Vinci uses traditional construction for Georgian work

The complex refurbishment of a 250-year-old hotel and spa, perched above natural thermal springs, required meticulous planning and a return to traditional construction methods. Stephen Cousins reports. Photography by James White.

Next time you drink a bottle of Buxton mineral water, take a moment to consider the fact that the water was drawn from a natural spring located just a few metres below one of the most complex and demanding refurbishment projects currently under way in the UK.

The £50m redevelopment of Buxton Crescent will see main contractor Vinci Construction restore and expand a Grade I-listed Georgian-era masterpiece into an 80-bed five-star hotel, and reopen the adjoining Grade II-listed Natural Baths to the public for the first time in almost 50 years.

The buildings stand directly above a network of ancient thermal springs, including the only source of natural mineral water used by Nestlé’s state-of-the-art bottling plant, which means that the majority of construction work in close proximity to underground springs, including digging and demolition, must be carried out by hand to minimise vibration and avoid a catastrophic fracture. Work in the most sensitive locations must adhere to detailed method statements prepared by a team of four dedicated hydrogeologists.

Cary Hadfield, senior project manager at Vinci Construction, told CM: “Without proper precautions, demolishing a structure could release the burden on the ground, causing it to pop open and shower water out. As a result, we are literally breaking out a section of slab at a time, then leaving the rubble in-situ until we have cast a new suspended slab above it to put the weight back on. It really is as piecemeal as that.”

As if that wasn’t enough of a challenge, the team is engaged in constant redesign as ongoing survey work inside the main Crescent building reveals an array of unexpected structural anomalies and extensive timber rot.

“It is constant surprises with this building,” says John Ferguson, senior architect and contract administrator at studio Curious Worldwide. “You think you know it, then you peel away the plasterwork and see that the wall has apparently been filled with whatever previous builders had to hand, and you think: my God, how did it ever stand up?”

Fashionable spa town

Buxton Crescent, designed by the renowned architect John Carr, is the most imposing structure in the centre of Buxton, a tourist hotspot in the heart of the Peak District in Derbyshire.

Arc-shaped in plan, the building stands at the foot of a steep landscaped hillside and was originally constructed between 1780 and 1789 for the Duke of Devonshire, as part of a scheme to establish Buxton as a fashionable spa town to rival Bath.

In its current raw state of ongoing structural repairs, the building is very atmospheric, characterised by dark hallways pierced by occasional shafts of dusty sunlight, dank cellars and peeling paint. The head foreman relates the story of a site security guard who, standing alone on the grand staircase to the Assembly Room one night, felt a ghostly presence pass right through him like a chill wind. It’s like an eerie encounter from a Mary Shelley novel.

Above: The highly decorated Assembly Room. Below: The Crescent as depicted in a 19th century lithograph by G Rowe

The Crescent building originally comprised two hotels – the St Ann’s Hotel, to the west, and the Great Hotel, later the Crescent Hotel, to the east – separated by six lodging houses. By the mid-19th century, the hotels had expanded to take over the lodging houses.

Fast forward to the 1970s and the Crescent Hotel was used by Derbyshire County Council (DCC) as offices and a public library, but had to be closed in 1992 due to structural defects. The St Ann’s Hotel carried on trading until 1989 when it was closed pending a full refurbishment.

The building’s owners, High Peak Borough Council and DCC, struggled for many years to find a viable commercial use for the building, until, in 2003, developer the Trevor Osborne Property Group and CP Holdings/Danubius Hotels (which together formed the Buxton Crescent Hotel & Thermal Spa Co) put forward an ambitious plan for a luxury spa hotel intended to restore Buxton’s status as spa capital of Britain.

The company will fund just under £20m of the scheme, with the rest coming from the public sector – including £23.8m in Heritage Lottery funding. Andrew Ryan, director of the Trevor Osborne Property Group, told CM: “It’s one of the biggest Heritage Lottery grants given to any project, which is a result of the massive conservation deficit on the scheme [the amount by which the cost of repairs exceeds the market value of the asset when repaired]. As a result we are working hard to conserve as much of the historic fabric of the building as possible.”

However, the project was thrown into financial crisis in 2010 when the East Midlands Development Agency ceased operation and had to withdraw £5m. Historic England subsequently stepped in with a £500,000 grant and provided professional assistance to help review costs and ensure the scheme achieved consent. Work on the main construction contract started on site in March 2016.

Above: Traditional building techniques prevent vibrations from machinery. Below: Construction is complicated by the patchwork of earlier repairs

The complex project involves interconnected elements of refurbishment, conservation and new build. The Crescent building will be transformed into a single hotel with 80 luxury bedrooms, six specialist retail units at ground floor level, and the Assembly Room, a refurbished 18th century ballroom with ornate painted plasterwork, marble fireplaces and Carr’s original priceless chandeliers, in the East Wing.

The Natural Baths, which dates from 1853, will have its two spa pools refurbished and brought back into use, and a new-build extension added with an “inside-to-outside” rooftop pool. The new concrete structure will be clad in limestone, from the same quarry as the stone used on the original building.

Going with the flow

Working in close proximity to protected natural springs raised the risk stakes considerably. The Crescent faces the site of St Ann’s Well, where warm spring water has flowed up through geological strata for thousands of years, and up to a third of the supply is extracted for use at Nestlé’s bottling plant.

Any work with a potential impact on the springs, such as digging or releasing existing structures, had to be assessed by the team of four hydrogeologists – two employed by Nestlé, one by the councils, and one by the developer – which developed precise step-by-step method statements of how the work should be carried out. One critical method statement covers the delicate procedure to tap into an existing spring, build a “capture chamber” around it, and install a pump to take spring water to one of the refurbished pools.

The main concern of any potential damage is not just contamination, but an interruption to the natural flow, says Hadfield: “If we were to open up an excavation and groundwater started seeping in the first thing we would have to do is monitor the temperature.

Cold water is normal groundwater and not a big issue, but a temperature above 26 deg C means thermal groundwater and is very serious. Nestlé has built a multi-million-pound business on being able to secure a set amount of water at a set time, at a set temperature and location. Any disturbance might not be a quick fix.”

Above: Work is continuing on the Crescent exterior. Below: The grand staircase leading to the Assembly Room

The trickiest part of the operation, forming a 6.5m-deep new concrete basement directly below the Natural Baths on top of Nestlé’s spring, was hived off into a separate £2.5m enabling works package, carried out in 2013, in an effort to de-risk the main contract.

The main contract is in the form of a traditional JCT contract, with most of the construction risk borne by the client. Ryan comments: “After signature of the contract, in spring 2016, we went through a very thorough process of tendering packages to subcontractors and direct meetings with Vinci and the design team to look at elements of the scheme to see how best to construct it.”

The danger of vibration to the springs put severe limits on the use of plant and heavy machinery inside the Natural Baths and hotel, and resulted in a return to traditional ways of working, such as breaking out with hammers and removing debris in wheelbarrows.

What lies beneath

Natural springs are not the only hidden hazard the team has to deal with. A decision, by Historic England and the two councils, to limit building investigation work to non-intrusive surveys until the main contract was awarded, meant a full understanding of the structure was not possible until the project got to site.

All the hotel’s floor beams and roof structures are timber, and the level of rot uncovered was concerning, says Curious Worldwide’s Ferguson: “When we started to tackle the timber repairs the project grew arms and legs – much more of the structure was rotten than we had expected.”

Large amounts of the floor structure had to be removed; ends of joists supported in external masonry walls were often rotten, particularly on the Crescent’s convex rear elevation.

Elsewhere, internal walls and ceilings often looked uniform but the underlying structure varied wildly, as a result of a patchwork of alterations over the past two centuries. In one case, a section of the ground floor ceiling was removed to reveal that the chimney breast had been cut at ceiling height, leaving the 2.5m of heavy masonry above supported precariously on just two small pieces of timber.

The alteration was made to create a new corridor through the building, which pushed through the radial “spine” masonry walls that separated the lodging houses.

In another instance, the team uncovered a suspended timber floor construction, totally uncharacteristic for the period, hung from a timber truss suspended below main roof trusses. The truss was supported by two timber hangers from which the chunky “bressumer beams” that take the weight of the floor were hung. "We were standing on the suspended floor looking up at the ceiling and all of a sudden we were thinking: there’s a rotten truss supported on dodgy brickwork, and corroded iron fixings for the bressumer beams… this probably isn’t safe!” Ferguson jokes nervously.

Efforts to date previous alterations are an ongoing challenge. The building is almost 250 years old, so even alterations from over a century ago could be considered “new”. “There’s a bit of a Time Team aspect to this building. We keep discovering alterations, then there is intense discussion between the experts over what is original and what isn’t, what is important to retain and what isn’t,” says Ferguson.

Despite the high degree of rot, the focus has always been on conservation, where possible, while taking into account the needs of a five-star hotel, such as fire-rated compartment floors, and supports for new service runs.

Structural acrobatics

The huge amount of timber structure needing replacement, coupled with restrictions on access and machinery, resulted in designs for complex new hybrid structures and working methods.

Many floors comprise a mixture of original and new structure, such as structural bressumer beams that span between internal walls, timber floor joists and steel beams. Some retain original “pugging”, a traditional method of sound- and fire-proofing floors by covering joists with lath and plaster then covering it with a screed-like material.

“The intent was to try to retain all the original ceilings, floors and pugging, but unfortunately many were sandwiched between rotten timber structures, which could not be removed without destroying them,” says Hadfield.

Above: View along the colonnade, with historic signage. Below: Peeling paint and plaster is stripped back

The floor of the Assembly Room was in a critical state after an incident in the 1990s, when it partially collapsed as a result of being overloaded with heavy bookcases when used as a library. Large new steel beams needed to support the floor had to be split into discrete sections that could be carried in by hand, lifted into position by Genie hoist, then connected with intricate bolting and splicing details. Similar steels are being used throughout the building.

Some of the stone staircases that lead up to attic level were leaning precariously. The masonry and mortar supporting the individual stone treads, which cantilever out from the wall, had weakened over time. The solution involved installing a jacking system on props underneath to lift the treads so operatives can rebed and repack mortar around each one to increase its strength.

The project team now has its eye set on practical completion, currently scheduled for late 2018, although delays may result from the extensive structural repairs still being uncovered.

“We have just found a 200-year-old unstable arch that supports a gigantic chimney and have had to put in place an exclusion zone,” says Ferguson. “Every day there’s a new challenge. Everyone on this job, including contractors, designers, consultants, has said it is the most challenging project they have ever worked on,” he concludes.

Too much stress, too little strength

Overstressed trusses in the attic required an ingenious solution

Efforts to design support for the hotel’s attic level required an ingenious structural solution involving a “web” of oriented strand board (OSB) timber beams.

The existing roof is supported on a series of queen post trusses that span from the front of the Crescent to the back, with floor joists spanning across the bottom chords to support the attic level.

The Georgian-designed trusses were extremely overstressed by modern engineering standards, says John Ferguson, senior architect and contract administrator at studio Curious Worldwide: “A truss only acts as a truss if it spans from one side to the other, but the Georgians had installed a supporting wall right in the middle, which makes the truss behave like two separate beams and removes a lot of the strength.”

The initial plan was to install heavy steels below the trusses in order to increase support, but getting the steels in position was problematic. The floors below were weak, which would have made it necessary to install crash decks to prevent the steels from smashing through if they were dropped.

“It was a complete headache,” says Ferguson. “In the end, myself and the structural engineer devised a timber solution to take the load off the trusses so they wouldn’t require any strengthening.”

A system of custom-built JJI-Joists – similar to I-beams made of OSB timber – was installed above the ceiling line of the floor below, to reach up to the base chord of the truss and support the attic floor.

The structure had to be designed as an intricate web to enable it to pass around a series of massive balancing stones that form the cornice on the facade and project back into the ceiling void below of the attic.

“We had to build up the joists in tiers in a criss-cross pattern to snake around the stones and reach the bottom chord of the truss, ready to install the floor,” says Ferguson. “It’s a totally unique and innovative system.”