Raw earth: an ancient building material with a bright future

Myriam Duc, Soils, Rocks and Geotechnical Structures laboratory (SRO/GERS), and Erwan Hamard, Aggregates and materials preparation processes laboratory (GPEM/MAST), Gustave Eiffel University.

A thousand year old building material, raw earth has regained its place in modern construction in recent years thanks to its many qualities, particularly its ecological benefits.Two specialists from the Gustave Eiffel University, Myriam Duc and Erwan Hamard, discuss the interests and prospects of this unusual material.

Like the surprising town of Shibam in Yemen, earth has been used as a building material for thousands of years around the world. France is no exception, with its extensive raw earth heritage: adobe buildings in Auvergne-Rhône-Alpes, cob in the north-west, adobe in the south-west and Champagne, and  wattle and daub in Normandy... Often rural and built more than a century ago, earthen buildings in France account for around 1 million homes and 15% of France’s built heritage. It’s important but often little-known heritage, like the many adobe buildings in Lyon.

Earth was a much-used material in the past, but was gradually replaced by "all-concrete" construction in the 20th century. In recent years, however, it has regained its place back into modern construction. "Raw earth is on the rise, , often on the initiative of public authorities who are commissioning large-scale buildings like : gymnasiums, schools, theatres, etc." notes Myriam Duc, a researcher at Soils, Rocks and Geotechnical Structures laboratory (SRO/GERS).

In today’s context of climate change and ecological, energy and social transition, raw earth has a number of qualities that justify this renewed interest. "This material is very interesting for many reasons," says Erwan Hamard, researcher at the Aggregates and Materials Preparation Processes laboratory (GPEM/MAST). For me, the first interest is that we could replace some of the concrete with earth. If you ask what resources are available to build the city, the answer is by far earth. Every year in France there is a surplus of 40 million tons of excavated earth: using this abundant resource, which is considered to be waste, would reduce quarrying. "In Brittany, for example, all the region's single-family homes could be built using excavated earth that is sent to storage facilities. And this is probably also the case in other regions like: Auvergne-Rhône-Alpes, Occitan, Champagne, Île-de-France..."

Another advantage of using earth is that it reduces the environmental impact of construction. Unlike the production of concrete, which generates significant CO2 emissions (one ton of cement, which is essential for concrete, generates between 800 kilos and one ton of CO2), earth requires no chemical reaction, and the processes used are low-carbon. "According to residents, earth also provides acoustic and hydrothermal comfort. However, this is an aspect that remains to be studied, ," stresses Erwan Hamard.

"Although raw earth and its qualities are being rediscovered, there is still work to be done that the material finds its rightful place between wood and concrete" warns Myriam Duc. "The main obstacle today is regulation: there is a lack of responses to regulatory constraints, particularly seismic constraints," adds Erwan Hamard. This issue is at the heart of a French National Project on earth building, co-sponsored by the Gustave Eiffel University, which brings together all those involved in developing the raw earth sector.

Another difficulty is that earth is not a standard material. "It varies from one region to another, and you can't build with the same techniques (cob, adobe, rammed earth, etc.), or make the same constructions everywhere. You need to know the area, its geology and what you can do with it ," explains Erwan Hamard, whose work focuses on the subject of land suitability.

Finally, the development of the sector requires training of masons, architects, engineers, etc. Here again, the university is making its contribution with the professional training "Objectif Terre Course". "We need to raise awareness of the use of this material, because we can only prescribe what we know," observes Myriam Duc. Other issues, such as transport - earth loses its environmental value over distances of more than 30 km - and the storage of earth from extraction to the building site, still needs to be refined before we see more and more buildings constructed using raw earth. "In fact, the management of areas or platforms for storing soil over more or less long periods needs to be optimized by anticipating the location and timing of soil needs and resources," stresses the researcher.

Raw earth: Mixture or finished material made from of unbaked earth not mixed with hydraulic binders such as cement or lime.

National Project: A plan run by the French Ministry for Ecological Transition and Territorial Cohesion. "National Projects" aims to implement collaborative research in the field of construction in order to advance knowledge, practices, tools and methods.

Adobe: Moulded earth bricks that are masoned after drying to form a load-bearing wall.

Rammed earth: Earth compacted in a formwork to form a monolithic load-bearing wall.

Cob: Earth kneaded in a plastic state, shaped then stacked to form a monolithic load-bearing wall.

Wattle and daub: A mixture of plastic earth and fibres used to fill a load-bearing structure, often made of wood.

¹Source : Projet National Terre 

The interview was conducted by the Kogito agency.