Crumbling foundations threaten many overbridges on the M5 motorway and the problem calls for a long-term rescue effort.
Back in the 1960s when most of the M5 was built, no one had heard of thaumasite sulphate attack (TSA), a chemical reaction which attacks and erodes concrete structures. Concrete pier foundations for up to 30 overbridges in Gloucestershire were constructed on Lower Lias Clays; excavations were backfilled with the excavated material – normal practices back then and fully in compliance with existing codes and standards.
It was not until the late 1990s that the first cases of TSA were reported on the piers and foundations, which are 30 or more years old. Since then, several more affected bridges have been found, and there is a good chance that all the overbridges along this stretch of the M5 are likely to be suffering from the same form of deterioration.
Once the full magnitude of the problem was finally revealed, Highways England faced a massive challenge. Surveys showed that the superstructures of the overbridges were in good condition, making demolition and replacement an unattractive option. Apart from the costs involved, M5 traffic would face years of lane restrictions and overnight closures, a nightmare scenario.
Repairing and strengthening the damaged piers and foundations, however, an attractive option on paper, also involved some daunting technical and logistical challenges.
M5 repair plan
Southampton-based R&W Civil Engineering is now five bridges into an estimated £20M, three- to four-year repair programme as a tier 2 supplier to Highways England Area 2 asset support contract owner Skanska.
“Innovation was needed, and innovation doesn’t scare us,” says R&W director Howard Hutchinson. “Nor are we reluctant to invest in new technology.
“So far the biggest problem has been accessing pier foundations in the central reservation. The motorway has stayed open under a narrow lane running traffic management layout. So have the overbridges. It all makes for very congested working areas.”
M5 thaumasite problem
To date five bridges have been repaired and four – two at the junction 13 Stroudwater Interchange, one each side of the interchange at Eastington and Standish Lane – are undergoing remedial works at the moment. All were originally built to the same basic design, with the main difference being the presence or otherwise of piers in the central reservation.
Hutchinson says that Standish is the odd one out of the current four. “Each set of five piers sits on a single pad foundation, as opposed to the individual pads on the other three.
“And the foundations are much deeper, at 6m rather than the 2m to 3m elsewhere. It requires a different solution.”
On all the bridges, work begins with the clearance of vegetation and the removal of revetments and their pitch fibre drainage to form a working area. At Standish and Eastington access stairways have to be installed in the central reservation, and the open box beam (OBB) safety barriers removed to create a working space straddling both fast lanes.
Where the existing foundations are reasonably shallow, as at Stroudwater and Eastington, the next stage is the installation of cofferdams to allow the foundations and lower piers to be exposed and the TSA damage to be evaluated.
These cofferdams were one of the many challenges the R&W team faced from the outset. “We had to come up with some innovative solutions for the sheet piling, given they had to be 4.5m long and we only had 3m headroom beneath the bridge deck,” says Hutchinson.
M5 thaumasite restricted headroom
“At Easington and Standish Lane we had to excavate slit trenches to get the sheet piles into position before we could start driving. Stroudwater was a tougher proposition.”
Here there was a challenging combination of hard ground and a high water table that required interlocking sheet piles. “[Piling contractor] Suttle Projects brought in a Movax Side-Grip pile driver fitted to an excavator, which solved the problem,” he says.
Sampling of the exposed concrete was the next phase. “So far every foundation we’ve exposed had TSA to some degree, with up to 100mm of penetration,” Hutchinson reports. “Thankfully the rebar was in reasonable condition.”
Before any concrete could be broken out, the bridge deck had to have temporary longitudinal restraints installed between the deck soffit and the abutments. These were to cope with the hypothetical case of a convoy of HGVs performing simultaneous emergency stops on the carriageway above, Hutchinson explains.
Hydrodemolition blasts away the affected foundation concrete. Special measures have to be taken to deal with the wastewater from the hydrodemolition, which is very high in sulphates. Extra anti-crack reinforcement will be installed, and a C32/40 DC-3 sulphate-resisting mix pumped in to encase the foundations, increasing their dimensions by 140mm overall.
Once the new foundation concrete has gained enough strength, a deck-propping system can be installed – something of a logistical nightmare, says Hutchinson, with little room for error, particularly in the central reservation.
With the deck supported as well as restrained, work can begin on removing the deteriorated concrete from the piers.
M5 thaumasite deck restraint system
“On the earlier bridges the cofferdams became very congested, with all the bracing, propping and access scaffolding,” Hutchinson reports: “We cut back to 50mm behind the two layers of reinforcement, around 140mm in total, to get good penetration of the new concrete.”
Site workers fixed anti-crack reinforcement around the piers from the foundation to ground level, installed shuttering and pumped the DC-3 concrete into place. Again, overall dimensions were increased by 140mm. Finally, an asphaltic waterproofer was applied to all below ground surfaces, and the drainage was reinstated.
Rather than backfill with the weathered Lias Clay, foamed concrete supplied by Hanson was used. “This ensures good compaction,” Hutchinson says.
Standish Lane’s deep foundations would have been uneconomic to repair in the same manner.
Instead, Skanska opted for a radical, piled, solution, already proven on an earlier bridge project further down the M5.
Work will begin with the installation of 10, 600mm diameter augured piles up to 17m deep either side of each of the existing pier foundations.
Then cofferdams will be installed and excavated, the exposed sections of the piles cut back, and new 900mm deep pile caps poured at depths of approximately 1m to 1.5m below ground level.
This time the temporary propping will have to take all the deck loads, as the piers will be cut away at pile cap level. New reinforcement from the pile caps will be coupled to the existing pier reinforcement. Fresh concrete will fill the gap.
“The old foundations and the lower section of the piers will then be redundant,” says Hutchinson. “So we just leave them in the ground. This is much more economic than excavating all the way down and then having significantly more concrete to repair.”
Temporary propping and lateral restraints will be dismantled and removed, cofferdams will be extracted.
Waterproofing, drainage reinstatement and foamed concrete backfill will follow as on the other bridges.
Any above ground concrete repairs will be carried out, before kerb replacement and the slipforming of concrete step barriers and resurfacing is carried out where necessary.
Total cost of restoring the current quartet of bridges to health is somewhere north of £6M – a much more acceptable outlay than the estimate of up to £10M to completely replace a single overbridge.
The fact that all bridges so far have been repairable is also reassuring for Highways England, and makes the target of rehabilitating at least half of the 30 bridges by 2018 look realistic.