Heriot Watt University and Arc Architects

Cob construction and its response to flooding

Executive Summary

Earth building is a sustainable method of construction that has been used in Scotland and worldwide for many centuries. It is estimated that ½ a million earth buildings currently exist in the United Kingdom with this number being liable to increase.

These forms of construction are perceived to have good ‘green’ credential and more specifically have very low energy input, and create virtually no pollution . Other advantages include the material low toxicity, high thermal mass, and high hygroscopic capacity, which is believed to regulate the internal humidity of the building . Additionally, upon demolition, earth construction simply reverts back to earth . 

Earth construction can be subdivided into various forms, with cob being one of the most frequently occurring types utilised within the UK.

There are a number of behavioural uncertainties with these structures, such as the effect of changing moisture contents within the material. In recent years, climate change has been attributed to be causing an increase in the average air temperatures and the occurrence of extreme weather events. According, to the UK Meteorology Office, precipitation levels during the summer of 2007 were in some days the amount expected for one month, causing many incidents of flooding.

Many of these buildings are constructed using earth materials that are naturally deposited on flood plains. Worryingly, many of these structures are constructed on the flood plains were the materials are sourced.

It is clear that the likelihood of flood events are on the increase. However, what is not understood is the affect of submerging cob walls in water. It is obvious, even to the lay person, that an earth wall would lose its structural integrity if submerged under water for a long period, however, we do not understand the duration to which these forms of construction retain their integrity (and therefore their safety characteristics) under such conditions or if signs of deterioration manifest themselves before failure occurs.

The performance characteristics of the materials exposed to these conditions have been little studied and this research attempts to investigate these broad questions, examining them using data collected via experiments, case studies and current literature.

More specifically, the research has discussed the following;

• the influence of the climate change on cob structures by the laboratory simulation of the flood behaviour and subsequent drying;
• study the micro-structure of materials for cob construction and undertake a preliminary investigation of wetting and drying of wall/samples using Environmental Scanning Electron Microscopy (ESEM) analysis techniques;
• assess cob wall modes of failure when subjected to prolonged submergence under water and sustained load conditions;
• document the modes of failure in an attempt to better understand survey and defect analysis of these structures;
• analyse the impact of construction processes and materials selection for cob construction

Preliminary findings
From preliminary laboratory work undertaken at Heriot-Watt Univeristy and the site investigations on cob buildings, the research team and the industrial partner concluded:

• A correlation appears to exist between the rate of cob materials compaction and the duration to which the structural integrity of the walls was retained when the samples were submerged in water.
• The use of straw reinforcing significantly increased the duration to which the wall could be submerged before failure.
• The materials for cob construction exhibited both hydrophobic and hydrophilic characteristics. This would have an influence upon the materials ability to saturate and dehydrate, and also have an impact upon moisture transfer mechanisms.
• Unsaturated cob wall/samples developed surface tension between hydrophilic surfaces and this is believed by the authors to increase inter-particle bond strength within the material by the suction effect. 
• Un-reinforced cob walls that were submerged in flood waters, exhibited an undercutting pattern of deterioration prior to failure.

Final considerations

It must be emphasised that the project is currently on going and further information will be disseminated in due course.