this document - Rural Access Programme

advertisement
Briefing Note – Gamgadi, Bridge Reconnaissance
Briefing Note –Visit to Gamgadi for Bridge
Feasibility Assessment
OVERVIEW
 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
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.
Page 1 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance

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.
Page 2 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Site Reconnaissance
The RAP3 Bridge Specialist visited the site on the 22 nd 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 Location
On 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:-
Page 3 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance

This location has clear similarities to Location A.

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.
Page 4 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Consideration of Bridge Layout
Assumptions

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
Bridge
Layout
Advantages
Disadvantages
Technology for the erection of long span
steel trusses has been demonstrated as
feasible under Rap2
Single 70 metre
Span
No foundations need to be constructed in the
river bed so construction time is likely to be
shorter
Construction risk is also minimised as there
would be little delay during an extreme flood
event
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
Environmentally more sensitive as minimal
adverse effects on the river water flow and
associated fish life in the river
Page 5 of 22
Comments
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
Briefing Note – Gamgadi, Bridge Reconnaissance
Three spans of
25 metres
Common form of construction in Nepal so a
wide range of contractors could tender for
the work
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
Standard bridge designs available for this
type of bridge
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
Page 6 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Page 7 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Page 8 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Page 9 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Page 10 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Page 11 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Page 12 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Contract and Construction Considerations
The 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 Costing
There 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
Page 13 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
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.
Page 14 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
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
Road Kilometre:-
Mug Kanali Nadi
126 Kms from Kalikot
General structural Type
GPS E 3270104
N 0613176 Option B
Proposed:- Single Span Steel Truss
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,
Substructure Type:-Open foundations onto existing
rock strata
Alternatives:-
Alternatives:-
Stayed Bridge
Well foundation at shallow on south bank or
caisson foundation
Bailey Bridge
Page 15 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Superstructure Materials:-
Substructure Materials:-
Options:-
Options:-
Galvanized steel truss to minimise maintenance
Concrete cantilever abutments with short wingwalls
perpendicular to line of the river
High yield steel to minimise section size
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/SEASONA
L
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.
Page 16 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
RIVER 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.
SKETCH OF SITE
See Appendix 2 Photographs for reference
Page 17 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
APPENDIX 2:- Photographs of the Bridge Site at Gamgadi, Mugu District.
Approx. Bridge Location B
Approx. Bridge Location A
General View of the North bank of the Mug Kanali Nadi near Gamgadi, Mugu District showing possible
bridge locations A and B
Page 18 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
View of the Mug Kanali Nadi near Gamgadi, Mugu District looking downstream from southern bank
Page 19 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Approx. Bridge Location A
Here the North Slope is flatter making alignment of approach road
easier but the bridge foundation may be on slip material
Page 20 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Existing DDC Road alignment
Bridge Location B
Foundations on North bank can be founded on rock band
Exposed bank 2.00 metres
approx.nment
Page 21 of 22
Briefing Note – Gamgadi, Bridge Reconnaissance
Bridge Location C
River is slightly narrower but North slope, although flatter, is clearly
less stable with loose scree material apparent
Page 22 of 22
Download
Related flashcards

Towers

17 cards

Toll bridges

78 cards

Create Flashcards