Right on Kew: Graham's glasshouse restoration | Construction Buzz #211
Rare plants, exotic flowers, towering trees – Kew Gardens provides the most unusual of construction sites for Graham on its £50m framework. Stephen Cousins reports.
“To come out here, on a day like this, with the sun blazing, it’s a glorious place to work,” says James Sands, framework manager at Graham Construction, as we scoot along in a golf cart past giant redwood trees, lush beds of exotic flowers and perfectly manicured lawns.
He is taking CM on a tour of the famous Royal Botanic Gardens in Kew, west London, to explore the varied and challenging construction works the contractor is carrying out under a £50m five-year framework. The Kew Capital Development Programme, funded by the Department for Environment, Food and Rural Affairs (DEFRA), includes new buildings, major refurbishments and site-wide infrastructure upgrades.
Among works under way are the high-tech £4.9m Arboretum Nursery glasshouse, the £4.6m Japanese-inspired Pavilion Restaurant, and refurbishments to a modernist aluminium glasshouse, the Herbarium Complex and Palm and Water Lily Houses. A connected job at Kew’s sister estate, Wakehurst in West Sussex, involves the delivery of roof repairs to the Grade I listed mansion.
Such a logistically complex array of works would test any construction manager’s mettle, but the stakes are higher at Kew where UNESCO World Heritage Site status, proximity to some of the world’s rarest plants, and throngs of tourists, has placed an unusually strong emphasis on safety and protection. Precautions include restrictions on the speed, size and frequency of vehicle movements, special driving licences and protective measures for the horticulture.
“Everything at Kew is about the flora and fauna,” says Sands. “We are not allowed to touch the plants or drive on the grass. The Pavilion Restaurant is home to 100-year-old vines and a unique horse chestnut tree, the first of its kind planted in the UK, that must be retained as part of the build. There has been a huge focus on planning to ensure the wildlife is sustained and not damaged during construction.”
The golf cart pulls up for the first stop on our tour: the Pavilion Restaurant, a high-end dining destination at the southern end of the Gardens, near the huge Victorian Temperate House recently restored by ISG. The two-storey Japan-inspired building, designed by Ryder Architecture, replaces a demolished Victorian restaurant on the site. It will provide 400 covers, around three-quarters of which will be located outside under a 2.5m-wide canopy entwined with the century-old vines.
The steel-framed building is set out on a minimalistic 6m square grid which is repeated in the layout of the curtain walling. The structure was relatively straightforward to erect, says Sands, apart from certain details that had to look “crisp”, such as a sharp edge on the floating flat roof which extends out over the courtyard.
The discovery of an undocumented basement level and structures containing asbestos resulted in delays when the project was coming out of the ground. “Records of buildings at Kew are often insufficient, drawings are either out of date or haven’t been updated, and finding out where information is archived is a challenge,” says Sands.
A former tea-room building on the site, burned down by suffragettes in 1913, increases the likelihood that historic artefacts will be uncovered. Although none have been found so far, an archaeologist remains on call to screen any interesting finds.
After a five-minute drive, we pull up alongside a giant pile of manure the size of a three bedroom house. “This is the largest non-commercial compost heap in Europe: it even has a dedicated viewing platform for visitors,” says Sands, holding his nose.
Just around the corner is the Arboretum Nursery, a circa 13,000 sq m climate-controlled building, thought to be the most high-tech in the UK, where Kew horticulturalists will care for rare or endangered plants and flowers.
The glasshouse replaces three smaller greenhouses that stood on the same footprint and offers additional space for potting in six distinct areas that can be programmed to simulate different climatic conditions. Such sophisticated engineering is critical to the survival of certain species: one plant being propagated is thought to be the only specimen in existence outside of a small area in Spain.
The Arboretum Nursery was a turnkey package by Belgian company Deforche Construct, a world leader in glasshouse design. It features a unique viewing area, positioned between the potting spaces and an energy centre, where visitors can see the scientists at work through a glass wall.
The project was modelled in Level 2 BIM to ensure coordination of the complex services and the glass and steel structure. All components were prefabricated at Deforche’s factory in Belgium, delivered “just in time”, and assembled as a series of standard 12.5m bays that can be expanded in future.
We walk through a cloud of fine mist into an incubator bay where the most sensitive specimens will be cared for. Above our heads, a cyclometer device records data on humidity and temperature. This is relayed to the BMS, which controls two boilers and a series of compressors that heat and pump water through diffusers into the spaces.
Effective plant propagation requires moisture on leaves and heat on roots so plant pots are placed on bespoke benches with integrated heating. The benches run on rollers and can be pushed together to maximise space.
With the structure complete, Graham is assessing a proposal to build an adjacent two-storey staff welfare block, either in cross-laminated timber or a less expensive modular system. Sands comments: “CLT is the client’s preferred option but the contract is budget driven so we are also speaking to modular contractors. One concern is the modular units may require tall delivery lorries and Kew has lots of overhanging trees.”
The final stop on our tour is the Davies Exploration House (formerly the Australian House), a refurbished aluminium-framed glasshouse located behind the Temperate House.
This modernist structure, constructed in 1952 by the Crittall Manufacturing Company, features large arched trusses that span the entire space to remove the need for columns or cross beams. It is one of the earliest examples of aluminium frame construction and Crittall went on to install aluminium windows in almost every post-war British home, as well as London buses.
Graham took possession of a derelict building and its internal fit out involved constructing new retaining walls, for drainage and planting, and a polished slab. New timber benches line the space and a new corner office is covered with pre-finished Sapele cladding.
“Exploration House was a logistical challenge – all our deliveries had to arrive and leave via the Temperate House site, where ISG was completing its refurbishment; if they had important work underway, our hands were effectively tied,” says Sands.
Graham Construction’s work here is only just beginning. After completion of the Pavilion Restaurant, expected at the end of April, there is a proposal for a restaurant near the north entrance, with a construction value of around £5m, starting in September or October. Funds for refurbishment of the mighty Palm House are to be confirmed soon, and multiple works are required to improve pathways and electrical services on site.
This is welcome news for the many staff who enjoy this unique environment. “Having been here for over a year, I’ve seen the full cycle of nature and the many colours of different seasons,” says Callum Sanderson, design manager with Graham. “Sometimes I have to remind myself to take a step back and take it all in – you don’t get to work on a site like this very often.”
Working at Kew brings its own unique logistics challenges. Callum Sanderson, design manager at Graham Construction, explains that Kew runs its own police constabulary which requires all contractor employees to complete a bespoke driving test.
Construction traffic runs on the same paths as tourists, so each vehicle must adhere to a strict 5mph speed limit and be accompanied by two banksmen, one at the front and one at the rear, to walk it in and out of the site.
Sanderson comments: “The speed limit is incredibly slow and the programme had to be developed to reflect that. It can be frustrating when you have a large concrete pour. As a result, there have been a couple of 13-hour days.”
Delivery lorries are limited in size to the width of the path, so some construction components must be broken down into shorter lengths. For example, the steel frame for the restaurant was delivered in 3m-long sections.
The trees, including the historic horse chestnut which stands at the main vehicular entrance, also have to be protected. Around £100,000-worth of 400mm-thick vehicular matting was installed on the ground as part of the enabling works to protect the roots from being crushed. The porous design allows rainwater to penetrate and reach the roots.