Last November, the Somerset town of Bridgwater was rocked by the sudden collapse of its river wall. Mark Hansford describes the complex rebuild.
Imagine the scene in Somerset’s historic Bridgwater. It is Friday 4 November. The town is waiting expectantly for its annual carnival, the main event in the 40,000 population town’s calendar and famous for being the largest illuminated carnival in Europe. It is late afternoon and crowds are already gathering along the main route through the town for the best vantage points. But tonight there is to be an epic side show in the adjoining riverside street called West Quay.
“It was very much like a sand castle on the beach. The incoming tide would erode the foot of the exposed earth slope and more material would collapse from above.”
Peter Radford, Somerset County Council
The air is dank and heavy after a protracted downpour. There is standing water in the road and more water is spouting from manholes serving the drainage system. The tide is falling quickly in the adjacent River Parrett, which runs out to the River Severn with its huge tidal range.
Suddenly, the earth starts to move. Around 500t of masonry quay wall and material behind it makes its bid for freedom and begins an eastward V-shaped march into the river. Water bursts through fractures in the wall and a 6m deep, 35m long chasm appears in the edge of the road. At its apex the abyss is 7m wide.
Thankfully no-one is injured in the collapse. Crowds of onlookers race to the scene, expectantly waiting for gravity to deliver the coup de grace. Everyone wants to see the inevitable big splash. Local media diverts from the carnival to cover the collapse.
Meanwhile, emergency, local authority and utility services swing into action. The road is closed and around 25 people are evacuated from the listed buildings facing the collapse. Businesses also have to close, along with two property development sites. Efforts are hampered by the crowds that have arrived for carnival night.
Engineers converge on Bridgwater overnight as the dispersing carnival traffic and motorway chaos permits. A dawn meeting is convened between the authorities and the utility companies. It is quickly concluded that West Quay is in a parlous state and that all primary utilities must be diverted for an extended period.
West Quay hosts a plethora of buried services, including 1m and 300mm diameter sewers, an 11kV electricity cable, a 125mm diameter gas main and water, gas and electricity supplies. Wessex Water and its contractors had to perform heroics to safely divert the flow from its sewers with more than 1km of temporary pipework and substantial temporary pumps. A serious pollution risk was duly averted.
At a follow-up meeting between officers of Sedgemoor District Council (SDC), Somerset County Council (SCC) and the Environment Agency, each authority took on emergency and recovery tasks. The county council’s county structures manager Peter Radford was handed the job of stabilising the situation.
It was clear from observing the river bank that each retreating tide was taking more exposed fill from behind the part-collapsed masonry.
“It was very much like a sand castle on the beach. The incoming tide would erode the foot of the exposed earth slope and more material would collapse from above,” recalls Radford. “This was very worrying because this process would soon erode support to vital buried services. Meanwhile, tall, old buildings were only a few metres further away.”
A crucial concern was also the 1970s spun concrete trunk sewer that had become exposed in the chasm. The sewer was in danger of fracture from the large slabs of asphalt road surfacing that rained down on it from 3m above after each tide. “Dithering and delay was simply not an option,” says Radford.
Kenny Higgins, director of Crestmoor Construction, SCC’s framework contractor with the brief for structures emergency works, received a call on the Saturday afternoon. “The council’s plan was to coat the exposed and eroding fill and the exposed sewer with sprayed concrete delivered from a remotely supported platform,” says Higgins. “This was to avoid any unnecessary loading of the [riverside] and to place the concrete where it was needed through a tube.”
Procurement of specialist support continued into Sunday night. Sprayed concrete specialist Shotcrete joined the fray at dawn on the Monday morning, while an 80t telescopic crane from local company Marsh Plant was selected as the platform and diverted to the site. Emergency consents were secured and a CDM health and safety plan was put together at record speed enabling the Crestmoor operation to start in earnest with concrete spraying on Tuesday 8 November.
By the evening of 11 November all eroding earth faces had been coated in 250mm to 600mm of fibre and mesh reinforced concrete with a drainage layer behind. This was predominantly in the three crescent shaped bays between exposed (but previously buried) concrete columns associated with the sewer.
The exposed trunk sewer was similarly protected, but more to resist fracture from falling debris and the real risk of flotation given it that was likely to be empty and submerged in 2m to 3m deep water during high tides. Behind each panel of sprayed concrete was a sheet of filter drain material and a porous pipe feeding out at the base of the chasm through a rubber non-return “blabbermouth” valve.
“It was clear pent up water had a lot to do with the collapse, and we didn’t want the same problem destabilising our new line of defence,” says Radford.
Many notched up a 100-hour working week during the intense stabilisation effort. “To get it done in just seven days, when new cracks were opening up all the time, was an impressive effort,” says Higgins.
“An unfamiliar team, brought together within hours by Crestmoor, performed like clockwork,” adds Radford. “This was a brave and skilful piece of contracting work with everyone focused on getting the job done quickly and well in difficult conditions. In just one week after the initial collapse the continuing erosion and disintegration of West Quay was arrested and time was bought to enable permanent solutions to be considered.”
Two proposed solutions were being simultaneously developed. The Environment Agency commissioned its consultant Black & Veatch to carry out a feasibility study. This deemed West Quay, in its part-collapsed state, a non-viable platform for staging the reconstruction work. This meant an attack from the river, which would necessitate large-scale temporary works and, essentially, the construction of a new wall in front of the old one. This scheme was costed at £3.8M to £4.2M with a design and construction schedule potentially taking 18 months.
The other solution, developed by the county council, proposed a multi-stage repair conducted largely from West Quay itself. This scheme involved the deployment of arrays of ground anchors and piles supporting a concrete reinstatement and backing wall, which would then be finished with stone-faced reinforced concrete
panels, restoring the wall to its precollapse appearance. This proposal was costed at £1.2M-£1.4M with an ambitious construction programme of eight months and, in effect, an immediate construction start.
“There is a clear commitment to understand the causes and to put that knowledge to good use in the future”
Robin Sanders, Capita Symonds
A key feature of the SCC scheme was the objective of enabling the withdrawal of all hazard notifications and temporary restrictions on the site at the end of the first phase. Radford’s scheme would also include a trial stage, where one portion of the collapse area, Bay 1, would be used as a proving ground to confirm methods and finalise the design.
Radford was aware that the county council’s proposals carried a higher level of risk and uncertainty, but that this was offset by the potential to substantially reduce time and cost. “The primary focus was on time and getting West Quay and its community back to normal as quickly as possible,” says Radford. “Since the emergency work in November we had been keeping a very close eye on the stability of the site. This gave me confidence we could proffer this solution, but there have been a few sleepless nights.”
The two options were considered by senior officers from SDC, SCC and the Environment Agency on 19 December. It was agreed that the SCC structures team should beasked to take its scheme forward, but that the Black & Veatch proposal would be kept in the wings in case trials showed the SCC solution to be unviable.
SCC and Crestmoor agreed on a system of electronic structural monitoring supplied by Strainstall from Bath. “We had an excellent response from Strainstall,” says Radford. “Our requirements were quickly understood and precision movement gauges were built and installed within days.”
Crestmoor mobilised on 3 January. Further input came from Somerset crane specialist Marsh Plant and Cornish drilling, ground anchoring and piling specialist Saxton was brought in to deliver the piling. The scheme also calls for the deployment of helical ground anchors supplied by ABC Anchors of Wiltshire.
The local community will be in no doubt as to the work in hand and its difficulty. Buildings on West Quay look down on a scene reminiscent of First World War trenches, with the contractor contending daily with extraordinary new volumes of silt left behind with each outgoing tide.
“Every time the tide comes in it deposits a lovely load of new silt for us,” bemoans Higgins. The site team daren’t attempt to make the wall watertight lest the activity affects its stability.
Amid the filth, the site team is expertly threading mini piles and ground anchors safely between and around vital buried services, the key objective being secure anchorage into the Mercia Mudstone, 8m below the road surface.
While the mudstone is appearing at a consistent level, everything above it is highly variable: medieval works and buried masonry, including the hidden remains of an adjacent 12th century quayside development.
The team had a fright with its first lateral anchor drilling, when the arisings from the hole were soft silt with what appeared to be fragments of long dead river reeds. “We were not optimistic about the ground but this was worse than we expected,” says Radford. “Then we took stock again of the orientation of this first drilling and realised it was on the axis of the old water gate that used to enable waterborne craft to enter the town’s castle, and that we had drilled into the long buried water gate channel. Somewhat better material has been found on either side.”
The risk to heritage surrounding the site is well understood. English Heritage has visited the site and various council heritage officers are closely involved. County archaeologist Bob Croft says: “We support the whole principle of the SCC project which is stabilising what is there and replacing only the collapsed material with something more competent. We also approved of the commitment to reface the rebuilt wall with the original stone. We are now anxious to stand back and let the engineers get on with it, this remains an emergency where every interest, including heritage, is being served by the ‘fix’ making the best possible progress.”
In the background SCC, SDC the Environment Agency and Wessex Water have collaborated on the co-funding and commissioning of an independent collapse investigation. This is being led by Capita Symonds director of forensic services Robin Sanders. All information of possible use is being collated and forwarded to Sanders, including statements from local people who witnessed the collapse.
“There is a clear commitment to understand the causes and to put that knowledge to good use to ensure such problems are headed off here, and elsewhere, in the future. I am sure there will be things for the sector at large to learn from,” says Sanders.
The potential for high groundwater pressure behind the wall is nevertheless featuring strongly in SCC’s design parameters. “We are designing to substantial hydrostatic pressures along with the range of earth pressures and possible surcharges,” says Radford. “All drillings have shown the ground under the road is saturated so we also plan to provide new subsoil drainage.”
SCC’s plan to do this is a novel one. The large volume of backfill concrete in the collapse bays will accommodate a series of interconnected cylindrical voids designed to capture and store groundwater until it can be safely allowed out into the river through non-return valves at low tide. “This will reduce ground water pressure, save handling and the need to place around 100t of concrete, save money and be more sustainable,” says Radford. “A delightful ‘poacher turned gamekeeper’ feature of this proposal is that any accumulation of ground water in the tanks before low tide release actually contributes to the stability of the repair.”
It would seem that observers hoping to see any sort of splash where tonnes of old masonry fall in the river may be disappointed - and Radford hopes they are. The final activity of SCC’s phase one is the careful and staged nibbling down of the wall from the stabilised and reconstructed quayside. “We plan to drill core holes into its back face and use hydraulic bursters to gently prise off convenient and neatly catalogued chunks which will then be taken away for the stone to be salvaged. This stone will then face the precast concrete panels that will finish the job off in phase two,” says Radford. “When we have finished we want people to struggle to see the joins with the original wall.”
Work is progressing well, with the team hoping to complete its £400,000 phase one by the end of April. The £800,000 work to reinstate the wall will then be let as a separate contract.