Earthen Structures

Cob (also known as cobb, clay-lump or clom) and other forms of unbaked earth wall construction can be particularly vulnerable to water damage. Earth buildings are nearly always constructed on a masonry plinth of flint, stone or brick to protect them. Plinth levels have changed throughout history and can range from 300mm to 600mm in height. If moisture rises above the plinth for a prolonged period, or if it is trapped behind an impermeable surface, the cob will begin to disintegrate via slippage, collapse or rodent infestation.

Earthen structures can withstand a certain level of floodwater, but only if they are in good condition. Some of their resilience is in the thickness of their walls. However, as some cob blocks or brickettes are made up of clay deposits, they have the potential to expand, which can result in localised cracking. The presence of clay in earthen structures will depend on the regional soil. Where the subsoil reaches its plastic limit, it can deform or collapse.

Case study evidence suggests that where walls are subjected to submerged conditions, they can tolerate a level of undercutting and loss of earth material from the wall face.

It is important to identify the compaction rate (the stress applied whilst making the cob to remove air from the pores and improve density) and material composition, and whether the wall is reinforced with straw. This will be hard to do with an existing building without destructive testing, but could be assessed after a flood has occurred to determine if wholesale replacement is needed.

Where straw is not present, the building is at risk of collapse when saturated for more than 24 hours. Where it is suitably compacted and made of the correct materials, with straw reinforcement, the building is more likely to withstand initial flooding.

Ultimately, it is vital to understand how a cob wall is constructed to identify the building's vulnerability to floodwater, and whether its structural integrity is at risk.