Soil-based construction materials are of interest as structural building materials due to their green credentials, as well as being present in many historical structures. For effective conservation of the latter, and to motivate greater uptake for new construction, understanding of the mechanical and hydraulic properties of these materials is in need of improvement. Earthen construction materials can be considered to be manufactured unsaturated soils, and advances in understanding can be made by considering them from a geotechnical point of view. This paper presents initial results from a major programme of testing, seeking improved properties for earthen construction materials, where unusual organic compounds have been employed as stabilisers. Two gums (guar and xanthan) used as stabilisers for a soil mixture are shown to have significant effects on certain mechanical properties, some of which can be explained, and other aspects which are in need of further investigation.

A bounding surface compression model with a unified virgin line for saturated and unsaturated soils.

This paper investigates the mechanical behaviour of a hypercompacted unstabilized earth material manufactured by compressing a moist soil to very high pressures up to 100 MPa. The hypercompaction procedure increases material density, which in turn improves mechanical characteristics. Samples were manufactured at the scale of both small cylinders and masonry bricks.The effect of ambient humidity on the mechanical characteristics of the material was investigated at the scale of cylindrical samples, showing that both strength and stiffness are sensitive to environmental conditions and tend to increase as ambient humidity reduces. The strength of the bricks was instead investigated under laboratory ambient conditions by using different experimental configurations to assess the influence of sample slenderness and friction confinement. Additional tests were also performed to evaluate the influence of mortar joints and compaction-induced anisotropy. Overall, the hypercompacted earth material exhibits mechanical characteristics that are comparable with those of traditional building materials, such as fired bricks, concrete blocks or stabilized compressed earth.

A geotechnical perspective of raw earth building.

The paper presents a model that takes into account the influence of void ratio and hydraulic hysteresis on soil water retention. The model is based on the definition of two bounding surfaces, i.e. a main drying surface and a main wetting surface, that delimit the region of admissible soil states in the space of degree of saturation, suction and void ratio. An auxiliary variable, named scaled suction, is introduced to combine the effects of suction and void ratio into a single quantity, so that the main surfaces are recast as curves in the plane of degree of saturation and scaled suction. An increase of scaled suction corresponds to drying of the soil while a decrease of scaled suction corresponds to wetting of the soil; hence the drying/wetting behaviour is governed by changes of both suction and void ratio. The model assumes that the derivative of degree of saturation with respect to scaled suction depends on the distance of the current soil state from the main curves, which ensures a smooth transition of the drying and wetting paths towards their respective main curves. One advantage of the proposed model is that all wetting and drying paths can be integrated in a closed form and are described by means of two explicit equations (one for drying and one for wetting) with different constants of integration. The model requires seven parameters whose values can be obtained from as little as a single drying-wetting test. Predictions are validated against two different data sets published in the literature, showing a good ability of the model to capture the behaviour observed during laboratory tests on fine grained soils