4m to 5m. Presence of slightly weak to strong quartzite is indicated after 6m depth below the location of the proposed abutment.

The continuity and strength of the rock increase as we move down depth below the traversed lines.

Tomographic Inversion on Tshokhi Ri bridge sites are presented below>

Traverse LB T1 along axis (Tshokhi Ri): (Traverse is along bridge axis and 0m located at the proposed abutment side and 26m along road at Dewathang side): -4m lies at the river edge side and 0m lies near the marked stone (borehole point).


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Tshokhi Ri LB T1 across axis:(traverse running along foot path which is 11m away from borehole point marked at a stone) and -4m is at upstream and 26m downstream.

Possible formations: Bedrock of strong quartzite present at the river edge parallel to the traverse.

Traverse RB T1 (Tshokhi Ri): (Traverse is along bridge axis and 0m located at the proposed abutment side and 26m along road at Khalatsho side): -4m lies at the river edge side and 0m lies near the marked tree (borehole point) and 26m lies backward towards Khalatsho.

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-4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

( m)Di st ance

(m/s)

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Scal e = 1 / 25

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30045060075190110511201135115011651180219522102225224022553270328533002

Scal e = 1 / 25


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Tshokhi Ri RB T2 (Traverse across bridge axis with -4m at the downstream side of abutment, and 26m location at upstream side and 11m crossing the 11m point of above Traverse RB T1):

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( m)

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( m)Di st ance

(m/s)

30045060075090010501200135015001650180019502100225024002550270028502999

Scal e = 1 / 25

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( m)Di st ance

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3174415656898139371060118413081432155616801804192820512175229924232546

Scal e = 1 / 25


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Result Interpretation on Tshokhiri bridge site:

Left Bank: Considering the result of seismic analysis and the geological assessment it is concluded that strong bed rock of quartzite is likely below 1m-1.5m from the surface at river bed level below the existing abutment.

Right Bank: The seismic result indicates the presence of soil cover and/or colluviums of quartzite boulders and gravels and or weathered rocks below the proposed abutment for a depth of 4m to 5m. Presence of slightly weak to strong quartzite is indicated after 6m depth below the location of the proposed abutment.

The continuity and strength of the rock increase as we move down depth below the traversed lines.

The surface geology and seismic study indicates favorable bridge foundation on both the banks of Tshokhiri at the proposed location. Down depth continuity of bed rock for both banks is more than 5m.

Diuri Bridge Site:

Seismic results for Diuri bridge site at its left bank indicate bedrock below the proposed abutment. Strength of the rock increases as we move deeper. Depth of top soil and colluviums is 2m and hard formation is indicated below it. Bed rock observed is strong Diamictite and quartzite with phyllite.

The surface geological study indicates thin soil covered flat to gently sloping left bank. The presence of outcrop of quartzite at the riverside and below the abutment and also at the flat top surface indicates that the site is stable and suitable for bridge construction. The rock presence is seen for at least 20m both along and across the bridge axis at the left bank bridge site.

At the right bank, two traverses were made along and across bridge axis. Presence of strong rock is indicated after 2.5m to 5m depth underneath of the traverse line. Bed rock observed is strong


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Diamictite and quartzite with phyllite. The continuity and strength of the rock increase as we move down depth below the traverse lines

Tomographic Inversion on Diuri bridge site

Traverse LB T1 along axis: The traverse is along bridge axis and 0m located at the proposed abutment side and 26m along the road alignment towards Dewathang side. Tomography along bridge axis LB Diuri (0m at river side with 3.5m of this traverse intersecting the 11m point of Traverse line across bridge axis. Hard rock is indicated below 2m at the bridge abutment site.

Diuri LB T2 across bridge axis:

Tomography across Left Bank along bridge axis (26m at upstream and -4m at hill downstream side with 11m point of this Traverse intersecting the 3.5m point of other traverse along bridge axis. Hard rock expected below 3m to 5m depth and rock strength increases as we move down depth.

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0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

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(m/s)

500639778917105611941333147216111750188920282167230624442583272228612999

Scal e = 1 / 25


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Traverse RB T1 (Diuri)

Tomography along bridge axis at the right bank (0m at river side and 20m at last chainage point at hill side). Here 4m point of this traverse line intersects 11m point of the traverse done across the bridge axis. Presence of hard rock is expected 2.5m to 5m underneath the traversed surface.

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80092210441167128914111533165617781900202221442267238925112633275628782999

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Traverse RB T2 (Diuri)

Tomography of Seismic Traverse across Diuri Right Bank: Here 11m point of this traverse line intersects 4m point of the traverse done along the bridge axis. Presence of hard rock is indicated from the surface at the bridge axis point and upstream of the bridge axis.

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1000111112221333144415561667177818892000211122222333244425562667277828892999

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Result Interpretation on Diuri bridge site:

Left Bank:

The surface geological study indicates thin soil covered flat to gently sloping left bank. The presence of outcrop of quartzite at the riverside and below the abutment and also at the flat top surface indicates that the site is stable and suitable for bridge construction. The rock presence is seen for at least 20m both along and across the bridge axis at the left bank bridge site.

Seismic results for Diuri bridge site at its left bank indicate bedrock 2m below the proposed abutment. Strength of the rock increases as we move deeper. Depth of top soil and colluviums is 2m and hard formation is indicated below it. Bed rock observed is strong Diamictite and quartzite with phyllite of Deo Ri Formation.

At right bank presence of strong rock is indicated after 2.5m to 5m depth underneath of the traverse line along the bridge axis. Bed rock observed is strong Diamictite and quartzite with phyllite. The Continuity and strength of the rock increase as we move down depth below the traverse lines. Surface geological observation and seismic results across the bridge axis too indicates strong bed rock exposed at the bridge site.

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(m/s )

1 1 3 71 2 5 71 3 7 71 4 9 61 6 1 61 7 3 61 8 5 51 9 7 52 0 9 52 2 1 42 3 3 42 4 5 42 5 7 32 6 9 32 8 1 32 9 3 23 0 5 23 1 7 13 2 9 0

Sc al e = 1


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V SHUGURI SLIDE

Geographical coordinates of this slide is 26° 50’ 11.70”N and 91° 16’ 37.80”E. The general features around this unstable area are indicated in the Google Earth image below.

Shuguri slide is caused due to loose soil cover (mostly black) and very highly weathered weak phyllite rock underneath and poor water management along the highly weak formation. The other attributes are the steeper terrain slopes and absence of harder formations both at hill and valley sides of the road. Roughly the slide occupies a length of 200m (150m below the road and 50m above the road) and a width of roughly 60m along the road.

Tomographic Inversion of Seismic Traverse along the hillside of existing road at the Shuguri slide.

Seismic Traverse along road


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Result Interpretation: The seismic traverse indicates loose soil cover for a depth of 2m to 4m and then highly weathered rock or compact colluviums underlie the loose soil cover.

The surface geology and the seismic result indicate loose soil cover for about 2m to 4m depth and then very highly weathered rock (dark grey to black phyllite) and some patches of weathered rocks at the valley side (refer photos above).

Proposed mitigation measures for Shuguri Slide

1. Gabion walls are recommended at hill side immediately above the road and at valley side from a depth of 20m where slightly weathered rocks are exposed in patches. The gabion wall length should cover the actual width of the road that has been affected by the slide. About 30m stretch was affected when the study was conducted during this monsoon. The hill side height of gabion can be 3m to 5m. The height of wall at valley side may be extended up to the location of the weathered bedrock of phyllite where the gabion may be anchored.

2. Better water management area is essential to prevent the rain water and seepages especially at the hill side. The slide affected stretch of the road should have concrete lined drains (about 70m long).

3. The area may be planted with local species of grasses and plants over the surface of the affected zone; at least 150m across road (50m at hillside and 100m at valley side) by 50m along the road.

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VI GEOLOGICAL STUDY AT PASAKHA ROAD & BHAWANIJHORA SLIDE

Surface Geological investigation and subsoil investigation by seismic refraction technique has been carried out at Pasakha Access Road, across Bhawanijhora debris bed and at Bhawanijhora Slide.

Surface Geology

The geology between Bhawanijhora slide and the stretch of Pasakha road that passes through this slide debris is as described in the subsequent sections that follow.

Sandy gravels and boulders of dark grey and pink quartzite and phyllite (which are dominantly the slide debris of Bhawanijhora slide) occupy this stretch. The distance between the road and the base of the slide is almost 1000m.

The Bhawanijhora slide roughly covers 500m length by 400m width through the active zone.

The hill slope between the Pasakha road and the Bhawanijhora slide is relatively stable along the eastern flank. Bed rock of phyllite and quartzitic- phyllite are seen exposed at the base of this hill slope.

The western slope lying upstream of Mr. Pradhan’s house has highly active slide for a stretch of around 300m till the confluence of Bhawanijhora stream with a smaller stream further

Bhawanijhora Slide

Seismic Traverses along Pasakha

Bhawanijhora Slide


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upstream.

Seismic Refraction Survey Traverse along the road: Six traverses were made along the road passing through the Bhawanijhora slide debris.

Pasakha road: Traverse 1

Pasakha Road: Traverse T2

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Traverse T3

Traverse T4

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Traverse: T5

Traverse T6

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30039047956965874883892710171107119612861375146515551644173418241912

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30039448958367877286796110561150124413391433152816221717181119061999

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30039448958367877286796110561150124413391433152816221717181119061999

Scal e = 1 / 25


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Traverse -T1 (below Pradhan’s House Alay side): -4m at Alay side at foothill

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30039448958367877286796110561150124413391433152816221717181119061999

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Traverse -T2

Traverse -T3 (Pasakha side – 50m before reaching foot hill)

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30039047956965874883892710171107119612861375146515551644173418241912

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30039448958367877286796110561150124413391433152816221717181119061999

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Seismic Traverse Above masonry wall (Bhawanijhora slide)

Above 2nd Gabion wall at Pasakha side

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30039448958367877286796110561150124413391433152816221717181119061999

Scal e = 1 / 25

Traverse above Masonry wall

Traverse above 2nd Gabion wall

Masonry Wall


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Interpretation of the results of seismic refraction :

The seismic result between Pasakha road section and base of Bhawanijhora slide indicate loose top soils with debris of alluviums/colluviums at the widespread stream bed for a depth of about 5m. Below this layer there is possibility of weathered rock (possibly phyllite) and compact colluviums and alluviums of quartzite and phyllite with gravels and silt, and sand.

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( m)Di st ance

(m/s)

346449551653755857960106211641266136914711573167517771880198220842185

Scal e = 1 / 25


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Proposed Mitigation Measures

Following mitigation measures are recommended to control the impact of debris slide to the road crossing below and the slope protection on the unstable western slope at Alay side below main Bhawanijhora slide.

1. Construction of check dams (5m high by 50m to 80m long walls)- 4Nos (at spacing of 150m to 200m at the locations as shown above between existing masonry at the base of Bhawanijhora slide and the Pasakha road. This would control the debris flow especially during monsoon.

2. Construction of around 300m of Protection wall at the base of slope starting from the confluence of main Bhawanijhora stream with its tributary from Alay side to the start of the active slide at this slope (below Pradhan’s house-refer photos above). The slope is very unstable due to presence of highly weathered talcose phyllite.

3. Bio-engineering of the unstable area with local species of plants and grasses where ever possible especially in the stabilizing zones to prevent reactivation of the slide.

STABILITY ANALYSIS OF LANDSLIDES

1 GENERAL

Slopes largely remain in an equilibrium situation where resisting forces, forces resisting movement, exceed those forces creating movement. Instead of slope instability slope failure is often used to indicate that the slope is no longer stable and that movement occurs.


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The result of instability is the landslides: this is a general, non precise description for a whole group of mass movement phenomena. A more formal definition of a landslide is: the movement of a mass of rock, debris, or earth down a slope. (Cruden,1991).

Terzaghi (1950) made a distinction between external and internal factors causing mass movement.

Internal causes are the mechanisms which bring about a reduction of its shear strength to a point below the external forces imposed on the mass by its environment, thus inducing failure (Bell, 1983) (weathering, pore pressure).

External causes are those mechanisms outside the slope mass involved, which were responsible for overcoming its internal shear strength, thereby causing it to fail (overloading, removal of support of the toe, earthquakes, shocks, etc).

Both internal as external causes may affect the equilibrium conditions of the slope by:

• increase of shearing stress

• decrease of cohesion

• decrease of frictional resistance

• liquefaction

THE FACTOR OF SAFETY: A DETERMINISTIC APPROACH In construction of infrastructure works and in civil engineering it is standard practice that

uncertainties in circumstances, conditions, and properties and structure of materials that

influence a civil engineering construction, are compensated by overdesigning, i.e. applying a

design more safe than strictly necessary. This surplus for these parameters is usually

quantified as a multiplier larger than unity: the factor of safety. The factor of safety is also a

deterministic measure for “the” stability of a slope; here, we use it in that sense. The factor of

safety is expressed as the ratio of forces or moments resisting movement to the forces or

moments promoting movement. In a formula:

//s

forces moments RESISTING movementFforces moments PROMOTING movement

=

At a factor of safety Fs of 1 we have limiting equilibrium; when Fs>1 a slope is stable, when

Fs<1 a slope is unstable. Selby (1993) gives the following guidelines for the factor of safety:


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Factor of safety: Stability of slope:

<1.0 Slope considered unsafe

1.3-1.5 Slope considered satisfactory for routine earthworks

1.5-2.0 Slope considered satisfactory for earth retaining structures and dams

2.0-3.0 Slope considered satisfactory for foundation structures

SLOPE FAILURE IN CIRCULAR MODE:

Rotational failure: total stress analysis for cohesive soils

If a cohesive soil body fails in a perfectly circular mode, then a total stress analysis is most

easily done by looking at the moments resisting and promoting movement.

When we consider a typical cohesive soil, we can assume its friction angle to be 0°. That

means that any resistance to movement on the circular failure plane is derived from the

cohesion only. Per meter width of a slope, this cohesion, c, would act on a length

corresponding to the length of the circular arc of the failure plane, La. The lever of this resting

force with respect to the centre of rotation is equal to the radius of the slip circle, r. The force

promoting movement is of course the own weight of the rotating mass, W. The lever of this

force is the horizontal distance from the centre of gravity to the centre of rotation, d. combining

this into an equation gives:

. ..a

sc L rFW d

=

ROTATIONAL FAILURE: METHOD OF SLICES

The method given above is of course limited since it assumes the friction angle to be 0° and a

perfectly circular slip surface; besides, it does not take into account groundwater pressures. A

far more versatile solution is presented by the method of slices, which really is one of the most

basic applications of numerical analysis; it allows for non-uniform soils and groundwater

conditions.

The slope is divided into slices as shown in the figure below. In the simplest approximation we

assume the resultant of interslice forces zero. The other forces acting on every individual slice

are calculated, including the groundwater pressures at the slip surface (u).


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By summating the resulting partial forces resisting and promoting movement we find the

overall factor of safety for n slices as:

( )( )( )

1' . tan ' cos .

sin

n

i i i i i i ii

si i

c l W u lF

W

φ α

α=

+ −=∑

∑

Figure 1 - Basics of the method of slices in a material with cohesion c’ and friction angle φ

Note that αi is the angle of the slip surface for every individual slice, not a slope angle. This is

equal to the angle between the vertical and the line from the centre of rotation to the midpoint

of the base of the slice. In the figure above, slices to the right of the centre of rotation have a

negative value for αi whereas slices to its left have a positive value.

2. STABILITY ASSESSMENT (SHUGURI SLIDE)

The slope at Shuguri Slide (approximately 6 kilometres from Nganglam at the Nganglam-

Dewathang road corridor has been analysed for its stability using SLOPEW software from

Geostudio 2012.


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The slope has been analysed by two different methods but the result of the analysis from both

methods is almost same which is as follows.

I. Bishop Method

This method is widely used in analysis. The minimum factor of safety for this slope by this

method is 0.874 which is much less than the equilibrium limit. Other minimum F.S also is

below the limiting equilibrium (highest being 0.985). This failure slip circle is shown in the

figure below depicted by the white colour with slices. The detail of the analysis is appended at

the end of this report.


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Figure 2: Figure showing stability calculation by Bishop Method.

Road level


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Janbu Method: This is also another commonly used method for slope stability analysis.

Figure 3: Figure showing stability calculation by Janbu Method.

The minimum factor of safety for this slope even by this method is 0.805 which is much less

than the equilibrium limit. The other minimum F.S is 0.927 which is also lower than the limiting

equilibrium. Other factors in the grid are also below the limiting equilibrium which suggests

that the slope will not be in limiting equilibrium. This failure slip circle is shown in the figure

above depicted by the white colour with slices. The detail report of this analysis is appended at

the end of this report.

Road level


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HAZARD DUE TO LANDSLIDE

The hazard study was mainly based on the observation of slope, the landscape, and landslide

areas, minutely analyzing any indications of ground movements in and around the study

areas, water seepages and marshy land. Hazard due to the above mentioned factors are

analysed and suitable mitigation measures proposed accordingly.

This unstable slope across the highway falls in a high hazard zone. The high level of hazard is

due to the high slope angle, and creation of landslide from the road construction works,

presence of slide mass on the slopes, loose top soil and a highly weathered underlying rock

mass. It is possible that if the slide is not mitigated now, there is a possibility of this slide to

extend further endangering the stability of the road. To mitigate or minimise this high hazard,

certain measures are recommended like the construction of retaining structures on the slope

and bio-engineering. It has been observed that there are already few existing remedial

measures like gabion walls at the hill side of the road but further more needs to be built for the

complete safety of the road. The details remedial measures are as discussed below.

REMEDIAL MEASURES

Minimizing influence of ground and surface water

Water plays a very important role in the stability of slopes. The influence of groundwater on

the factor of safety is significantly reduced by the presence of groundwater. To minimize

groundwater influence, and to decrease infiltration into a slope after rainfall, drains can be

installed in a variety of ways to stabilize slopes. However, in many cases we will have to

incorporate different drainage measures in the design of a slope. A surface lined drain at the

hill side of the road will have to be constructed to provide an easy flow path for rainwater and

thus preventing the rain water to run over the slope. This will eventually lead to achieving

minimum infiltration in the fragile slopes below the road.

Strengthening or supporting

When a slope with a given geometry has to be stabilized we can do this by increasing the

shear resistance of the soils involved (for soil slopes) or by fixing in place individual rock

blocks (for rock slopes). The most obvious options are:

• Rock or soil anchors

• Soil nailing

• Grouting


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• Retaining walls

Among these various stabilisation options, construction of retaining structures is proposed.

The shear resistance is increased by a retaining wall construction at the hill and valley side of

the road in the slide affected stretch (about 60m). The details on dimensions of the retaining

walls are provided in the designed drawings.

Bio Engineering Works: After certain stability is achieved after construction of the walls, bio

engineering works preferably with local species may be carried out in the slide affected area.

RECOMMENDATION AND CONCLUSION

From this study, it is concluded that the slope across the road has become unstable due to

factors as discussed above and the indication is that the slide mass resting on the slope will

move down. Therefore the method proposed will have to be adopted soon to stabilize the

slope. The proposed further expansion should also include slope stabilization measures

wherever required in order that instability is not created affecting the road.

It is also recommended that the muck from the construction site should not be disposed along

this slope. The muck should be collected and transported to an identified disposal site.

3 STABILITY ASSESSMENT (BHAWANIJHORA SLIDE)

The highly active area as shown in the photo below is the most unstable slope at the Bhawanijhora slide.


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This most unstable slope at the Bhawanijhora landslide zone has been analysed for its

stability using SLOPEW software from Geostudio 2012 as done for Shuguri Slide. The slope

has been analysed by two different methods but the result of the analysis from both methods

is almost same which is as follows.

I. Bishop Method

This method is widely used in analysis. The minimum factor of safety for this slope by this

method is 0.753 which is much less than the equilibrium limit. Other minimum F.S also are

below the limiting equilibrium. This failure slip circle is shown in the figure below depicted by

the white colour with slices. The detail of the analysis is appended at the end of this report.


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Figure 4: Figure showing stability calculation by Bishop Method.

Janbu method: This is also another commonly used method for slope stability analysis.


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Figure 5: Figure showing stability calculation by Janbu Method.

The minimum factor of safety for this slope by even by this method is 0.675 which is much

less than the equilibrium limit. The other minimum F.S is 0.91 which is also lower than the

limiting equilibrium. Other factors in the grid are also below the limiting equilibrium which

suggests that the slope will not be in limiting equilibrium. This failure slip circle is shown in the

figure above depicted by the white colour with slices.

The detail report of this analysis is appended at the end of this report.

HAZARD DUE TO LANDSLIDE

The hazard study was mainly based on the observation of slope, the landscape, and landslide

areas, minutely analyzing any indications of ground movements in and around the study

areas, water seepages.


65

The slope falls in a high hazard zone. The high level of hazard is due to the high slope angle,

presence of slide mass on the slopes, loose and highly weathered rock mass.

RECOMMENDATION AND CONCLUSION

From this study, it is concluded that the slope has become unstable due to factors as

discussed above and that the slide mass resting on the slope will move down until the angle of

repose is maintained. Remedial measures like soil/rock, nails, retaining structures and water

management by constructing catch drains in the body of the slope can be some possible

measures for stabilization but will be expensive. Alternatively, the arrest barriers/check dams

as proposed in the designed drawings is a solution to negate the adverse impact of the slide

debris to the road further below the Bhawanijhora slide.

VII OVERALL CONCLUSION

1. The surface geological study and the seismic refraction result on Shuguri slide indicate loose soil cover for about 2m to 4m depth and then very highly weathered rock (dark grey to black phyllite) and some patches of weathered rock outcrop at the valley side.

2. The seismic refraction result and surface geological study between Pasakha road alignment and base of Bhawanijhora slide indicate loose top soils with debris of alluviums/colluviums at the widespread stream bed for a depth of about 5m. Below this layer there is possibility of weathered rock (possibly phyllite) and compact colluviums and alluviums of quartzite and phyllite with gravels and silt, and sand.

3. The recommended remedial measures especially for major unstable areas (Shuguri slide) on Nganglam-Dewathang section and Bhawanijhora slide at Pasakha road may be followed to control the impact of these landslides.

4. Laboratory test results of bed rock samples collected from bridge sites at Nganglam-Dewathang road indicate an UCS (Unconfined Compressive Strength) value of more than 80Mpa and Water Absorption value of less than 1% indicating that the rocks are strong and competent.

5. Based on the results of seismic analysis and observed surface geology of all the bridge sites on Nganglam-Dewathang road are located on stable geological formations.

6. The location of the bridges and major landslides and its detailed topographical features are indicated in the designed drawings.


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ANNEXURES

GOOGLE EARTH IMAGES: GENERAL & GEOLOGICAL FEATURES


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68


69


70


71

LOCATION MAP OF NGANGLAM-DEWATHANG ROAD

GEOLOGICAL MAP OF BHUTAN (REPRODUCED FROM BHARGAVA ED. 1995)


72

GEOLOGICAL FORMATIONS THROUGH ROAD CORRIDOR:

GEOLOGIC MAP OF BHUTAN SHOWING MAIN THRUSTS


73


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LABORATORY TEST RESULTS TEST RESULTS OF SOIL SAMPLES FROM UNSTABLE SITES Soil samples were collected from unstable sites and following tests conducted

• Sieve Analysis • Density • Direct shear

The test results are presented below;


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TEST RESULTS OF ROCK SAMPLES FROM MAJOR BRIDGE SITES. POINT LOAD TEST: Value of Point Load Index Strength (Is50): value)

Name of Bridge & its Bank

Point Load Index Value in MPa

UCS in MPa

Kirungri (Left Bank) 3.661 87.864

Kirungri (Right Bank) 6.319 151.656

Menchudrang Ri(Left Bank)

6.603 158.472

Menchudrang Ri (Right Bank)

13.001 312.024

Tshokhiri(Left Bank) 4.590 110.16

Tshokhiri (Right Bank)

6.319 151.656

Note: UCS (Unconfined Compressive Strength value is 24 times the Is50 value.

VALUE OF WATER ABSORPTION TEST ON ROCK SAMPLES FROM MAJOR BRIDGE SITES

Name of Bridge & its Bank

Water Absorption Value in %

Remarks

Kirungri 0.47 Water absorption value < 10%

Menchudrang Ri Bank)

0.70 Water absorption value < 10%

Tshokhiri 0.39 Water absorption value < 10%

Sample No Direct Shear Test Bulk Density Remarks

Sl. No.

c = Cohesion (in Kpa)

ø = Internal Friction Angle(in °) In gm/cc

1

After Chokhorling Mobile Tower 0.015 30°57’50” 1.861

2 Near 31Km 0.190 33°41’21” 1.920


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REFERENCES 1. Feasibility Study Report: RNP II (AF), Dept of Roads, Thimphu 2. "Final Report - Geotechnical Study for the Rural Access Programme"

by Scott Wilson and Kirkpatric, 2005. 3. Geotechnical Report on Jangchucholing-Tashidingkha Road", RAP II

Final Report 4. Bhargava, O.N., (1995): The Bhutan Himalaya: A Geological Account.

In: Geol. Survey, India. Sp. Publ. No. 29. Pp

5. In-country Course DGM-ITC 2004, Department of Geology & Mines, Thimphu.

6. Seismic Manual and Documentation for ES-3000 Seismograph by Geometrics

7. Geological Map of Bhutan by Sean Long, et-al-2011 8. Bromhead, E.N. (1986): The stability of slopes. Surrey University

Press, London. 9. Gansser, A. (1964). Geology of the Himalaya. In: Regional Geology

Series edited by De Sitter, L.U. 10. Selby, M.J. (1993): Hill slope materials and processes. Oxford

University Press, Oxford

appendix iii: geological investigation along road … · geological& subsoil investigation...

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