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Briefing Note – Gamgadi, Bridge Reconnaissance

Briefing Note –Visit to Gamgadi for Bridge Feasibility AssessmentOVERVIEW

The RAP3 programme is investigating the option of providing a connection of the District Headquarters at Simikhot in the Humla District with the strategic road network utilising existing DoR and DDC constructed

roads. The Mugu District Headquarters has recently been connected to Kalikhot following the construction of a fair

weather road from Kalikhot to Gamgadi. This road becomes unusable during the monsoon and winter months due to numerous sections that become impassable to even 4 by 4 vehicles or trucks due to snow, ice and water from springs and poor drainage management. However the majority of the road has a reasonable alignment and an acceptable road formation and there are proposals by the DoR to upgrade the road over the next few years.

This will result in traffic being able to reach Gamgadi for a greater percentage of the year. In order to achieve the desirable objective of providing vehicle access from the strategic road network to the District Headquarters of the western RAP3 districts it requires the construction of a bridge over the Mug Karnali River running in the valley to the north east of Gamgadhi. This would enable the connection of the existing Kalikhot to Gamgadi DoR road with the DDC constructed roads running along the eastern ridge to Darma and then along the Humla Karnali Nadi to Simikhot.

A visit to evaluate potential bridge locations and produce a preliminary feasibility report was carried out by the RAP3 Bridge Specialist between March 19th and 26th 2014. The Bridge Specialist flew into Jumla airport and walked to Gamgadi and was subsequently able to visit and assess possible locations and span layouts for a bridge.

This briefing note summarises the findings.

Several initial alternatives were considered for the bridge ranging from the construction of a bailey type bridge developed initially for military use but now commonly used as emergency replacement bridges to more sustainable steel through truss. The common feature of the alternatives was that the river be spanned without the use of erosion susceptible piers that would need to be constructed at a predicted substantial depth in the existing river channel.

The recommended bridge, subject to the carrying out of topographical, geotechnical and hydrology studies to confirm the voracity of the initial feasibility study inspection, is a 70 metre single span steel trough truss founded on open foundations at a suitable depth in the rock on the northern bank and a caisson foundation again either founded on a suitable rock strata or a well type foundation on the southern bank.

The recommended bridge offers a number of advantages both from the consideration of the ease of construction in this remote area with a relatively short working season and from consideration of its sustainability and resistance to the effects of climate change and unpredictable flood events arising from the river.

Several of these are:-

Construction of the foundations would take place outside the existing river channel and would therefore not affect the river flow or be affected by normal river morphology. The environmental effects on the river

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itself would therefore be minimised. The construction of a concrete bridge superstructure would necessitate the use of pre-stressing to achieve the required span. This requires the production of reliable high strength concrete that has already been identified as a problematic quality control issue.

The technology exists in Nepal for the successful fast construction of a single span bridge over a fast flowing mountainous river as has been demonstrated by the RAP2 Bridges over the Arun River at Leguwa and Sabha Khola near Tumlingtar. The erection can be carried out using a travelling crane supplied as part of the truss steelwork and only requires an excavator to load the individual steel sections onto a trolley running on the previously constructed deck cross transoms.

The erection of the steel truss can continue during the monsoon and winter periods as the height of the river flow will not affect truss erection works above it.

The use of a factory manufactured steel truss ensures that a high standard of Quality Control can be maintained during the entire bridge construction cycle

The use of a steel truss will maximise the whole life performance of the structure

The steel truss itself is 100% recyclable at the end of its design life although there are examples of steel structures continuing to perform satisfactory for an extended period over their expected lifespan subject to suitable maintenance interventions.

The use of concrete with its problems of labour intensive large scale production in remote areas coupled with the difficulties in sourcing suitable and reliable quality sand and aggregates, importation of high quality cement, reinforcing bars and mixing equipment is minimised to the construction of the foundations.

The effects on the local environment arising from quarrying larger quantities of suitable rock for concrete aggregate and sand for both the substructure,(foundations, abutments) as well as the superstructure (beams, deck slab and deck diaphragms), is minimised.

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Site ReconnaissanceThe RAP3 Bridge Specialist visited the site on the 22nd March 2014. He was accompanied for the visit by the RAP3 District Team Leader Mr Tulsi Neupane and the District Citizens Network Chairman Mr Chitra Bahadur Malla whose land is adjacent to the proposed bridge sites on the south bank of the Mug Kanali River.

Mr Bahadur advised the RAP team that the DoR had previously visited the site and had identified three possible locations for the bridge.

Mr Bahadur showed the team the approximate locations of the bridge centreline and a GPS position was taken by the RAP team for each of these locations on the southern bank of the river.

The Preliminary Bridge Site Reconnaissance inspection form is Included in Appendix 1

Photograph 1 shows the approximate positions of these three locations.

Bridge LocationOn visiting the site it was apparent that there was, broadly, three locations near the existing alignment of the existing DDC constructed approach road running up from the river, approximate elevation 1770 metres to approximately 2100 metres that would be feasible to locate the bridge crossing.

Location A

GPS Co-ordinates:- 0612980 3270033

Main Features:-

The location was considered as the North Slope is approximately 40 degrees making the approach road alignment less steep. However the existing slope angle is clearly the result of some previous slope movement. The slope is therefore likely to be unstable and require additional, perhaps extensive soil stability engineering.

The northern abutment foundations are also likely to be quite deep so as to reach undisturbed strata thereby giving more homogenous soil parameters to ensure the stability of the substructure.

Location B

GPS Co-ordinates 0613176 3270104

Main Features:-

The location was considered as there is clearly an extensive rock outcrop exposed in the north slope dipping steeply towards the river.

It would be expected that the foundation on the north bank could be founded on this rock band at a relatively shallow depth with savings in abutment construction and therefore cost.

The existing line of the DDC constructed approach road runs very close to this rock outcrop and can be tied into the bridge deck with minimal alignment adjustment, again offering potential overall cost and construction time benefits.

Location C

GPS Co-ordinates 0613344 3269991

Main Features:-

This location has clear similarities to Location A.

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The location was considered as the North Slope is approximately 35 to 40 degrees making the approach road alignment less steep. However as in Location A the existing slope angle is clearly the result of some previous slope movement. The slope is therefore clearly likely to be unstable and require additional, perhaps extensive soil stability engineering.

The northern abutment foundations are also likely to be quite deep so as to reach undisturbed strata thereby giving more homogenous soil parameters to ensure the stability of the substructure.

The existing approach road is slightly further away from this location and would be slightly more expensive to construct across an unstable slope and require additional slope stability engineering.

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Consideration of Bridge LayoutAssumptions

Existing river width between its steep banks varies between 50 to 60 metres along the length between Location A and Location C

North abutment would be founded on suitable rock strata close to the existing river bed level.

South abutment would be founded on a suitable rock strata at a deeper depth than the north reflecting the dip of the exposed rock strata

The minimum bridge soffit level would be 2 metres above maximum 100 year return period flood level taken as approximately 5 metres above the water level at the time of the site visit. I.e. approximately 2.5 metres above existing top of bank level.

Spread (open) footing founded on soft to medium strength rock with a foundation from projection from the abutment of approximately 8 metres

Required bridge span from centreline of bearing to centreline of bearing is 70 linear metres

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Bridge Layout

Advantages Disadvantages Comments

Single 70 metre Span

Technology for the erection of long span steel trusses has been demonstrated as

feasible under Rap2

No foundations need to be constructed in the river bed so construction time is likely to be

shorterConstruction risk is also minimised as there would be little delay during an extreme flood

event

Environmentally more sensitive as minimal adverse effects on the river water flow and

associated fish life in the river

Limited to steel truss type structure as concrete bridge alternative would require pre-stressing

technology and associated high strength concrete requirement not readily available in Nepal and not in remote area such as Mugu

Single span deck types that can realistically be considered are:-

Bailey bridge type prefabricated panel bridges

Steel through truss bridges Composite plate girder and

concrete deck

Three spans of 25 metres

Common form of construction in Nepal so a wide range of contractors could tender for

the work

Standard bridge designs available for this type of bridge

Construction of two pier and associated foundations in the river will have a high

construction risk and would be expected to require an extended construction period due to

the short working season

Potential long term stability problems arising from general scour

Potential catastrophic foundation failure from scour under extreme flood events

Deck types that can realistically be considered are:-

Insitu concrete beam and slab decks

A concrete arch bridge layout is not considered feasible at

any of the bridge locations due to the flat flood plane on the

southern bank


Extracts from Messrs Reid and Sons Technical literature on prefabricated steel bridges

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Briefing Note – Gamgadi, Bridge ReconnaissanceContract and Construction ConsiderationsThe bridge location is in a remote region of Nepal.

It is understood that the area can be affected by both snow in the winter period, from January to March as well as the monsoon period from the end of June and September.

This reduces the time when the Kalikhot to Gamgadi road is likely to be able to be used by heavier vehicles to around 4 months a year.

It is critical for the movement of plant materials and labour for the construction of the bridge within a realistic contract period and within the Tender Cost that this fair weather road from Kalikhot to Gamgadi remains usable.

If this road becomes unserviceable for any period during the dry season it is likely that the Contractor would submit a justifiable claim for a delay.

It is therefore suggested that a provision item is included in the Contract Bills of Quantities for the spot improvement/maintenance of the road from Kalikhot to Gamgadi so that the Engineer can instruct the contractor in consultation with the DoR Maintenance Authority now responsible for the road so as to have a mechanism to quickly address any problems in pliability of the road.

It is suggested that the minimum contract period should be three working seasons.

It is suggested that the Contract is let under the FIDIC Yellow Book, Design and Build form of contract as the bridge erection must be considered as part of the design loading considerations and the Contractor can be given the maximum opportunity to incorporate innovations that address the major constraints associated with this bridge.

It is essential that the Contractor and designer are highly technically competent and capable of assessing and mitigating the numerous constraints that this bridge throws up.

One solution would be to fast track the procurement by negotiating directly with the contractor that demonstrated such technical competence for the construction of the RAP2 bridges at Sabha Khola and Leguwa.

Budget CostingThere are a number of factors that make a realistic estimation of the likely cost of the bridge structure difficult.

The depth of the foundations and the cost of the transportation of materials are two key elements that it is difficult to quantify at this stage

However for a preliminary estimate based on available ex-works costs of a 70 metre span closed through truss on spread footings at a depth of 3 metres would be broken down as follows:

GBP

Steel truss steelwork costs 550,000

Transportation to site 150,000

Erection cost at 50% of steel price 225,000

Construction of Substructure 350,000

Sub-Total 1,275,000

Preliminaries at 20% due to remote area 255,000

Contingency Item 20% 255,000

Estimated Total Cost 1,785,000

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Recommendations The bridge should be a single span structure to minimise the construction and serviceability issues

associated with construction of intermediate supports in a fast flowing seasonal river.

The construction of the bridge is feasible providing a technically competent contractor is willing to tender for the work in this remote location.

The use of the FIDIC Design and Build contract is recommended so that the design can take full account of the erection methology and so as to minimise the transportation of plant equipment and steel sections.

The design will need to take full account of the existing transportation constraints especially the tight curves of the Kalikhot to Gamgadi road alignment and this may require a maximum length of steel section of between 5 and 7 metres.

The contract period should take into account the short working season in this region of Nepal and a minimum contract length of three working seasons is therefore recommended.

The bridge should be designed to current DoR loading standards as a minimum, or any higher appropriate standard, e.g. UK BS5400 loadings standards

The minimum width of the deck cross-section should be six metres carriageway with a one metre segregated footpath

Abutment river training works should be incorporated into the design.

The bridge should be designed to resist an appropriate level of earthquake restraint

The Contractor must carry out a comprehensive soil investigation, possible a resistivity survey to confirm his foundation design assumptions.

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APPENDIX 1: Preliminary Bridge Site Reconnaissance Inspection Forms

PRELIMINARY BRIDGE SITE RECONNONISANCE INSPECTION FORM

Bridge Number: New Bridge site Date of Inspection:- 22nd March 2014

Bridge Name:-Gamgadi Bridge Inspector:- Tim Stiff/Tulsi Neupane/ Chitra Bahadur Malla

District:- Mugu Road Section:-Gamgadi to Simikot

Obstacle Crossed:-Stream/River/Road

Mug Kanali Nadi

Road Kilometre:-

126 Kms from Kalikot

General structural Type

Proposed:- Single Span Steel Truss

GPS E 3270104 N 0613176 Option B

Span Lengths:-

River width varies from 50 to 60 metres Adopt initial span of 70 metres to found on river bank

Articulation:- Simply Supported Deck on sliding Proposed bearings with earthquake restraint

Superstructure Type:-Steel Truss either through or under truss,

Alternatives:-

Stayed Bridge

Bailey Bridge

Substructure Type:-Open foundations onto existing rock strata

Alternatives:-

Well foundation at shallow on south bank or caisson foundation

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Briefing Note – Gamgadi, Bridge ReconnaissanceSuperstructure Materials:-

Options:-

Galvanized steel truss to minimise maintenance

High yield steel to minimise section size

Substructure Materials:-

Options:-

Concrete cantilever abutments with short wingwalls perpendicular to line of the river

Photograph Ref. No’s:-

See Appendix 2

APPROACH ROADS

It is recommended that the approach roads will be designed to feeder road standard.

It is recommended that a width of 6 metre carriageway is provided on the immediate road approach to the bridge to ensure that vehicles can safely pass on the bridge and facilitate the usual nepali practice of parking adjacent to a crossing to be catered for without blocking the road.

GRADIENT North side at maximum gradient of 12% for Feeder road standard

South side minimal gradient dependant on required deck soffit level and deck slab thickness.

ALIGNMENT North side:- the existing line of the approach road runs in a westerly direction on a straight alignment from the level of the river bank to the DDC constructed road alignment

On the south side an approach road runs from the Gamgadi Bazaar in a series of hairpin bends down to the exiting flood plane

RIVER FLOW CHARACTERISTICS

DEPTH/ALIGNMENT/SEASONAL

Fast flowing approx. 3 metres/sec at time of visit

Seasonal flow with maximum depth appearing to be about 2 -2.5 metres higher than water surface at time of visit Maximum depth at time of visit is 1 - 2 metres at the middle of the river.

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Briefing Note – Gamgadi, Bridge ReconnaissanceRIVER CHANNEL/BED

(size of boulders, bank scour rapids/ discontinuities)

River constrained in well-defined steep sided river banks with numerous well rounded boulders sized between 500 to 1000 mm visible in river bed. See Photos for more details

Evidence of small rapids where rock bands are exposed in the river channel

River is running against the north bank but has a series of shallow curves along the valley alignment.

FOUNDATIONS

North Side

South Side

Observations

It is possible, subject to a more detailed soil investigation that the North foundation can be founded at a relatively shallow depth on

the exposed rock strata evident at Location B

The Southern bank is currently a flood plain area that is used for arable farming. There is evidence that the river has changed its

channel several times, and near location B there is a single lines of gabions that provide erosion protection to the arable area. These

gabions appear to be adequate to maintain the existing river alignment.

It would be expected that the flood plane has a large amount of boulder of similar size to those seen in the existing river bed and

any foundation should be founded on a competent rock strata which is likely to be at a greater depth from the north side due to

the dip of the rock band. There may be some difficulties in excavating for the south foundation unless it is an open

foundation.

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SKETCH OF SITE

See Appendix 2 Photographs for reference


APPENDIX 2:- Photographs of the Bridge Site at Gamgadi, Mugu District.

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General View of the North bank of the Mug Kanali Nadi near Gamgadi, Mugu District showing possible bridge locations A and B

Approx. Bridge Location A Approx. Bridge Location B


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View of the Mug Kanali Nadi near Gamgadi, Mugu District looking downstream from southern bank


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Approx. Bridge Location AHere the North Slope is flatter making alignment of approach road

easier but the bridge foundation may be on slip material


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Bridge Location BFoundations on North bank can be founded on rock band

Existing DDC Road alignment

Exposed bank 2.00 metres approx.nment


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Bridge Location C

River is slightly narrower but North slope, although flatter, is clearly less stable with loose scree material apparent

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