At 7.2 metres, the circular walls of the Sheppard Theatre are the highest rammed earth walls in the UK. To make rammed earth walls, loose, moist subsoil is compacted in thin layers between shuttering or formwork. Mechanical compaction forces clay molecules to bond with the aggregate (a physical rather than a chemical bond), giving the wall its strength. Particle size and grading, moisture content, and clay content are all critical to the performance of the finished wall.
Llynclys Quarry near Oswestry was identified as source of suitable material (the earth for the walls in our Information Centre also came from Llynclys), and samples of several different materials were taken for testing. The material chosen was a waste product that had already been processed and had a suitable grading - particles of 6mm and less.
A proprietary circular shuttering system with an adjustable radius was used for the theatre walls. Earth was added in 150mm layers and compacted using a hand held pneumatic ram. Four sections of wall were built, with 2 full height gaps for doors. The walls are 500mm thick, incorporating 320 tonnes of earth.
The rammed earth provides structural support for the roof, but does not form the external walls – these are timber-framed glazing (on the south side) and hemp and lime. There is a corridor around the theatre, between the rammed earth and the external walls. The high performance double glazing uses a low-e coating, argon gas fill and insulated spacers in FSC-accredited redwood & oak frames, and has a U-value of 1.1 to 1.4 W/m2K.
We also used earth blocks for internal partition walls on the ground floor, adding extra thermal mass to the building. Made of un-stabilised subsoil similar to that required for rammed earth, blocks are manufactured by compressing the earth in a mould. They can be laid like conventional masonry, using a thin layer of clay slurry as mortar. In the past, we’ve manufactured blocks at CAT, but it is now possible to buy them ready-made.