Making Earth Construction Great Again
Earthen building techniques
Nowadays it is estimated that the world population currently living or working in constructions built from raw earth is in the range of 30–40%. Earthen building techniques considerably differ in material composition and construction methods, adobe masonry, rammed earth and cob are the most representative.
Adobe masonry is a modular construction technique consisting of earthen blocks and mortar, usually an earthen mortar. Rammed earth is a construction technique in which moistened earth is compacted in consecutive layers within a wooden formwork. With this technique, monolithic elements are built. However, the layered structure due to the compaction process has influence on the crack mechanism.
Cob technique consists of earth mixed with water to a plastic consistency, then straw fibres are added. Walls made of cob can be considered as fibre-reinforced monolithic structural elements. It is worthwhile to mention that some vernacular earthen building techniques are no longer in practice, and the knowledge of how to build with these materials has been lost.
Structural behaviour of earthen materials and seismic regulations
Although earthen materials are among the oldest building materials, they are currently the least understood in terms of structural behaviour. The correspondence between mechanical behaviour and the physical properties of these building materials are strictly related to mineralogical and granulometric features. For this reason, the scatter of mechanical property values can be large. In comparison with stone and brick masonry, they show low strength values and are highly sensitive to changes of hygrothermal conditions.
While some guidelines and standards for building with earthen materials do exist, like in USA and Peru, these often lack design charts. Moreover, values specified do not consider the high variability of earthen materials in terms of mechanical properties, which is dependent on a number of parameters affecting physical and chemical bonds at micro-structural level.
Seismic regulations for countries like Morocco and Pakistan where earthen buildings are still present are often based on those of developed countries and exclude earth as a building material. When seismic regulations for earthen buildings do exist, like in New Zealand, these tend to group all earthen materials into one category.
The conservation of earth constructions
The conservation of earth constructions demands high maintenance as they are vulnerable to several factors. Damage to structures can occur for several causes, such as rainwater, soluble salts from the material itself and/or contained in the water that enters the structure through capillary ascent, oscillations in temperature and, in desert regions, by airborne sand particles.
Historic earthen materials are often affected by material loss caused by erosion or by the activity of insects or smaller mammals. Back erosion and animal burrows reduce the load bearing capacity of the walls and are clearly a weakening factor during earthquakes. Termite infestation, for instance, was assumed to be a key factor leading to the collapse of some parts of the Bam Citadel in Iran in 2003.
Earthen constructions are also susceptible to cracking both under low tensile and low compressive stresses. When they are located in regions with high earthquake risk, their intrinsically low resistance to dynamic actions is further worsened by such durability issues. A considerable number of earthen constructions are present in regions suffering severe seismic events as China, New Zealand, South America and the Mediterranean area. Although damage to dwellings and their collapse is usually the cause of human losses, earthquakes are as well devastating to the built cultural heritage in these regions. As a matter of fact, it is often overlooked that a considerable amount of heritage sites, of which many are endangered, are built from earth.
A number of construction and repair practices negatively affect earthen buildings and make them susceptible to high damage even under low seismic forces. A few typical recurring examples are lack of box-type behaviour due to poor wall-to-wall connections. No floor stiffening, the presence of heavy roofs not supported by ring beams, and also roofs often not connected to walls contribute to increase the vulnerability of earthen constructions to seismic actions. Frequently cracks in earthen structures are insufficiently or inappropriately repaired because of lack of knowledge and/or technology.
When the size of the crack width does not permit grouting, to re-establish structural continuity of earthen buildings usually is used earth as a repair material by stitching or exchange of earth blocks (like in adobe structures). The repair of cracks is traditionally done by stuffing manually mortar into the gap. Naturally, this method is only usable for cracks with large widths. Another disadvantage is that cracks going through thicker walls cannot be completely reached by the tools used for stuffing the mortar into the crack.
Grouting is often used to repair cracks but can also be used to fill smaller voids and gaps. Due to the nature of earthen materials grouts have to meet considerable demands on a variety of properties, which are good workability, low shrinkage, a good bond to the original material, chemical and mechanical compatibility and durability. Grouting is usually complementing other strengthening techniques such as the introduction of reinforcement meshes or tie-rods. In particular, the behaviour of cracks repair by grouting poses a challenge in earthen materials and demands specific requirements for the grouting mortar.
The grouting methodology for earthen materials is mostly based on gravity flow or on low-pressure pumping systems. A practical example for structural grouting of earthen structures is the Pio Pico Adobe in California, USA. This building was repaired in 1991 with an earth-fly ash grout and suffered limited damage during the Northridge Earthquake in 1994.
While recent studies point out the effectiveness of grouting for historic brick and stone masonry, the structural performance of grouted earthen buildings depends on the shrinkage/swelling behaviour and the bond of the grout. Stone or clay brick masonry can be wetted prior to grouting, which prevents loss of moisture in the grout and which greatly increases the bond between grout and stone. This pre-wetting cannot easily be done with earth due to its sensitivity to liquid water. For this reason, for lime and hydraulic grouts is crucial the determination of suitable water/binder ratios and water retention values. On the other hand, the definition of the water/soil ratio for earthen grouts represents a key factor as far as shrinkage behaviour, bond and mechanical performances.
As is the case with other types of masonry, the use of highly rigid mortars or grouts may create stiffness discontinuities in the masonry, which, in case of seismic loading, may result in stress concentration and crack formation. It is therefore recommended not to use very stiff materials in the repair or strengthening of earthen buildings, since even small stress concentrations suffice to cause damage.
Challenges in the grouting of earthen structures are good bond, long-term performance and the restoration of long lasting structural integrity and continuity of a damaged structure.