Case Study: Hard Pad Grouted Foundations for Precast Pier Bases – Confederation Bridge, PEI, Canada (1993–1996)
Contractor: Vinci Grand Projects
System: Grouted Fabric Formwork (Grout Bags) for Hard Pad Foundations
Project Context and Foundation Requirement
During construction of the 12.9 km Confederation Bridge—spanning the Northumberland Strait between Prince Edward Island and New Brunswick—each 4,000 t precast pier base needed to be placed precisely onto a stable foundation. The seabed, formed of weathered mudstone, had an irregular profile, and conventional levelling methods were unsuitable for this scale and depth.
To address this, grouted hard pads were used beneath each pier base. These pads, fitted with grouted fabric formwork, created a controlled bearing interface that could adapt to seabed undulations while ensuring reliable load transfer.
Foundation Technology: Grouted Fabric Formwork
Fabric-formed grout bags were pre-attached to the underside of each hard pad in a condensed state. Once installed on the seabed, the bags were filled with neat cement grout, expanding to mould precisely to the seabed surface. This provided full contact, avoided voids, and established a consistent, engineered foundation without the need for large-scale underwater concreting.
Design and Installation Process
Each pad was installed using a custom three-pronged frame, which handled placement, levelling, and grouting. The frame operated from a jack-up barge and allowed the process to be conducted with minimal diver involvement.
- The grout bags were divided into five compartments, allowing local adaptation to the seabed and limiting the risk of overpressure or blowout.
- Surface-controlled grouting ensured accurate filling, with vent lines confirming full expansion and contact.
- Grout included anti-shrink additives to ensure long-term stability under load.
Once the hard pads were fully grouted and cured, the precast pier bases were lowered onto them by the Svanen heavy lift vessel.
Foundation Performance and Advantages
The grout bag system provided a moulded, high-strength contact between each hard pad and the seabed, offering:
- Adaptability to seabed tolerances of ±150–450 mm
- Consistent load distribution across 64 pier locations
- A fast, controlled installation sequence that minimised marine risk
The ability to standardise and repeat the foundation system under challenging marine conditions was critical to the bridge’s modular construction approach.
Project Outcome
The grouted hard pad system played a key role in enabling the efficient, high-precision installation of pier foundations for one of Canada’s most ambitious infrastructure projects. The success of this method contributed to the project’s rapid progress and long-term durability, supporting a structure designed to withstand heavy loading and ice impact for over 100 years.
This project remains a landmark example of how fabric formwork foundation systems can solve complex geotechnical challenges in large-scale marine works.