Construction of Foundation & Sub-Structure of Bogibeel Rail-Cum Road Bridge Over River Brahmaputra- An Engineering Marvel

Anupam Das, Sr. General Manager, Gammon Engineers & Contractors Pvt.Ltd.

The former Prime Minister of India Shri H D Devagowda laid the foundation stone of the Bogibeel Bridge on January 22, 1997 at Kulajan (Assam). After 5 years of laying of the foundation, on April 22, 2002 the Prime Minister of India Shri Atal Behari Vajpayee inaugurated the construction work. However, work on the main bridge (foundation & sub-structure) of the Bogibeel Bridge started only after a passage of six years, that is, on April 04, 2008.

The Bogibeel Bridge is the fifth bridge across river Brahmaputra, approximately 17km downstream of Dibrugarh Town, located in the eastern part of Assam. It is being constructed for Double Line BG Track and three lane road. This bridge is going be the lifeline of the northeastern part of the country. It will facilitate connectivity between North and South of river Brahmaputra in the eastern region of Assam and Arunachal Pradesh. National Security of the eastern region of India gets further strengthened, after commissioning of the Bridge. The Government of India has also given it the utmost importance by declaring it as a National Project in the year 2007.

The Bogibeel Bridge Project is a part of Assam Accord 1985. It is the longest Rail-cum-Road Bridge in India as on date, having a total length of 4.94 km.

The Rail-cum-Road Bridge of N.F.Railways connects the Dibrugarh Railway station and Moranhat Railway station of south bank with New Sisibargaon in the District of Dhemaji on north bank, between Rangiya - Murkongselek (presently Meter Gauge Section and proposed for Gauge Conversion along with commissioning of the bridge at Bogibeel) with proposed rail link of 74 km. The bridge also provides the road link of 29.45 km between the NH-37 on south bank and NH-52 on north bank which connects KuIajan on NH-52 on north bank and Durgapur Tea Estate on NH-37 on south bank. This proposed road link has been designated as NH-52(B). The estimated cost of the Project is around Rs. 3230.02 crores. The Bogibeel Bridge was commissioned for Rail and Road traffic on 25th December 2018.

The Project
Gammon India Ltd was entrusted with the construction of the Well Foundation & Sub-structure of the Rail cum Road Bridge across the River Brahmaputra by N.F.Railways. Since the bridge falls in a seismically active zone (Zone-V), special care has been taken in the design of the bridge to mitigate disasters. For this, the depth of the wells has been increased by 25% and the thickness of the steining of wells has been increased to 3m instead of 1.5 or 2.0m. Two out of three caissons (P2 & P3) are in the main deep channel (water depth 14m to 18m), with water velocity ranging from 3m/sec to 5m/sec. Also, the Caissons were to be constructed in a limited working season of 4 months (November to February) including Fabrication, Erection, Launching, Grounding and Sinking of Caissons up to a safe depth (atleast 20m below bed level).

Scope of work includes 42 double ‘D’ (16.2m x 10.5m) well foundations of 58.6m depth for P2 to P39, 68.75m depth for P1 & P40, and 42m depth for A1 and A2, including 3 Caisson foundations along with Pier & Pier Caps.

The total concrete involved is nearly 3.5 lakh cum. Caisson foundations are one of the most difficult deep foundations to construct in any river and more so in the mighty and ferocious Brahmaputra. Sudden flash floods due to untimely and unprecedented heavy rains in the upper reaches of the river made this task even more challenging. The fact is that working on a river like Brahmaputra one cannot follow a strict methodology. This is because the river takes new forms and changes its course every year, so the depth, bed level etc. keep changing, leading to forced changes in the planned methodology. The working season is just four months (i.e. Nov to Feb) as workable water level recede only by end of Oct and start rising from the 1st week of March.


Salient Technical Features

Name of Job : CONSTRUCTION OF WELL FOUNDATION AND SUB-STRUCTUREOF BOGIBEEL RAIL CUM- ROAD BRIDGE ACROSS RIVER BRAHMAPUTRA NEAR DIBRUGARH
Contract Value (including 5 additional Wells)
Work Done	Rs. 361.37Crores
Escalation	Rs. 55.09  Crores
Total	Rs. 416.46 Crores
Type of contract	Item Rate
Contract Period	42 months
Project Start Date	04.04.2008
Completion Date As per CA	03.10.2011
Completion Date  after 1st EOT	30.06.2013
Completion Date  after 2nd EOT	30.04.2014
Completion Date  after 3rd EOT (Actual completion date)	30.04.2016

Well Foundations
42 Well Foundations were placed namely A1, P1 &P2, P39, P40 & A2. The distance between A1 - P1 & P40-A2 is 32.750M and the rest 39 span is of 125m each.

Type : Double ‘D’

Depth : 58 m (for wells P2 to P39) & 46 m (for abutment wells A1 & A2) & 68.75m for Wells P1 & P40).

Span Arrangement: 1 x 32.75m + 39 x 125m +1 x 32.75m =4940.50 m

Cutting Edge & Steel Strakes
The cutting edge is made up of ISA 200 structural steel plates 20 mm thick and 12 mm thick in outer face. The total qty one cutting edge is 40.52 MT. These are placed on a dry surface after leveling the required area.

Well Curb
The well curb is of double D shape of height 5.1m from bottom. The inner and outer side of well curb 2.0m from bottom is lined with steel strakes to sustain the impacts of blasting / grabbing etc. during sinking operation. The well curb is concreted with M35 grade.

Sinking
Sinking of wells was carried out by means of conventional sinking method using cranes and grabs. The well was sunk up to a depth of 58.60M from RL 98.6M. The strata met with during sinking of the wells included fine sand, sand mixed clay, and sand mixed gravel.

Steining
The steining heights of wells are A1 & A2 – 36.9M, P1& P40 – 63.65M and P2 – 55M P3 to P40 – 53.5M, with thickness 3.0 M. The steining was cast in lifts with M35 grade of concrete.

Bottom Plug
When the cutting edge of well reached the founding level, well was plugged under water with M25 grade concrete and 10% extra cement for under water concreting.

Excavated material filling & top/intermediate plug After concreting of bottom plug, sand filled inside the wells up to bottom of top plug / intermediate plug. At A1 & A2 wells top plug of 15.15 M3 of M25 grade concrete was cast at bottom of well cap. P1 to P40 wells intermediate plug of 15.0 M3 of M25 grade concrete was cast under water.

Excavated material filling & top/intermediate plug After concreting of intermediate plug. At A1 & A2 wells, top plug of 15.15 M3 of M25 grade concrete was cast at bottom of well cap. P1 to P40 wells intermediate plug of 15.0 M3 of M25 grade concrete was cast under water. Then fresh water filled between top of intermediate plug & bottom of well cap for stability of wells.

Well Cap
Well cap of 2.2 M depth was cast with M35 grade concrete. The quantity of concrete of each well cap is 252.0 cum.

Caisson Details

Sl. No	Well No	Length Caisson	Bed Level	Depth of water	Grade of Concrete
2.1	P2	40.3m	78.35	18.55	M-35
2.2	P3	21.9m	88.	8.70	M-35
2.3	P38	16.3m	85.54	11.47	M-35

Pier / Pier Caps & Abutment / Abutment Caps
There are 2 rectangular abutment A1 & A2 & 40 circular hollow piers P1 to P40. The dimensions of pier, abutments & caps are as follows:

Sr. No	Description	Size of Pier / Abutment		Height of Pier / Abutment	Size of Pier Cap   /Abutment Cap
		Outer Dia/Length	Inner Dia/Breadth		L x B x H
3.1	A1 &A2	3m	1m	4.215m	13.9x5.480x1.2
3.2	P1-P40	5.3m	3.3m	14m	13.9x5.480x1.2

The grade of concrete was M35 for abutment, pier, abutment, cap & pier cap.

Fender Wall
After completing the Pier Cap, Fender Wall was cast from the top of Well cap. Height of Fender wall is 5.720M, Bottom thickness 600mm upto 1.50m height, then it inclines up to top. Top width is 350mm.

Sand filling
After completion of Fender wall concrete, sand fill inside the fender wall portion at top of well cap. On top of it 300 mm thick bitumen/sand mixed layer was laid. Major Quantities of Prime Activities

SNo.	Description	Quantity	Unit
1)	Well Foundations:	42	Nos
	Fabrication of cutting edge	42	Nos
	Cutting Edge placing / erection	39	Nos
	Floating, lunching & grounding of caisson	03	Nos
	Well Curb	42	Nos
	Well Steining	2235.5	RM
	Well Sinking	2378	RM
	Bottom plugging after reaching the founding level	42	Nos
	Sand filling inside the dredge hole	42	Nos
	Intermediate Plugging	40	Nos
	Top Plugging	2	Nos
	Non recoverable beam and slab placing for sealing the  dredge hole of well foundation	40	Nos
	Well cap	42	Nos
2)	Substructures:
	Piers	42	Nos
	Pier cap	42	Nos
	Fender Wall	38	Nos
	Sand filling in the annular space between piers and fender wall & bitumen laying.	38	Nos

Sl No	ITEMS	Unit	TOTAL QUANTITY
1	Concrete.	Cum	3,40,335
2	Reinforcement Steel.	MT	20260
3	Cement.	Bag	2740128
4	Coarse Aggregate.	Cum	294959
5	Sand.	Cum	175110
6	Sinking.	RM	2378
7	Structural (For Fabrication of Cutting Edge, Caisson etc.)	MT	2340

Well Foundation & Sub Structure Construction Sequence

1. Cutting Edge placing / erection
2. Well Curb
3. Well Steining / Well Sinking
4. Bottom plugging after reaching the founding level
5. Sand filling
6. Intermediate plugging
7. Non-recoverable beam and slab placing for sealing dredge hole of well foundation
8. Well cap
9. Piers
10. Pier cap
11. Fender Wall
12. Sand filling in annular space between piers and fender wall.

Method of Constructing Well Foundation

Cutting Edge placing / erection:
Fabrication as per drawing up to required height at fabrication yard
Bending of cutting edge angle as per the radius mentioned in the drawing by Hydraulic Jack at Fabrication Yard. Placing of bent angle and straight angle over the already made pedestals.


Fixing of skin plate on the outer and inner side on the bent angles.

Erection of Cutting Edge at location

• Cutting Edge is generally placed in Natural Island or in built up Island.
• Natural Island is formed naturally after siltation of soil / sand in the river. It is most suitable for placing of cutting edge of well.

Built up Island

• Where the depth of water is within 3m to 5m and where the water is stagnant, in which case the island is made for placing of Cutting Edge.
• The island is constructed by driving two rows of Bamboo / Balli piles at a spacing of 200mm for Bamboo and 400mm for Balli piles.
• A minimum distance of 0.75m is kept between two rows of Bamboo/Balli.
• Each pile should be driven at least 1/3 of water depth below bed so as to have a sufficient grip.
• Required nos. of horizontal and cross bracings are fixed with vertical piles for strengthening the Island.
• After that Bamboo matting is fixed along the inside faces of the ballies and the space between the matting is filled with puddle clay or sand bags.
• Sand bags are dumped on the outside of the outer row of piles to provide support to the piles.
• The space enclosed by the bamboo or ballie walls is filled with sand upto the proposed top level of the Island.

Well Curb

• Fixing of inner shuttering above Cutting Edge up to 5.100 mtr height.
• Reinforcement steel fixing up to the required height.
• Outer shuttering plates fixing.
• Concreting

Well Steining / Well Sinking

• After concreting of well curb, it is kept undisturbed for 3 days. Then inner, outer shuttering plates are removed and manual sinking is done.
• Fixing of formwork and reinforcement for well steining (3 mtr lift).
• Concreting (3 mtr lLift)
• After setting of concrete, deshuttering (3 mtr lift) is done.
• Sinking (3 mtr. lift) is executed by using crane and grab.

Bottom plugging after reaching the founding level

• After the cutting edge reaches the founding level, bottom plugging is done by Tremie pipe method.

Sand Filling

• After bottom plug concreting, the well is kept undisturbed for 72 hours for setting of concrete.
• Sand is filled inside the dredge holes up to the RL. 70.070m i.e. a height of 22.970 m.

Intermediate plugging

• After the completion of sand filling, the well is left undisturbed for at least 48 hours so that the sand settles down.
• After the waiting period, intermediate plugging is done by Skip Bucket or Tremie pipe method.

Non recoverable beam and slab placing for sealing dredge hole of well foundation

• After completion of intermediate plugging, the rest of the well steining is filled up with water upto the bottom of the well cap.
• Well cap soffit shuttering to be done by placing non recoverable beam and slabs.
• After completing the soffit shuttering, reinforcement and outside shuttering concreting is done.

Well Cap

• After completion of non-recoverable soffit shuttering, fixing of well cap reinforcement and shuttering is done, followed by concreting.

Pier

• After completion of well cap casting, reinforcement and shuttering works for twin piers is done.

Pier Cap

• After completion of piers, pier cap soffit shutters, scaffolding, and supporting arrangement to be erected.

Fender Wall

• Fender wall is cast in two lifts. The height of fender wall is 5.720 mtr.

Sand filling in the annular space between piers and fender wall
After removal of Fender Wall shuttering, sand filling is to be done. The last 300 mm layer is to be filled with bitumen layer mixed with sand at a ratio 1:10.times required for sand filling including bitumen layer.


Methodology Adopted for Caisson Foundations

• Fabrication of K-lift (5.1m + 1.6m = 6.7m) and required Modules of 2.4m height in the fabrication yard during the monsoon period.
• Shifting of Prefabricated Modules of Caisson at Launching Bed after cutting in segments as per pre-approved cutting plan.
• Reassembling of Caisson Modules at Launching Bed up to 6.7m.
• Floating and placing the Caisson at location with the help of wire ropes and pulley as per tethering arrangement drawing.
• Addition of required pre-fabricated modules and placing of stage wise concrete as per sequential drawing of Grounding.
• Ground the Caisson as per drawing.

Fabrication & Erection of Caisson at Fabrication Yard

• Bending of Angle (ISA 200X200X20) by Hydraulic Jack.
• Placing of bent angles and straight angle.
• Fixing and welding of 8mm thk. outer skin plate up to 0.8m height.
• Fixing of central diaphragm angle (10634mm long 1no.) in its position.
• Placing of vertical frames as per drawing and align the same with the help of staging support.
• Fixing of inner & outer skin plates in layer up to 5.1 m. height throughout along with diaphragm portion including fixing of Hoop angles and cross bracings with the skin plates.
• Complete welding of caisson up to K-Lift (5.1+1.6m), except the joints where caisson were cut for making pieces for shifting.
• Fabrication and erection of further required Modules at fabrication yard.
• Making of Coffer Dam for Launching of Caisson. Location was selected by proper survey and considering the depth of water along the towing path.

Erection of Caisson at Launching Bed

• Three nos. of high power Tugs were used for towing and bringing the caisson to location.

Grounding Sequence of Caisson

• 150T concrete was uniformly placed in curb portion.
• Additional lifts were built, reinforcement was extended, winches were lifted, and concreting was done after checking the bed levels. This process continued till the caisson reached nearer to bed level. During the last stage, the free board is kept minimum 5m and C.E at least 0.5m above bed level.
• As the cutting edge reached nearer to bed, the preparation of grounding started.
• Safe working platform were constructed above Caisson for easy access to all hoppers.
• Bed level was checked and necessary grabbing done for leveling the bed.
• Sand bags were dropped around the Caisson to prevent scouring.
• The alignment of Caisson was checked by total station placed in pre-established control points.
• Caisson was readied for grounding.
• Concrete was poured faster continuously. Caisson started to sink down and the C.E reached below the bed level.
• In this above operation alignment of Caisson was checked in every half an hour by total station and control points. The final adjustment of alignment is done with the help of winch arrangements.
• In all stages the free board was kept not less than 2.0m.
• Caisson Modules were added further, concrete was placed and sinking done until the Caisson reached up to safe gripping length.
• After reaching the safe grip length (Minimum 1/3 of total depth of water), concreting was done up to top of Caisson and all tethering arrangements were removed.

Challenges Encountered and Remedial Measures Adopted
Change in Bridge length due to shifting of Guide bund in North and South bank by N.F Railways

• Since the Guide bunds were not feasible for construction in its original location by N.F Railways, due to high water current, shifting of Guide bunds in North and South bank by N.F Railways was necessitated. Due to this shifting of Guide bunds, the length of the bridge increased from 4315.5M to 4940.50M, which resulted in increase of 5 additional well foundations. Also, shifting of Guide bunds from original locations by NF Railway resulted in change in course of river, for which change in concreting methodology had to be adopted by us.

Seasonal Change in the course of river and water depth at different locations resulted in change in construction methodology and materials / plant & machineries movement / logistics to desired locations:

Change in Course of River
Before Commencement of the Work

1st Working Season

Additional service road making:
South Bank:
An additional service road was made from staff establishment area to the service bank, which was made for material loading-unloading and barge movement at South Bank location, nearly 1.0 km downstream from the bridge center line.

An additional service road was made for concrete transportation from Batching Plant location to concrete loading point location along the bridge center line.

Buoy Bridge Making for Concreting
North Bank & South Bank:
Buoy Bridge (made of structures like MS plate 8 mm thick and ISMC 125) has been constructed on both sides of the river bank (South Bank & North Bank), for transporting concrete from the batching plant to Island. The Buoy Bridge was anchored by large anchors weighing 10MT and buoys, to keep the buoy bridge as straight as possible. However, due to heavy flow in the river and excessive rise in water level during monsoon, we had lost our substantial portion of Buoy Bridge and had to rebuild the Buoy Bridge again for the next season.

The buoy bridge construction was greatly hampered by the construction of Guide Bund at South Bank. Initially the Buoy Bridge was made at a distance of a few meters from the Batching plant, but due to construction of the Guide bund and shortening of the river width, the buoy line had to be reconstructed again at a distance of approximately 300m towards the north bank side.

Methodology of concreting of 1st Working Season (DIBRUGARH & SILAPATHAR SIDE)

Concreting method:

• Concrete transported from batching plant to pump-1 by T.M. then from Pump-1 to Pump- 2 by pipeline laying over Buoy Bridge placed at island. Pump-2 to location trough pipeline i.e. well P4, P5, P6, P7 & P8.
• Concreting operated from Batching Plant to Transit Mixer to Concrete Pump-I near River Bank to Pump-2 at island and then Pump-3. Pump-3 to location P35 & P34 by direct pumping. For this purpose, one main floating Buoy line laid at 100m D/S side of Bridge center line for crossing the river channel.

2nd Working Season
Buoy Bridge Making for Concreting
North Bank & South Bank:
Buoy Bridge (made of structural like, MS plate 4 mm thick and ISMC 125) has been constructed again for the 2nd season on both sides of the river bank (South Bank & North Bank), for transporting concrete from batching plant to Island. The Buoy Bridge was anchored by same process as it was stated earlier in the 1st season. However, due to heavy flow in the river and excessive rise in water level during monsoon, we had lost again substantial portion of Buoy Bridge and had to rebuild the Buoy Bridge again for the next season.

The Buoy bridge construction work at North Bank of 2nd season was greatly affected by the construction of Guide bund at North bank side. The concrete loading point location was variable due to this construction activity.

Additional Service Road Making:
South Bank:
Additional service road which was made in the 1st season for concreting became completely unusable as guide bund was constructed. The other road for transporting construction materials was completely damaged due to flood and submergence during flood. Hence another service road was made, nearly 1.0 km upstream from bridge center line for transportation of all the necessary construction materials and plant-machinery to Island.

Methodology of concreting of 2nd Working Season (Dibrugarh & Silapathar Side)

Concreting method:

• Concrete operated from batching plant to Transit Mixer to concrete pump-1 above guide bund to concrete pump-2 above crate near river bank to Main Island. From Main Island to P6, P7, P8 P9 by directly pumping placing the pump at river bank / loading point. For wells P10, P11, P12 Transit Mixer used for transporting concrete from loading point to pump-3 placed near well location and then pumping of concrete to actual pouring location.
• Concrete operated from batching plant to Transit Mixer to concrete Pump-1 above guide bund to concrete Pump-2 above crate near river bank and then to main island by laying pipeline laying over Buoy Bridge. From main island to wells locations T.M. used for transporting concreting. From loading point to Pump-3 & Pump-4 placed near well location and then pumped to actual pouring location.

3rd Working Season
Buoy Bridge Making for Concreting:
North Bank & South Bank:
Buoy Bridge (made of structural like, MS plate 4 mm thick and ISMC 125) has been constructed again for the 3rd season on both sides of the river bank (South Bank & North Bank), for transporting concrete from batching plant to Island. The Buoy Bridge was anchored by same process as it was stated earlier in the 1st season. However, due to heavy flow in the river and excessive rise in water level during monsoon, we had lost substantial portion of Buoy Bridge and had to rebuild the Buoy Bridge again for the next season.

Additional Service Road Making
South Bank:
An additional service road has been made in the 3rd season for transportation of concrete to the 1st concrete loading point location. The road has been built up from batching plant location. The road was almost 5 km, for carrying the entire concrete as well as construction materials for shifting to locations.

Methodology of concreting of 3rd Working Season (DIBRUGARH & SILAPATHAR SIDE)

Concreting method:

• Work had been bifurcated into two groups: Group – I = P-2 to P-5 Group – II = P-6 to P-16. Under Group – I, Concreting operated from B.Plant to Transit Mixer to Concrete Pump-I placed above crate then direct pumping to location i.e. P2, P3, P4 & P5 locations. For this purpose, one main floating Buoy line laid at about 140m D/S side of Bridge center line and one sub line to be connected from main line to P2, P3, P4 &P5 location. Under Group – II, Concreting operated from B.Plant to Transit Mixer to Concrete Pump-I placed above Crate near River Bank to Pump-2 at mid river and then to Main Island. From Main Island to wells location Transit Mixers used for transportation of concrete from River Bank to Concrete Pump-placed near Well location and then Pump to actual pouring location.
• Work had been split into two groups: Group – I = P-40 to P-38 ; Group – II = P-37 to P-27. Under Group – I, Concreting operated from B.Plant to Transit Mixer to Concrete Pump-I above Crate near River Bank to P40, P39 & P38 locations. For this purpose, one main floating Buoy line laid at about 140m D/S side of Bridge center line and one sub line to be connected from main line to P38 location. Under Group – II, Concreting operated from B.Plant to Transit Mixer to Concrete Pump-I placed above Crate near River Bank to Pump-2 at mid river and then to Main Island by laying pipeline laying pipeline over Buoy Bridge. From Main Island to wells location Transit Mixers used for transportation of concrete from River Bank to Concrete Pump-placed near Well location and then Pump to actual pouring location P35 to P27. For this purpose, one main floating Buoy line laid at about 140m D/S side of Bridge center line and one sub line is connected from main line to P37/P36 location.

4th Working Season
Buoy Bridge Making for Concreting
North Bank & South Bank:

• Buoy Bridge (made of structural like MS plate 4 mm thick and ISMC 125) has been constructed again for the 4th season on both sides of the river bank (South Bank & North Bank) for transporting concrete from batching plant to Island. The Buoy Bridge was anchored by same process as it was stated earlier in the 3rd season. Due to heavy flow in the river during monsoon, we had lost substantial portion of the Buoy Bridge and have rebuilt it for the 4th season. In addition, the length has also increased in the North Bank due to the drastic change in the River’s profile. We have made 900m long Buoy Bridge for laying concrete pipeline at North Bank and 450m long Buoy Bridge at South Bank.

Additional Service/Approach Road Making
South Bank /North Bank:

• After complete construction of Guide Bund, the river profile has changed again in this season. In this season there was formation of two live river channels with enormous water currents including few dead channels which was not envisaged. One live channel was formed between P15 & P16 and the other one was formed between P27 & P29. As the working season is very less and there was no approach for material shifting, we had hired additional plants for closing all the live & dead river channels and for making of Approach Road on time at Island for carrying construction materials and concrete. We have made around 3.500km Approach Road from P5 to P32 location and around 700m long approach at downstream of P22 for shifting of materials and for transportation of concrete through Transit Mixers. Total earth work involved for closing the River channels and for making of Approach Road was around 47250 cum (4200mx7.5mx1.5m) cum. We have used 3 Excavators (2 Poclain & 1 JCB), 2 Hyva, 4 Tractors and 1 Dozer for completing the work.

5th Working Season
Buoy Bridge Making for Concreting
North Bank & South Bank:

• Buoy Bridge (made of structural like MS plate 4 mm thick and ISMC 125) has been constructed again for the 5th season on both sides of the river bank (South Bank & North Bank) for transporting concrete from batching plant to Island. The Buoy Bridge was anchored by the same process as it was stated earlier in the 4th season. Due to the devastating flood in the River on June’12 & Sep’12, we had lost a substantial portion of Buoy Bridge & Anchor Buoy so we have dropped the Anchor Buoy again for this 5th season. Due to the flood, the starting of work was delayed by about one month. We have made a 600m long Buoy Bridge for laying a concrete pipeline at the North Bank and a 450m long Buoy Bridge at the South Bank.

Additional Service/Approach Road Making

• In this season, there was formation of one live river channel between P16 & P18 including few dead channels. As the working season is very less and there was no approach road for material shifting, we had hired additional Plants (3 Hyva Dumpers, 1 Dozer, 1 Excavator) for closing the live & dead river channels and for making the Approach Road on time at the Island for carrying construction materials and concrete. We made around 3.750km long approach road from P5 to P35 location and a 500m long approach road at downstream of P30 for shifting of materials and for transportation of concrete through Transit Mixers. Total earth work involved for closing the River Channels and for making of Approach Road was around 23906 cum (4250mx7.5mx0.75m) cum. We have used 3 Excavators (2 Poclain & 1 JCB), 3 Hyva and 1 Dozer for completing the work.

6th Working Season
Buoy Bridge Making for Concreting
South Bank:

• Buoy Bridge (made of structural like, MS plate 4 mm thick and ISMC 125) has been constructed again for the 6th season on Dibrugarh side of the river bank (South Bank) for transporting concrete from batching plant to Island. The Buoy Bridge was anchored by same process as it was stated earlier in the 5th season. Due to the devastating flood in the river on June’13, July ’13 & Sep’13, we had lost a substantial portion of Buoy Bridge and Anchor Buoy so we dropped the Anchor Buoy again for this 6th season. Due to the frequent rise in water levels and heavy currents in the water in the months of Sept’13 and Oct ’13, the starting of work was delayed by almost one month. Due to frequent rise in water levels and heavy currents we are not able to fix the Buoy Bridge for laying the concrete pipeline at the South Bank.

Additional Service/Approach Road Making:
South Bank /North Bank:

• In this working season the Brahmaputra River has unprecedently and drastically changed its course as compared to previous working seasons. This has created substantial hindrances in execution.

As per a river survey conducted on 11.11.2013 and after sudden rise in water level on 30.10.2013 (water level rose up to RL 99.15m) the profile of the River changed again and we found enormous scouring of bed levels in all channels between P12 to P20 location. This time the situation had worsened. We found a further increase in the depth of channels due to heavy scouring.

As per the river profile, we found two major flows of water/channels between P12 to P14 (avg depth of water-2.5m, and in earlier river profile the depth was-1.60m), between P16 to P17 (avg depth of water-2.75m, and in earlier river profile the depth was-1.90m) and one channel between P6 to P7 (avg depth of water-0.80m, and same in earlier river profile).

In the month of Nov’13 we made an approach road by hiring 1 Dozer, 2 Hywas, 2 Excavators of JCB, 3 Water Pumps etc, Since the approach road had washed out from P18 to P21, we again rebuilt a 400m road from P18 to P21 location of 12800cum.

In the month of Nov’13, we filled two major channels and made an approach road between P18 to P21 (400mx10mx3.2m) and between P5 to P7 (300mx10mx2.5m) of 20300cum. Due to the major formation of a channel between P12 to P17 and due to the changed conditions we planned to lay an additional Buoy bridge between P12 to P17 on 400m d/s side of the bridge and accordingly started the jointing and shifting of Buoys to P12 locations from 23.11.2013.

In the month of Dec’13, due to more depth of channel and heavy currents in water between p12 to p17 we were unable to close the channel and after discussing with NFR officials we laid the additional Buoy Bridge of 350m on 400m downstream side of P15 location and we made an additional approach road also from P12 to one end of the buoy bridge of 1000m length and another end of buoy bridge up to P18 of 1000m using additional hired plants.

We have made approach road from P5 to P12 and from P12 to one end of second Buoy bridge of 1875mtr in addition to filling of channels between P5 to P8 locations and we found huge earth filling between P17 to P18 locations and around P17 location of 6 to 7 mtr depth of water.

Between P17 to P18 (125x10x4.0), filling was around 5000cum and extra width was kept due to continuous scouring of road. Around P17, huge earth filling was done (30x20x7m) of around 4200cum and by the end of Dec’13, we made an approach road of 1875mtr plus another end of Buoy bridge to P24 location of 1875mts with total of 3750mts of approach road in addition to major filling of channels by using additional hired plants and our company’s machines.

Up to 15th Jan’14, we made approach road from P24 to up to P34 location using hired plants and equipment and company owned machines. In the month of April’14 due to rise in water level up to RL 98.20 on 01.04.2014, all approach roads submerged under water and loading point at pump3 got damaged and channel extended up to P7 location and P5 loading point was also completely damaged. The concrete of P24 14th lift stopped with poured quantity of 210 cum. We completed this concrete work using barges and tugs by taking round about on the downstream side of the bridge of around 6 to 7km length, and incurred huge expenses. We have completed concrete of P24 14th lift but when water levels further raised up to RL 98.350 and damaged the road between P18 to P19 location and when movement of TM completely stopped, then we stopped further concrete in this season.


The plan showing the status of profile as on 11.11.2013 after sudden rise in water level on 30.10.2013 (water level raised up to RL 99.15m), the profile of the River changed again and we found enormous scouring of bed levels in all the channels between P12 to P20 locations. This time the situation had worsened compared to the previous River profile taken on 21.10.2013 and we found increase in depth of channels due to heavy scouring.

As per the River profile, we found two major flows of water/channels between P12 to P14 (avg depth of water-2.5m, and in earlier river profile the depth was-1.60m), between P16 to P17 (avg depth of water-2.75m, and in earlier river profile the depth was-1.90m) and one channel between P6 to P7 (avg depth of water-0.80m, and same in earlier River profile).

Work was badly affected due to formation of major channel between P12 to P17 locations and to transport the concrete we had laid additional buoy bridge of 350 m length.

Due to above hindrances we faced problems in shifting materials at island (we had to take boat up to P18 location to shift the materials on second island), delay in approach road making on above location, and delay in starting concreting activity at island locations.

7th Working Season
Buoy Bridge Making For Concreting
South Bank:
Buoy Bridge (made of structural like, MS plate 4 mm thick and ISMC 125) has been constructed again for the 7th season on Dibrugarh side of the river bank (South Bank) for transporting concrete from batching plant to Island. The Buoy Bridge was anchored by same process as it was stated earlier in the 6th season.

Additional Service/Approach Road Making
South Bank /North Bank:
In this working season, the Brahmaputra River has unprecedently and drastically changed its course as compared to previous working seasons. This has created substantial hindrances in execution.

As per river survey conducted on dt. 15.10.2014, the profile of river has changed and found enormous scouring of bed levels in all channels between P18 to P22 location and this time the situation has worsen compared to previous season and found further increase in depth of channels due to heavy scouring.

As per river profile, we have found two major flow of water/channel between P18 to P22 (avg depth of water-1.0m) and between P24 to P24 (avg depth of water-2.75m) and one channel between P6 to P8 (Avg depth of water-2.50m)

The Plan showing the status of profile as on dated 15.10.2014. after sudden rise in water level on dt 30.10.2013 (Water level raised up to RL 99.15m), the profile of river has changed again and found enormous scouring of bed levels in all channels between P12 to P20 location and this time the situation has worsen compared to previous river profile taken on dt 21.10.2013 and found increase in depth of channels due to heavy scouring.

As per river profile, we have found two major flow of water/channel between P12 to P14 (avg depth of water-2.5m, and in earlier river profile the depth was-1.60m), between P16 to P17 (avg depth of water-2.75m, and in earlier river profile the depth was-1.90m) and one channel between P6 to P7 (Avg depth of water-0.80m, and same in earlier river profile).

Work is badly affected due to formation of major channel between p12 to p17 location and to transport the concrete we had laid additional Buoy bridge of 350 m length.

Due to the above hindrances we aced problems in shifting of materials at island (we had to take boat up to P18 location to shift the materials on second island), delays in the making of the approach road on the above location, and delays in starting concreting activity at the island locations.

8th Working Season
Additional Service/Approach Road Making
South Bank / North Bank:
In this season, there was formation of 4 live river channels and a few dead channels, excluding 2 major river channels at both ends of the river Brahmaputra. The 1st channel was near P8-P9 D/S, 2nd channel was between P15-P16, the 3rd channel was between P20 to P22 and the 4th was near P25. As the working season is very less and there was no approach for material shifting, we hired additional plants (4 Hyva Dumpers, 2 Dozers) for closing the live and dead river channels for making of the Approach Road at the Island for carrying construction materials and concrete. We made around 3.750km of main approach road from P5 to P32 location and two additional service roads (one near P9 D/S to P15 and other P19 D/S to P21 and P16 via P19) around 2.2 km, for shifting of materials and transportation of concrete through Transit Mixers. The total earth work for closing the River Channels and for making of Approach Road was around 29500cum (3750mx7.5mx0.75m+2250mx 5.0mx0.75m). We used 2 Excavators and 1 JCB, 4 Hyva and 2 Dozers for the road making work.

Early onset of monsoon, prolonged monsoon & unprecedented flash floods resulted in constant disruption in the works. The peak working season practically reduced to 4 months only (i.e. from 15 Nov to 15 March)

Risk: Floods in October – November and mid-March.

Outcome of the Risk:

1. Work stopped
2. Loss of Turnover and Time
3. Idling of Establishment
4. Possibility of heavy scouring if the well foundations are not taken up to safe grip length, resulting in tilt and shift beyond permissible limit and control.

Mitigation strategy:
Utilized available time by optimizing the resources for which planning/methodology had been freezed.

Targets achieved:

1. Placing of well foundation (Cutting Edge) by 1st week of Nov.
2. Taking the well foundations to safe grip length (38.225) by 1st week of March.

Caisson Grounding & taking the Caisson Foundation to safe depth within 4 months of working season
Caisson foundations are one of the most difficult deep foundations to construct in bridges, especially in the mighty and ferocious River Brahmaputra, along with the arrival of unwelcomed guests – the floods!

The Brahmaputra changes its course every year and the working season is only four months (from Nov to Feb) as workable water level recedes by end of Oct and starts rising from the first week of March.

Since the bridge is made in a seismically active zone (Zone-V), special care was taken in its design to mitigate possible disasters. For this, the depth of the wells was increased by 25% and the thickness of the steining of wells extended to three meters, which is generally confined to 1.5 – 2 meters. We had to build two caissons (P2 & P3) in the main river channel (water depth 14m to 18m), having water velocity, ranging from 3m/sec. to 5m/sec. Besides, we had to construct both the Caissons in a very short period i.e. only in 4 months (from November to February) including Fabrication, Erection, launching, grounding and sinking of Caissons up to safe depth (atleast 20m below bed level), since the water level and velocity recedes in the month of October and again the water level and velocity starts increasing from March onwards.

Methodology adopted in 2nd working season (2009-10) during construction of Caissons
Coffer dams for P2 and P3 caisson launching bed was made at 1000 mtr upstream side of bridge center line in Oct’09. This was made after a detailed survey. During the selection of location; the water depth was around 5 m. Both caissons were erected up to 5.1m ht. at Launching Bed. There were two flash floods on 10/10/09 and 19/11/09, after which, heavy siltation took place near the launching bed.

Caissons P2 and P3 were taken to the actual location on 17/01/2010 and 21/01/2010, respectively, after dredging and making the channel. There was a delay of around 22 days in shifting of caisson because of the heavy siltation.



There was a sudden increase in water level from RL 96.990m to 97.870m on 04/03/2010 due to heavy rainfall in Arunachal Pradesh and in the upper region of Assam. Due to increase in water velocity, the total force applied on the caisson was drastically increased.

Two of the three anchors of P2 caisson gave way due to heavy winds and water currents. Caisson P2 gradually drifted and stabilized at about 130m downstream side from the original location.

Since the draft of Caisson was 9.7m, it was not possible to shift to a safer place at that time. Therefore, it was secured in that position with four 32 mm dia wire ropes tied with well P1 and two from the downstream side anchors. But in that location due to heavy scouring the caisson started tilting. The scouring was uncontrollable due to heavy currents in the water at that location and it was not possible for any boat or person to approach the caisson. Ultimately, all the wire ropes anchored with the caisson were snapped one by one and slowly the P2 Caisson submerged into the water.

It was the same situation at location P3, where caisson drifted away very fast at about 1.2 km d/s to the centreline and finally got submerged into the water.

To conclude, we could infer that we lost both the Caissons due to:

• Late grounding of Caissons due to the delay of around 22 days in shifting of Caissons, because of heavy siltation enroute.
• Early onset of floods with heavy water velocity in the first week of March.
• Sudden floods and change in direction & velocity of water caused angular force/ impact on the Caissons, which snapped the tethering arrangements.

After adopting the revised methodology as mentioned above, we successfully grounded both the Caissons by 30th Dec’10. We had erected, floated and grounded both the Caissons (P2 Caisson ht. = 19.5m and P3 = 14.7m at the time of Grounding), within 74 days only and saved 53 days compared to the earlier season.

Total fabrication and erection involved: For P2 Caisson 390MT (40.3m ht.), for P3 Caisson 225MT (21.9m ht). We had achieved 7 days’ time cycle for a 2.4m lift module including erection, fit up, welding, reinforcement fixing, winch lifting and concreting, whereas in the earlier season it took 12 days per 2.4m lift.

Detailed break up of time saved due to revision in methodology:

Sl. No.	Description of Work	Time taken in  2nd Season (in days) as per Original  Methodology	Time taken in  3rd  Season (in days) as per Revised Methodology	Time Saved (in days)
1	Erection of Caisson up to Well Curb Reinf. fixing and Fixing of Winches @ Built up Launching bed.	51	30	21
2	Launching arrangement and floating	22	8	14
3	Erection of Modules, Reinf. fixing, Concreting at location till Grounding	54	36	18
TOTAL		127	74	53

Tilt Rectification of Well P37

After construction of Guide Bund by another agency under N.F.Railway at SPTR side, the main water flow diverted from P39 to P37. Previously, P37 was in the Island and the main water flow was between P38 & P39 at SPTR side.

General arrangement of Well P37

• 6th lift steining was casted on 25.05.2010.
• 6th lift sinking was completed on 31.05.2010 and average C.Edge R.L was 78.764m.
• Total Height of Well (incl. well curb) = 23.225m
• 7th lift steining was almost ready for casting on 6.06.2010

P37 Well Before Rise of Water Level

• Suddenly water level started rising due to heavy and incessant rainfall from 06.06.2010 and reached up to R.L 101.85m from 100.40m (1.45m in 48 hrs).
• Due to construction of Guide Bund by other agency under N.F.Railway at SPTR side, the main water flow diverted from P39 to P37. Previously the well P37 was in Island and main water flow was between P38 & P39.
• This sudden increase in water level from R.L 100.40M to 101.85 in 48 hours and coupled with heavy water current at P37, there was heavy scouring and the Well P37 tilted towards U/S & DBRT side on 08.06.2010.
• At present, Cutting Edge is approx. 5.625m at U/S and 6.625m at D/S below the present Bed Level.

WELL P37 got tilted due to heavy scouring during rise of water level

Change of C.G. Due to Tilt

• Total weight of well
• Structural : 40MT
• Reinforcement : 177MT
• Concrete : 5990MT
• Total weight=6210MT (say)
• Volume of well under water: 2480cum
• Weight of the water displaced by well = 2480MT
• Effective weight of the well = (6210-2480) = 3730MT
• Max. Tilt of the well 1447mm
• Tilt angle = 5o7’28”
• The weight acting towards the tilt =3730 x sin5o7’28” = 333MT
• Force acting towards the tilt =3263KN/m²
• Force acting downwards =36408KN/m²

Tilting Force

Date	Direction	Time	Maximum Tilt (M)
08.06.2010	U/S	3.30 pm	1.285
09.06.2010	U/S	3.50 pm	1.395
10.06.2010	U/S	10.25 am	1.406
11.06.2010	U/S	8.00 am	1.438
12.06.2010	U/S	12.00 am	1.438
13.06.2010	U/S	4.00 pm	1.443
14.06.2010	U/S	7.00 am	1.447
MAXIMUM Tilt is 1447mm (1:11.20) towards U/S side and 450mm (1:23.33) towards DBRT side.

Observation of Tilt
Immediately, as a preventive measure, we started sand bag dropping at U/S side to stop further scouring and inside the dredge hole for preventing further tilting.

To rectify the tilt, concreting was executed at D/S side up to 3m ht. for increasing the well height and for providing additional weight at D/S side.

After that, concreting at U/S side of well steining up to 1.2 m ht. was executed, since the steining top at U/S side was 500mm below water level. (water level =100.1m @ 12.06.2010). After concreting, the D/S side shuttering was removed and the crane was mounted on the barge as controlled sinking was brought to the location.

Continuous grabbing from extreme end of D/S side dredge hole was executed.

Tilt was observed and recorded in every 30 minutes interval and sinking continued till the tilt was completely rectified.

Concreting of 7th lift for Eccentric Loading


Difficulties to Work in Mid River Under Heavy Water Current

There was no such working platform except for the barge. It was very difficult to keep the barges in position against high water currents.

• Tug 1 (Ganga) of 200HP coupled with two marine engines and Jalashree of 90HP with two marine engines, faced difficulties in shifting empty material barges against heavy water currents.
• Difficulties and high risks in concreting of well P37 due to heavy water flow coupled with high velocity.

Long distance pipeline concreting using Batching Plant, Concrete Pump, Transit Mixer & Pipeline laid over Buoy bridge.

A chute arrangement was made keeping all other arrangements same on the top of Guide bund that minimized pump requirement at that location. This step increased the concrete pouring rate.

During floods, the Concerting Methodology adopted for Wells in river (nearer to river bank) comprises of Transit Mixers mounted on Barges & towed by Tugs/Self-propelled Double Boats to Well locations:


Managing the Supply of Cement in remote sites in peak working seasons, when consumption is around 8,00,000 bags of Cement.
Risk:
Non-availability of cement due to shortage of Rakes from Railways during peak working seasons.

Outcome of the Risk:
Work gets stopped. Loss of Turnover and Time. Idling of establishment.

Mitigation strategy:
For increase in storage for at least two Rakes at each bank (North & South) had been identified and finalized. One rake at our own godown and another at the client’s godown at each bank. Storage was done from October.

Additional alternative cement brands like Ramco, Zuari, Bharthi, Penna, Birla Gold has been approved by client.

Managing Supply of Coarse Aggregate of 1,00,000m³ & Sand of 45,000 m³, during peak working season in 4 months, where there is an acute shortage of aggregates, boulders and sand due to their non-availability in the surrounding areas of the site.
To mitigate the huge requirement during 4 months, the supplier had installed 4 crushers of 20 TPH capacity at the site for better control on the quantity and the quality of the materials. The boulders had to be procured from quarries around 190 km away from the site.

In addition, at least one month buffer stock was generated. Average consumption was about 835 m³ per day during peak season. The sand supply was ensured as per the schedule, with at least one month buffer stock. Average consumption was about 375 m³ per day in peak working season.

Frequent stoppage of work during working season due to Law & Order problem & frequent bandhs / strikes by local parties.
Risks:
Law and order problems.

Risks encountered:

1. Abduction threat by anti-social elements.
2. Firing incident at our Project Manager’s vehicle on Silapathar side by anti-social elements.
3. Firing at Island by police at Dibrugarh side resulting in causalities of anti-social elements.
4. Insecurity of staff & workers and the effect on their morale, which led to mass exodus and resignations.

Mitigation strategy:
Round the clock tight security at site by deploying substantial number of Armed Police force by local Police and Administration.

After persuading the SP of Dibrugarh and Silapathar, Armed PSO was provided to the Project Incharge for his personal security and four armed police provided at the site.

Innovations & R&D
Stabilising Concrete pipeline carrying Buoy Bridge.
Construction methodology of concrete carrying buoy line during 2nd Season (2009-10).


The buoy had been finally changed to a stable structure by altering its shape, which eliminated chances of toppling over or any other stability problem during concreting.


Modified Anchor:

Considering heavy scouring and water currents, anchors of 10 tons were used instead of the anchors of 2.5 tons that were used in the earlier season. This provided extra stability to the buoys and the pipeline, ensuring better and safer concreting process against unexpected river behavior.

Value Engineering
Increase in diameter of screw conveyor
To increase the production capacity of the Batching Plant, we changed the dia of screw conveyor from 168mm to 219mm.

This change was done for the screw conveyor from the Cement Silo to the Batching plant only. Due to this change, the production capacity increased from 22 cum/hr to 28 cum/hr and we also saved costs.

Increase in pouring rate by Chute Arrangement

By normal/routine movement of vehicles we have to cover a 6km (up & down) stretch to transport concrete by TM from batching plant to Pump-1 location (placed on top of crate/apron). In order to reduce the time and cost, we introduced the chute arrangement technique, which provided a self-route to concrete from top of guide bund to pump-1 location. We also changed the movement of TM from top of the Guide bund instead of the routine route.

This step increased the concrete pouring rate and reduced TM movement from 6 km to 2 km thereby saving 4 km per trip, and also reduced the number of TMs and maintenance cost.

With this arrangement, we saved 30 minutes per Transit Mixer (loading, unloading & back to batching plant) and HSD cost.

Value engineering adopted at site	Value creation	Value Captured
Customised length of 9 mtr Rebar 32 mm dia instead of 12 mtr were used in well foundation	Saving in wastage of reinforcement	Saving in cost of reinforcement
Fabrication  and erection of Caisson in modules	Saved time, fuel for shifting material from fabrication yard to site	Saving in extra time for executing steining and early grounding of caisson.
Erection of cutting edge by fabricating in modules.	Saved  time, fuel for shifting material from fabrication yard to site	Saving in extra time for executing steining.
DGs at site were used for centralised lighting	Reduced unwanted running of DG with less load	Increased utilisation of DG set and Fuel.
At Dibrugarh Side, Concrete used to be transported via road by TM upto the foot of the Guide bund from batching plant. To eliminate long distance transportation of concrete, a chute from the top of the guide bund was erected and thus made the concrete to fall directly into the hopper of the concrete pump in such a way, so that the concrete does not get segregated and the desired quality is maintained.	Reduced movement of the transit mixture	Reduction in requirement of the TM resulting in surplus of 2 nos of TM in Jan 12
The screw conveyor of batching plants for cement feeding was increased from 168mm to 219 mm dia.	Actual production capacity increased from 22 Cum/hr to 24 Cum/hr	Cost saving by increased production capacity.

Utilization of scrap reinforcement steel
Due to shortage of reinforcement steel at site we used scrap reinforcement steel for main construction work after welding as per IS 9417 – 1989.

The scrap reinforcements joined by welding extended up to the required length.


30% of the welded reinforcement was fixed at location and mixed up with the new reinforcement. We utilized scrap steel of around 90mt.

Quality Initiatives and Assurance
We made a well-established Quality Assurance Plan which is being implemented at site by adequate Quality Control of the incoming, in-process and the final products.

The critical materials of this job (Cement, Reinforcement Steel, Structural Steel, Sand, Aggregates and Admixtures were tested thoroughly). Testing of properties of the materials which was beyond our scope was done by an external authority.

We had to establish new concrete mix proportions every year to suit the work methodology.

We put aside difficulties like variable quality of aggregate, frequent change in cement brands, and unpredictable nature of the River, which forced us to change the mix proportioning every year. We achieved 1400 mtr relay pumping at Dibrugarh side and 1250 mtr relay pumping at Silapathar side.

Despite all the drawbacks, we achieved a standard deviation 1.946 N/sq mm at Silapathar side and 2.505 N/sqmm at Dibrugarh side (for M35 grade of concrete).

We have set up well equipped quality control laboratory on either side of our establishment.


Conclusion
The completion of the Bogibeel Bridge has ushered an era of prosperity and development for Assam and Arunachal Pradesh. It is of strategic importance and has further strengthened the nation’s security in the North-Eastern region of India.

“All Construction Bottlenecks Can Be Won By Engineering Solutions”