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BLOCK PARTY: THE SECRET OF HOW TERMITES BUILD THEIR MOUNDS

Researchers have pinpointed the methods termites use to create their impressive mounds, using a building technique that gives the structures incredible strength. 

To the untrained eye, a termite mound may appear to simply be a pile of soil, cleverly shaped to form a protective home for termites. However, termites build mounds for a number of reasons, such as to regulate the temperature of the nest and to provide protection from predators. The nest plays a crucial role in the termites’ survival. While much research has looked at the structure of termite mounds, there has been little exploration into the ‘building blocks’ of soil they use, or how those blocks are stuck together to create these structures.

A study published in Earthen Dwellings and Structures has shown that a termite structure can, in fact, be ten times stronger than the soil surrounding it and can remain standing for decades to centuries.1 The question is, how do termites achieve this?

Researchers from the University of Cambridge, UK, and the Indian Institute of Science have identified not only how termites create their tiny ‘building blocks’ but also their composition and overall strength when assembled to create a mound.

In order to understand how termites create their building blocks, the researchers caused a breach in a live Odontotermes obesus mound (a species that is widespread across Southeast Asia). They then observed how the termites repaired it. Three different castes of termites were found at the site of breach repair: major workers, minor workers and soldiers.

They observed that termites mix moist soil with their secretions (saliva) to make small, irregular spheres, or ‘blocks’. These blocks act as the basic building blocks of termite mound construction, much in the same way that humans use bricks in building construction (main picture above shows Odontotermes using soil spheres to build a mudding sheet). This process of mixing a building material with organic matter (in this case, termite bodily secretions) to form a stronger building substance is known as biocementation.

At the site of the breach, only major and minor worker termites were found to carry blocks, not the soldiers. Interestingly, the blocks they created were also of different sizes; the blocks made by major workers were on average 3.7 times larger than those made by minor workers (Figure 1). The researchers then asked the question, how do termites build (and repair) such strong structures with blocks of different sizes?

They observed that the different sized blocks were interspersed during the process of breach repair i.e. the smaller blocks filled the voids between the larger blocks. By using a mixture of small and large blocks, the termites achieved tight packing of the soil – similar to how we would pack a suitcase, with a number of large and small items, ensuring no gaps. This construction method was shown to give great strength to the mound.

If termites could choose the material from which to construct their blocks, what would they choose? The researchers also investigated this element of mound construction. In order to understand how material properties affect the process of mound construction, termites were offered a wide range of materials ranging from fibrous materials to polymers and granular materials. Out of 24 materials offered to the termites, 21 were used for making blocks. The materials with the highest ease of handling were granular, attracted water but did not absorb moisture from the air, and had a rough surface. Red soil, the type found in warm, temperate, moist climates, was the most popular, with crushed hydrogel and sand also being amongst the materials frequently selected by the termites.

Figure 1: a) Blocks made with soil by major workers and b) by minor workers c) electron micrographs of blocks made up of glass beads d) arrows indicate termite saliva deposits where glass beads meet

The research also, importantly, quantified the strength of the termites’ mound construction by subjecting samples from different parts of an Odontotermes obesus mound to compression testing. Control soil from the surrounding area was subjected to a similar test. The compressive strength of termite mound soil was found to be 1200-1800 kPa, while for the control soil it was 125-150 kPa, indicating that termite mound soil had ten times greater strength than reconstituted mound soil.

This could only be caused by two factors: firstly, the way the termites shaped their blocks and fitted them together and secondly, the way termites changed the properties of the soil by applying their excretions. Understanding the characteristics of these biocements is the next step in this research.

To achieve a tenfold increase in compressive strength of soil is an incredible feat. It is perhaps no surprise that we are turning to termites to seek answers the global question of how we can construct buildings in a more cost-effective and eco-friendly way. The area of biocementation is one that is already gathering interest amongst engineers; in fact, it may not be long until we see concrete being strengthened with the use of microbiology.2 In September 2019, the South Dakota School of Mines and Technology in the US was awarded nearly half a million dollars from the National Science Foundation to examining termite mound construction, with the purpose of furthering the advancement of materials suited to sustainable construction practices.

1 Zachariah, N. et al. (2019). Strength and Cementation in a Termite Mound. Earthen Dwellings and Structures: Current Status in their Adoption, ed.1, chapter 12.

2 Charpe A., Latkar M. V. and Chakrabarti, T. (2018). Biocementation: an eco-friendly approach to strengthen concrete. Proceedings of the Institution of Civil Engineers – Engineering Sustainability. Volume 172, no. 7.

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