The main activity over the weekend was the placement of two sections of bridge deck for the Countess Wear pedestrian and cycle bridge.
The installation of this structure is the most complex element of the project – it involves nine separate sections of the cantilever walkway being craned and fixed into position alongside the Countess Wear river bridge.
The first two sections (spans one and three) were lifted into place using large cranes positioned on the road, which was the reason for the weekend road closure.
One of these initial sections is positioned beneath overhead power lines which operate at 132 kV. To achieve a safe lift, the power lines had to be turned off, which required planning with Western Power Distribution (WPD).
WPD shut off this section of the power grid while maintaining power to its customers, requiring a three-hour period for various other parts of the grid to be switched to establish alternative power routes.
Due to the presence of the power lines, the bridge deck lift also required a ‘tandem lift’ where a crane is used at each end of the item being lifted, rather than in the middle.
These sections of bridge deck are largely hidden by the stone parapet and won’t be visible to drivers who use Bridge Road, but these photos show the two sections of bridge deck in position.
Other operations carried out over the weekend were the placing of fill material behind the retaining wall which runs from the railway bridge to Matford roundabout. Work was also carried out on kerbing, lamp columns and verges. Bridge Road was re-opened to traffic at 11pm on Sunday evening.
The remaining seven spans of the pedestrian and cycle cantilever bridge will be installed during future road closures.
The contractor has also experienced a number of technical issues that have impacted on the overall scheme programme.
The original steelwork fabricator proposed to make this new structure and the rail bridge went into administration in April 2016, some three months after the start of the main contract. This meant the procurement process for this significant subcontract had to start again. Although the new sub-contractor hoped to meet the original dates with an accelerated fabrication programme, it has proved too difficult to achieve while meeting the exacting standards required.
The flood relief structure constructed in the mid-sixties is being widened on both the north and south side of the road. Hydro demolition has been used on the existing concrete of the structure to expose the original reinforcing steel. This uncovered that the 50 year old “as-built” drawings were inaccurate which created complications. More of the existing steel had corroded than anticipated and had to be replaced after water ingress through a faulty construction joint. New reinforcing steel has been tied to the structure to allow the widening to take place.
In order to widen the structures it has been necessary to install approximately 250 piles across the site. Some of the piles are up to 18 metres in length, which has presented difficulties. Additional piles have had to be installed to achieve the necessary standard and this has required a redesign of foundations. The foundations for the new structures have been completed and the works are now up out of the ground. Importantly, this means they are no longer subject to tidal interference as two flood events earlier in the scheme programme also delayed progress.