Building with Earth Historical Revision and Improved Characteristics by Adding Supplementary Materials
This thesis by Hind El Houari aims at exploring the huge potential earth architecture has, by revising earth historic construction techniques, understanding the specificities of the material, to contribute in proposing alternative solutions for the upgrade of its mechanical properties, in order to respond to the economical and environmental problematic.
Earth, a historical construction technique
Earthen architecture is a great example of how human beings, for centuries, have managed to put their environments under their mercy, sculpting the raw materials into various creations. Humans around the globe have developed ingenious techniques to turn the four elements that were in their disposition to reshape their environment, and transit from a nomadic life to a more sedentary one.
Archaeological excavations have unveiled evidences of millennium cities that were entirely built with earth, going from the earliest found human settlement Göbeklitepe (Turkey), to Çatal Höyük (Turkey), Jericho (Palestine), Babylonia (Iraq), Bam (Iran). Figure 4 shows an excavated site from Göbeklitepe in Turkey. It is considered to as one of the first human settlements from the Neolithic period.
Through earthen architecture, our ancestors were able to get the inspiration from natural caves and other structures, to build shelters and dwellings that were more secure and stable. And thus have started the first human civilizations.
Early civilizations have created not only simple dwellings, but also granaries, citadels, palaces and fortresses, religious buildings, urban centers and cultural landscapes. Figure 5 showcases an example of citadels made completely with earth, the citadel of Bam, in Iran, listed as the largest adobe building in the world. It was nevertheless destroyed after an earthquake in 2003.
Earthen architecture, in fact, plays a distinctive role in revealing the local identities and in ensuring the sustainability of construction techniques and their aim to unleash a powerful cultural and artistic expression.
Figure 6, represents one the most renowned examples of earthen architecture from Pre-Saharan Morocco. Aït Ben Haddou is an outstanding example of a southern Moroccan ksar illustrating the main types of construction to be observed in the valleys of Dra, Todgha, Dades and Sous. Not only is this fortress a witness of an ancestral knowledge in earth construction techniques, but it is also a guardian of the culture and the community spirit it encloses, as it celebrates the tolerance and art of living together, gathering Muslims and Jewish, Arabs, Amazighs and Sub Saharan Africans, in one single sanctuary.
It is therefore by understanding the specificities of the local building techniques with earth as a raw material, that it is possible to contribute into the social and cultural development, with respect to the environment and with a focus on the sustainable development.
An inventory effectuated by CRAterre-ENSAG under the supervision of UNESCO (CRAterre-ENSAG, 2012), suggests that in 29% of the constructions around the world, earth material constitutes more than three quarters of the construction. In Africa and the Arab World, this proportion is respectively of 40 and 35%, while proportions of 15, 22 and 25% respectively are in the region of Latin America, Pacific Asia, Europe and North America.
Figure 7 and Figure 8 show respectively the actual distribution and typologies of earthen historic constructions in Europe, and then in Middle East and North Africa.
These building typologies and techniques vary from a region to another due to climatic and environmental factors, but they are generally in both cases buildings with rammed earth or adobe.
Other techniques might be included, but they depend on the specificities of each region, the materials available, the stabilization techniques, and are not generalized.
For 24% of those buildings, earth material represents less than a quarter of the construction. This situation is the most frequent in Europe and North America (50%) but also in Latin America (35%). This shows construction typologies that are using other complementary materials, and that are widespread in the mentioned regions.
Techniques of construction in Iberian Peninsula and Morocco
The Iberian Peninsula is a rich mine for examples of earthen architecture. The availability of a wide range of adequate soils, as well as the climatic conditions of South Western Europe has made Portugal and Spain notable for the large number of ancient monuments, churches, fortresses but also examples of vernacular architecture in urban and rural settlements.
In Morocco, on the other hand, an ancestral knowledge of earth construction has been developing for centuries, and has been carried out from master (maâllem) to apprentice (mtaâllem). The techniques translate the balance between the man and his environment. It is by using the available elements: earth, water, fire and plants, together in an homogeneous way, that Moroccans have been able to create shelters that face the adversity of the climate.
It is also notable that the cross cultural influence between North Africa and the Iberian Peninsula has enabled the enriching of a panoply of techniques in earth construction, that are now characteristic of these regions.
The same examples of molded adobe blocks, for instance, found in the imperial cities of Fes and Marrakech, are also present in the Andalusian cities of Granada and Seville. The same goes for the rest of the different building techniques.
In order to catalogue them, these techniques are classified in Figure 9 according to the way in which the earth is used in construction.
For the case of Morocco, the examples presented are mainly focused on the monumental architecture rather than the vernacular one. This is mainly due to the fact that monumental architecture is better documented and preserved. Although the use of earth construction is most present in regions like Morocco as vernacular architecture, it is through the study of monuments that is possible to examine them on a larger scale, as more efficient structural solutions are employed and better results are reached.
Vulnerability of historic traditional earth construction techniques
In order to take better preservation measures for the conservation of historic and traditional earthen architecture, it is important to understand its weaknesses and observe the factors that lead to its deterioration. This step is important in targeting the necessary material parameters and properties that need to be upgraded in the earth constructions and structures.
The following is summary, from various references, and from the observation of earthen constructions, to the principal factors that cause damages in earthen structures. These are mainly attributed to construction deficiencies, natural factors.
It is to note, nevertheless, that different factors can simultaneously be causes for the degradation. An anthropogenic factor for example, generally leads to the acceleration process of further natural factors.
- Material deficiency
These are principally related to the texture of the soil that is selected for the construction, or to the composition of the earth mixture.
A soil with a high content of stones and gravel, but a low content of clay, has an effect in the mechanical properties of the earthen material. It will have a low compressive strength and water resistance. A soil with a high content of clay will cause cracking due to shrinkage.
Although adding different traditional admixtures to upgrade the properties of the soil, these can sometimes have a negative effect. Adding straw, for example, enhances the tensile strength of the mixture, but is prone to drying and decompositions. This leaves void in the structure, which lowers its mechanical performance.
- Structural defects
The lack of a proper design that is generally incorrect or even inexistent, due to the empirical and intuitive nature of the construction methods of earthen architecture, lead to major structural defects that cause severe damage to rammed earth or other construction techniques.
For example, timber A-frame trusses are often used as support structure of the roof of rammed earth constructions, but if incorrectly designed they may transmit horizontal thrusts that cannot be absorbed by the walls, resulting into cracking and leaning.
This said, the thoughtful design of the earthen structures, regarding the depth of the walls and the use of the adequate materials, is important to have better structural performances.
- Foundation problems
Having a foundation that is not adequate for the type of construction, where it doesn’t have the necessary bearing capacity to transfer the loads, causes major damages to the structural integrity. These are mainly noticed in severe cracks and even collapse.
- Direct action of rain water
Observing earthen constructions suggests that rain water attacks their unprotected parts, mainly the top and base of the walls. Erosion also occurs on the vertical direction of façade surfaces (this decay is accelerated by the wind action and the effects of the oceanic climate). (Figure 38)
- Punctual erosion
Some parts of the earthen building are vulnerable to the erosion. These weak points are where the leaking of water converges.
- Water settlement and infiltration
Rain water is generally confined in localizations where there was a previous collapse of a wall, a loss of material due to water leakage (Figure 39 and Figure 40), or because of waste deposit which blocks the natural way for the water drainage. Errors in the design and construction such as the lack of a proper roofing system, with no adequate slope, can also cause water settlement.
- Thermal movements
It is generally thought that the softness and pliability of earthen structures gives them immunity t o problems of thermal. However, this is not true. These movements cause vertical cracks, found through the walls length (spaced in regular intervals) and at walls junctions. As a result, the monolithic behavior and stiffness of the all is affected.
- Biological activity
In conditions with a high moisture content, it is possible to see signs of biological colonization. This results in the growth of plants that cause cracking in the structure due to the tensile stress caused by the expansion. The biological activity is not only linked to the flora, but is also manifested in the colonization of animals and insects that drill tunnels within the structure and feed from the organic matter in it.
- Natural disasters
Earthquakes and floods are all responsible for severe damages and may even need to collapse of the earth constructions. In particular, earthquakes inflict the highest catastrophic effect. This is both a consequence of the fact that earth constructions are usually built on places with moderate to high seismic hazard. and that these constructions present high seismic vulnerability.
In fact, the seismic performance of earth constructions is very deficient when compared with contemporary structures, due to their low strength and high deadweight. The deficient constructive dispositions also greatly contribute for the poor seismic behavior of earth constructions. Such deficiencies are typically related with the lack of connection between structural elements (walls, arches, domes, vaults, roof frames, etc.) composing the earth construction. Therefore, a strong earthquake may lead them to collapse or may inflict severe structural damage, by originating harsh cracks and reducing the overall structural stiffness.
- Wind action
Wind has mainly an erosive action over earth constructions. However, it can also affect other decay agents, like shrinkage or rain.
SEE ALSO: Making Earth Construction Great Again
This thesis research was deemed to explore the potential earth architecture has. This was done by revising earth historic construction techniques, understanding the specificities of the material, and finally the proposal of alternative solutions for the upgrade of its mechanical properties, in order to respond to the economical and environmental problematics.
In the inventory that was conducted to detail the common historic construction techniques, focused on the categorization of examples from the Iberian Peninsula and Morocco, several categories and subcategories were defined as follows:
- Construction by removal;
- Construction by addition;
- Earth used as aggregate;
- Stacked earth. This category encloses: cob, or stacked earth with subsequent shaping and piled earth with no susequent shaping;
- Piled earth in blocks. This category encloses: cut blocks without grass roots, cut blocks with grass roots, hand-shaped blocks, and finally moulded blocks (adobe, poured earth and rammed earth).
These categories are common two both mentioned regions, but some of them are only limited to archaelogical sites, and are not practiced anymore. There was therefore more focus on the adobe and rammed earth techniques, that are still widespread and used in many rural regions as a vernacular architecture.
With all the positive aspects the earth construction presents, it is perceived as a vulnerable material, and therefore generally unfairly considered as a poor material, and the vernacular architecture that emanates from stricly reserved for the poor. This misconception has led to a loss of interest in the use of the earth material in the modern context.
It is not until the last 45 years that light was shed on the importance of the reconsideration of the earth material, as it is affordable, widely available, and most importantly sustainble. And although this reconsideration came with several norms and regulations, these standards are not exhaustive, and fail to cover all the specifities of the earth material.
Another raised issue is the use of physico-chemical stabilzers with the raw earth, which are not always adequate to the initial material. Although cement and lime, for instance, have positive effects on the mechanical properties’ enhancement and he water resistance, which improves the durability of the earthen structures in arid climatic conditions, this stabilization technique presents major drawbacks.
Firstly, this stabilization has a considerable influence on the price of the construction. Moreover, the use of cement and lime make the recycling process of the used composition impossible, and therefore, the possibility for the reuse of the material, which characterizes earth as a sustainable material, is omitted.
The ambitions that contemporary humans have to further stretch the limits of a material, threatens in this case the ecological and environmental interest that the earth material presents.
The conducted research experiments investigate the possibility of using alternative stabilizers, issued from recycled waste materials (coal mining waste) or from non synthesized and non industrialized fibers (algae Posidonia Oceanica). Other additives were nevertheless used, such as calcium aluminate cement ad Portland cement but with lower percentages than the usual applications.
The results have shown that the unstabilized earth blocks, as well as the blocks stabilized with coal mining waste activated with Portland cement and the blocks reinforced with Posidonia Oceanica, have a better performance than those with hydraulic stabilizers. Not only this, but these earth blocks have a lower energy consumption and life cycle environmental footprint.
A revision of historical earth construction techniques, as well as an experimental investigation of the mechanical behavior of stabilized and unstabilized earth blocks were carried out within the framework of this thesis, but these topics still need further investigation. In addition, a set of questions regarding the development of other types of reinforcement solutions for earth constructions should be addressed in further research. Therefore, a listing of future works is proposed as follows:
- Application of the stabilization of earth material for different construction techniques, such as on large- scale models in laboratory of rammed earth;
- Further investigation on the mechanical behaviors under tensile strength and shear strength, with the same dosage rates;
- Investigation of the behavior of loam stabilized with Posidonia Oceanica with the use of different water content and different dosage rates;
- Investigation of the behavior of loam stabilized with coal mining waste activate with Portland cement, with the use of different water content and different dosage rates;
- Experimental testing to assess the durability of the earth blocks, and resistance to erosion, freeze and thaw effect, using the Geelong test for instance;
- Study of the environmental impact of these stabilized earth blocks and investigate in the possibility of further recycling of the blocks after the end of their life cycle.