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Local planning authorities will often allow structures to be built around trees within a root protection area on the proviso that the structures are supported by screw piles. Where the foundations are affected by tree roots (or their previous removal), you may be required to employ a fairly deep trench filled with concrete but with a compressible material to one or both sides of the external trenches to counteract any heave or expansion in the ground. the prototype arch is a listed structure at Pennsylvania Castle on the Isle of Portland, and will be closely matched stone-by-stone! A printout of a photograph taken square-on helps gauge the size of the stonework, as well as general dimensions and information obtained from the Historic England listing. Note how I’ve emphasised the mortar joints on the printout to aid visibility, and written notes to help speed up the process.
Short bore piles are typically 2–3m long and can be reinforced with steel (Image credit: Homebuilding & Renovating) Clays with the highest plasticity (and so the highest risk) are generally found in the South East of England, stretching up through the East Midlands to the Humber in the North and down to Bath in the WestThe first 900-1,200mm layer of clay is subject to movement due to expansion and shrinkage from seasonal variation in moisture content, so it is generally necessary to excavate foundations to a depth where the amount of moisture present remains stable. Michael is Homebuilding & Renovating's Director of Content, Vice Chair of the self build industry body, the National Custom and Self Build Association ( NaCSBA), presenter of multiple property TV shows and author of Renovating for Profit(Ebury).He also runs an architectural and interior design practice, offering design and project management services. He is one of the country's leading property experts and has undertaken over 30 building projects including two self-builds and the renovation of a Grade-II listed farmhouse. In this practical, I’ll be focusing on a few methods you can use to construct some of these prototypes. For best results, I’ll be using materials suitable for scribing; a harbour wall made from rigid insulation and plaster (with another using DAS air-drying clay), an ashlar arch and various rubble walls made from styrofoam, and finally, a dry stone wall made out of more air-drying clay. Please note; as the painting technique I’ll be using will be similar for every practical, I shall share this at the very end. WHAT YOU NEED To keep the stones in our wall as horizontal as possible, multiple level trenches are dug on the steep sections of the hill. The first layer of stones can then be laid flat, working your way up the hill. Slowly build up the layers, but do try to copy the prototype where stone size and placement are concerned; larger stones are less common than smaller stones. Strip, trench fill or pad foundations must be cast at a minimum depth of 750mm in low plasticity clays, 900mm depth in medium, and British Standard 8004 recommends a minimum depth of 1m for foundations in the highest risk areas. If there are, or were, trees nearby, depths of up to 3m may be necessary. The depth necessary will depend on the type of tree, as species have different water demands, with tall broad-leafed species such as poplars having the greatest impact.
The rest of England and Wales generally have low plasticity clays but even these still carry some risk. To cover the entire quay, I use household filler/plaster as it’s cheap, available in large quantities, and you can make it to the viscosity desired by varying the amount of water in the mix. Apply a thin layer of PVA to the quay walls, followed by a generous coating of plaster. Once left to harden (which can be up to 48 hours), sand the plaster until the required smoothness is achieved. Water pipes must enter the building at a depth of at least 750mm but no more than 1.35m below ground. If that means that they pass through a concrete foundation then they must either be laid prior to pouring or, better still, a duct installed for them to be pushed through later. The capping stones feature a slope either side of a raised and rounded middle point. On a length of styrofoam 7mm wide, mark two parallel lines in the centre 1mm apart. Using a flat file, form slopes on either side.The raised section that’s left canbe rounded off with more light filing. Cut to the lengths required, then glue them onto the parapet using PVA.Tiny stones are glued along the top to reduce its ‘stepped’ nature; ahead of adding the capstones. The inset photograph shows one of three large clay ‘stones’ being roughened with a craft knife. Two of these are end stones (one visible on the bottom right of the main photograph), and the one left of the pliers would be used by people to hold onto while climbing over the stile. The rear of the arch differs in three ways. Firstly, the crenellations are only half-depth except at the outer ends; which wrap around the sides. Secondly, a slope joins the bottom of the crenellations to roughly the same level as the top of the cornice on the front. Thirdly, the entire archway has a rebate where the gates once sat. These all require careful trimming with a sharp blade. These are a relatively new foundation solution consisting of slender, hollow steel shafts with a small number of steel helices (or screw threads) welded to them. The piles are screwed into the ground until they achieve sufficient friction to support the required load. If the chalk is soft it will need to be excavated until firm chalk is reached. Chalk soils can be prone to erosion, so be wary of the possible presence of hollows or caves.
The process of making stone walls using styrofoam is very similar, but there is no need for plaster, nor to scribe any joints; the pressure from the mechanical pencil on the soft foam while marking the courses will achieve the same result. As such, the Styrofoam rubble walls on the diorama (the ruined barn shown here, and the retaining walls) don't require further explanation, but the construction of the ashlar arch certainly does…
Natural Paving
Medium plasticity clays are found in the rest of the South East, across the Midlands and up beyond the Humber Estuary towards the North East. They also turn up in some isolated areas in the North West of England near the coast Diagrams of strip foundations (left) and trenchfill foundations (Image credit: Homebuilding & Renovating) There are three types of clay in the UK, classified by their plasticity, which is how much their volume can change due to their water content. Where the soil is soft or of a low loadbearing capacity, wide strip foundations can be used to spread the load over a larger area, reinforced with steel so that the loading per square metre is reduced.
Where there are or have been trees (removing trees does not remove the risk of expanding clay) engineers may recommend reinforcing foundations with embedded steel. Foundations in clay soils can also be protected to some extent from damage due to expansion (heave) by lining the trenches with compressible material such as clayboard. Michael has presented over 150 property shows for BBC, ITV1, Channel 5, UK TV Style, and Discovery RealTime, including I Own Britain's Best Home; Don't Move Improve; Trading Up; Good Bid, Good Buy; Build, Buy or Restore?; How to Build A House; and Hard Sell.The painting technique for the dry stone wall is slightly different from the rest. To ensure the paint reaches the crevices, spray a fine mist of water, then apply a very weak dark grey wash. Next, apply a slightly watery colour that matches the prototype stone in its un-weathered freshly-cut state. Finally, add additional dark grey washes as necessary, along with dry-brushed yellowy-green ‘lichen’ spots. Providing the chalk is not too soft, foundation widths of 450mm for low-rise buildings are generally acceptable. The depth of the foundation must be below any frost action, a minimum of 700mm. Now use a flat needle file to create chamfered edges along the archway. With the styrofoam walls complete, apply a layer of gesso to everything. Gesso is an acrylic paste made from chalk, binder, and white pigment. It’s used as a primer to prepare surfaces for painting, but it also provides a little more strength and texture. Simply apply it with a brush, and leave it overnight to dry. If sewage pipes leaving the building have to be deeper than the top of the foundation concrete then they should also be ducted; they cannot be trapped within the concrete and must be able to move freely. Sand holds together reasonably well when damp, compacted and uniform, but trenches may collapse and so sheet piling is often used to retain the ground in trenches until the concrete is poured.
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