Context90.htm

CHRIS WOOD

Soft capping: justifying a return to the picturesque

English Heritage is investigating the use of soft capping with soil and turfs as a more effective way of conserving exposed walls than the usual mortared, hard caps.

The appearance of vegetation on ruined masonry sites has long been a vexed issue. In the past two decades criticism of the 'traditional' approach of English Heritage and its predecessors in removing anything green has focused on the sterile appearance of these ancient monuments. But during that time English Heritage has been trialling soil and turfs (soft capping) as a more effective way of conserving exposed walls than the usual mortared, hard caps. Research is now in hand to test whether these soft caps will indeed achieve the hoped-for benefits and

what effect they will have on the masonry below. The work is primarily aimed at providing technical answers but it should also have very important implications for the monument in terms of appearance, presentation and biodiversity.

The picturesque appeal of flora as a quintessential feature of the gentle decay of ruins has long been debated and written about. Much attention has of course concentrated on aesthetic, spiritual and philosophical issues. In the Victorian era these ultimately spawned the conservation principles which today we rather take for granted. Matters came to a head when the government took the most important monuments into guardianship and set about conserving them. The row over the works at Tintern Abbey made front-page headlines in the national press in 1922. The Office ofWorks' objective was to minimise the intervention, but also to make it safe for visitors and perhaps for posterity. This meant removing ivy, but also the soil and grasses that colonised the wall tops. Removing flora from monuments has essentially remained the policy throughout the 20th century.

In the early 1980s regional teams in English Heritage began experimenting with soft capping by laying turfs and soil on the tops of low walls, particularly where these were constantly being damaged by frosts and people walking on them. The best-known example is Wigmore Castle in Herefordshire¹, where a previously unconsolidated ruin was repaired and soft capping reinstated. However, despite this work little or no monitoring or scientific analysis was carried out to evaluate the performance of these treatments.

Why do 'traditional' hard caps cause such problems? In essence, it is very hard to make them work effectively given the exposed nature of many of the sites. They are often prone to extremes of weather, in particular suffering from harsh frosts. During much of the past century cements were used to consolidate wall tops, which tend to crack. The impervious nature of the material prevents moisture escaping with the result that when it freezes it expands and exacerbates the problem. The widespread use of cements has left a difficult legacy for those now charged with maintaining these monuments. Lime mortars were tried but initially these were weak putty limes which did not survive the frosts.

Another problem with hard caps stems from the run-off of rainwater. The exposed core is deliberately built up in the middle to encourage water to shed quickly. This tends to concentrate run-off in distinct paths, with the result that it wettens the face and encourages disfiguring algae to colonise. More important, those who have observed this over the

Okehampton Castle: frost has caused further damage to the masonry.

Byland Abbey: the use of cement has helped to crack the ruins.

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years report that this run-off has hastened the erosion of important decorative and tooled stonework. Given these problems, soft capping is being tested to see whether it could achieve three objectives. First, to act as a thermal blanket; that is, to keep the temperature above freezing point at the head of the wall. Second, to see whether the roots, soil and grass could hold the moisture until it could then transpire naturally in the wind and sun. Third, to test whether this sponge effect could prevent concentrated run-off down the face of the wall.

Four years ago the Building Conservation and Research Team at English Heritage commissioned a series of pilot laboratory experiments² to test these objectives at the University of Oxford's Department of Geography, which specialises in stone deterioration and weathering. Locally sourced turfs and soils of different types and depths were placed in perspex containers with temperature, moisture and humidity probes within the soil and at the wall head. These were compared with bare stone and hard caps. For the thermal blanket tests these containers were placed in an environmental cabinet that simulated the extremes of hot and cold found the previous year at Hailes Abbey in Gloucestershire. These conditions were continuously cycled.

The results were very promising. No freezing took place under a soft cap, even with only 50mm of soil, but it did with the hard cap. Water retention was generally good. Only with extreme rainfall and a minimal 50mm soft cap did any penetration occur.

In late 2003 a three-year research programme was initiated. The aims were to test whether soft capping works effectively, to compare it with hard capping, and to determine best practice for installation, management and maintenance. The emphasis of the research is on the technical issues, so aesthetics, presentation, conservation philosophy and biodiversity have largely been ignored.

The research project is being run by a steering committee of four which meets regularly to discuss progress. Expertise is also drawn in from related fields, such as soil science, botany, stone deterioration, climatology, moisture movement and monitoring. The work is being coordinated at the University of Oxford, which is also carrying out the laboratory testing. The outputs will include articles and scientific papers, an English Heritage Research Transactions volume containing all details of the research, and a Technical Advice Note for those contemplating using soft capping. These publications are planned to be ready for a conference in autumn 2006.

Experiments have been set up at Byland, Rievaulx and Whitby Abbeys and Kirkham Priory, all in North Yorkshire, Thornton Abbey, near the Lincolnshire coast, Hailes Abbey, and Howbury moated site in Kent. The sites have been chosen deliberately for specific tests and all could potentially benefit from soft capping. There are a wide range of exposures, heights of walls, types of stones and local micro-climates.

The most intensive testing is being carried out at two of the sites. A turf cutter which was used to get material from a corner of the site produced a consistent depth of soil. The topsoil used was a medium loam clay mix to the current British Standard. An original objective was to find an optimum grass with very tight rooting system that minimised water penetration, supplemented with a clay soil that maximised moisture retention. Advice from ecologists and soil scientists was that this was neither sensible nor practical. In any event, the research aims to produce practical advice, so it is of no benefit to test an ideal turf which a commercial grower will not produce because of limited sales potential, and which probably will not survive in the environment on which it is being placed.

There are six different test areas, with soil depths of two, four and six inches, each 3.5 metres in length. A model 'best practice' hard cap using a mix with a feebly hydraulic lime as the binder has been placed in another 3.5 metre length alongside the soft capping. Moisture and temperature probes have been placed on and below the wall tops and within the soil.

Two soft caps have been installed at Kirkham Abbey. This is a tough limestone with a different exposure, the walls having a 50-foot drop to one side. Turfs and

Kirkham Abbey: removing hard caps proved difficult.

Kirkham Abbey: soft cap being installed.

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References

1 Coppack, G 'Conserved in the gentle hands of nature', Context 73, March 2002

2 Viles H, Groves Q Wood C (2002) 'Soft walling capping experiments', English Heritage Research Transactions,Volume 2, Stone

soil were deliberately laid on to the wall head to follow existing contours. These soft caps are being monitored and progress is being recorded. A 'best practice' hard cap using a moderately hydraulic lime has also been placed alongside one of the soft caps.

All field sites are photographically recorded every quarter and the condition of masonry and flora are noted. Under the sample areas on the main test site are two methods of testing moisture and temperature at the head of the wall and within the core.

The range and extent of the laboratory tests have been expanded. The results are confirming what was learned following the pilot work, particularly with regard to the thermal blanket tests. Experiments on water holding and transmission properties of soft capping have yet to be started. Tests are also proposed to measure the rate of moisture transpiration. The drying out rate is likely to be a key feature of the success or otherwise of soft capping. The maintenance regimes and costs are to be evaluated this year and the longer term site monitoring is set to continue.

One new laboratory test has been added as a result of difficulties encountered in removing the existing hard cap at Kirkham to make way for one 'model' cap in lime. The masons found it was very difficult to remove the concrete capping. A kango hammer proved to be inadequate for the task, so plugs and feathers were needed. This is a quarrying technique used for splitting stone. On inspection a high proportion of the facing stones were found to be cracked, probably because the normal thermal movement was constrained by the concrete, which was harder than the stone.

In the longer term this could well prove to be a significant problem, although it is impossible to

Byland Abbey: installed soft capping with model hard capping being put in alongside for comparison.

estimate just how long it would be before drastic intervention would be required. What does seem certain is that if this is typical over the whole site, then it will be extremely expensive to rectify in a new hard cap. But it may be possible to install quickly and avoid drastic intervention if soft capping reduces the range of thermal movement. This would not only help to preserve precious fabric, but it could also save English Heritage a great deal of money. Laboratory tests are being carried out to help answer this.

So far, positive results are emerging. As with all research projects, progress is being continuously reviewed and this sometimes initiates a wider range of experiments. The Kirkham example is typical.

Although soft capping may appear to provide technical answers to the problems posed, they will not be effective in every instance. It is still essential to understand what is causing the masonry to decay. At Byland Abbey for example, stone which remains wet seems to be fairly stable, whereas in drier areas there is quite dramatic deterioration. While this can be explained partly by the use of cement in previous repairs, it is not the only answer. Almost certainly there is salt damage caused by the fluctuating moisture contents. Tests are being carried out on the salts, stone and mortar to establish the reasons.

But if soft capping does work effectively and leads to the masonry below being much drier, with stone such as this calcareous sandstone it may prove to hasten the decay rather than slow it. So whenever soft capping is contemplated it is still essential to gain a thorough understanding of what is happening on site and the factors that determine the decay mechanisms.

Hailes Abbey: moisture levels below the capping are monitored.

Chris Wood is senior architect conservator at English Heritage and a member of the IHBC technical subcommittee.

CONTEXT 90 : JULY 2005



References 1 Coppack, G 'Conserved in the gentle hands of nature', Context 73, March 2002 2 Viles H, Groves Q Wood C (2002) 'Soft walling capping experiments', English Heritage Research Transactions,Volume 2, Stone	soil were deliberately laid on to the wall head to follow existing contours. These soft caps are being monitored and progress is being recorded. A 'best practice' hard cap using a moderately hydraulic lime has also been placed alongside one of the soft caps. All field sites are photographically recorded every quarter and the condition of masonry and flora are noted. Under the sample areas on the main test site are two methods of testing moisture and temperature at the head of the wall and within the core. The range and extent of the laboratory tests have been expanded. The results are confirming what was learned following the pilot work, particularly with regard to the thermal blanket tests. Experiments on water holding and transmission properties of soft capping have yet to be started. Tests are also proposed to measure the rate of moisture transpiration. The drying out rate is likely to be a key feature of the success or otherwise of soft capping. The maintenance regimes and costs are to be evaluated this year and the longer term site monitoring is set to continue. One new laboratory test has been added as a result of difficulties encountered in removing the existing hard cap at Kirkham to make way for one 'model' cap in lime. The masons found it was very difficult to remove the concrete capping. A kango hammer proved to be inadequate for the task, so plugs and feathers were needed. This is a quarrying technique used for splitting stone. On inspection a high proportion of the facing stones were found to be cracked, probably because the normal thermal movement was constrained by the concrete, which was harder than the stone. In the longer term this could well prove to be a significant problem, although it is impossible to
Byland Abbey: installed soft capping with model hard capping being put in alongside for comparison.
			estimate just how long it would be before drastic intervention would be required. What does seem certain is that if this is typical over the whole site, then it will be extremely expensive to rectify in a new hard cap. But it may be possible to install quickly and avoid drastic intervention if soft capping reduces the range of thermal movement. This would not only help to preserve precious fabric, but it could also save English Heritage a great deal of money. Laboratory tests are being carried out to help answer this. So far, positive results are emerging. As with all research projects, progress is being continuously reviewed and this sometimes initiates a wider range of experiments. The Kirkham example is typical. Although soft capping may appear to provide technical answers to the problems posed, they will not be effective in every instance. It is still essential to understand what is causing the masonry to decay. At Byland Abbey for example, stone which remains wet seems to be fairly stable, whereas in drier areas there is quite dramatic deterioration. While this can be explained partly by the use of cement in previous repairs, it is not the only answer. Almost certainly there is salt damage caused by the fluctuating moisture contents. Tests are being carried out on the salts, stone and mortar to establish the reasons. But if soft capping does work effectively and leads to the masonry below being much drier, with stone such as this calcareous sandstone it may prove to hasten the decay rather than slow it. So whenever soft capping is contemplated it is still essential to gain a thorough understanding of what is happening on site and the factors that determine the decay mechanisms.
Hailes Abbey: moisture levels below the capping are monitored.
Chris Wood is senior architect conservator at English Heritage and a member of the IHBC technical subcommittee.

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