Underpinning solution safeguards historic National Trust property

National Trust estate Dunham Massey is a site of special scientific interest (SSSI), home to deer, owls, and birds as well as a Georgian mansion. The site’s Deer Barn had suffered with subsidence for 40 years, threatening its structural integrity. As traditional remediation would endanger the legally protected brown-eared bats residing there, the National Trust turned to geotechnical contractor Geobear, who used a minimally invasive underpinning technique to preserve the historic structure and protect wildlife.  

In the heart of Cheshire, Dunham Massey is a 3,000-acre National Trust estate featuring to a stately home, extensive gardens and a vast deer park. Playing an essential role in local conservation efforts, the estate’s Deer Barn was built in 1740 and is home to a diverse range of protected bat species.

The barn remains largely unchanged from its original design. Despite its enduring presence in the estate, the barn has suffered with structural instability due to a combination of soft grounds and burrowing rabbits beneath the foundations. The most significant concerns focused on the colonnade — the series of arches supporting the low-level roof around three sides of the building — which has faced ongoing pressure from previous structural movement.

Stabilisation efforts date back to 1984 when engineers identified the colonnade roof was spreading outwards, putting lateral pressure on the walls and causing rotation of the foundations.

Works were undertaken in 2013 including crack repairs and underpinning of two brick internal columns, but they failed to fix the uneven settlement. In 2020, construction consultancy Thomasons conducted further investigations which confirmed the structural movement was still ongoing. 

“The restraint systems installed to the front and left-hand elevations, though well-intentioned, were not fully effective,” explained Jane Entwistle, Technical Director at Thomasons, and past President of the Institution of Structural Engineers. “The anchor bolts had started pulling out under minimal pressure, making the current restraint system unreliable. Further intervention was needed.”

Protecting wildlife while maintaining heritage

Brown long-eared bats had roosted within the barn for decades. Their presence brings legal responsibilities under the Conservation of Habitats and Species Regulations, which legally protects the bats. Therefore, any heritage conservation efforts must carefully consider the winged inhabitants when conducting ground remediation.

Traditional methods for stabilisation, such as excavation, were not a viable option here. Heavy machinery could risk irreparable damage, while vibrations and noise could disturb Dunham Massey’s ecosystem.

Geobear proposed a ground improvement solution which utilises their pioneering geopolymer injection technology. Rather than deepening the foundations, the technique involves injecting geopolymer resin into the ground beneath the structure. Once injected, the material rapidly expands, filling voids, compacting the soil and increasing its load-bearing capacity.

“Traditional repairs at Dunham Massey would have involved hand-dug pins and some machinery to remove spoil and deliver concrete,” explained Andy Lee, VP of engineering at Geobear. “Instead, we only need to drill to depth according to our treatment plan, using technology that didn’t disturb the parkland or the historic structure.

“Our approach removed the complexity and kept the site quiet,” Lee continued. The geopolymer cured quickly, reaching 95 per cent strength in just 30 minutes, allowing the team to complete the work in a fraction of the time required for traditional underpinning.

Stability without complexity

Geobear’s technique improved the ground beneath the Deer Barn without disrupting the fragile ecosystem or damaging the estate’s deep history. For heritage structure, public buildings and infrastructure projects facing similar challenges, Geobear provides sustainable, non-disruptive ground improvement solutions tailored to sensitive environments.

To find out more about the National Trust visit www.nationaltrust.org.uk