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  • 4DHeritage team

A celebration of earthen architecture

A celebration of mud architecture, its past, its future, and its conservation

Mud, mud, glorious mud. It has been used since prehistoric times for moulding objects to building shelters. It has been mixed with materials like straw, fermented and used to create some of the world’s most glorious buildings such as the iconic Great Mosque of Djenne.

There is a huge variety of mud or earthen designs around the world. They range from the Native American earth lodges or shelters, simple dwellings implanted in the ground or covered in soil; to ‘wattle and daub’ houses that used "a 'wattle' of interwoven poles to provide a lattice structure filled by mud or 'daub'".

Mud, soil or earth architecture. What is it?

Mud is perhaps a too simplistic term and often such architecture is described as earth or soil architecture. Earth and soil imply something more complex and include the ‘Sod’ houses which use turf and were built on the northwest coast of Europe, and later by European settlers on the North American prairies.

Adobe or mud-brick buildings are prevalent throughout the world and include houses, and much larger communal structures such as apartment buildings, mosques and churches. One example of these are the Fujian Tulous in South-Eastern China which are large fortified rammed earth buildings that may be home to up to 80 families. Another example are the great mosques of the Sahel and more about that later. Other earth structures include mounds and pyramids used for religious purposes, levees, forts, trenches and embankment dams.

The structural properties of earth

We were delighted to welcome one of the world’s foremost experts on this field in the webinar earlier this summer. Richard Hughes is a trained engineer and building conservator who has undertaken projects for many international and national organisations including the Aga Khan Trust for Culture, UNESCO/UNDP, ICOMOS, Ove Arup and Partners, and the Egypt Exploration Society. He is a specialist in the structural evaluation and conservation of historic buildings and archaeological sites and has given scientific advice to many firms of engineers and architects on engineering practices as well as correct use of traditional building materials, especially wood, soil and stone. He has particularly been involved in assessing traditional structures in hazard prone areas (affected by earthquakes and floods) and on the new use of soil as a structural building material.

Who better than Richard to explain the complex sciences behind both these often-simple structures and how local customs and traditions can preserve a finely evolved approach to harnessing this most accessible of building materials?

Earth buildings and sustainability

One architect who recognised the complexity and potential of earth building and raised its profile in the UK at the beginning of the 20th Century was the Welsh Architect Clough Williams-Ellis. He expounded on the possibilities of building with soil. William-Ellis’s book ‘Building in Cob, Pise and Stabilized Earth’ first published in 1919 generated a special interest in sustainable building. At that time traditional building resources such as brick and timber were in short supply, and there was a need for alternatives. It was published again after the second world war. He was also the designer of the holiday town of Portmeirion which was an early experiment in sustainable tourism.

Renewed interest in these early experiments has been kindled by the sustainable architecture movement as well as an awareness of the challenges of earth building conservation.

At first glance earth as a material seems simple, but compared with other building materials there is huge variation. Earth is hugely diverse. Its properties change laterally, vertically or depending on the temperature and humidity. It really does matter where it is dug from – which bit of the river bank, at what depth, and when.

The idea of mud as a material can be misleading as that has connotations of a slurry, when many earth buildings use it in its dry form. When dry it can be like a powder and compacted, when its building properties become very different.

The variety of the material and diversity of local geography, along with the anthropological customs means that great diligence is needed to understand local traditions which may encapsulate an earth alchemy refined over many, many generations. Restoration projects require a detailed understanding of both the soil geophysics and chemistry as well as the traditions which may capture the tricks and techniques for making the most of the earth resources; not just for reconstruction, but also to improve its resilience going forwards. Durability will require ongoing and constant maintenance.

The complexity of soil as a material

Soil can be a product of river erosion, from flood plains or even the breakdown of rock. It a mixture of different sized particles, minerals and water. The ideal soil for building is a sandy silt with a bit of clay. These are three very different materials. Clay has a buttery texture, silt is silky and can spread smoothly, whereas silt has distinct grains in it. In the right proportions these materials complement one another. Clay is very plastic and sticky, but when it dries it shrinks a lot. Silt resists compaction.

The particles are held together by different processes.There are the electrical charges. For example the clay has a negative charge. Sand has a positive charge, attracting one another together. This can be enhanced by soluble salts which act as electrolytes. Whether adhesion is created by the suction within the voids or pores between the particles. There is usually a lot of air trapped between the particles, when this is forced by compression, the material becomes much harder.

Water also can reduce the suction between the particles, making the material more malleable and easily spread. However, if the material is overworked and loses these pores, it can lose its stickiness or adhesive properties. This is illustrated by plasterwork which sticks when it is first ‘thrown’ onto the wall or ceiling, but too much working it will cause it to fall away.

The Atterberg limits are the basic measure of how the water content of soil affects its plasticity, the quality that defines how easily it is moulded or shaped. These parameters define when the soil changes from solid to a plastic mud-line state to a liquid state. As the earth changes from one state or another, its volume changes; and this shrinkage is a really crucial factor when building or restoring earth buildings. The higher the proportion of clay in the soil, the more plastic it is, but the greater the shrinkage – and ultimately the strength. However, soil with a much higher sand will show much less shrinkage but is much more fragile.

The compaction and strength

Soil compaction occurs when soil particles are pressed together, reducing pore space between them. Heavily compacted soils contain few large pores, less total pore volume and, consequently, a greater density.

A compacted soil is much less prone to water infiltration and drainage, so that it is much better for building on, with much less settlement. Compacting increases the strength of the material by driving out the air between the particles and thereby increasing the pore suction between them.

Magical additives

The stability and strength can be enhanced by additives. The science of soil stabilisation evolved quickly during World War 1 with the need to stabilise earth on the runways around the world. But there is a long history of additives to improve strength of earth structures.

Sea Salt has been used to stabilise dirt roads in Australia. Other local additives include cactus juice is used in Peru; oxblood is used in Nigeria, cheese in Yemen, eggs are used in Oman; and pig urine in India. All the above additives produce fermentation products which create polysaccharides. These polysaccharides or starches can be both insoluble and very strong – similar to the kind of polysaccharides secreted by barnacles to fix themselves to rocks beneath the sea.

How these elements all come together in the Great Mosque of Djenne

Many of the strands of earth architecture as described by Richard Hughes are illustrated by the construction of the Great Mosque of Djenne at the heart of the island city in the Bani River, and possibly the largest earth-built building in the world.

The Mosque is widely regarded amongst architects as ‘one of the greatest achievements of the Sudano-Sahelian architectural style’. This region includes the grassland regions of West Africa, south of the Sahara, but north of the fertile forest regions of the coast. The style incorporates mud bricks and adobe plaster. Large wooden-log support beams often jut out from the wall face of large buildings such as mosques or palaces. These beams strengthen the structure, act as scaffolding both for the construction and maintenance done at regular intervals by the local community. These beams are sometimes made bundles of rodier palm (Borassus aethiopum) sticks, called toron, that project about 60 cm (2.0 ft) from the surface, as in the case of the Great Mosque and also serve as decoration.

The walls of the Great Mosque were constructed of sun-baked earth bricks (called ferey), held together by a sand and earth-based mortar which is then coated with a fermentation of mud and straw that is applied each year by the masons of the town, giving the building its smooth, moulded look.

The earliest examples of Sudano-Sahelian style probably come from the nearby iron age site of Jenné-Jeno around 250 BC, where the first evidence of permanent mudbrick architecture in the region is found.

The current mosque incorporates ceramic half-pipes which extend from the roofline to direct rainwater from the roof away from the walls, improving the resilience of the building during the rains.

We asked people to think of words they associated with mud architecture at the start and at the end of the webinar. The difference between the two is very revealing as it highlights the human quality of earth architecture and the way in which it is a living structure, sustained by deep knowledge, tradition and a collective responsibility.

The Annual festival of the ‘Crepissage’

One of the best illustrations of the way in which earth buildings are living structures, often with deep links into the heritage of their communities is annual festival of the ‘Crepissage de la Grand Mosquée’ that takes place each year in Djenne , when the entire community becomes actively involved in the maintenance of the mosque. There is music, food and celebrations, but the main purpose is to make good the damage inflicted on the mosque in the past year. This may be the erosion during the annual rains or cracks caused by changes in temperature and humidity.

In the weeks leading up to the festival, the mud is prepared in pits, incorporating the fine clay and minerals from the alluvial soil of the Bani river. The mud is so rich in minerals that it is also used for dyeing the local textiles to create the distinctive Bogolan designs. It takes several days for the mud to ferment or cure, and during this time it needs to be periodically stirred. This is a task usually assigned to the young boys who play in the mixture, kneading into it a fine plaster-like substance.

During the festival one group of men carry the plaster from the pits to the mosque. A race is held at the beginning of the festival to see who will be the first to deliver the plaster to the mosque. Another group of men pass the plaster up to a third group standing on the torons as a ladder or platform who then plaster the sides of the mosque. Women and girls carry water to the pits before the festival and to the workmen on the mosque during it.

Members of Djenné's Guild of masons direct the work, while elderly members of the community, who have already participated in the festival many times, sit in a place of honour in the market square watching the proceedings; ensuring the fidelity with which knowledge and traditions are passed down between the generations.

During the annual festival there is a fusion of the inanimate and animate, earth is transformed into a plaster in an alchemy unique to Djenne, the structure of the Mosque is renewed and the whole community comes together sustaining a centuries old tradition. The annual festival is a fusion of the inanimate and animate. Mud, mud, glorious mud.

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