BEBO Precast Concrete Arch Bridge on NH-30 at Bakhtiyarpur
|Location||NH-30 between Patna and Bakhtiyarpur|
|Consultant||Consulting Engineer Group|
|Application||Precast Concrete Segmental Arch Bridge|
|System Offered||System Offered: BEBO® Type E 60T/0|
An Indian Oil Corporation Limited (IOCL) gas pipeline crosses NH-30 between Patna and Bakhtiyarpur at Ch 800+296, skewed at 49°. The alignment of the carriageway is also curved at the crossing. With the gas line in place below the ground, IOCL disallowed any surcharge on the ground at that location. This eliminated adoption of the convention cast in situ or precast concrete systems which require temporary supports. These factors favoured accepting the BEBO® precast concrete arch bridge.
The semi-elliptical Type E 60T/0 BEBO® precast concrete arch bridge was selected to provide the required geometry. Clear span of the bridge at the top of pedestal is 18.252m. Ground clearance up to soffit at crown is 4m, which was acceptable to IOCL.
The bridge comprises of arch elements of thickness 356mm and width 1816mm. Owing to logistics issues related to large span and profile of the elements, each element is cast as two leaves as standard. The two leaves are connected at the crown and concrete in situ, to structurally provide an arch element monolith at the crown and pin-jointed at the supports. The thickness increases at the crown to 508mm to accommodate coupling details between the two leaves of each element. Several elements are placed together to cover the required carriageway width at the top.
Considering the elaborate profile of each leaf, a suitable area for casting yard was rented. The leaf was cast with M45 concrete using 20mm downgraded aggregate. The grade of reinforcement steel was Fe500. Simultaneously, piles were cast at the site to carry the grooved pedestals for supporting the arch elements. Each support had 44, 1,000mm diameter bored cast in-situ friction-cum-end bearing piles, 27m deep. The pile caps and pedestals were continuous at each span. Piles, pile caps and pedestals were of M35 grade concrete with maximum aggregate size 20mm. To ensure perfect level of the arch elements, levelling pads of grout were cast within the grooves with level tolerance of ±3mm.
The first pair of leaves, one of the end elements was erected. Erection of the first pair required precision locating since these set the starting-point for erecting other elements progressively. Timber shims were used for horizontal alignment. After all elements were erected, the grooves within which the arch elements were supported were grouted.
Gaps between the arch elements were treated to prevent seepage of water from the fill that could cause piping of fines. Coir rope was placed within the gaps and covered with jute saturated with cement slurry. This was topped with mortar and covered with nonwoven geotextile. Thereafter dowels were placed, tying exposed reinforcement within the crown. The crown was concreted with M45 concrete.
Since the essence of analysis of a BEBO® arch is soil-structure interaction, the type of soil used for backfill of the arch is significant. Soil gradation must essentially fall within the recommended envelope. Local soil fitted within the envelope and was found suitable. Backfilling was done on both sides of the arch in rises of 400mm at a time on each side to ensure balanced horizontal thrust on the structure. Compaction was done in 200mm lifts to 97% of maximum density as per the modified compaction test. Backfill was thus done to crown level. The pavement crust was constructed thereafter. The bridge was completed within 8 months.
Value addition to our Clients
- The BEBO® system meets the challenges of cost and time constraints and superior quality requirements of highway development.
- Construction is speedy. Leaves of the arch can be positioned within hours without disturbing the system being spanned. Once the backfilling is complete to 500mm above the crown, heavy construction equipment and normal traffic can ply over the bridge.
- The BEBO® solution is eco-friendly with lesser quantities of vital building materials required with virtually no maintenance.
- The System offers economy in design. Savings in concrete is possible up to 50% and almost 40% in reinforcement as compared to a convention bridge.
- Compliance with BEBO® specifications for design and construction guarantees a quality product. Longevity of the concrete structure is assured. The structure requires virtually no maintenance considering that there are no bridge bearings, approach slabs, or decks to be maintained.