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. 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. 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. Short bore piles are typically 2–3m long and can be reinforced with steel (Image credit: Homebuilding & Renovating) 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.

The rest of England and Wales generally have low plasticity clays but even these still carry some risk. If the water table is high (for instance if the gravel is submerged), the bearing capacity will be halved, so it’s important to keep the foundations as high as possible. A shallow, reinforced, wide strip foundation may be suitable.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.

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.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 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. 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. 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.