New technologies, designs and construction methods are being increasingly deployed in order to mitigate the challenges in bridge construction. Before a bridge can be built, an appropriate method of construction must be chosen. At a recent Bridges in Asia conference organised by Indian Infrastructure, C. Sankaralingam, advisor, former vice-president and head, special projects, L&T Construction, discussed the latest techniques being deployed in bridge construction, their benefits, key challenges and the lessons learnt from ongoing and completed projects.
Techniques deployed in bridge construction
The planning, design and construction of bridges and flyovers are very challenging. However, there has been rapid development with the availability of high strength steel and concrete. Many bridges are being constructed especially using new techniques such as segmental and full span construction and erection.
Precast segmental construction technology gained popularity as it accelerates bridge construction, provides better quality control and lowers the life cycle cost. Moreover, elements with difficult shapes and congested reinforcement can be cast at ground level. Under this method, precasting of segments/spans is done in the casting yard and the cast segments/spans are transported to the erection location. The segment/span is erected using a launching girder/bridge builder. Precast segmental construction technology also provides better environment protection at the construction site as limited construction activities take place at the location. Segmental casting is primarily carried out using two methods – long-line and short-line.
Under the long-line method, casting of segments for the entire length of a span is done without repositioning the segments each time. The diaphragm segment is cast independently on separate standalone beds and is then shifted to the long-line bed. Meanwhile, the placement of the rebar cage (of infill segments as per the casting sequence) is done with the sheathing duct. The bulkhead shutters are fixed on one side of the segment or on both sides. For the concreting of segments, curing by water or application of curing compound to the exposed surfaces is carried out.
In the short-line method, each segment is cast next to the previous segment in a special adjustable casting machine. This ensures that the interface between the two segments matches exactly when erected. Each successive segment is then cast next to the previous one. The soffit form is positioned near the fixed bulkhead and the placement of the rebar cage is done with the sheathing duct. The match cast segment in the short-line method is shifted with a manipulator and curing compound is applied on the exposed surfaces.
The long-line method is preferable when construction has to be done in straight lines and there isn’t much curvature. In case there is a long curvature, it becomes difficult to align adjustments and in such bridges the short-line method is preferable. The long-line method is cheaper than the short-line method as not much movement of trolleys is required.
Precast segments are usually erected using the span-by-span method or balanced cantilever method. In the span-by-span method, an entire span is assembled, post-tensioned and erected so that it is self-supporting before the next span is erected. Span-by-span erection is the most common, simplest and cost-effective construction method for precast segmental bridges. This method of construction is suitable for spans up to 60 metres. It is compatible with both simply supported and continuous spans. A typical launching girder has front support, middle support and rear support as well as a lifting winch/movable gantry for segment handling.
In the balanced cantilever method, the pier head segment is erected or cast in place on the pier by taking support from the pile cap and the lifting frame is erected over the pier head and anchored to the pier head segment. It is erected on both sides of the cantilever. The cantilever in the lifting frame is fixed based on the length of the segment and a gap of 300 mm is provided between the erected and the erecting segment. The segment is transported using a trailer after which vertical and horizontal positioning is done. Hydraulic jacks are used to adjust the transverse and longitudinal gradients of the segment. The segment is positioned and the stitch concreting between the pier head and the segment is done. Once permanent stressing is completed, the lifting frame is detached from the erected segment.
Similarly, the segments are added either at the same time or alternately to each cantilever to maintain a relatively balanced system. It is suitable for longer spans above 60 metres (and up to 250 metres) in length and varying depth spans, and marine operations. The deck is erected segmentally on each side of the pier in a balanced sequence to minimise load unbalance and longitudinal bending in piers and foundations. The segments are erected individually or in pairs with cranes on the ground or barges, lifting frames or cranes on deck, and self-launching gantries. The advantage that balanced cantilever erection brings is that the segments are handled individually, and the erection equipment is lighter and less expensive than a span-by-span construction.
Full span method
The full span method is one of the fastest techniques that is primarily used in the construction of high speed rail, the bridges and viaducts. It involves casting the whole bridge span in the casting yard and transporting the whole span with a multi-axle tyre trolley to the bridge site. Suitable gantry cranes are used for tandem lifting of the span. A custom-made span transporter is placed over the erected span. The span is loaded on the transporter and shifted over the already erected spans until reaching the erection location. After this, the span is lifted from the transporter and handled using the launching gantry. After completion of the span, the launching gantry is auto launched and is ready for the next span erection.
Despite the deployment of new technologies, the construction of bridges still faces many challenges. A key issue is the heavy investment required for purchasing equipment. Equipment such as launching gantries, bridge builders and moulds require investment that cannot be re-covered in one project. Another issue during the implementation process is the delay in shifting of underground and overhead utilities; this causes delay in foundation works, thereby affecting the progress of a project.
Further, the bridge construction segment faces tough market competition as more and more players are entering the market which makes the bidding competitive. Apart from the above, availability of skilled human resources is also a challenge. Providing continuous training to manpower and training engineers on the latest construction techniques is an essential requirement in these bridge construction projects.
In order to successfully execute any bridge construction project, it is essential that there is proper planning to finalise the precast yard and land availability (as the land requirement can be as huge as 50 to 60 acres). Further, the number of moulds, straddle carriers and launchers to be used should be planned properly and well before time.
It is essential to understand that these projects are time-bound and hence it is important to finalise the methodology of construction, the equipment to be deployed and manpower in the initial stages as these are long lead items. Further, it is important to achieve the desired cycle time with utmost attention to quality and safety to avoid any rework.