Soft capping our heritage ...

Laurence Gale MScin Public Places

The natural weathering process plays havoc with many of the historically important sites under the stewardship of English Heritage and, often, innovative methods are required to try and halt the ravages of time. A recent trip to Gloucester gave our editor an insight into the hard work the organisation carrys out to preserve these treasures.

Alan Cathersides, Senior Landscapes Manager, leads the Gardens and Landscape Team at English Heritage. The team specialises in the management of historically important sites and the landscape and ecology around them. He is particularly interested in management methods which benefit both historic and natural heritage.

Alan was keen for me to see the restoration work being undertaken at one of their most important sites - both in terms of Christianity and the number of visitors it attracts - Hailes Abbey in Gloucestershire.

He and his colleagues have been instrumental in evaluating and developing a strategy for protecting old ruins using a technique known as soft capping, and has been involved with the research since its inception, working closely with Zoë Lee, BA, M.Sc (Oxon), Heather Viles, MA (Cantab), DPhil (Oxon) and Chris Wood BSc GradDip Cons (AA), MRICS, MRTPI, IHBC.

Their work has resulted in the publication of a valuable document detailing the benefits of soft capping historic walls - Soft capping historic walls: A better way of conserving ruins? This seventy page document outlines the research and projects undertaken at several English Heritage sites, including Byland Abbey, Kirkham Priory, Thornton Abbey, Howbury Moated Site, Rievaulx Abbey, Whitby Abbey and Hailes Abbey.

Alan introduced me to the team of specialist monumental contractors (Nimbus Conservation Limited Somerset) who, for a number of years, has worked on many English Heritage sites. The five man team is headed up by Kristan Short who oversaw the month long project at Hailes Abbey which was completed in March of this year.

At the time of my visit in late February, eighty percent of the work had been completed, so I was able to see for myself the scale of the work involved. The project, which is expected to cost around £50,000, will help preserve the ancient ruins.

The concept is relatively straightforward and involves the laying of harvested natural turf over a layer of loam soil placed on top of the walls and structures to be protected. This natural layer effectively acts as a thermal blanket, reducing the effects of frost damage. One of the biggest costs is the erection of scaffolding to access the walls, plus the fact that all materials have to be walked onto the extensive site using wheelbarrows.

Alan now explains the process and benefits of soft capping.

"It is important to remember that all of the old castles, abbeys and other historic buildings we now see as ruins in the landscape were once fully functional buildings, with a distinct inside and outside and a roof over the top. They were certainly not designed to be open to the elements from every direction, and the deterioration problems which occur are a direct result of this.

The two principle causes of fabric deterioration on ruins are the 'Freeze/Thaw Cycle' and the 'Wet/Dry Cycle'.

Freeze/Thaw Cycle - when water freezes, it expands in volume by 9%. If this water is in the surface layers of stone - particularly porous limestone and sandstone - this expansion can cause the surface layers of the stone to break off, a process termed 'spalling'. If entire blocks of stone are soaked, it is possible for the whole block to shatter.

Wet/Dry Cycle - Porous stone can hold water within its structure, especially if it is open to the elements during wet periods. Then, just like plants, stones can lose water when the weather is dry and, again like plants, particularly when it is breezy. Water evaporates from the surface of the stone, and water within the stone matrix is 'sucked' to the surface to be lost in turn.

Some stones contain various mineral salts, and these can be dissolved into solution if the stone is soaked, travel to the surface of the stone and then, as the water is evaporated, they come out of solution. This sometimes happens at the surface and shows up as a white crusting on the stone, but it can also happen just below the surface. Some salts can expand in volume by 40% or more as they come out of solution, which can cause spalling even worse than that caused by frost.

From the turn of the 20th Century (but particularly from the 1920s onwards), many historic sites were excavated and opened to the public. The belief, at the time, was that all vegetation was bad for the stonework (not to mention untidy!) and was, subsequently, stripped off and replaced by 'hard capping' - wall tops made from hard, cementaceous mortar designed to shed water as quickly as possible.

This work was done with the best possible intentions but, by the 1990s, it was clear that fabric deterioration was not being slowed and, in many cases, appeared to be accelerating. Looking closely at these problems, it became apparent that hard capping was not improving the situation and actually seemed to be making it worse because;

- Hard caps always cracked, often within a few years of installation, and even hairline fractures allowed water to penetrate into the core of the wall. Once in the core, water took the easiest way out. This should have been the mortar joints (lime mortar allows some water movement) but, along with hard caps, many monuments had been repointed with hard cement mortar and, so, the water was forced out through the stones exacerbating the problem cycles described above

- Hard caps gave the stonework below no protection from thermal changes, so frost could penetrate from the top as well as the side of a wall
- Hard caps held the historic fabric below in a tight grip, and expanded and contracted at a different rate to the historic stone below and, in some cases, shattered the stone

- Hard caps shed water directly down the face of the wall, in the same direction, every time it rains. Not only does this increase the potential for damage by the problem cycles, but led to a clearly defined growth of algae within the stone surface in certain areas of the wall which are repeatedly wetted and which appear, even on dry days, like a stain on the wall

Because of these growing concerns, English Heritage commissioned Professor Heather Viles and her team at Oxford University to look into the possibility that soft capping - the use of soil covered in grass and other herbs - on wall tops could be beneficial.

They have clearly shown, by both laboratory experiment and field testing that soft capping;

- Acts like a 'thermal blanket' on the wall top, moderating both the upper and lower limits of air temperature changes. This prevents damage from frost, but also reduces the amount of thermal movement below a soft cap - both of a hard cap and/or historic fabric

- Sheds water in different directions each time it rains and, where the soft cap is well established, the longer grass stems often act like an overhang, directing water away from the wall face

- Appears to stabilise water movement within the core of the wall - although this is much less clear because of the problems of monitoring water movement within walls

Because soft capping clearly offers protection to exposed stonework, it does no harm to the historic fabric and could be easily removed in the future, if necessary. English Heritage believe that this option is a good way of protecting ruins from environmental damage.

Trials have been carried out on small areas of wall tops in various locations throughout the country but, because it is a major change in the presentation of monuments to the public we have decided to completely soft cap an entire site whilst, over the next two years, monitoring the response from visitors. Hailes Abbey was chosen for this trial because it is located in a frost pocket and has suffered increasing and serious damage to the stonework over the last few years.

Technical notes

The soft capping being installed is turf cut from an adjacent field, which is then laid over a medium loam soil to create a soft cap approximately 100mm thick. Wherever possible, we have been using local turf for the following reasons;

- It contains a wide range of grasses and herbs which grow naturally in that area. The exposed position and limited rootzone make soft caps a very stressed environment, which some grasses/herbs can survive and others cannot. Using a turf with a wide mix helps to ensure some will thrive

- The turf can be cut thicker than is usual for commercial turf (virtually no transport costs to worry about) and this helps aid establishment, particularly on higher wall tops where any sort of aftercare is impossible once the scaffolding has been removed

- The soil taken with the turf has a good seed bank within it. Whilst this would be undesirable on a sportsfield, it is a positive benefit in soft caps and helps aid re-establishment after dry summers (forget last year - we do get them occasionally!!) when even the most drought tolerant species can succumb

We try to avoid soil with a high clay content, as this is likely to crack during dry spells and allow water to get into the core of the wall when it rains.

The edges of the soft cap are the most difficult to establish, because they are most exposed to drying out, particularly on the south facing side of a wall, and we are currently experimenting with planting Stonecrop (Sedum anglicum) plugs into the sides of freshly installed caps in case the grasses die and, on older caps where the sides have already died back, to try to prevent the exposed soil eroding away."

Back on site, Alan was assessing the progress being made and ensuring the contractors had enough turf to complete the job; to date, they had harvested over 2000 square metres of turf from a nearby field. The field belongs to English Heritage, but is rented out to a local tenant farmer who grazes sheep on the land as part of the management plan. All the turf was lifted using a pedestrian turf cutter, with turf being rolled by hand and transported by wheelbarrow into the abbey ruins.

The turf had been cut in small rolls at a depth of 100mm to retain as much soil as possible. It was then placed in situ on the top of the imported loam soil to cap the walls and any exposed masonry.

Alan was also keen for me to see the technique of planting the sedum into the sides of the soft capping to provide some additional protection during dry periods. He had actually propagated the plants himself.

He remains confident that, once the work is completed and the turf becomes established, the soft capping will come into its own in the coming years and slow down the rate of erosion.

It is only when you see, at first hand, the work being undertaken by English Heritage that you get an understanding of how important these wonderful historic sites are to the nation and how vital the organisation's work is across England.

Hailes Abbey

Hailes Abbey is two miles northeast of Winchcombe in Gloucestershire.
The abbey was founded in 1246 by Richard, Earl of Cornwall, the younger brother of King Henry III (1207-1272). It was settled by Cistercian monks (known as 'White Monks' from the colour of their habit) from Beaulieu Abbey in Hampshire.

Hailes Abbey became a site of pilgrimage when Richard's son, Edmund, donated to the Cistercian community a phial of Holy Blood, purchased in Germany, in 1270 and allegedly the blood of Christ. Such a relic of the Crucifixion was a considerable magnet for pilgrimage, and the shrine built for the 'Holy Blood of Hailes' was probably the most famous pilgrimage site in England after the tomb of Saint Thomas à Becket in Canterbury. From the proceeds, the monks of Hailes were able to rebuild the abbey on a magnificent scale.

During the Dissolution, the commissioners of King Henry VIII declared the famous relic to be nothing but the blood of a duck, regularly renewed, and the last Abbot, Stephen Sagar, admitted that the Holy Blood was a fake in the hope of saving the abbey. However, Hailes Abbey became one of the last religious institutions to be closed following the Dissolution Act of 1536. The Abbot and his monks finally surrendered their abbey to King Henry's commissioners on Christmas Eve 1539.

Soon after the Dissolution, Hailes Abbey was granted to Katherine Parr (sixth wife of Henry VIII), and subsequently passed through a succession of family connections. It was during this time that the abbey church was demolished and some of the monastic buildings were converted for use as a family home. By the end of the 18th century, the site had suffered extensive destruction, and it was left in an overgrown and decaying state until it was excavated some 100 years later.

Holy Blood

Hailes was not the only establishment with a claim to have 'holy blood', although it is generally the most well known. During the medieval period, it was generally accepted that the blood at Hailes and these other sites was genuine.

Other establishments with claims include Westminster, Ashridge and Glastonbury. The various histories of these rival claims are superbly documented in The Holy Blood - King Henry III and the Westminster Blood Relic by Nicholas Vincent.

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