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Title of Paper Raw earth construction: is there a role for unsaturated soil mechanics ? D. Gallipoli, A.W. Bruno & C. Perlot Université de Pau et des Pays de l'Adour, Laboratoire SIAME, Anglet, France N. Salmon Nobatek, Anglet, France ABSTRACT: “Raw earth” (“terre crue” in French) is an ancient building material consisting of a mixture of moist clay and sand which is compacted to a more or less high density depending on the chosen building tech- nique. A raw earth structure could in fact be described as a “soil fill in the shape of a building”. Despite the very nature of this material, which makes it particularly suitable to a geotechnical analysis, raw earth construc- tion has so far been the almost exclusive domain of structural engineers and still remains a niche market in current building practice. A multitude of manufacturing techniques have already been developed over the cen- turies but, recently, this construction method has attracted fresh interest due to its eco-friendly characteristics and the potential savings of embodied, operational and end-of-life energy that it can offer during the life cycle of a structure. This paper starts by introducing the advantages of raw earth over other conventional building materials followed by a description of modern earthen construction techniques. The largest part of the manu- script is devoted to the presentation of recent studies about the hydro-mechanical properties of earthen materi- als and their dependency on suction, water content, particle size distribution and relative humidity. A raw earth structure therefore consists of com- pacted moist soil and may be described in geotech- 1 DEFINITION OF EARTHEN nical terms as a “soil fill in the shape of a building”. CONSTRUCTION Depending on the initial moisture content and manu- facturing process, the dry density and degree of satu- “Raw earth” (“terre crue” in French) is a building ration of a raw earth structure can be more or less material consisting of compacted moist soil (clayey high. In general, the degree of saturation is highest at sand), with the possible addition of reinforcing the time of construction but rapidly drops due to the agents such as fibres or chemical binders. Unlike combined effects of solar radiation, temperature and “cooked earth” (e.g. conventional masonry bricks), relative humidity. the soil-water mix is subjected to the least possible Because of the nature of earthen structures, ge- transformations and is used in a form very close to otechnical principles should be employed for their its natural state. analysis and design. This is, however, seldom the Several construction techniques making use of case and the wealth of knowledge accumulated over raw earth have been developed over the centuries. the years about soil behaviour and, more specifically, These techniques adopt different manufacturing pro- about unsaturated soil behaviour has been little ex- cesses and are known under different names but all ploited so far by designers of earthen structures. of them use moist soil as the base construction mate- Earthen buildings are still conceived by resorting rial. to empirical rules rather than engineering science. The strength, stiffness and durability of raw earth Yet, it is indubitable that a transfer of knowledge are primarily governed by the inter-granular bonding from geotechnical engineering to raw earth construc- generated by soil water capillarity. Fibres and chem- tion would greatly benefit the development of this ical binders, such as lime or cement, may also be building technique. added to the soil mix to enhance mechanical proper- ties. If lime or cement are used (typically in the pro- portion of 6% in weight), the term “stabilized raw earth” is employed to mark a difference with respect to standard raw earth that does not make use of chemical binders. 2 RECENT RENAISSANCE OF EARTHEN CONSTRUCTION Figure 1 shows the energy consumption of the build- ing, transport and industry sectors in Europe, the US and Japan as estimated by the OECD (2003). In Fig- ure 1, only the energy for the operation of dwellings is attributed to the building sector while the energy used for the transportation of building materials to the site and the energy required for the construction and demolition of the structure are included under the transport and industry headings, respectively. This means that the total energy consumed by all ac- tivities related to building is even greater than that shown in Figure 1. In a recent publication, Szalay Figure 1. Energy consumption by sectors. Source: European (2007) arrived to similar conclusions as those shown Commission, US Department of Energy, Japanese Resource and Energy Agency (OECD 2003). in Figure 1 and estimated that the operation of resi- dential buildings is responsible for about 40% of all 2 Reduction of operational energy (hygro-regulator energy consumed in Europe. effect). In a wet atmosphere, an earthen wall ab- The building sector is also the largest consumer sorbs vapour due to the presence of clay in the of raw minerals and produces about 33% of the earth mix. The absorbed vapour is released again waste annually generated in the European Union when the surrounding environment becomes dri- (EEA 2010). This waste is usually not recyclable and er. This helps controlling the hygroscopic condi- is disposed in landfills, resulting in loss of land, pol- tions inside a closed environment by absorbing lution and social alienation. excess moisture, storing it and returning it when From the 1970s, researchers started to quantify necessary and, therefore, reduces the need for air the environmental costs of construction. These stud- conditioning to regulate relative humidity. ies were at the origin of a renewed interest in alterna- 3 Reduction of operational energy (thermo- tive construction materials with more eco-friendly regulator effect). The hygro-regulator effect de- characteristics than concrete and steel. Earthen mate- scribed at point 2 has also a significant role in rials appeared as a viable option because earth is controlling temperature inside dwellings. This is harmless to human beings and can be easily sourced because water evaporation from the earth mass is in the vicinity of the construction site with conse- an endothermic process that takes away heat (i.e. quent reduction of transportation costs. Earth is also latent heat of evaporation) from the surrounding recyclable, and therefore inexhaustible, and when atmosphere during the hottest hours of the day; properly manufactured can offer high strength, ex- while water condensation inside the earth mass is cellent thermal properties and low embodied energy an exothermic process that releases heat (i.e. la- while remaining a low-cost material. The use of tent heat of condensation) during the coolest earthen materials can therefore reduce consumption hours. With respect to thermal properties, the of natural resources not only during construction but conductivity of raw earth is relatively high and of also during the life of a structure by cutting heat- the order of 10-1 W/mK while most insulating ing/air conditioning bills and by recycling demoli- construction materials have conductivities of the tion waste. order of 10-2 W/mK. The volumetric thermal ca- The advantages of earth as a building material pacity of earthen materials is of the same order of have been known for centuries as demonstrated by magnitude as that of concrete, i.e. around 2×103 the number of historical earthen structures world- kJ/m3K (Houben & Guillaud 2006). However, the wide; however, these advantages have started to be volume of an earthen wall is considerably larger quantified only recently and are summarized below: than that of a concrete frame. This confers to earthen structures a greater ability to store heat 1 Reduction of embodied energy. The preparation, during hot times and return it during cold times transportation and construction of earthen materi- with a daily phase shift of 10-12 hours. als require only 1% of the energy needed for ce- 4 Recycling or disposal of demolition waste. Ac- ment based materials. Similarly, the manufacture cording to Bossink & Brouwers (1996), the waste of compressed earth blocks requires, at most, one generated by construction and demolition of third of the energy of fired bricks, namely 440 3 3 structures accounts for between 13% and 30% of kWh/m compared to 1300 kWh/m (Little & all solid landfill waste world-wide. Of this Morton 2001). amount, demolition waste represents the largest share, with an estimated ratio of demolition to construction waste of about 2:1 (Bossink et al. 1996). In this respect, waste from the demolition dismantling, but also stronger to resist vibrations of earthen structures consists of plain earth that from tougher ramming. The movable small shut- does not need to be disposed in landfills but can ters of the past have been replaced by rolling or be easily recycled or safely released into the envi- sliding modular formworks, similar to those em- ronment. However, this advantage is partly lost if ployed in concrete construction. chemical binders are used, in which case the eco- 2 Prefabricated rammed earth. Building of rammed friendly credentials of raw earth are compromised earth structures often requires continuous assem- and disposal of demolition waste into the envi- bly and stripping of formworks. When this is not ronment becomes more problematic. possible, larger prefabricated rammed earth pan- 5 Acoustic insulation. Raw earth presents excellent els can be used instead. Prefabricated panels are acoustic characteristics and provides good sound produced on-site or off-site by compaction of a insulation due to its high dry density (often in ex- wet soil mix into formworks of variable shapes cess of 2000 kg/m3) and thickness (often in ex- and sizes, e.g. a panel can be up to 2.5 m height, cess of 0.25 m).
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