The Structural Conservation of Ruins: the structural assessment of masonry walls in ruined structures

Ella Seed: Thank you. Good afternoon and welcome to today's technical Tuesday webinar on the Structural Assessment of Ruined Masonry Walls. The structural engineering team at Historic England will be publishing new guidance on this topic which we hope will be available to download next week. The document has been developed with the support of Cadw and the care panel and focuses on historic methods of construction, mechanisms of decay, material properties, assessment techniques, and structural repair strategies. I'm joined today by Alex Evans, Senior Structural Engineer at Historic England. Alex will provide a brief overview of the guidance. On the 27 October, we'll be hosting a second webinar with David Wiggins. This will focus on the engineering properties of lime mortar and the structural behaviour of historic masonry. David has authored the chapter on the structural behaviour of masonry constructed with lime mortar in the guidance. And as Matt said, please submit any questions using the separate questions box. We will aim to answer as many as possible at the end of the presentation. Thank you and over to you, Alex.

Alex Evans: Thank you very much, Ella. So, let's kick off the Structural Conservation of Ruins. Like Ella said, this is a guidance document that's to be published shortly.

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Alex Evans: So, I'm going to start with an Introduction to Ruins and what we actually mean when we talk about ruins. We're going to cover Deterioration and Failure, Initial Assessment, Surveying and Monitoring, Numerical Assessments, Repair Methods, and we're going to briefly touch on Major Structural Interventions at the end.

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Alex Evans: So, an introduction. So, what do we mean when we talk about a masonry ruin? So what we mean is a building or structure which used to be complete and in use but is no longer in active use, and that's because it has already undergone some extent of deterioration or failure, and so it's no longer complete. So, for the purposes of this guidance document and this presentation, we're going to assume that walls are of solid masonry, typically stone or brick. It doesn't have a roof. It's not a retaining structure and we're not going to cover arches or vaults because that in itself is a very large subject and worthy of a separate guidance document. So, many ruins appear to be untouched or not modified, whereas in fact many have been repaired and subject to interventions several times.

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Alex Evans: So, looking at the typical construction, if we took a brick masonry wall in a ruin, brick was more widely used since the Tudor period and especially the Great Fire of London. So, historic brick work is not always fully bonded even though it may appear to be from the outside. It's commonly used just as a facing to some rubble core or alternatively as a core to an ashlar-faced wall and one shouldn't assume that there are many, if any header stone header bricks. There is a large variation in size and material properties to historic brickwork so you shouldn't assume that the material properties information that is available for new bricks today should be applied to historic brickwork. And then, looking at stone masonry, solid stone walls in ruins are generally two faces of skins encapsulating a rubble core. Stone masonry has been refined over time to incorporate large openings and a typical example of this would be in churches and cathedrals. And there is a huge, huge variation, especially in the UK, in the type and material properties of stonework.

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Alex Evans: So, looking at mortars as well which are an integral part of masonry ruins, mortar is essentially a binder with an aggregate and water and sometimes admixtures. Most mortars up to the early 19th century were lime or earth clay or earth-based. Cementitious mortars did become more widely available, but they do have very different material properties and their use on historic ruins can lead to accelerated decay. Aggregates can be subsoil sand or gravel crushed stone. They were generally whatever was available locally. Additives can be included in mortar. These do change the properties. For example, you can use pozzolans to modify setting times.

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Alex Evans: So, I'm going to touch briefly on Lime Mortars. Like Ella said, there will be a separate, far more in-depth presentation on this, but for the purpose of just giving an overview, there are generally three main categories of lime mortars. We've got the Air Limes, we've got the Hydraulic Limes, and we've got the Gauged Limes. So, very simply, Air Limes are essentially mortars made with a Limestone that is pure and that has no impurities and this sets by reaction with carbon dioxide in the air. It will not set if it's immersed in water and can actually in some cases, you know, in can be stored by immersing it in water to stop it from setting. Hydraulic Limes, also called Natural Hydraulic Limes or NHLs, these are made using Limestones which in their natural state, include clay or other impurities. They react with water and are able to set underwater. These lime mortars are stronger and more durable than Air Limes but very, very critically, especially for historic stonework, they are much less porous. If you're specifying a new mix, you could use a modern pre-bag lime. These are available in NHL 2, NHL3.5, and NHL5. The number represents the value of the cube strength at 28 days. It's important to remember that NHL mortars do continue to cure for much longer than cement mortars and the final strength of the mortar is often far in excess of the stated value. This is why NHL mortars are not always compatible with historic buildings because they are significantly stronger and less porous than the mortars used in the original construction. And then finally we have gauged lime so this would generally be an air lime mixed with the measured quantities of another additives, that additives could be itself hydraulic lime to alter the properties, and this was commonly used in the 20th century for brick construction and for the repair of monuments.

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Alex Evans: So for lime mortar, the main focus is actually on moisture management and protecting the stone or brickwork from deleterious materials. The compressive forces in a ruin are usually much lower than the capacity of the mortar. And the outcome of this wicking behaviour is that soluble salts are gradually transported out from the masonry units through the mortar joints and the salts are the principal agent of decay in masonry buildings in the UK. When in a solution, salts themselves are harmless but when the long-term drying front occurs, they crystallize and damage the fabric. So, the sacrificial weathering of traditional lime mortars is a direct consequence of this wicking behaviour. Thanks to its fine micro structure, airline mortar makes a very effective pus to a wide range of traditional masonry materials. But after decades or centuries of drawing harmful salts and moisture out of the walls, the mortar joints may have worn back to the depth of the joint, much like the tread on a worn tire. This is why periodic repointing is needed. So, repointing replaces that missing structural material and reinstates the full wall profile for carrying the load. And at the same time, it's refreshing to use the tire analogy, the depth of tread for future sacrificial protection of the masonry, and it continues to stave off decay.

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Alex Evans: So, other materials you might find in a ruined masonry structure, bonding timbers. These can be longitudinal timbers built into brick courses. They are found in the late 17th to mid- 19th century walls. One of the purpose was to hold brick work together so you could build above it. You can also find timber lintels over windows and doors and iron or bronze cramps. Sometimes these are set in lead to provide restraint or to fix stone units together. Timber decay and rust jacking of steel or iron elements can cause significant deterioration to the condition of a masonry wall.

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Alex Evans: And then we come to foundation so large parts of a structure, especially on schedule monument sites can be found below ground. Typically for masonry ruins, shallow spread foundations are pretty common. It is common for settlement to have occurred due to the ground consolidation during construction and then rectified in subsequent lifts as construction progressed. So, you very often see joints that are not straight and then built above them joints which were then constructed during the next construction season having been corrected and a horizontal. So, it's important to differentiate between this initial movement which has resolved and any ongoing or progressive movement. You do find timber piles which are used in very poor ground like the photo on the right. These do remain sound while they're fully saturated but then they can decay if the piles dry out or they are subject to cyclical wetting and drying.

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Alex Evans: So, now we have an idea of what we mean when we're looking at a ruin. We can look at the methods of deterioration and failure.

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Alex Evans: So, how does a ruin become a ruin in the first place? So, ruins have already undergone some kind of deterioration or failure. They've lost their basic environmental protection such as a roof or windows and they've lost structural support maybe from losing lintels or supporting walls or floors. So, this could be for a number of reasons. The original causes could be a change in political or religious culture. The obvious one being dissolution of the monasteries which led to the ruination of a huge number of abbeys and priories. Then you could have war, conflict or disaster. So many fortifications were damaged or destroyed during the Civil War and World War II. You could have redundancy. So buildings which ceased to have a viable use and were left to deteriorate slowly over time and this could be because of depopulation or pandemics or social changes or they became too expensive to maintain or they no longer met requirements. And then we have stone robbing, so stone was often taken from redundant structures just to save the labour-intensive job of sourcing new stone buildings.

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Alex Evans: So, after that initial ruination, we have ongoing decay and deterioration. The main one is water ingress, but we also have wind, the effects of vegetation, load path alteration, ground conditions, we do have climate change increasingly, there's previous interventions, and lack of maintenance.

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Alex Evans: So, water ingress is the leading cause of ongoing deterioration. Originally, the structure would have benefited from the protection of a roof, copings, pointing render or line wash, but the loss of these elements allows water to freely enter into the structure resulting in the wash out of fines and binder material. The water ingress also allows soluble salt precipitation to occur. This is particularly noticeable in salt gritting zones at the base of walls especially in areas where the stone is softer. You find you see it quite often in places like Cheshire, Herefordshire and Shropshire. Water can freeze and expand which then leads to the cyclical freeze damage and once you have a source of water in a wall it becomes more attractive for vegetation to establish.

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Alex Evans: Wind can also cause decay to structures. Localized wind effects can be quite damaging. You can have trapped vortices accelerating decay. You can have wind funnelling due to the monument geometry, so if you imagine a castle in a gatehouse to a castle, you've got a lot of wind hitting that face, but it can't escape. It gets funnelled through the openings and it's quite common to see quite a lot of wind damage in gatehouses and opening areas around castle walls. The porous and soft stones are particularly vulnerable. Now, wind damage mainly affects the surface but it can be significant enough to undermine and destabilize some stonework. So, the image you can see on the right here this is a castle in Herefordshire and you can see from that one stone near the top of the image where the original face of the wall was and how much stone has been eroded away because of the effects of wind.

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Alex Evans: And then we have vegetation, so vegetation is very often an indication of water ingress in a monument. You get the seeds establishing a voids and the loose core. Woody plants are particularly damaging. Plants like red valerian or if you get saplings establishing and these roots can jack the stonework and break joints apart. If you've got a lot of top growth, you can have wind rocking and this can displace stonework. And then as the organic matter builds up, you get a, it creates nutrients and compost for further plants to grow and you end up creating this cyclical process.

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Alex Evans: And then we have load path alterations. So, ruins are three-dimensional structures and they need to be able to resist loads in every direction. So, if you have material loss, the stresses can be increased or decreased and the loads be redistributed around the monuments. So, if the support is lost, this can happen if the support is lost either laterally through the loss of maybe floors or roof structures or vertically through the loss of elements like lintels.

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Alex Evans: And then we have ground conditions so the levels of the ground can rise or fall around a ruin. Levels rising could be due to build-up of organic matter or like in the photo on the right, the falling of masonry debris or tree roots which can exert a lateral pressure on walls and also trap moisture and vegetation behind the wall. Falling levels could be due to spoil removal or like the image on the left. This is badger burrowing damage to a masonry ruin. You can have soil erosion or paths being cut or worn into the ground, but then these falling levels because a lot of ruins do have shallow structures; they can undermine the base of the wall. Changes to the water table and moisture content of the ground can also cause soils to shrink or swell, and increased saturation can also weaken soils.

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Alex Evans: And then we have climate change so we're seeing this increasingly we have more frequent storm events. You've got much higher levels of rainfalls within shorter periods of time. This can exacerbate existing defects and increase water ingress and accelerate the erosion making material loss more likely. So, the image on the lower part there that's Chichile College in Northampton showed and that was there was a sudden failure of the masonry skin there after a storm events where a lot of water entered the structure in a very, very short space of time and burst the stone. We also have coastal erosion. So you can see in the top image there with Reculver Towers in Kent that the coastal erosion has progressed around the monument and the extra measures have been taken to try and protect the ruin from coastal erosion.

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Alex Evans: And then we have previous alterations and a really common example of damage to a monument is previous repointing in a mortar that's too hard. And this does include like I said earlier some of the natural hydraulic line mortars. Alterations can also include re-profiling of wall heads to try and manage water runoff. You can end up with ponding in some of these areas if it hasn't been done very well. You can have underpinning and that might lead to differential settlements. There's insurgence of beams or ring beams within a wall and also especially on publicly accessible monuments, alterations will very often have been made to enable access.

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Alex Evans: And then we have lack of maintenance. So it should be expected that ruins will undergo some kind of deterioration even when they're maintained and especially if the stone is soft or friable and this is certainly the case at Sawley Abbey in the image on the right here. So it's a managed site but it is undergoing ongoing decay and deterioration because the stone is so soft. Inadequate maintenance will accelerate any deterioration and exacerbate any issues. Basic maintenance would include things like periodic repointing, rebidding of loose stones, removal of vegetation, but it's worth remembering that if the site is scheduled, then it's quite likely that schedule monument consent may be required for these works.

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Alex Evans: So then we come to the initial assessment. So undertaking a structural assessment, this is often to review the condition of the ruin and to assess any risk of deterioration, loss or failure. Normally carried out by structural engineers. Many reasons it could be done then it might be commissioned as a response to a periodic survey by an architecture surveyor or it may also be done if there's any safety concerns, if there's been any noticeable movement or even partial collapse.

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Alex Evans: So, before undertaking an assessment, it's important to have an appreciation of the conservation principles. So, schedule monuments are of great importance and they are protected under law. They may also be listed and they also may maybe be within an area that's also protected by another designation such as a world heritage site. So, the ultimate aim is to protect and stabilize the monuments. Some deliberate change might be acceptable but the presumption is it will be handed down in the same state or better than it was found. Minimum intervention is the overarching primary aim of good conservation engineering. So that's why it's important to investigate defects thoroughly to understand the cause of deterioration and not just the symptoms that are presenting on the ruin. It's important to use compatible repair materials so any new mortar and stonework should be compatible with the existing materials and also suitable for the current environmental circumstances. So, as we mentioned earlier, these ruins no longer benefit from the environmental protections that they used to have and so in some cases, it might be that the original mortar materials aren't necessarily the ones that are appropriate for the current environmental conditions that the ruin is being exposed to.

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Alex Evans: So, then we've got the appraisal. So we'll initially do a desktop study and this is doing a background check on the ruin essentially and how it may have changed over time. It's really useful to glean any information on previous repairs because these might be affecting the structural behaviour of the ruin. It's useful to see whether cracks or defects have been picked up before if they're ever longstanding and also any changes to the monument whether this is through ground levels, trees or geological conditions. So once you've got that information, you can look at doing an initial visual survey. This is typically done from ground level, but increasingly using drones to look at things at the tops of walls or more difficult to access areas or maybe additional access equipment if you need to look a bit more closely at something. During the initial visual survey, you might take some basic dimensions and a measure of lean if there's a lean. But the main aim is to assess a condition, identify structural issues and assess possible causes of deterioration. At this stage, calculations are not usually required, understanding the structural behaviour at this stage is usually the most important thing.

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Alex Evans: So, after the initial assessment, you might find that further information is needed. So, this can include monitoring for movement, opening up or ground investigations. Some structural analysis may be required at this stage, although again a thorough understanding of structural behaviour is still really important because it's important to remember these are not designed structures. They will unlikely conform to codes. They were not designed to any kind of code or guidance like you would design a structure today. And so skills are required to assess these buildings beyond that of what is used for modern codes.

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Alex Evans: And then you might begin then to consider design and repairs. So, after you've done the assessments and you've maybe undertaken some further investigation work, you might find that the structure is actually sound and its simply undergoing material deterioration. So in this case, less intrusive repairs and consolidation can be applied. So, this might include repointing or vegetation clearance. If, after the assessment, you think the structure is unsound and a structural repair is needed then at this stage, it's important to have early discussions if the site is scheduled with the inspector of ancient monuments to begin to discuss what might be acceptable.

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Alex Evans: So then we come on to going into a bit more depth about surveying and monitoring. So, it's important then that you start by trying to establish what is the monitoring for and is it even necessary, the required accuracy, what is the frequency? You might only want to take measurements or monitor for movement once every 3 months over the period of a year or so or you might need something more immediate. What natural variations might be expected? This will help differentiate between natural seasonal movements and any ongoing progressive movement. Access restrictions. Many locations and monuments are not easy to get at so you should consider whether the monitoring should be in person or does a remote system need to be used and also how critical is any movement? If any movement could be considered dangerous, it might be that you need to put in a system where you can have trigger levels and warning systems.

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Alex Evans: So, there's different techniques for further investigations. You could do close-up visual surveys using rope access, cherry picker or tower scaffold. A crane basket can be used where actually getting close to the monuments or within the fall zone of a monument might be considered too dangerous. So, a crane basket off a telescopic crane can sometimes enable you to get close without having without needing anybody or any equipment to be within the fall zone or even sometimes simply just a ladder is sufficient. So, you would have a measured survey if you need to define a geometry so this is particularly useful on leaning walls if you want to do some calculations on lean or bulging face work. And again, it's useful to look back on archive information to look for any evidence of any historic interventions or defects. GPR or ground penetrating radar can be used. It's bit more of a high-tech technique to determine the location of maybe buried timbers or metal work or voids within a masonry core. Very cost- effective and simple methods to use would be using hand tools so things like plumb bobs or even tapping to identify delamination and if there's any deterioration because of maybe inappropriate materials, material testing might be used.

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Alex Evans: So, typical systems of monitoring, high-precision target monitoring so this is where you can put set targets monitored using a total station. If you've got high-level areas or places that are difficult to access, maybe the targets can be put in once and then any subsequent monitoring can be done without actually having to directly access the location you want to look at. There's 3D laser scanning, maybe a bit more appropriate for something where you've got overall trends you want to look at. If you've got something in a very difficult to access location or something that's difficult to get to, you can use remote tilt meters or crack meters or vibration monitors. So, these can read very small changes and the advantage of these is they can take live readings for remote viewing at frequent intervals. So, if you have something where movement might be any small amount of movement might be critical, you can set trigger levels and alerts. And then you've got Tell tales, crack width gauges, callipers, these are methods of measuring by hand. They are cost-effective and very simple and other methods like photogrammetric comparison or satellite monitoring for you might want to use for things like ground movement.

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Alex Evans: So, let me come on to numerical assessment. So, I'm not going to go into too much depth on this. It's a lot more detail in the guidance document but we'll have a brief overview. So, the need for assessment is the first thing to consider so where you've got a wall that the loading is environmental where the structure hasn't substantially changed for a significant period. It's not leaning certainly not significantly and it's been well maintained. It can usually be deemed that a stability analysis is not critical or not required. But there will be some instances where a stability assessment will help make an informed choice about a ruin. So again, it should be remembered that modern structural design codes, it's not straightforward to apply these to historic ruins. And again, so this is why it's important to get the right expertise so maybe a recommendation from a conservation professional or the easiest thing to do would be to refer to the conservation accredited register for engineers. Inspectors if there's a schedule monument application or a listed building application involved or if it's maybe a heritage at risk site, the historical England structural engineering team might be able to offer some technical advice as well.

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Alex Evans: So in terms of looking at the mechanisms of failure and what would suit a numerical assessment if you've got something like unravelling or maybe face collapse, this is difficult to assess numerically because it's more of a material deterioration problem. The mechanism suited to numerical analysis would be cases where you might have overturning or sliding, bending or bearing or it might be a combination of all of these.

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Alex Evans: So very simply, one of the numerical assessments you might use for a simple wall would be the middle third rule, where you've got a wall of uniform thickness and freestanding. So, if the centre of gravity is within the middle third then the wall is in compression. If it moves outside the middle third, then the tension cracks may appear on the back face as that back face begins to go into tension. And then if the centre of gravity then moves beyond the footprint of the wall, it would become unstable.

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Alex Evans: So, principally the loads acting on the ruin the main one is self-weight. That will be the largest force acting on a ruin masonry wall. Then it does undergo environmental loads. The obvious one being wind. The lack of verticality can then induce eccentricity and eccentric loads into the wall. Variable actions would only be applicable if you had floors, walls or any access onto the monument.

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Alex Evans: So, once we've done all our assessments and considerations, we can move on to repair methods. So, things to consider before you determine on a repair method would be the conservation principles. The overarching one being minimum intervention, the repair should be reversible wherever possible. It should be compatible with the original structure and not cause further damage. Alien materials can be used if it reduces loss or disturbance of historic fabric but it should be applied with caution and a thorough understanding of the compatibility. It's very often that a single solution cannot satisfy all of these points. So, you might have using an alien material which does involve minimum intervention but that is not reversible for example or you might have something using more traditional method but that would result in more loss of fabric. So, it's important to have these conservations with other conservation professionals. It's important and I've put that in capital letters because it is really important to understand the cause of the defect before applying a permanent repair because if you don't do this and you just treat the symptoms without understanding the cause, you could lead to unnecessary work or you could even exacerbate damage and deterioration to the monument. Understanding the significance can affect what repair method is adopted and this is why it's important to collaborate with inspectors or archaeologists and other conservation professionals to help find the most appropriate solution.

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Alex Evans: So typical repairs to common defects. You could have overhanging masonry, so this could possibly be due to deteriorated sport or localized collapse. Some of the methods that are available could be stainless steel bar plate or beams, so these could be hidden or exposed. Hidden repairs do require good recordkeeping so that any work on the monument done in the future, as we've said, can be better assessed if there's a record of any previous works that have been done. Exposed repairs can be more aesthetically intrusive, even though they might involve using more traditional materials.

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Alex Evans: Overhang masonry, it could be due to also deteriorated timber lintels, so you could put back a new timber lintel, but again it comes back to these understanding that the environmental exposure is now different to what the original timber lintels had to tolerate. So, lead trays or lead linings can be used to try and slow the decay. If you've got a really difficult to access location and it's in a more environmentally aggressive location and you've got something that is fairly discreet visually, you could consider the use of concrete lintels, but again, you would have to carefully discuss that with the inspector archaeologists. Corbelling masonry can be an alternative to using an alien material and it's pretty durable but there's a visual impact and it can confuse the readability or archaeological of the structure.

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Alex Evans: And then if you could use pinning or securing loose stones with things like helical ties, wires or threaded bars and these are secured using grout or resin. They're not easily reversible but they do result in a greater retention of original fabric. So again, it's applying those conservation principles and having those discussions to see what's most appropriate.

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Alex Evans: And then you've got bulging or separation of facing masonry. So, this is where you might have separation due to voiding of the core. So, there's a number of different ways you could address this. You could tie the facing stones back using sock anchors or helical bars which are normally installed in a grid at regular sensors. You could grout the core to fill the voids and try and reinstate a connection with the facing masonry but you may still require some stitching or tying back of the stones. And if with the grouting, you may require temporary support so that you don't have any bursting of the face. It's possible to introduce new bonding stones, this would use original and compatible materials, but it's quite an intrusive repair and would result in further damage to the monument. And then if you've got some small areas, sometimes one of the options is to locally dismantle and rebuild.

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Alex Evans: So then we come on to leaning walls. You could jack it back into position if the lean is uniform and it doesn't have any other structures connected to it which would be adversely affected by jacking. If the rotation is due to poor foundations or inadequate bearing, you may need to do this in conjunction with underpinning or some sort of foundation alteration. Another option would be to take down and rebuild the wall but this does result in the loss of archaeological value so you may only want to consider this where maybe architectural form or the silhouette is significant. If you've got return points, you could anchor the wall back if it has parted from those return walls to give it a bit more capacity.

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Alex Evans: And then you could also add structure to allow the wall to more effectively span horizontally. So, this could include the addition of anchors or helical bars across the wall. It may be possible to introduce a ring beam or a head beam at the top which could enable the wall to span vertically. Or you could add buttresses so these can be constructed in masonry, concrete or steel. They do have a very significant visual impact, but they're less intrusive to the original fabric. If you were to in install a buttress, you should just be aware of the buttresses itself settling and the possibility of that and that it doesn't pull the ruin over any further. So, careful consideration has to be given to the foundation and the support of those buttresses.

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Alex Evans: And then we've got crack repair so usually but not always, cracking usually indicates the structure is experiencing a tensile force perpendicular to the eye of the cracking but this is not always the case. And so this is why proper diagnosis is important. If you repair cracks without understanding the root cause, then it can result in failure of the repair, it can also result in new cracking establishing elsewhere at the next weakest point along the wall.

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Alex Evans: So, some options for repairing cracks. You could just repoint them without any further intervention because it may be that the movement is ceased, but if you're not quite sure, you could use this repointing as a baseline for further movement. So, it's a very low intrusive method to close the crack, but then also assess for any further movement should it occur and can be used in itself as a monitoring technique. You could tie the cracks with helical bar. This is also called crack stitching and this re-establishes the stress distribution across the crack but it might be difficult with rubble coursing. These helical ties don't work in in bending. They are designed to work in tension. If you tie you could tie with masonry sock anchors. These are larger bars set in a sleeve with either grout or resin. And these are really for significant cracks or maybe open joints. They have a much higher capacity than stitching and they also have a bending capacity. Or it might be that if it's a very localized area, rebuilding could be an option possibly the area can be locally taken down and rebuilt and then tying stones could be incorporated in the rebuilding.

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Alex Evans: And then we've got temporary repairs and support. So in some cases, you might need some temporary support or repair to provide immediate stability ahead of any permanent works or further investigations. So, this might include scaffolding or shoring, propping or strapping but it's important to understand the legislative framework around the scaffolding or shoring to a monument because you might well still need schedule monument consent to install propping to a scheduled monument.

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Alex Evans: So, repairs should never be carried out in isolation so you should only apply a repair after full consideration has been given to the cause of any deterioration or movement and after consideration's been given to the unique sight specific conditions and significance of the ruin as a whole and also individual elements of that ruin. So, the structure needs to be reviewed holistically possibly including work by surveyor architect or archaeologist and remedial measures to the cause of the defect should also be applied as well as the symptoms. This does commonly include addressing water ingress and vegetation. So, if you were to repair a crack without addressing the root cause of the water ingress, the deterioration is going to just keep happening and you may end up with further deterioration or causing further cracking in a new location.

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Alex Evans: So just touching briefly on major structural interventions so major structural interventions aren't really covered in the guidance but we do mention them just that they are a bespoke a situation that requires a bespoke design following a very, very thorough period of investigation. It will always involve a collaboration with an inspector or planning authority and other professionals such as a conservation architect, QS, ecologist, and archaeologist. Major structural interventions are difficult to define but it could include things like major underpinning works, permanent propping, installation of walkways or bridges, and any other intervention which would constitute a major structural change. So, if you are considering a major structural change to a monument in order to safeguard it, it is also worth considering or looking at whether that major structural intervention may cause more harm than allowing the ruin to go through a managed decline process.

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Alex Evans: So, I think that's given a fairly brief overview of the guidance. So yes, we'll come back now to Ella as well and run through any questions you might have.

Matt: Alex, thank you so much. What a blistering presentation that was, absolutely amazing. And I can see clearly you have a plethora of questions in the Q&A there. I totally anticipate that we will not be able to address all of those questions given the time we have left. That said, we will record the questions in the Q&A and provide written answers to them if we haven't addressed them in the webinar. I should say we will provide written answers which we will publish along with the recording. So that said, over to you Ella.

Ella Seed: That's great. Thanks Matt and thanks Alex that was really brilliant. Yes, we have had quite a few questions. So, I will start with it's quite difficult to find examples of works to scheduled ruins as SMC's aren't made public. Will the guidance include case studies and examples which have been carried out?

Alex Evans: Yes. There will be case studies in the guidance document, there's a collection of case studies at the back.

Ella Seed: Brilliant. Yes. And I think that covers quite a different range of different construction types and interventions so hopefully that would be really useful. We've had a question on what happens if the mortar is less porous than the brick work or stone. I think you did cover this a little bit in.

Alex Evans: Yes. So what happens if it's less porous is that the water then essentially the water will always follow the path of least resistance. So, if your masonry unit whether that's brick or stone is more porous than your mortar then what will happen is you will get the movement of moisture through the masonry unit and the drying face and the salt crystallization will then occur on the face of the masonry unit rather than the mortar and that's when you get that sacrificial erosion of the unit rather than the mortar which is not what we want to achieve.

Ella Seed: Thank you. Yes and obviously we see quite a lot of issues with that with historic sort of repointing from the 20th century. Given the unusually dry summer we've had the ground will have shrunk leading to movement, there any way to help buildings. I think Yes we are going to see quite a lot of cracking really aren't we? That's over the next sort of six months or so that we may have not have seen before. I think you know you covered in a lot of detail all the different types of monitoring and I guess we would just say that we that would be the first port call on that really.

Alex Evans: Yes, and also talking to professionals that have been around a long time because there's nothing new under the sun and we will have gone through I'm especially thinking of the summer of you know I think it was 1976, there's lots of well documented examples of building movement that occurred during previous hot spells. So, I think the thing is not to rush into anything on that score and do the background research, talk to other professionals who may have had experience in previous very, very hot summers. And like I said, monitoring is always a good thing to do.

Ella Seed: Brilliant. Thank you and now on lime mortars, we've had a question. Typically, how often do you repair using airline rather than NHL mortars?

Alex Evans: It does depend on the environmental exposure of your site in a short term. I mean you can if you have a very sheltered site which is not necessarily you know it's not coastal the mortar will last a lot longer than it would do on the seafront somewhere. So, a lot of it is down to its where it is and what it has to put up with.

Ella Seed: That's great. Thank you. We've had one on GPR coming in. Does it need a homogeneous wall or a relatively flat surface? And my understanding is that it works a lot better on a flat surface such a sort of batch stone something like flint for example, which is very rough.

Alex Evans: I think so. Yes.

Ella Seed: And I think on that it's also important to make sure that the person interpreting it is sort of experienced on that really because it can be quite difficult to interpret.

Alex Evans: Yes. Your baseline measurements and your range of readings needs to be carefully set up so that you're getting, yes you're interpreting it correctly. I think a lot does come down to the interpretation and the skills of the person doing it.

Ella Seed: Thank you. More of a sort of wider question I guess is whether or not to intervene on a ruin. Should we allow it to deteriorate or should it always be repaired? I think that's quite a big question, isn't it? But I think it is one that you know we do consider.

Alex Evans: Yes, we do. And I think I've seen another point about you know when a ruin is on a public site, there's always that you've always got to consider that risk element of where as well. Is the deterioration going to involve some sort of sudden failure that's going to put someone the member of the public at risk or is it something if something's maybe on a private site that has no public access where any interventions would outweigh letting it deteriorate. It's one of those I think it's got to be a sight specific consideration and also involving archaeologists, conservation office, you know, inspectors. It's quite it's got to be a very thoroughly discussed yes sight specific bespoke decision I think.

Ella Seed: Yes, absolutely. Thank you and then one more perhaps. Has the approach to soft capping exposed warheads changed? Is rough racking a suitable alternative? I think both are valid and have their place in different locations and.

Alex Evans: Yes, and its maintenance as well. Yes, and again comes down to environmental exposure. How easy it is to maintain. I've seen some sites where they've put soft capping in and but they've unfortunately they've also kept goats and the goats have eaten the soft capping. So, yes it's again it's one of those things where they're both viable options and it's having that discussion and investigation works to consider the options the pros and cons of each option and yes coming to that informed decision I think on a sight specific basis.

Ella Seed: Brilliant. Thanks Alex. I think that's probably all we have time for today. So, yes, thank you very much for joining and for all your questions. We hope you can join us again on the 27th.