Rammed earth walls: natural and sustainable living

 

‘Rammed earth’ is a technique for constructing foundations, floors and walls from natural raw materials such as earth, chalk, lime or gravel. With the lowest enviromental impact of all building techniques currently used in the commercial construction industry, rammed earth is receiving growing attention and becoming more and more popular with architects, builders and clients alike. In this short post, we tell you a little more about the technique.

Rammed earth walls RISE Design Studio architect west london

Mixing it up

Rammed earth walls are made by ‘ramming’ a mixture of materials into place between flat, temporary panels called ‘formwork’. The panels are normally made of wood or plywood and they are clamped around the earth to make sure it doesn’t bulge when compressed. The materials rammed into the formwork generally include gravel, sand, silt and a small amount of clay.  An additive like lime, cement of asphalt is often added to make it extra strong.

The mixture is compressed inside the formwork by a mechanical ram and compacted to approximately 50% of its original height. The compression takes place in stages, usually in depths of 10cm to 25cm, and this is repeated until the top of the formwork is reached. This is the part of the process that results in the beautiful layers sometimes seen in rammed earth walls.

After the wall has been completed, the formwork is removed and a surface texture can be applied, e.g. by wire brushing or carving. The wall is normally too hard to work on after about one hour. As the walls dry and harden (ideally in warm weather), the compression strength of the rammed earth increases to a maximum of 4.3MPa (620 psi). This is less than concrete but sufficiently strong for domestic buildings.

Environmentally-friendly

Cured rammed earth has a high thermal mass. This means that it absorbs heat during the day and releases it at night, which reduces heating and cooling costs. Environmental impacts are also kept low if the amount of processing of rammed earth materials is kept to a minimum (i.e. artificial additives) and material is sourced locally where possible. Most of the energy used in the construction of rammed earth is in quarrying the raw material and transporting it to the site.

Healthy home

The composition of rammed earth allows it to absorb and release humidity from the inside of a building. This improves air quality, particularly for people who suffer from asthma. Rammed earth is also inorganic, which means that it won’t decay and does not support the growth of mould.

Back to our roots

More than 30% of the world’s population uses earth as a building material, hence this technique is not new. Properly constructed rammed earth can also last for thousands of years, demonstrated by the many ancient structures built using this technique which are still standing around the world.

It has received growing attention in recent years as the construction industry and its customers have a growing sense of the need for environmentally friendly and sustainable building practices. Not only does the technique offer opportunities for carbon-neutral building projects, the technique can be employed in a wide range of climatic conditions and for developing housing that would otherwise be constrained due to expensive construction techniques.

It is a technique that we have been exploring at RISE Design Studio, particularly in our drive to use local, sustainably-sourced building materials.

History of a building: foundations

 

Constructing foundations is one of the oldest of human activities. Foundations provide support for structures by transferring their load to layers of soil or rock beneath them. Over 12,000 years ago, neolithic inhabitants of Switzerland built houses on long, wooden piles that were driven into the soft beds of shallow lakes, keeping people high up above dangerous animals and hostile neighbours. A few thousand years later, the Babylonians raised their monuments on mats made from reed, and the ancient Egyptians supported the pyramids on stone blocks which rested on the bedrock. It was in ancient Rome that foundation engineering really leapt forwards, with rules created and concrete used. In the first of a series of posts that chart the history of modern building elements in the UK, we look at how foundation engineering has changed over the past century or so.

Foundations building Muhlhofen

Byelaws for healthy buildings

The Public Health Act, introduced in 1875, was the first legislation that required byelaws to be set by the authorities. These byelaws were focussed on the development of new streets, ensuring the structural stability of houses, preventing fires, providing adequate and efficient drainage, and ensuring air space around buildings.

In 1878, the Building Act provided more detail regarding house foundations and wall types. For foundations, the byelaws stated that walls should have stepped footings (twice the width of the wall) and that nine inch (225mm) thick concrete should be placed under the footings unless the building sat on gravel or rock sub-soil (‘solid ground’). At that time, Portland cement was seen as making the best concrete, with hydraulic lime as ‘the next best thing’. Common (hydrated) lime was seen as inferior.

It is not known how many authorities adopted these byelaws outside London – many produced their own, less onerous rules. This meant that the nature and quality of foundations varied somewhat (concrete foundation, brick footings, rubble/flagstone)  with depths varying according to circumstances and, in general, shallower than modern foundations.

Raft vs strip foundations

Raft foundations are created from reinforced concrete slabs of uniform thickness, covering a wide area. They spread the load over the whole area of the foundation, in effect ‘floating’ on the ground. In the 1920s and 1930s, this type of foundation was common, with brick footings also permitted.

In the late 1940s and throughout the 1950s, most new houses were built on strip foundations, although raft foundations remained popular. Strip foundations are particularly suited to light structural loadings, such as those found in many low-to-medium rise domestic buildings, with minimum strip widths applying to different ground types and total load. These regulations were set out in the National Building Regulations in 1965 and applied generally throughout England and Wales, with the exception of London which had its own Building Acts.

Today, raft foundations are quite rare, except in former mining areas. An overview of modern types of foundations can be found here.