The Reburial of Archaeological Sites

Jim: So, thank you very much, Matt. Welcome to today's session. As Matt says, I'm Jim Williams, Historic England Senior Science Advisor, and I've been involved in work on preserving archaeological remains and construction impact for over 20 years, and have coauthored guidance on piling in archaeology and preserving archaeological remains. And I've spent about 13 years working closely with HS2, which explains why quite a few of the case studies I'll be talking about come from that project. So, I would just like to thank everyone for coming along today to hear about this. If you've come to a regional meeting and heard me talk about this, it may be very familiar, and I won't mind if you then slip away realising that you've come to watch something twice. Maybe you're just really excited by what you just...

I'm going to turn the camera off for the presentations, and I'll put that back on again once we get on to the questions session. Right. So, today I'm going to be talking about reburial based on the updated guidance that we published in 2024. And that document was part of a suite of guidance documents on preserving archeological remains that we published in 2016. The guidance on reburial was coauthored with John Stuart, who was an architectural conservator working for Historic England until he retired recently, and Matt Canty, our geo-archeologist. So, the outline of today's session, I'm going to start with some definitions, talk a bit about sand and terram, introduce our reburial checklist, cover reburial objectives and our three assessments, talk about the fundamental criteria of reburial, look at materials, then cover reburial design, stakeholders, monitoring, maintenance, and documentation, and finally end with case studies.

But I want to start with some definitions, by looking at the difference between the terms reburial and backfilling. When we started writing the guidance, we couldn't find any clear definitions of either term. So, we've written some ourselves. We've defined reburial as the purposeful placement of excavated soil or other appropriate material into an archeological excavation area to protect remaining archeological deposits, finds, or structures, and to ensure their future survival. By contrast, backfilling is about putting material back into the trenches or an area once excavation has been completed, subject to a WSI and signed off by relevant authority. Hand or machine is appropriate, and there's no specific methodology involved. If you find something interesting or significant, for example, in the evaluation, and you decide to leave it there undisturbed, or perhaps partly excavated, and come back to it at a later mitigation stage, then that's probably a form of temporary reburial rather than backfilling. And we'll cover that later on.

So, in the guidance, we've defined three situations where reburial might take place. The first of these is opportunistic preservation. For example, where unexpected remains turn up in strip, map, and record, or mitigation excavation, which are of high significance, evidential, or esthetic value, and you want to incorporate them into a development. The second example is perhaps planned mitigation preservation. This might occur when you have a site that you already know is there, but you need to uncover it or revisit it as part of mitigation works, and then it's reburied. And then, finally, we've identified research excavations, those repeated visits to the same site by universities, local societies, or grassroots archaeological organisations over a number of years. Those excavating these sites might not think that what they're undertaking is any reburial, but given the level of surviving archaeology usually left behind, and the fact that temporary reburial or covering over parts of the site between seasons can and does happen, it can be useful to think about what goes on at these research excavations as reburial.

Now, this webinar and the guidance that it's based on is not about me telling you what the right reburial materials are for particular sites and situations, and instead it's about a process and a range of assessments that need to be carried out to identify what the right reburial materials are for particular sites and situations. And I'm going to talk you through those in a bit. But before doing that, I just wanted to talk a little bit about sand and terram. Using these materials has become almost a default reburial methodology, used ubiquitously regardless of the situation. But how did we get to that point, and why? I think the history of reburial begins in London with the Rose Theatre, which itself was one of the key triggers for the start of planning guidance on archaeology. As a site in London, it was constrained for space.

Therefore, much of the excavated material had been taken away and couldn't be used for reburial and protection of the site, so imported materials need to be brought in and to be as inert as possible. And sand was chosen for the job. And similar approaches were taken at other sites, such as the Globe Theatre, and before long, an approach taken for one site becomes a standard for all sites going forward, even when existing material from the excavation is available. An additional reason for terram's popularity is because people want to use something during temporary reburial to make next season's excavation easier to start. And as I'll explain later on, it should only be used with certain caveats in place. Finally, there's a sense that putting down a marker layer is useful so that future archaeologists know where previous excavation is reached, which I find fairly surprising given that archaeologists usually managed to locate evidence of earlier excavations on their sites, even if they haven't been backfilled with sand, terram, or plastic.

I'd argue that our properly recorded and archive site data should usually be sufficient to inform future archaeologists about the depth of previous excavations. Equally, there are probably better ways of protecting your site from future accidental damage than burying huge quantities of materials derived from fossil fuels whose long-term stability we don't really understand. Do you want to be the one who contaminates a future archaeologist's site by leaving behind something that breaks down into thousands of pieces of microplastic? So, having disparaged the use of sand and terram, I'm not going to tell you what reburial materials to use just yet, otherwise you might all leave now. The next thing I want to talk about is the reburial checklist, which takes you through all of the key questions you need to ask in producing a reburial strategy.

It can be used by those writing, or those reviewing and assuring these documents. It's in the guidance document. And it's also available as a separate downloadable template. And hopefully Matt can put a link to those in the chat for you now. The idea is that those creating a reburial design would complete these boxes here, taking out the questions and putting in their own answers, and then put a tick in the final column when that part was completed. The checklist starts with reburial objectives. Defining reburial objectives is a key part of any scheme. They help to determine the design and the materials. The primary objective is likely to be to create an environment as close as possible, or better, than that which existed prior to excavation. Programmatic considerations are also likely to feature within the objectives.

For example, is it going to be a short or long-term reburial, protecting from temporary or permanent threats or changes to land use? It could be a reburial aimed at improving the existing burial environment. The checklist then needs to cover three assessments for reburial, that is, the need to assess the significance, to understand what's being reburied and why it's important. Looking at the nature of significance, what aspects are important and why, the extent and the level of significance. You also need to look at the condition. This is an important aspect of the guidance, and a key part of making decisions about reburial that's previously not really been carried out very effectively, and one of the reasons why we decided to update this guidance. So it's important, when undertaking a reburial design, to look at the state of preservation of archaeological and environmental remains, to look at and undertake a condition assessment of architectural features if they're present, for example, walls, floors, mosaics, and to look and investigate whether or not they need stabilisation or consolidation.

The final assessment is one of threats and risks. For example, are there any environmental threats to water, vegetation, livestock, burrowing animals, erosion, or human threats, from theft, vandalism, accidental damage from subsequent use? For example, if there's any agricultural activities, construction, or post-construction maintenance activities that might take place, after reburial. Or is there a risk for financial threats, for example, a lack of funding that might impact the reburial program? So, a full list of these threats and risks is given in the threat and risk assessment table, which is also in the guidance. And there's a copy of that available to download and to fill in as well. Having considered your three assessments, you're now able to start producing your reburial design.

The starting point for producing your design is to work through three sets of criteria that, collectively, we've called the fundamentals of reburial in the guidance. The information from these criteria is then combined with your reburial objectives to help inform the final reburial design. The three fundamental criteria are environmental, functional, and programmatic. Not all will be relevant to your site, but they should all be considered as you set out your objectives and design. And I think the important thing to emphasise with this checklist and with the guidance is that very much of it is scalable. So, if you've got a small sample site, it'll take you just a quick run through of these. Think about what some of the questions are, and what some of the issues are. If you've got a big, complex site with multiple different types of information, then obviously you're going to need to take a lot longer.

But I think the point is that you should be able to use this guidance almost regardless of the size, scale or complexity of the site you're trying to rebury. So, let's think about environmental criteria. It's important that you have good water movement. Reburial material should not significantly change the way water moves through the archaeological layers it features. There needs to be intimate contact between the fill material and the buried feature. This helps to promote water movement. We need to ensure there are no voids in the reburial fill. There needs to be chemical and physical compatibility between the film material and the buried feature, and the fill needs to provide good thermal protection to protect against environmental impacts, ranging from freezing to excess thermal gain.

The reburial design also needs to consider burrowing animals, managing vegetation growth, and reducing erosion so your reburial materials don't get washed away. In terms of functional criteria, does the reburial design need to facilitate any subsequent re-excavation, or to protect the site from theft, vandalism, or accidental damage? Wherever feasible, reburial designs should require minimum maintenance so they're less likely to fail. And finally, after excavation and reburial, many sites may continue to be used for farming, as open spaces, or be integrated into development sites, so they need to have capacity to allow some kind of activity to take place over the buried feature. And then, programmatic criteria. Is your reburial going to be something that's happening just seasonally between excavation seasons on those types of archaeology?

Or is it short or medium-term, temporary reburial, perhaps while planning a longer-term reburial, or is it a long-term reburial for good? There is a strong potential for short-term reburial solutions to become long-term conservation problems. So, it's good practice for any reburial to fully consider all relevant reburial criteria, particularly when materials other than the excavated soil is being used. Where a short-term or seasonal reburial has taken place using inappropriate materials, but has become, by default a permanent reburial, the site may need to be revisited, and the reburial materials updated. To get us thinking about designing a reburial strategy, I'm going to share a couple of quick case studies to show how different aspects of the fundamentals of reburial and reburial objectives influence reburial design.

So, the first of these is a site called Culverwell, where the design needed to provide thermal protection and not exacerbate long-running animal management issues. It's a Mesolithic site, an important building, Dorset, dating from around 8000 BP. The site was discovered in 1966, and volunteers began excavating in 1967 and continued over the years, the final excavation taking place in 1996. After this, most of the site was backfilled, except for one area which was left exposed and protected by timber shelter so the public could see it. The site was added to the Heritage at Risk Register in 2008 because the exposed part of the site was degrading from animal burrowing and erosion, and the cover structure was ageing and in need of repair or replacement. In the end, the decision was taken to rebury the site.

So, from a programmatic perspective, this is a long-term permanent reburial. In terms of the environmental criteria, the reburial needed to provide sufficient thermal protection from freeze-thaw, and it was also important to manage the risk of burrowing animals by not making the reburial material too deep, and in fact creating a modern-day pillar mound. And it was also critical not to create a new mound in the landscape with existing prehistoric earthworks. The second example is of a temporary reburial of remains identified during evaluation in London. The evaluation of this site, known to be over the Roman Forum, identified surviving Roman deposits and structures. The evaluation trenches, which were dug inside an existing building, needed to be filled in...

Sorry, quick sip of water. Need to be filled in once the work was complete, but the archaeologists will be coming back to do a full excavation in this area once the building has been demolished and the perimeter piling installed. So, this is a good example of a site with a need to protect temporarily between the evaluation and mitigation of construction works in the presence of remains of high significance. It is planned as a temporary reburial, but it's also important to consider what guarantees you've got that it will be built as planned in 2 to 3 years’ time. What if the funding falls through, or the developer goes bust? So, there is a need to balance having the convenience of re-excavation with some kind of separator layer, with the risk of introducing inappropriate materials into the ground in the long term. Coming back to that point I made about short-term solutions becoming long-term problems.

Sorry, I just need a quick cough. Okay. The next thing I want to cover is reburial materials, as the choice of fill materials for any reburial scheme is critical. As we know from our environmental criteria, reburial materials need to be inert or chemically compatible with the buried feature, so have a similar pH. It's also important that they're compatible and provide good continuous physical contact with the buried feature. It's also likely that for most sites, fill materials will need to provide good capillarity. That's the free movement of ascending and descending moisture. Soil from the excavation is usually the ideal fill material. That is the one key message in the guidance, and a change from current practice. It's not really a change from the previous version of this guidance, but it is, I think, a much clearer statement of that.

Nothing else can be specified which so effectively minimises new impacts on surviving archaeological remains. Some soil management may be needed to ensure that it's usable, for example, sieving to remove large or sharp stones. The use of sandbags to support features should be avoided, as it's hard to place these without creating voids, and difficult, therefore, to ensure the backfill is properly compacted. Soil for reburial needs to be free from upper organic humus-rich topsoil material. Where it is possible, and as soon as it's proposed that site remains may be reburied, soil from features being excavated should be separated and stored for later reuse. Covering the soil will help it retain moisture and reduce plant growth. There are some sites where space constraints have traditionally led to soil being removed from site and deposited elsewhere.

For these sites, retaining that soil for subsequent reburial on site, or even off site, will represent an additional challenge. However, this needs to be balanced with a subsequent cost and effort required to source an appropriate replacement material for reburial that can satisfy the criteria within this document. If there isn't enough existing excavated material for reburial, for example, it's been taken away, or if it's contaminated, or if fill materials must meet specific geotechnical requirements, other reburial materials will need to be used. The properties of any material used need to be assessed. Most material suppliers will have datasheets for their products, which should be consulted for precise details. The location within the reburial scheme where the film materials are used also needs to be considered.

Will the material be used in the primary fill in direct contact with the archaeological remains, or is it providing secondary fill further up the sediment sequence? The mechanical properties of sand make it suitable for reburial. Frequently, it's the cheapest readily available material in most locations. However, poorly graded sand or sand used in too great depths can impact on liquid water capillarity. Not all sands are suitable for reburial projects. They need to be high-silica, low-iron sands, free from soluble salts and organic materials that are chemically inert. Ideally, they should have a consistent sand size, particle size. It is beneficial to source the sand from as close a location as possible, as transport costs are a large part of any bill. Moving on to geosynthetics. This is a term used for a wide range of fabric and sheet geosynthetic materials such as geo textiles, geo grids, and geo membranes that typically end up just being called terram.

Geo textiles are the most common. Usually they are permeable meshes or fabrics made from polypropylene or polyester, which can last for centuries in the ground. They're designed to separate, protect, and reinforce layers, filter materials, or assist drainage in many geotechnical or construction situations, the numerous types available have a range of strengths and permeabilities. As I mentioned earlier, we've got to the point where there is almost a default usage of geosynthetics for reburial projects. Usually they're not required and can cause damage. They should never be placed in direct contact with archaeological or architectural features, as this can promote root growth beneath the geotextile, or cause mineral precipitation that adheres the geotextile to the feature. If unsuitable materials are used, they can become a barrier to the movement of water.

They are all derived from fossil fuels and have an environmental and financial cost if left buried in the ground. So, if used, their installation needs to be justified and based on a full understanding of their material properties. Manufacturers' technical data sheets should be consulted in advance of specification and purchase, and it's particularly important that they provide good permeability. So, this slide here shows a list of some examples where they could be used, but I'm not going to cover that in much more detail today. That information is available in the guidance. So, just to recap, so far I talked about reburial objectives, three assessments, three fundamental criteria, and also talked a little bit about reburial materials. So, I just want to highlight a few more key points from the guidance as I draw to a conclusion.

The first is around stabilisation. Where condition assessment has identified the need for structures or surfaces to be stabilised prior to reburial, it's important that all mortar repairs need to be specified and applied by an accredited conservation specialist. Any plant growth may need to be removed prior to reburial. Where plants have rooted into a structure, careful removal and further stabilisation may be required. From the significance assessment, you might have concluded that the significance of the site is high or very high. This tends to make people think, therefore, that the site needs some sort of special treatment or materials. And mosaics, I'm afraid, are a classic example of this phenomenon. Mosaics are relatively uncommon on archaeological sites in England, and they are often of high significance.

And this makes people want to do different things to them because they're special, like add imported materials and geotextiles. And I know I have to be careful with what I say here, because I can see we've got a couple of participants from sites with mosaics. However, unless there are clear conservation requirements for additional materials to be used, reburying mosaics using clean soil from the excavation is the best option. Sieving the soil, if necessary, to remove any large stones or rocks for the immediate area in contact with the mosaic is sufficient special treatment. When dealing with the reburial of mosaics or wall plaster, a conservator with relevant experience in these should be involved in planning the work during the excavation and when carrying out the condition assessment to advise on the best approach to reburial.

In some instances during development and construction work, there may be proposals to build something over what you're reburying. The basic procedures and process to follow for preservation decision-making and understanding piling impacts are set out in the Preserving Archaeological Remains Guidance and Piling in Archaeology Guidance. Sometimes the load from what's being built is transferred to the ground through piles or ground beams. In other instances, the load is directly on the ground. For example, with embankments or areas of soil storage. In these cases, the best way to understand potential risks and impacts is to ask a developer's engineer to calculate the load and potential ground settlement based on the design and geotechnical information. This assessment should identify the amount of settlement and the depth of which the settlement will occur.

The level of harm to buried or reburied archaeological remains will be governed by the nature of the remains and the surrounding deposits. Where the archaeological remains robust and the predicted settlement is limited, the future understanding of the site is unlikely to be compromised. However, where the site contains fragile remains and the predicted settlement is large, reburial may not be possible. Another option, such as excavation or redesign or the development may be required. And there are two case studies looking at embankments constructed over archaeological remains in the guidance, and I'm going to talk about one of them later on. Three final considerations as set out in the checklist around stakeholders, maintenance, and monitoring, and documentation and archiving.

There's a section on stakeholders, and this is really important, because relevant stakeholders need to be involved in the development of the reburial design. The landowner or site manager is a critical stakeholder because the long-term management of the site is ultimately going to be their responsibility. Most sites will require some form of maintenance and monitoring after reburial, even if that's just grazing of grass or an occasional visual inspection. The aim of monitoring is to ensure that film materials are continuing to provide adequate protection to the reburied remains, and the site is not subject to any harmful erosion, vegetation growth, or unacceptable human activity. Maintenance and monitoring require staffing and funding. The duration needs to be discussed and agreed with relevant stakeholders.

Maintenance and monitoring proposals should be included within the project documentation, and where necessary, secured through management agreements. Undoubtedly, the simpler the maintenance and monitoring regime, the greater the chance of success, and it's likely that stakeholders are either going to be responsible for this maintenance and monitoring, paying someone to do it, or providing access. So again, their buy-in is essential. And regardless of the scale and type of archaeological site or how it's funded, whether it's a commercial development or community research excavation, a reburial design and detailed specification documents should be produced. They should be kept somewhere so they can be referred to in the future. Recommendations are given in the guidance of where they should be archived, included in the HER, given to the landowner or site manager, and included within the site archive.

So, two main things I think it would be great if you all took away from today, if nothing else, and that is that the key to designing a good reburial strategy is about following a well-defined process as set out in the guidance and the checklist, and that as a reminder that in most cases, importing fill or other reburial materials like geotextiles or marker layers isn't necessary, and the existing material is usually the best solution. So, I'm going to stop there and just pause to give the chance to answer any questions. And then, when we've done that, I'll move on and talk about a couple of case studies. Okay. Thank you.

Matt: Thank you, Jim. We have one question at the moment. But for all our attendees, please feel free to type in any questions you have at the moment. So, Jim, the one question- maybe this is going to be a very quick Q&A. Do you have any examples using non-marine clay as a reburial material? Or maybe you could talk about that in any shape or form?

Jim: I don't have very many... I don't have any examples of using non-marine clay, or even marine clay as a reburial material. Obviously, you will have sites that are excavated within clay. And therefore those sites, one would expect the first option for reburial materials would be to choose the clay that has been excavated. Clay can be harder to place as a material than the other more free-running materials. So, I think in part it would require a little bit of time just to work through what some of those considerations might be. And it might be that you aim to use some clay within the immediate vicinity of the material you're reburying, or the features you're reburying, and use other material from the site for, for sort of filling in further up a sequence or something.

Matt: Okay. Yeah. I can imagine clay being a problematic material to work with. Another question just popped in here. Most of your examples focus on the inorganic structures. How do these recommendations address oxygen and microorganism-related damage for both inorganic and organic features?

Jim: Okay. Well, I think the majority of cases that we traditionally have encountered where we're talking about reburial are situations where someone finds substantial architectural remains on an archaeological site, and there's then an interest and a desire from, a variety of different directions to rebury them rather than to excavate and to lose them during the development. So, undoubtedly, the focus is around inorganic materials and structures. That said, most excavations on waterlogged and organic sites would require quite rapid backfilling or reburial, because you don't want it to dry out, and you don't want to encourage the introduction of oxygen for any longer than you possibly can. Not least because that also will lead to loss of soil carbon as well, which is something that's covered in a forthcoming webinar on soil carbon. I'm sure we can put some more details about that in the chat as well.

Matt: Fantastic. We're getting a few more questions in now. I'll read this one out for you. On a site I have, the excavation has been open, although covered with hesslan for some time. The excavated materials included concrete, which would, in my view, be broadly inappropriate. I'm assuming as a refill material. Would material from elsewhere on site as part of recent drainage works, be better, or inert well-graded sand?

Jim: I mean, this is... Without wanting to dodge the question, this is the point of working through the checklist and identifying the materials, identifying what your site is, and what the state of preservation of the various parts of the site are through your condition assessment. But assuming that the material from elsewhere on site is suitable, then it may be certainly better financially and from a climate perspective than bringing in inert sand from elsewhere on site.

Matt: Yeah. So, have there been successful cases where archaeology has been reburied but still allowed for public interpretation?

Jim: I mean, that largely depends on your definition of public interpretation. The other thing people sometimes like to do when they identify unexpected structural remains within an archaeological site is then expose them so that the public can see them, and allow them to deteriorate over the longer term, and then have to think about how they're going to deal with them. But oftentimes, then the public interpretation is, if you've reburied the site entirely, it's a question of how do you mark the presence of that site, or explain it with an information board? So, there's a Roman site up near Scarborough that was excavated five years ago, and the outline of the Roman buildings was marked out in the surface treatment of the playground which was constructed over the area, because they sort of moved the development around, took the houses out of that area, created an open space, and interpreted it that way.

Matt: Sounds like a really interesting project. Have you considered the inclusion of measures to deter burrowing animals? Are we still looking at layers of wire as an appropriate measure within preservation in situ?

Jim: I think that's a case-by-case question. What other ways are there of managing animals in the area? To what extent does the fencing take place around the site, rather than trying to bury metal within the site? Burying large quantities of metal that you know are going to corrode isn't probably going to be an appropriate measure most of the time, I would have thought.

Matt: Yeah, for sure. There is one other question, but I'm mindful of time. So, we'll go back into your case.

Jim: I can take that one and then do the case studies.

Matt: Yeah, fine. Okay.

Jim: Not that one.

Matt: Not that one. Okay. Are there case studies where there has been a trial of different materials matching pH, or whether panning has then been an issue? So, a consideration of the next use of a reburied site.

Jim: No.

Matt: Well, that's a very quick and concise answer.

Jim: Sorry. Apologies. No, I can't think of any sites where we've actively thought about trying different materials. It's normally been a case of trying to work that out during the design phase by assessing what you've got and what would be the most appropriate material to use.

Matt: Okay. Right. Let's crack on with the case studies, and there'll be another Q&A at the end of that if we have enough time. So, back to the presentation.

Jim: Thank you. All right. Just move these slides on. Okay. So, there are case studies on the Preserving Archaeological Remains Guidance page, where you can look at all of the guidance on preserving archaeological remains. The main document and its five appendices. And on the left-hand side, you can see four case studies. And what I thought I'd do is just talk through how that works. So, this is Culverwell, just using that as an example. And for each of the case studies, we have a site summary that just gives you a little bit of background. And then, we've broken down the case study using the same headings that were there within the guidance and within the checklist. So, just as a good reminder of the stages that one needs to go through looking at reburied objective significance, condition, and risk assessment, looking at the reburial design, and then the importance of stakeholder consultation, maintenance, monitoring, and documentation.

Of those, Culverwell is probably a good example of planned mitigation, preservation, thinking back to the types of reburial, and Harvil Road, Curzon Street, and Hunt's Green Farm are all examples of opportunistic preservation. And at both Harvil Road and Hunt's Green Farm, engineers considered the loading of embankments and the impact that might have on the material below. So, Curzon Street Roundhouse is a steam locomotive shed built in 1837-1838 for the London and Birmingham Railway to the design of railway engineer Robert Stephenson, and was the first built in the world to the roundhouse design. Its location was known from historic maps, but since it has been built there have been many further iterations of railway buildings right through to the 20th century, when prior to excavation, it was part of a surface car park.

Given this complex history, the level of survival was unexpected as a much higher level of truncation or loss was expected. This overhead view shot shows the proximity of the current train tracks to the site as well. This image is shot slightly from the north and obliquely, so it's quite strange if you then orient it the right way up with North at the top. But I kind of need to do that for you, so it makes sense with the rest of the maps. So, in the centre of this image, hopefully you can see my mouse. There's a foundation for the roundhouse turntable, which would have been used to move trains from the tracks that came in either side of this sort of triangular building here, and then be able to put the trains in any of these remaining bays around the edge of the shed over these rectangles, where they could then undergo maintenance and cleaning over these brick inspection pits.

And these multiple walls that you see over to the right are later additions to the train shed to accommodate longer trains. And I've got three close-up shots of some of those elements. So, the first of these, looking north, shows the turntable foundation in an early and quite friable concrete. And there's a much later drain, unfortunately, truncating that to the south side. And it also shows one of the inspection pits, which would have been deeper than the rest of the brickwork, because it had obviously been demolished at some point in the past. The second shot looking north east to give a bit more detail of that turntable material. And then finally, the turntable looking southeast, and a close-up of one of the inspection pits. And hopefully you can see some of the drainage going from the inspection pit into the drainage network more widely.

The next slide shows one of the original architectural designs for the building, which clearly shows some of that drainage network, and also the building to the left. You also see an elevation drawing of the building. And you can see there's a water tank sitting above that, and a basement area. And this plan just showing some of those elements in more detail with some of the excavated material. So, it's a pretty complex site. And just to show a plan of the roundhouse in the context of the railway terminus building over here to the left, and the Grand Junction Railway in this red outline squeezed in at the top of the site. This is the site where the new HS2 terminus in Birmingham is being built, which you can see in the image here, and over here on the bottom left is the roundhouse.

Unfortunately, the design of the viaduct in this location misses most of the roundhouse. And I've tried to superimpose a rough outline of the area here. So, eventually it will be within an environmental mitigation area. But as you can see in this image, in the top left, part of the ancillary building was going to be impacted by the location of one of the pile caps, this red blob in the top left here, for the viaduct structure, and it's the Y-shaped structure at the bottom of the image. There are also some much lighter brick elements, which also need to be recorded and reduced in height before reburial could start. But after much discussion, it was agreed that it was possible to rebury the rest of the roundhouse to preserve it. A landscape design, the final landscape design shows this area for proposed environmental mitigation.

And it's hoped that they'll pick out the shape of the roundhouse in some form of interpretation. Just picking up on that question earlier on. But prior to that, the space is going to be needed for temporary storage of construction materials. So, in order to inform the reburial design, HS2 commissioned a condition assessment to understand the state of preservation of the roundhouse remains. And you can see some of the challenges there in these images. The condition was variable, with areas of damage and failure of the brick culverts, and some deep shafts as part of the drainage network, and also a quite marked height difference across the site, and also highlighted the very fragile condition of the concrete turntable. After the condition assessment, it was agreed that the fragile areas, mostly the brick vaulted culverts and the turntable base, would need to be protected.

Concrete blocks were placed either side of the fragile features, with another reinforced slab placed over the top. They spread the load away from the fragile areas. The areas within the blocks were filled with uncompacted sand, and sand was also placed over the whole reburial site to thickness of about 150 mil above the top of the upstanding roundhouse features. The case study on the website outlines all of the other considerations, so please do go and have a look at that there. But as with any site where a reburial strategy was devised before we'd finished writing the guidance, there are some bits of the process that we probably did in a different order, or slightly different overall to the way we're recommending in this document. And in part, actually having the opportunity to work on quite a few sites within HS2 and elsewhere while this document was being authored provided a good test ground for ideas and procedures, as well as highlighting areas where draft messages in the guidance could be refined.

Okay. The second case study I want to discuss is another of those HS2 sites and another site that's featured on the website. So you can also look there for more details about this one. The site of Hunt's Green Farm contains evidence of an Iron Age boundary ditch called Grimm's Ditch, which itself is part of a larger monument that continues outside the site area shown on the map. It also contains evidence for Romano-British settlement, including industrial activities in the form of corn driers and iron working. The ditch shown in blue in this image is scheduled, and it's an upstanding earthwork of a ditch and bank, and I'll show you some images of that in a second. In the north of the plan, you can see the ditch here as a dark grey line. This part of the ditch wasn't known and was only identified during trowel trenching and subsequent topsoil stripping.

This green area here shows the reburial area, in this case, archaeological remains being reburied beneath an earthwork embankment. These images show the scheduled section of the ditch, which was the only scheduled monument on the whole route of HS2 to be physically impacted by the scheme. The shots here show the ditch during evaluation trenching, and also during final excavation. On the left, looking south across the ditch from the top of the bank, with the ditch continuing down to the right of the picture. The bottom right image is looking down the length of the ditch prior to its full excavation. After a substantial number of trees that had grown on it for at least 150 years had been felled. And the image on the top right is during excavation, when the full depth of the ditch had been exposed and the samples were taken.

It's not quite as neat as those Victorian images of deep archaeological excavations, as it had to be stepped for health and safety reasons. The Victorians, it seems, didn't have temporary works managers. There are other scheduled and designated sections of Grimm's Ditch located throughout Buckinghamshire, and it's generally understood to be an Iron Age boundary of some kind, perhaps some sort of territorial demarcation. However, its date and function have been much discussed and remain still largely undefined. OSL dates during HS2 excavations have indicated that a late Iron Age construction date is probable. The top right photo in this image here shows a first or second century AD Roman British infilling of the ditch, suggesting that by this point it was no longer recognised and respected as a boundary.

So, it's quite a short-lived monument in that regard, certainly, in this part of the landscape. I'll just overlay the earthwork again here because it helps to explain the archaeological approach taken on this site. The area at the North, C21051, was excavated as it was going to be impacted by temporary works, including a cycle road. The section of C21054 that you can see to the left of the green embankment was excavated because that area is part of the cutting for the railway. The rest of C21054 was left unstripped with the expectation that it would be possible to preserve it under the embankment. The proposal was also to preserve area C21052 under the embankment, but prior to doing that the topsoil was stripped, visible archaeological remains were mapped, and a limited number of additional sections dug through the ditch to help improve the understanding of this monument, and to better contextualize finds from either side of this section.

The significance of the monument was thought to be high, warranting its ongoing physical preservation. The decision to preserve the ditch and the area of Romano-British settlement and industry beneath the embankment, rather than to excavate these bits, was also in line with HS2's Heritage Memorandum, which states that the nominated undertaker, that's HS2, so far as is reasonably practical, will seek to reduce harm to the historic environment. The condition assessment for the site was very simple to undertake because the monument and the ditch was very finds-poor, and even in the areas of Romano-British settlement under the most robust find types were present, with little bone survival or other fragile material recovered during the evaluation or partial excavation of C21051.

In terms of the threats and risks, the main risks to the archaeological deposits were considered to be damage from construction and/or construction methodology, and excessive settlement due to the load imposed by the placement of fill material for the proposed earthworks. HS2 specialist supply chain undertook an assessment of settlement that would occur under the embankment. This demonstrated that with the right embankment design, the ground containing archaeological deposits, i.e. the upper two meters, was anticipated to experience a small amount of compression limited to 10 millimetres. The overall settlement caused by the earthworks was predicted to be 35mm over a depth of about six meters. Given the robustness of the material and the fact that a limited amount of settlement wouldn't affect the remains or the ability to understand them in the future, it was felt that reburial would be an appropriate option.

A protective starter layer of geo grid and large stone aggregate was designed to spread the subsequent embankment load and to reduce the risk of differential settlement between areas with archeological deposits and the surrounding natural. Construction control measures were designed to prevent damage from construction plant during the placement and compaction of earthwork materials, meaning that all materials had to be placed in front of the machines that then worked on to that area, rather than working directly on top of archeology. The images here show the overall design of the embankment at the top, and the specific design for embankment over areas of archeological remains. The geo grid helps to spread the load and lock the starter material into place.

A woven geotextile with appropriate breathability was placed above the starter layer for geotechnical purposes, to stop the fine grained earthwork fill material from washing down into the more coarse-grained starter layer and destabilizing the management. And that's the end of those two case studies. And I'm happy to take any further questions, either on those, or about the rest of the presentation.

Matt: Jim, thank you so much. We have had a few more questions coming in. I'll actually start with the one at the bottom, because it relates to Hunt's Green Farm.

Jim: Yeah.

Matt: So, what's the geo grid in this context?

Jim: So, geo grid is another type of geosynthetic. It looks a little bit like a wire mesh fence. Maybe the sort they used to have around tennis courts or school sports pitches or whatever. So, kind of that shape, sort of squares interlocking. And it's a more rigid form of geotextile. It has a slight load spreading function. And in this instance, the idea was to spread the load of the stone material that went on top. And also, because of its shape, it would help to lock that stone material in. And so, it's quite a heavily engineered reburial design, because it's also important that the four-metre high embankment doesn't fall down into the railway cuttings. So, there's a bit more- probably a little bit more engineering on this particular earthwork than you would normally expect for, say, a storage stack over archaeology.

Matt: Excellent. Someone's posted here. I have recently attended a talk about Boles Barrow, where they trialed specialized heavy-duty mesh to deter badgers, which seemed effective and survived well. Could this be an option elsewhere?

Jim: I don't know. There's- I'm not sure about that one. It's not something I'd be able to comment on how effective it would survive in the long term, or the effectiveness, really, for anything else on that. So, sorry, I can't add any more to that.

Matt: There you go. Is there any value in reexamining older, long excavated sites of high significance that have been reburied, but with materials that we now know are not the most conductive... I'm assuming that's conducive. To the long-term preservation of the archaeological remains, to remove that material to something more appropriate? Ignoring the obvious issue of funding.

Jim: Well, yes. Let's ignore the obvious issue of funding there, and obviously that would be excellent. But it's really just... I mean, it'd be useful to know if people have any specific sites in mind, and that they can think of ones where it would be useful to look at those. I don't think we're particularly keen to do research to see how effective materials that we wouldn't recommend have been, because I think we have a good idea about what we feel ought to be recommended, but it would be useful to know if there are sites out there that are actively degrading and people think that we ought to go out of our way to replace this.

Matt: That's a really good question, isn't it? How do you calculate the load capacity to protect the features of a reburial, backfill, or future use?

Jim: That's an easy answer. Easy question. Well, for me, it's an easy question. You ask an engineer who's capable of doing that in some incredibly complicated engineering software.

Matt: Fair enough. Okay, that last one is a comment, I think. So, that's all the questions we have from the Q&A. So thank you, Jim, for that fantastic presentation. Absolutely fascinating.