Hammersmith Flyover, constructed in the early 1960s, was an innovative design for its time and an early example of a precast segmental post-tensioned superstructure. However the details adopted for the post-tensioning system, particularly with regard to protection from water ingress at the anchorages and the segment joints, were such that over time there has been a severe deterioration, through corrosion, of the steel tendons.
The condition of the viaduct has been closely monitored by TfL over a number of years and the viaduct was closed for a short time in late 2011 / early 2012 during a detailed emergency assessment. The traffic load was then restricted for a few months after partial re-opening whilst strengthening was carried out. This localised strengthening (“Phase 1”) was carried out in a short timeframe to allow the flyover to be fully open in good time to be utilised as a ‘Red-Route’ into West London during the London Olympics in July 2012.
In late 2012 the process of design and implementation of the long-term solution to extend the serviceable life of the flyover commenced. This solution had to minimise the effect on the traffic and the strengthening was restricted to below and inside the deck to avoid any closure of the flyover. “Phase 2” strengthening commenced on site in late 2013 and is due for completion in Summer 2015. The works comprise a full replacement of the existing post-tensioning in order that it is rendered redundant along with other works such as deck waterproofing, central reserve reconfiguration and expansion joint and bearing replacement.
Project objectives had to bear this in mind and as such while working in a congested urban site environment, to a very tight programme we were required to innovate and develop solutions in order to meet Client’s key objectives which can be summarised as:
Minimise the disruption to traffic, local residents and businesses during the construction
Extend the serviceable life of the structure
Minimise whole life costs.
Space constraints inside the bridge mean the Strengthening of the deck has been designed to include a combination of tendons both inside the cells of the box and also on external faces to the soffit of the structure. The solution includes the attachment of a large number of short and long external PT tendons transferring load through 312No concrete anchor blocks (Blisters) each one accommodating high loads from both the tendon and also the Prestressing bars using to clamp it to the existing structure. This was achieved by designing new anchorage blocks utilizing Ultra High Performance Fibre Reinforced Concrete (UHPFRC). The solution included:
Manufacture of the precast anchor blocks using UHPFRC
Design and manufacture of unique lifting tools to install the blocks
In-situ use of UHPFRC placement inside the bridge deck (backslab) and in-situ anchor blocks
Design and development of a bespoke concrete pouring system.