Expamet Hy-Rib Supports Mine Stabilisation Programme

Expamet Hy-Rib® has become synonymous with the construction of some of the world’s most prestigious new structures. From Heathrow’s Terminal 5 to the Channel Tunnel Rail Link, Hy-Rib® provides an engineered solution to the construction industry’s needs.

Hy-Rib® was specifically developed by Expamet during the 1960’s for use as a permanent formwork for concrete stop-ends, slabs, retaining walls and columns and has been successfully used in numerous building and infrastructure projects throughout the world. 

So when Victor Camble,Technical Development Manager of Expamet was approached by construction company Hydrock at  Civils 07, about the possibilities of Hy-Rib® being intrinsic in a conservation and preservation project in Bath, he knew Expamet had to be involved.

“Stabilisation of old mine workings in the UK, has become a real issue in recent years, due to the possible danger to residents living above” explains Victor. “Expamet is now part of a project to stabilise the Combe Down Mines in Bath, and it’s an excellent opportunity to show exactly what Hy-Rib can do.”

The mines at Combe Down have been utilised since the 17th century, and were worked using a room and pillar method which literally meant hollowing out whole sections around a central pillar effectively producing a honeycombe below ground.  Like many such mines, irregular mining, and the theft of stone supporting the pillars has left the mines very unstable.

The mines were the source of almost all the stone associated with the city of Bath’s famous Georgian architecture. The Oolite Limestone – more commonly known as ‘Bath Stone’ has become an important part of Britain’s architectural history due to its use in buildings such as the Royal Crescent in Bath and Buckingham Palace. It was also favoured by Victorian architect Hans Price in his design of the areas first seaside ‘resort’ at Weston-Super-Mare.

Combe Down is an extremely important geological and archaeological site, designated as a Site of Special Scientific Interest (SSSI) and a Candidate Special Area of Conservation (CSAC) as well as being a home to the Greater Horseshoe Bat - one of the largest and rarest bats in the UK. The mines form one of the top 20 bat hibernation sites in Europe.  This means that any stabilisation project has to be sensitive to ongoing conservation needs. 

The stabilisation programme to protect the land, properties, church and school is a life-line for the 1500 residents currently living above the mines.  It has also had a surprising effect on the caves mammalian population, which has doubled since construction began thanks to the provision of long-term bat routes, including roosting and breeding sites with heated incubators.

The work that has started at Combe Down is the largest foamed concreting project in the UK.  After extensive trials and testing, foamed concrete was chosen as a suitable fill material for the mines’ caverns.  This concrete has the ability to be made very lightweight - essential when being pumped 1000m into the ground. It also negates potential settlement problems that may occur if filling with large volumes of denser materials and minimises environmental impact. Although the majority of the mines will be fully filled, the area occupied by the bats will retain a maze of passages and recessess created by the use of permanent formwork within the foamed concrete. Once complete this will then become a habitat for the bats to roost onto within the caverns and provide future access for monitoring and maintenance.

Expamet Hy-Rib® provides a rigid background (good bond and keying surface) for a sprayed concrete shell. This creates a permanent formwork for the foamed concrete to be pumped in behind without leakage, allowing the contractor’s production team to increase productivity and reduce time and costs.

The Hy-Rib® is fixed to a bespoke steel framework that creates the underground road way system enabling safe access. Due to the requirement to maintain the existing stone supports these roadways are rarely straight. The lightweight and easily manageable Hy-Rib® is readily adaptable to the myriad of profiles created within the existing mines.  A unique installation method allows Hydroc to speedily fix the Hy-Rib® to the face of the framework.

Hy-Rib® has an enviable portfolio of technical data. Independent bodies have undertaken full scale testing programmes to determine and verify the effectiveness of Hy-Rib®  in forming construction joints. The result is a product with well defined engineering properties, established formwork pressure characteristics and measured improvements to the joint’s performance. Hy-Rib® is certificated for use as permanent formwork by the British Board of Agrément (BBA).

Hy-Rib® is available in three thickness grades of galvanized steel to and stainless steel is also available where increased corrosion resistance is necessary. Factory curved sheets can be produced to customer specification. Hy-Rib® is supplied in 2m, 3m, 4m and 5m lengths, however to facilitate work in the Combe Down Mines, special lengths have been provided.