The Incremental Launching Method (ILM) has emerged as one of the most innovative and efficient bridge construction techniques used across projects especially in terrains where conventional scaffolding or falsework installation is challenging. Developed in the 1960s and refined over decades, ILM involves casting bridge superstructure segments behind one abutment and progressively pushing (or ālaunchingā) them longitudinally over the supporting piers using hydraulic jacks.
Unlike conventional methods that rely on extensive temporary works, ILM leverages a controlled, repetitive casting and sliding process that minimizes environmental impact and enhances construction safety. This method is particularly well-suited for multi-span continuous bridges crossing deep valleys, rivers, highways, or railway corridors.
Understanding the Incremental Launching Method
Under ILM, the bridge deck is cast in segments, on a casting bed or launching platform located behind one abutment. After each segment cures and prestressing is completed, the entire deck (including previously cast segments) is incrementally pushed forward over piers using hydraulic jacks and sliding bearings.
A steel launching nose, generally a lightweight truss, is attached to the front of the deck to reduce bending moments and shear forces at the leading edge during movement. This nose acts as a temporary extension, ensuring the deck safely spans between piers during launching.
This process continues until the entire bridge deck reaches its final position across all piersāeliminating the need for scaffolding, falsework, or access roads below the structure.
Advantages of Incremental Launching Method
The ILM provides a multitude of engineering and economic advantages:
- Elimination of Scaffolding: Ideal for valleys, rivers, or environmentally sensitive zones where falsework installation is impractical.
- Improved Safety: Construction activities occur primarily on the casting yard, reducing risk exposure at heights or over water.
- Continuous Quality Control: As all segments are cast in a controlled yard environment, concrete quality, curing, and alignment are maintained.
- Reduced Construction Time: Launching is repetitive and systematic, enabling faster project execution.
- Cost Efficiency: Saves labor and material costs associated with temporary works and access infrastructure.
- Minimal Environmental Disturbance: No need for temporary piers or formwork that might disturb riverbeds or ecosystems.
- All-Weather Construction: Since major works occur behind abutments, operations can continue even in adverse weather conditions.
Step-by-Step Process of ILM Construction
1. Setting Up the Casting Yard
A casting bed or stressing platform is constructed directly behind one abutment. It includes rails, formwork, and prestressing jacks. The casting yard typically accommodates one or two segments at a time, allowing continuous production while earlier segments are launched.
2. Fabrication of the Launching Nose
A launching nose, often made of a trussed steel structure about 60ā70% the span length, is connected to the first segment of the deck. This nose guides the bridge deck across piers and reduces negative moments during the launching process.
3. Casting and Prestressing Segments
Each segment is cast monolithically with the previous one using continuity reinforcement. Once the concrete achieves required strength, internal and external prestressing tendons are tensioned to create a continuous structure capable of resisting bending and shear during launch.
4. Incremental Launching Operation
Using hydraulic jacks, the entire bridge is pushed forward by a predetermined distance (equal to one segment length). Sliding bearings or Teflon-coated surfaces on the piers facilitate smooth movement. The process is repeated until the final alignment is achieved.
5. Final Alignment and Bearing Installation
Once the deck reaches its final position, temporary bearings are replaced with permanent bearings, and expansion joints, parapets, and deck finishes are completed.
Structural Mechanics Behind ILM
During launching, the bridge deck behaves as a continuous beam with support conditions that change progressively as it slides from one pier to another. The structure experiences alternating tension and compression zones, especially at mid-span and over piers.
To mitigate excessive bending moments during this transitional state, engineers employ:
- A steel launching nose to reduce cantilever moments.
- Temporary supports or launching saddles on selected piers.
- Optimized prestressing layouts to ensure uniform stress distribution.
Sophisticated finite element analysis (FEA) models are used during design to simulate these temporary load conditions.
Equipment and Systems Used in ILM
The success of ILM heavily depends on specialized equipment that ensures smooth, controlled movement of heavy bridge decksāoften weighing thousands of tonnes.
1. Hydraulic Jacking System
Hydraulic jacks are the core of the ILM process. They provide the required horizontal thrust to push the deck forward incrementally. These systems operate with synchronized control units to maintain uniform motion and prevent torsional stresses.
2. Sliding Bearings and Launching Devices
Teflon-coated or PTFEāstainless steel bearing surfaces are placed on the piers to facilitate smooth deck movement. These are lubricated periodically to reduce friction and prevent localized stress build-up.
3. Launching Nose (Steel Truss)
As the lightweight steel nose leads the deck across spans, it counterbalances the cantilever effect by distributing weight before the main girder reaches the pier. Its design ensures that deflection and bending remain within permissible limits.
4. Lifting and Alignment Equipment
Cranes, winches, and vertical jacks assist in deck alignment, especially during the transition from one span to the next. Sensors and laser-based monitoring systems ensure precise alignment within millimeters.
5. Temporary Bearings and Saddles
Specially designed temporary supports or saddles on piers handle the dynamic loads during launching. They are later replaced by permanent elastomeric or pot bearings.
6. Control and Monitoring Systems
Modern ILM projects employ automated computer-controlled systems for synchronized pushing, real-time monitoring of stresses, and control of displacement and speed.
Applications of ILM
The Incremental Launching Method is extensively used in:
- Highway and Railway Bridges over deep valleys and rivers.
- Bridges over existing roads or urban corridors, where access below the deck is restricted.
- Viaducts in hilly or forested terrains, where environmental impact must be minimized.
- Multi-span prestressed concrete or steel girder bridges with consistent cross-section.
Notable Indian examples include parts of the KatraāBanihal Railway Bridge, Ganga River Viaducts, and numerous National Highway Authority of India (NHAI) projects employing ILM for speed and safety.
Sustainability and Future Trends
Modern ILM techniques are being enhanced through digital modeling, Building Information Modelling (BIM), and automated hydraulic control systems. Environmental sustainability is further improved by using high-performance concrete (HPC), recycled aggregates, and low-carbon steel in launching noses and equipment.
Furthermore, robotic monitoring systems now track deck stresses, alignment, and launching speed in real-time, reducing human intervention and enhancing safety.
The future of ILM also lies in its integration with modular constructionāwhere prefabricated bridge units are incrementally launched using advanced jacking systems, combining the speed of precast technology with the flexibility of ILM.
Conclusion
The Incremental Launching Method (ILM) stands as a symbol of engineering precision, efficiency, and sustainability in modern bridge construction. By eliminating the need for extensive temporary works and optimizing construction from a single location, ILM has revolutionized how complex bridges are built in difficult terrains.
In India, as infrastructure expansion accelerates under national highway and rail corridor projects, ILM continues to demonstrate its effectiveness in achieving speed, safety, and superior quality, all while maintaining environmental balance. With continuous advancements in materials, automation, and design technology, ILM is set to remain a leading and reliable method shaping the future of bridge engineering.
Image Source: Doka.com, researchgate.net, wikipedia.org
