Anupam Das - Sr. General Manager, Gammon Engineers & Contractors Pvt. Ltd.
Situated amidst picturesque surroundings, Balason Bridge is located about 50 km from Siliguri Town, 17 km from Mirik Town and 80 km from Darjeeling Town. The Extension of Balason Bridge @ Dudhia is located 25 km from Siliguri Town in the state of West Bengal. It is one of the most aesthetically beautiful and technically challenging bridge constructed by Gammon India. The elegance of the bridge is enhanced by the piers, which are unusually slender, and blend with the ambience.
Balason Bridge’s girder proposal accentuated parabolic shape while the increase in root depth gave the visual impression of an arch to merge with the surroundings.
The construction of this bridge was difficult and equally challenging due to the logistics, hilly terrain, cold weather, and problems of accessibility. Despite the odds, construction of the bridge was taken up as a challenge and it emerged as an excellent structure. The Balason bridge was planned to solve the following major problems:
- To reduce the distance from Mirik to Kurseong town and vice versa by around 25 km and Extension of Balason Bridge @ Dudhia would reduce the distance from Dudhia to Bagdogra side by around 15 km. This has brought about substantial economic and social benefits to the population, centering both the towns and others as well.
- To create an access across the river Balason without disturbing the natural flow and river bed.
- To have a fast surface transportation facility.
Project Fact File
- Owner/Clients: Darjeeling Gorkha Hill Council, Council House, Lal Kothi, Darjeeling (West Bengal).
- Design & Construction: Gammon India Limited.
- Scope of work: Construction of Prestressed concrete bridge over river Balason connecting Kurseong to Mirik in the district of Darjeeling (West Bengal) & extension of Balason Bridge @ Dudhia.
- Completion date: 31/01/2008
|Scope of Work
Balason Bridge @ Darjeeling District
|Sl No||Description||Qty / Details|
|1||Well Foundations||3 nos|
|2||Open Foundations||12 nos|
|3||Pier & Pier Caps, Abutment & Abutment Caps||15 nos|
|4||Cast-in-situ continuous Pre-stressed Box Girder: The entire super structure was in 1:66.4 slope from Kurseong side to Mirik side to match the formation levels on either side||306 M||- Span of 88 M
- Span of 130 M
- Span of 88 M
|5||Cast-in-situ Simply Supported Pre-stressed Box Girder:||46 M||1 Span of 46 M|
|6||Total Length of the Main Bridge||352M|
|7||Cast-in-situ R.C.C. Deck Slab 750mm thick:||80 M||10 Spans of 8 M|
|8||Deck Width||10.100 M|
|9||Carriage Way||7.50 M|
|Extension of Balason Bridge @ Dudhia|
|1||Well Foundation||1 no|
|2||Open Foundations||4 nos|
|3||Pier & Pier Caps, Abutment & Abutment Caps||6 nos|
|4||Cast-in-situ Simply Supported Pre-stressed Box Girder||50 M 1 Span of 50 M|
|5||Cast-in-situ R.C.C. Deck Slab 750mm thick||32 M 4 Spans of 8 M|
|Total Project length = 514 Meter|
The Job involves the following:
- Well Foundations.
- Open Foundations.
- Pier / Pier Caps & Abutment / Abutment.
- Cast in-Situ RCC solid slab superstructure.
- Cast in-Situ Pre-stressed Box Girders of superstructure.
- Road Kerb / Railing kerb / Footpath slab / Approach slab / Railing.
- Wearing Coat.
- Expansion Joint.
|VOLUME OF WORK ON PRIME ACTIVITIES|
|Sl No||Items||Unit||Quantity of Balason Bridge @ Darjeeling||Quantity of Extension of Balason Bridge @ Dudhia||Total Qty|
|8||Structural (For Fabrication of Cutting Edge & Strakes, Gantry & Trestles, etc.||MT||387||13||400|
|ii||M.S. Finger Type.||M||10.1||NIL||10.1|
At Balason Bridge Darjeeling, 2 nos of Well Foundations were placed at Mirik side namely P1 & P2. The distance between P1 & P2 is 88 M. On the other hand at Kurseong side, 1 No. of Well Foundation namely P3 were placed at 130 M from P2. At extension of Balason bridge at Dudhia 1 No. of Well Foundation namely P5 were placed at 50m from existing P4 Foundation. The size of P2 & P3 Well Foundations are OD=10.5 M, ID=7.50 M & Steining thickness=1.5 M. The size of P1 Well Foundation is OD=7 M, ID=4.68 M & Steining thickness=1.16 M. The size of P5 Well Foundation is OD=8.5 M, ID=5.9 M & Steining thickness=1.3 M.
- Cutting Edge & Steel strakes: The cutting edge & strakes is made up of structural steel plates 6 mm thick, 10 mm thick, 12 mm thick & 16 mm thick in circular shape. The total qty for P1, P2 and P3 & P5 is 51 MT. These are placed on dry surface after excavating & leveling the required area.
- Well Curb: The well curb is of circular shape of height 2.8 M from bottom for P2 & P3, for P1 the height is 2.32 M from bottom & for P5 the height is 2.35 M from bottom. The inner side of well curb & part of steining from bottom is lined with steel strakes to sustain the impacts of blasting / grabbing etc. during sinking operation. The well curb is concreted with M30 grade.
- Sinking: Sinking of wells was carried out by means of conventional sinking method using crane, grabs & DD winch, tripod, grabs. The P2 & P3 well was sunk by means of Tata 320 Crane & grabs up to a depth of 21.60 M & 14.52 M respectively from placement of cutting edge level. The P1 & P5 well was sunk by means of DD winch, tripod & grabs up to a depth of 28.32 M & 5.91 M respectively from placement of cutting edge level. The strata met with during sinking of the wells were soft rock / boulders / sand mixed gravels. Though steel strakes, was provided as a liner for well curb & steining, blasting was carried out very carefully & safely at P3 & P5 locations.
- Steining: The wells P1, P2, P3 & P5 has 25.58 M, 15.90 M, 10.20 M & 6.49 M respectively height of steining, with thickness 1.16 M, 1.50 M, 1.50 M & 1.30 M respectively. Part of steining is lined with steel strakes to sustain the impact of blasting. The steining was cast in lifts with M30 grade of concrete.
- Bottom Plug: As soon as the well reached the founding level, well was plugged under water with M25 grade concrete with 10% extra cement for under water concreting. Concrete quantity of bottom plug of well P1, P2, P3 & P5 was 80.269 M3, 215.131 M3, 207.402 M3 & 130.5 M3 respectively.
- Top/Intermediate plugging: After concreting of bottom plug, excavated materials were filled inside the wells up to bottom of top plug / intermediate plug. At P3 & P5 wells top plug of 22.10 M3 & 14.00 M3 respectively of M25 grade concrete was cast at bottom of well cap. At P1 & P2 wells intermediate plug of 8.60 M3 & 22.10 M3 respectively of M25 grade concrete was cast, keeping vaccum between top of intermediate plug & bottom of well cap for stability of wells.
- Well Cap: Circular Well cap of 2.5 M depth of P2 & P3, 1.8 M depth of P1 & 2.0 M depth of P5 was cast with M30 grade concrete. The quantity of concrete of well cap P1, P2 and P3 & P5 was 70.1 cum, 218.7 cum, 218.7 cum & 114.90 cum respectively.
There are 16 Nos. of open foundations for abutment & piers. The design of E1, E2, E3, E4, E6, E7, E8, E9 @ Balason & E1, E2, E3 @ Dudhia foundations were similar. The AR, AL, E5, E10 @ Balason & E4 @ Dudhia were designed as Abutment foundations. At AR foundation a counterweight P.C.C. of 158 cum was provided for its stability. The grade of concrete was M35 for AR, AL & M30 for E1, E2, E3, E4, E5, E6, E7, E8, E9, E10 @ Balason & E1, E2, E3, E4 @ Dudhia. The sizes of footing are as under.
|Sl. No||Open foundation||Size of Footing||Grade of Concrete|
|1||AR||10.100 m||9.500 m||1.550 m||M35|
|2||AL||10.100 m||7.000 m||1.000 m||M35|
|3||E1||4.500 m||6.500 m||0.750 m||M30|
|4||E2||4.500 m||6.500 m||0.750 m||M30|
|5||E3||4.500 m||6.500 m||0.750 m||M30|
|6||E4||4.500 m||6.500 m||0.750 m||M30|
|7||E5||10.100 m||5.500 m||0.750 m||M30|
|Sl. No||Open foundation||Length||Breadth||Height||Grade of Concrete|
|8||E6||4.500 m||6.500 m||0.750 m||M30|
|9||E7||4.500 m||6.500 m||0.750 m||M30|
|10||E8||4.500 m||6.500 m||0.750 m||M30|
|11||E9||4.500 m||6.500 m||0.750 m||M30|
|12||E10||10.100 m||3.000 m||0.700 m||M30|
|13||E1||4.500 m||7.000 m||0.750 m||M30|
|14||E2||4.500 m||7.000 m||0.750 m||M30|
|15||E3||4.500 m||7.000 m||0.750 m||M30|
|16||E4||11.100 m||5.500 m||0.750 m||M30|
Pier / Pier Caps & Abutment / Abutment
There are 6 Nos. of abutment AL, AR, E5, E10 @ Balason & P5, E4 @ Dudhia & 17 nos of piers P1, P2, P3, E0, E1, E2, E3, E4, E6, E7, E8, E9 @ Balason & P4, E0, E1, E2, E3 @ Dudhia comprising of rectangular piers in each. The dimensions of pier, abutments & caps are as follows:
|Sr. No||Description||Size of Pier / Abutment (L x B)||Height of Pier / Abutment||Size of Pier Cap /Abutment Cap (LxBxH)|
|1.||AL||(3.5@bottom & 1.96m@top)x1.25m wide||5.700 m||5.95m x 2.51m x 1.0m|
|2.||P1||5.50 m x 3.50 m||1.610 m||5.5m x 3.5m x 1.8m|
|3.||P2||5.50 m x 3.50 m||13.540 m||5.5m x 3.5m x 2.3m|
|4.||P3||5.50 m x 3.50 m||15.500 m||5.5m x 3.5m x 2.3m|
|Sr. No||Description||Size of Pier / Abutment (L x B)||Height of Pier / Abutment||Size of Pier Cap / Abutment Cap (LxBxH)|
|5.||AR||(2.4m@bottom & 1.4m@top) x1.50 m wide||1.550 m||10.10m x 2.0m x 1.0m|
|6.||E0||5.45 m X 1.10 m||2.927 m|
|7.||E1||6.00 m x 0.60 m||6.500 m|
|8.||E2||6.00 m x 0.60 m||6.100 m|
|9.||E3||6.00 m x 0.60 m||5.700 m|
|10.||E4||6.00 m x 0.60 m||5.600 m|
|11.||E5||10.10 m x (1.2m at bottom & 0.605m wide at top)||6.100 m|
|12.||E6||6.00 m x 0.60 m||4.902 m|
|13.||E7||6.00 m x 0.60 m||3.759 m|
|14.||E8||6.00 m x 0.60 m||2.364 m|
|15.||E9||6.00 m x 0.60 m||1.931 m|
|16.||E10||10.10 m x 0.615 m||1.827 m|
|17.||P4||2.15 m x 5.50 m||1.450 m|
|18.||P5||1.75 m x 7.50 m||1.800 m|
|19.||E0||11.10 m x 0.45 m||3.647 m|
|20.||E1||7.00 m x 0.60 m||9.576 m|
|21.||E2||7.00 m x 0.60 m||8.973 m|
|22.||E3||7.00 m x 0.60 m||7.763 m|
|23.||E4||11.10 m x (1.2m at bottom & 0.605m wide at top)||6.087 m|
The grade of concrete was M35 for AL, AR, P4, P5, M45 for P1, P2, P3 & M30 for E0, E1, E2, E3, E4, E5, E6, E7, E8, E9, E10 @ Balason & E0, E1, E2, E3, E4 @ Dudhia.
Cast in-Situ RCC solid slab superstructure
RCC solid deck slab, 10 nos of 8 M span, was constructed at Mirik side beyond AL abutment. The width of the slab was 10.10 M while the depth varies from 0.750 M at centre & 0.624 M at edge for Extension of Balason Bridge (40M). The width of the slab varies from 10.10 M to 11.43 M while the depth also varies from 0.750 M at centre & 0.650 M at edge for Further Extension of Balason Bridge (40M) Part - II. The width of the carriage way varies from 7.50 M to 8.83 M & the longitudinal slope is 1 in 30. The grade of concrete was M30.
Cast-in-Situ Single Cell Cast In-Situ Pre-stressed Box Girder of Superstructure (Simply Supported Span)
There are 2 Nos. of single cell cast in-situ pre-stressed Box girder spans of 46 M & 50 M. The grade of concrete is M45. The dimensions of box girders are as follows:
|Sr. No||Description||AL-P1 Span||P4-P5 Span|
|1||Type.||Cast-in-situ simply supported pre-stressed box girder||Cast-in-situ simply supported pre-stressed box girder|
|2||Class of loading.||Two lanes of class A/One lane of 70R loading||Two lanes of class A/One lane of 70R loading|
|3||Span arrangement.||Single span of 46 M from Pier P1 to Abutment AL||Single span of 50 M from Pier P4 to Abutment P5|
|4||Depth of Box Girder.||Depth constant i.e. 3.250 M||Depth constant i.e 4.00 M|
|5||Width of Deck slab.||10.100 M||11.100 M|
|6||Width of carriage way.||7.500 M||7.500 M|
|7||Longitudinal slope.||1 : 31.75||NIL.|
|8||Width of soffit.||5.500 M||4.500 M|
|9||Pre-stressing Cables: (All cables consist of 12 strands of nominal diameter of 12.7mm class-II strands confirming to IS 14268:1995).|
|i) Soffit Cables.||20 Nos.||NIL.|
|ii) Dummy Cables.||02 Nos.||01 Nos.|
|iii) Web cables.||NIL.||22 Nos.|
Cast In-Situ Continuous Pre-stressed Box Girder of Superstructure
The total length of continuous pre-stressed box girder was 306 M. The grade of concrete was M45. The box girder had been cast by a specially fabricated form traveler gantry @ 4 M segments. There were 16 segments @ shore side & 16 segments @ river side of box girder over pier P2 covering a length of 128 M & similarly 16 segments @ shore side & 16 segment @ river side over pier P3 covering a length of 128 M, coupled with 24 M span on ground supports @ shore side of pier P2 & 24 M span on ground supports @ shore side of pier P3 & a central continuity module of length 2 M.
Road Kerb / Railing Kerb / Railing / Footpath Slab / Approach Slab
The grade of concrete for Road kerb, Railing kerb & Railing are M35. The height & width of road kerb is 325 mm x 300 mm for Balason & 300 mm x 200 mm for Dudhia. The height & width of Railing kerb was 350 mm x 300 mm & 225 mm x 200 mm for Dudhia. The railing comprises of cast-in-situ columns & pre-cast hand rails. In between any two railing columns there are three hand rails placed vertically. The size of railing columns are 260 mm x 265 mm x 1090 mm & the size of pre-cast hand rails are 2240 mm x 175 mm x 165 mm (typical) for Balason & 1720 mm x 120 mm x 170 mm (typical) for Dudhia. Provision for electrical poles has been provided in Railing for which a base plate of 310 mm square with u-bolts has been provided in the railing kerb. The grade of concrete of pre-cast footpath slab is M35.The size of pre-cast footpath slab is 500mm x 800mm x 70mm for Balason & 1000 mm x 600 mm x 75 mm for Dudhia. Underneath pre-cast footpath vacant space has been provided as service ducts in footpath. The width of footpath with railing is 1.80m having transverse slope of 2.50%. There are three nos. of approach slabs @ AR, E10 @ Balason & E4 @ Dudhia abutment locations. The size of approach slab is 9.50M x 3.20M x 0.300 M @ E10, 9.300 M x 3.50 M x 0.300 M @ AR & 10.50 M x 3.20 M x 0.300 M @ E4.
|Sr No||Description||Box Girder 306m (AR-P3-P2-P1 span)|
|1||Type.||Cast-in-situ continuous Pre-stressed box girder constructed by segmental construction method.|
|2||Class of loading.||Two lanes of class A/One lane of 70R loading|
|3||Span arrangement.||P1 to P2 : 88 M, P2 to P3: 130 M & P3 to AR:88 M|
|4||Depth of Box Girder.||Varies equally for first 65 M in both arm for P2 and P3 span i.e. reduces from 10 M at Pier head to 3.250 M at 65 M & then shore side arm further extended by 23 M where depth remain constant i.e. 3.250 M.|
|5||Width of Deck slab.||10.100 M|
|6||Width of carriage way.||7.500 M|
|8||Width of soffit.||5.500 M|
|9||Prestressing Cables: (All cables consist of 12 strands of nominal diameter of 12.7mm class-II strands confirming to IS 14268 :1995)
A) Pier P2
i) Web Cables
ii) Deck Cables
iii) Soffit Cables
iv) Dummy Cables
B) Pier P3
i) Web Cables
ii) Deck Cables
iii) Soffit Cables
iv) Dummy Cables
C) Continuity Span
i) Soffit Cables
The wearing coat on entire bridge deck & approach slabs consists of 75 mm thick concrete of grade M35.
There are 2 Nos. of strip seal type Expansion joint, which was installed @ AL & AR, locations of length 10.10 M each. The expansion joint @ P1 was M.S. finger type of length 10.10 M. M.S. angle type expansion joint was installed at P4 & P5 locations of length 11.70 M each. All the expansion joints are in full carriage width in single straight line & required slope & also over footpath in full width. 10 mm Expansion gap was provided at E1, E2, E3, E4, E6, E7, E8, E9 locations & 5 mm was provided at E5 & E10 locations @ Balason. 10 mm Expansion gap was provided at E1, E2, E3 locations & 5 mm was provided at E4 location @ Dudhia.
12 nos of POT/PTFE bearings & 1 no of PIN bearing was used in AL, AR, P1, P2, & P3 locations. 2 nos of rocker bearings were used in P5 location & 2 nos of rocker cum roller bearings were used in P4 location. At E0, E1, E2, E3, E4, E5, E6, E7, E8, E9 & E10 locations @ Balason and E0, E1, E2, E3 & E4 locations @ Dudhia 3mm thick bituminous pad was used.
The entire substructure & superstructure was painted with snowcem paint, moonstone color. The entire railing was painted with synthetic enamel paint, moonstone color.
Construction of continuous pre-stressed box girder
By constructing this bridge M/s. Gammon India Limited. had achieved a milestone of constructing the longest single span of 130 M, PSC box girder with Cast-in-situ segmental construction method (Segment length = 4 M).
- After the completion of piers at locations P2 & P3, arrangements were made in the well cap for supporting the temporary sand jacks over trestles which was required during cantilever construction.
- After placing the bearings on pier cap in position and freezing the same to arrest the seismic forces during the construction & placing the sand jacks over trestle on either side of the bearing, pier head unit at each pier location P2 & P3 was cast (consisting of two segments of the box girder, one on either side of the bearing).
- Thereafter, steel gantries were erected over them to commence cantilever construction.
- The steel Gantries known as CLC Gantries were around 58 MT weight each. In each P2 & P3 location, 2 sets of the same were erected (one at shore side & one at riverside) on the Pier head unit. With the help of these CLC Gantries segmental construction of 4 M segment of superstructure @ shore side & river side commenced simultaneously.
- After the concrete of the 4m segment unit already cast attains strength of 320 Kg/cm2, stressing of H. T. Cables done as per Drawing & Schedule.
- After stressing, CLC Gantry was moved to next 4 M segment unit.
- CLC Gantry was then leveled, aligned & locked as per level & Drawing.
- Reinforcement fixing, H.T. Cables laying/profiling, anchorage fixing, etc. completed as per drawing.
- Concreting done.
- The procedures 1, 2, 3, 4 & 5 mentioned above was repeated for construction of balance 4 M segment units similarly.
- There were 16 segments @ shore side & 16 segments @ river side of box girder over pier P2 & similarly 16 segments @ shore side & 16 segment @ river side over pier P3, coupled with 24 M span on ground supports @ shore side of pier P2 & 24 M span on ground supports @ shore side of pier P3.
- Suitable counterweights were provided to obtain the required factors of safety against overturning, during cantilever construction.
- As cantilevering was in progress for some length on either side, a temporary trestle support was erected at the shore side ends of the structure so that reactions on the sand jacks could be kept within low and manageable limits.
- Additional sand jacks were thereafter provided on the trestle support following which the sand jacks on trestles over well cap was released for transference of reactions on the sand jacks on temporary trestle, support at the shore side ends. The counterweights were suitably shifted on the decking above the location of the temporary support at trestle location and further cantilever construction was continued.
- Casting of the few end units (24 M span) of the cantilevers was taken up on staging support and after casting the central continuity module and stressing of remaining deck cables and continuity cables, the staging were released and trestle support dismantled to complete the structural system.
- Bearings frozen temporarily to arrest seismic forces were thereafter freed immediately to enable the deck to be free for the longitudinal translatory and rotatory movements.
- Casting of finishing items and fixing of expansion joints were followed, to complete the construction.
Details of Construction Sequence
Stage I - For Unit 1 Over Pier “P2” & “P3”
- Anchor bolt was left in Well Cap for temporary steel column.
- Pier & Pier Cap concreting was completed in all respect. Inserts were provided in Pier for supporting steel column.
- Temporary steel column were erected above Well Cap.
- The Main Bearings were placed over Pier in position and provided with freezing arrangement.
- Sand Jack were erected over steel column.
- Formwork system were erected for casting of First Unit i.e. Unit 1 on either side shore & river side of the Pier.
- Box Girder Unit 1 was cast on either side shore & river side of the Pier including Central Diaphragm.
- 63 dia short Suspenders were embedded during the casting of Unit 1 @ shore & river side.
- Main Bearings and all four Sand Jacks were placed in position while casting these units. These units were rested over Main Bearings and four Sand Jacks.
- After the concrete of Unit 1 attained a strength of 25 MPa, formwork supporting these units were removed & the decking cast had been supported over Main Bearings and four Sand Jacks.
- The Decking were firmly locked with 40 dia suspenders, with steel Column.
- CLC Gantry No.1 were erected on Shore side for casting Unit 2 shore side.
- CLC Gantry were anchored with Embedded Suspenders in Unit 1 shore side.
- Unit 2 was cast in two stages. In first stage, Soffit Slab along with 6 M height of Webs was cast. 4 – 25 MM dia Tor Steel (HYSD) extra reinforcement were embedded in Webs while casting Units 2 shore side. After the concrete in first stage attained Crushing Strength of 25 MPa, balance height of Webs and Deck Slab were cast.
- After Unit 2 shore side attained strength of 25 MPa, the Sand Jacks were lowered on River Side by about 25 mm, so as release their support to the Box Decking.
- The Sand Releasing Bolts of Sand Jacks, were retightened & these Jacks were left in position, throughout the CLC construction as a safety measure. However a Gap of about 25 mm were maintained between the top of Sand Jack and Soffit of the Box Decking.
- Gantry No.1 was moved further in position for casting the next Unit i.e. Unit 3 shore side.
- CLC Gantry No.2 were erected over Box Decking on River Side for casting Unit 2 river side.
- The CLC Gantry were anchored with Embedded Suspenders in Unit 2 shore side.
- Unit 2 river side was cast in two stages. In first stage Soffit Slab along with 6 m height of Webs were cast. 4 – 25 mm dia Tor Steel (HYSD) extra reinforcement were embedded in Webs while casting Unit 2 river side. After the concrete in first stage attained Crushing Strength of 25 MPa, balance height of Webs and Deck Slab were cast.
- After the concrete in Unit 2 river side attained the requisite strength, the corresponding cables were stressed.
- The CLC Gantry No.2 were moved on River Side further in position to cast the next unit on River Side.
- Unit 3 on Shore Side were cast in two stages. In first stage, Soffit Slab along with 4.5 M height of Webs were cast. 4 no – 25 mm dia Tor Steel (HYSD) reinforcements were embedded while casting Units 3 shore side in Webs. After the concrete in first stage attained crushing strength 25 MPa, balance height of Webs and Deck Slab were cast.
- The CLC Gantry No.1 were moved on Shore Side, further in position to cast the next Unit 4 on Shore Side.
- The lower working platform were introduced on CLC Gantry No.1.
- A Kentledge over Decking, exactly over the centre line of steel Column on Shore Side were placed.
- Similarly Unit 3 on River Side were cast & the requisite Cables were stressed.
- The CLC Gantry No.2 on River Side were moved further in position to cast the next Unit 4 on River Side.
- The lower working platforms on CLC Gantry No.2 were introduced.
- Unit 4 shore sides were cast in two stages. In first stage, Soffit Slab along with 5.0 m height of Webs was cast. 4 no – 25 mm dia Tor Steel (HYSD) reinforcements were embedded in Webs while casting Units 4 shore side. After the concrete in first stage attained crushing strength 25 MPa, balance height of Webs and Deck Slab were cast.
- Unit 4 river side were cast in stages similar to Unit 4 shore side.
- After the Unit 4 shore side & unit 4 river side cast attained the requisite strength, the corresponding Cables were stressed.
- The CLC Gantry No.1 were moved on Shore Side further in position to cast the next Unit 5 on Shore Side.
- The CLC Gantry No.2 were moved on River Side further in position to cast the next Unit 5 on River Side.
- Unit 5 shore side were cast in two stages. In first stage, Soffit Slab along with 4.2 m height of Webs were cast. 4 no – 25 mm dia Tor Steel (HYSD) reinforcements were embedded in Webs while casting Units 5 shore side. After the concrete in first stage attained crushing strength 25 MPa, balance height of Webs and Deck Slab were cast.
- Unit 5 river side were cast in stages similar to unit 5 shore side.
- After the Unit 5 shore side & unit 5 river side cast attained the requisite strength, the corresponding Cables were stressed.
- The CLC Gantry No. 1 were moved on Shore Side further in position to cast the next Unit 6 on Shore Side.
- The CLC Gantry no.2 were moved on River Side further in position to cast the next Unit 6 on River Side.
- Unit 6 shore side were cast in two stages. In first stage, Soffit Slab along with 4.5 m height of Webs were cast. After the concrete in first stage attained crushing strength 25 MPa, balance height of Webs and Deck Slab were cast.
- Unit 6 river side were cast in stages similar to Unit 6shore side.
- After the Unit 6 shore side & unit 6 river side cast attained the requisite strength, the corresponding Cables were stressed.
- Unit ‘7’ was cast on either side of pier simultaneously i.e. unit ‘7’ on shore side upto bottom of haunch below deck slab was cast first. Then unit ‘7’ on river side upto bottom of haunch below deck slab was cast, similarly casting of deck slab along with haunches at shore side was followed by the river side deck slab. Requisite cables were then stressed.
- After casting unit ‘7’ on both side first shore side gantry was shifted to cast next unit ‘8’
- River side gantry was then shifted to cast next unit ‘8’
- The procedure given in step Nos. 32,33 & 34 was repeated for casting unit No. 9 & 10 for arm over pier P2 & casting unit No. 9 to 13 for arm over Pier P3.
- Temporary trestle support was provided at a distance 34m. on shore side from c/l of pier for arm over Pier P2 & at a distance 46.0m on shore side from c/l of pier for arm over pier P3 with suitable sand jack system. 20 tonne kentledge was shifted & placed over the decking exactly over the c/l of temporary support ‘T’ from the original location of ‘J1’. Sand jack ‘J1’ (on shore side) was then released & ensured that cantilever arm rests on trestle support.
- Unit no. ‘11’ on shore side was cast.
- Unit no ‘11’ on river side was cast & requisite cable were stressed.
- Shore side gantry was first shifted to cast next unit no ‘12’.
- River side gantry then shifted to cast next unit no ‘12’.
- The procedure given in 37, 38, 39 & 40 steps was reapeted for casting units 12 to 15. for arm over pier P2 & for casting units 14 & 15 for arm over Pier P3.
- Unit No. ‘16’ on shore side was cast
- Unit No. ‘16’ on river side was cast.
- First river side gantry was dismantled.
- Then shore side gantry was dismantled.
- After completing abutment cap & pier cap in all respects, bearings were placed in position over Pier P1 (Mirik side) & over abutment A/R on (Kurseong side).
- Staging & shuttering for casting units between axis 9 to 18 on Mirik side & 84 to 93 on Kurseong side was erected.
- Units between axis 9 to 18 & 84 to 93 was cast including diaphragm over Pier P1 on Mirik side & over abutment A/R on Kurseong side.
- After the concrete attained sufficient strength, requisite cables were stressed.
- The staging & shuttering erected for casting these units were removed.
- Kentledge of 20 tonne and temporary column support with sand jack ‘T were removed’.
- Temporary column support with sand jack ‘J2 were removed’.
- Suspended platform on tip of the cantilever to cast continuity unit between axis 50 to 52 was erected.
- Continuity unit between axis 50 to 52 was cast.
- Continuity cables were stressed.
- Suspended platform were removed.
- General items were cast.
Chronological Photographs of Construction of Continuous Pre-Stressed Box Girder
The Grade of concrete used in foundations was M30 & M35 and in substructure & superstructure was M45. For substructure & superstructure coarse aggregate from Pakur (Jharkhand), 400 km from site was used and sand for all structures from Kartwa / Jalpesh (Jalpaiguri), 120 km from site was used. Ultra Tech 43 grade OPC Cement was used. The dosage of Cement was 360 Kg/m3 for M30 grade, 365 Kg/m3 for M35 grade & 400 Kg/m3 for M45 grade. In M45 Grade of Concrete, admixture Setplast 340 was used with a dosage of 0.7% by weight of cement.
The construction of the bridge has opened a new crossing to traffic and it enhanced the economical & social growth. It added the beauty in Darjeeling’s landscape & stood as an example of, one of the best Civil Engineering structure.
- The Project received the “ACCE AWARD 2007”.
- The Project received the “AWARD for Excellent Aesthetics Matching with Environment 2009 from IIBE”.
Owner: Darjeeling Gorkha Hill Council, Council House, Lal Kothi, Darjeeling (West Bengal). Design & Construction: Gammon India Limited, Mumbai.