Water main burst and flood damage resulting in extensive cracking and unstable foundations

Maidens Bridge, London Borough of Enfield

This picturesque bridge in the London Borough of Enfield was inspected by Goldhawk in order to assess the damage caused by a water main burst in November 2016. Remedial work hasn’t yet commenced on the bridge. However, the following case study gives Goldhawk’s recommendations. The carried road over the bridge has been reduced to a single carriageway controlled by traffic lights. This was a result of a lorry accident in 1968 which demolished the east parapet over the bridge span. This parapet has since been rebuilt.

The problem

The bridge and flood relief tunnels have suffered extensive cracking. Research into river levels showed that the river is subject to flooding and the bridge acts as a dam in periods of high water. The foundations of the flood relief tunnels and the spandrel walls follow the level of the sloping embankments and, when the river floods, the embankments become saturated. This has adversely affected the stability of the foundations of the flood relief tunnels which move vertically but are restrained at their ends by the vertical plane of the spandrel walls. This explains the cracking evident at the ends of the flood relief tunnels.

The main span of the bridge has suffered loss of mortar and loss of fill caused by the burst water main. There is also circumferential cracking between the barrel and the line of the spandrel walls. This is a common defect in masonry arches as the barrel is flexible in the vertical plane and the spandrel walls, if in good condition, provide a stiff vertical plane.

The Goldhawk solution

Interstitial grout injection is advised for the arch barrel and the flood relief tunnels to replace the mortar and fill lost by the flow from the burst water main. The more severe cracks should be stabilised using Helifix stainless steel Helibars and repointing should be carried out using lime mortar in the areas identified by Goldhawk.

Small diameter Helibars should also be inserted through the face of the barrel and anchored beyond the circumferential cracks that run under the spandrel/parapet walls. These will retain the flexibility between the barrel and the spandrel but prevent further outward movement.

The ReFORCE piling system is recommended under the flood relief tunnels to give support from below the water percolation level and thus prevent movement under flood conditions. These are installed, using a lance, through small apertures (40mm) drilled through the base of the tunnels. The resultant piles are typically 500mm in diameter once cured and up to 5m deep depending on ground strata. This system would enable the tunnels to be supported below the level of water flow but still allow percolation during flood conditions. The ReFORCE piling system is installed exclusively by StressUK which is a sister company of Goldhawk Bridge Restoration Ltd.