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Failure and shear and bending in earthen structural walls

Earthen structures are still widely used in the developing world, where they are economical and, properly constructed, durable buildings. They are also gaining renewed interest in the developing world, where low embodied energy, high thermal mass, recyclability, other sustainable characteristics, and increasing economic competitiveness are making them more attractive options. Still, from a modern engineering standpoint, there is still much to be understood about these materials.

We focus here on two earthen structural materials, cement stabilized soil block, also called compressed earth block, and rammed earth. The former is a mix of soil, water, and cement stabilizer, and is mechanically compressed and cured to create a brick. The bricks are often joined with standard or soil-cement mortar. Rammed earth may also have cement stabilization, though not always, and is compacted in a form without mortar.

We have developed a finite element model to simulate the plane behavior of these earthen structural materials. The bulk constitutive model is adapted from the Sandia GeoModel, developed for rock- and soil-like material. A smooth tension cap is added to help better predict the tensile behavior of these materials. A localization condition is used to predict the onset of bulk fracture. Weak existing interfaces, such as a brick-mortar interface or weak interface between compacted rammed earth layers, also can be added to the model. A traction-displacement relationship for the weakening along both new and forming fractures is created. Fractures may pass from the bulk material to the existing interfaces and back as they naturally progress through the material.

The models are used to examine the behavior of earthen structural elements in both in-plane shear and out-of-plane bending. The results are compared to experimental data, and finally applied to more sophisticated structural elements such as wall units with openings.

Author(s):

Craig Foster    
University of Illinois at Chicago
United States

David Weed    
University of Illinois at Chicago
United States

Adam Tennant    
University of Illinois at Chicago
United States

Mohammad Hosein Motamedi    
University of Illinois at Chicago
United States