1 INTRODUCTION 1.1 Structural Systems and Sustainability Structural systems and forms have developed in the last century to meet new challenges, with significant advances in minimizing material usage, withstanding extreme loads and reaching new constructible heights. But it can be argued that there have been no transformative changes in how we view structural systems or structural form with regard to integrated purpose or performance. As a primary example, engi-neers recognize the importance of developing sustain-able solutions to human needs, and yet structural sys-tems have had a limited role in sustainability of the built environment.

Sustainability in structural design has focused pri-marily on either innovation in materials or the decon-struction and reuse of structural elements (Kestner et al. 2010). While integration in design is the main fo-cus of green building design, true integration of the structural system with other building systems remains elusive. It may be possible that the structural forms on which we most rely, (e.g., beams, columns, trusses, shear walls) simply do not lend themselves towards seamless integration with other systems. Re-search in structural sustainability should go beyond re-engineering existing structural forms and systems to reimagining the role of the load-bearing structure in the overall building performance.

1.2 Biomimicry and Natural Habitats

Traditional structural design in buildings is a top-down approach in which the configuration is a re-sponse to architectural constraints and loads deter-mined from occupancy and generalized environmen-tal conditions. This approach is the diametric opposite of that used for most naturally built habitats, such as termite mounds, in which the occupants create a dy-namic habitat that is in direct response to the external environment. As described by Turner (2005), the re-sulting habitat is an extension of the physiology of the insects themselves. In short, the internal environmen-tal requirements for the existence of the organisms in the habitat are directly linked to and provided by the form of the habitat itself. Human construction gener-ally works to separate habitat from nature rather than work in harmony with it.

Obtaining inspiration from biological and natural systems, or biomimicry, has been successfully em-ployed in engineering and science to develop both new technology and new paradigms in design. While this field is not unknown in the building industry, it is typically a playground for architects rather than struc-tural engineers (e.g., Pawlyn 2011, Mazzoleni 2013, Dewi et al. 2013). There are some examples of bio-logical influences in structural systems. Waggoner and Kestner (2010) discuss biological cell growth pat-terns and their relationship to tensile structures, and Chen et. al. (2015) provides additional background and insight on how biomimicry is applicable to struc-tural engineering. Desirable biological models for structural applications include those with seamless in-tegration of the structure with other systems (e.g., sea-shells, tree roots), or those with exceptional weight/strength ratios, such as spider webs.

Biomimicry and locally responsive construction: Lessons from termite mounds for structural sustainability.

N. Claggett, A. Surovek, B. Streeter and S. Nam South Dakota School of Mines and Technology, Rapid City, SD

P. Bardunias Florida Atlantic University, Boca Raton, FL B. Cetin Iowa State University

ABSTRACT: Structural sustainability is examined from a new perspective in this research employing biomim-icry of termite mounds. The overall goal of the work is to develop structural topologies for use in low-rise buildings that that can significantly reduce energy consumption. These topologies would be organically devel-oped for local conditions and integrate structural and mechanical building systems. In addition, understanding how termites alter the physical and chemical properties of substrates will inform construction that leverages local and/or low-cost materials. In particular, this paper examines two facets of termite mound construction: manipulation of soil by termites to improve material properties, and the development of primary topology (e.g. mound shape) as a function of termite species, environmental conditions, and local soil properties

One application of biomimicry is to consider the adaptive interaction of habitat and environment. For example, the architectural engineering firm HOK was reported to be designing an 8000 acre city in India that mimics the original ecosystem of the area by using foundations that can seasonally store water; this de-sign mimics both the original forested landscape and the water diverting methods of local harvester ants (Gendall 2009). An oft-cited example of bio-inspired, integrated design is the Eastgate Centre in Zimbabwe. The ventilation of termite mounds was used as the in-spiration to naturally regulate the temperature of the building without the need for air conditioning by providing daytime heat sinks and a ventilation stack for the release of warm air. Ironically, although the Eastgate Centre is considered an exemplary model of biomimicry in building design, Turner and Soar (2008) illustrate that actual mechanisms for air ex-change in termite mounds is fundamentally different than what was employed in that design. Yiatros et al. (2007) develop a model of a building that, like East-gate, naturally ventilates the building based on the overall architectural configuration but is limited to a non-adaptable hexagonal building footprint.

The complex topology of termite mounds has been studied with respect to their applicability to multiple domains. For example, Perna et al. (2008) considered the topological efficiency of organically determined termite mound galleries as transportation networks when compared to random and planned networks. Guo et al. (2011) employed a termite mound analogy in mapping complex fracture systems. However, the most studied aspect of the mounds with potential ap-plication to human habitation is the manner in which the mound construction creates natural ventilation (e.g., Turner 2001, Korb 2003, Abou-Houly 2010, King et. al. 2015). The construction and resulting nat-ural ventilation of termite mounds directly ties to bi-omimetic principles described by Tsui (1999) includ-ing the use of local materials, the enhancement of air circulation, and energy efficiency without an external supply of power.

2 MOUND CONSTRUCTION While a wide variety of disciplines have benefited from natural inspiration, there are few that stand to gain more from biomimicry than the building indus-try. As nature’s foremost examples of structural engi-neers, building some of the largest and most complex structures built by any creature in the animal king-dom, termites may be a perfect source of inspiration for engineers to develop ever more energy efficient and stable structures.

An interesting phenomenon of termite construc-tion is the incredible diversity of the mounds. Ter-mites have been observed to create mounds ranging from small, simple domes to ornate “cathedral”

shaped mounds several meters in height. Much of the diversity evident in human construction, aside from that caused by cultural differences arises from envi-ronmental effects and material selection. An initial study was developed to determine if the same factors influence termite construction as human construction. Cultural influences are attributed to species; environ-mental factors and materials are here represented by climate and local soils. The study consisted of a meta-analysis of peer-reviewed literature and internet re-sources with information on mound topology, local soil profiles, local climate and, when available, soils in the mound structure. Over 65 mounds were con-sidered in the study.

Observed termite mounds were grouped into one of several different archetypal external topologies in-cluding dome, cone, cathedral, meridian (compass), and mushroom shaped mounds (see Figure 1). While not comprehensive, these labels represent the most common descriptors applied to termitaria in the field. Despite similarity in form between conical and dome shaped mounds, conical mounds were considered in-dependently, based on their size and a subjective es-timate of how strongly conical they appear (i.e. does the mound come to a point near the top). Due to the study’s emphasis on the structural aspects of the mounds, termitaria that don’t exhibit structural fea-tures, display no commonality even among species and location, or nest in trees were discarded.

Figure 1. Mound topology archetypes (Clockwise: Cathedral, Cone, Meridian and Mushroom)

While the body of literature available on termites

is quite large, few scientists make much note of the environmental conditions that affect the insects. As a result, a number of databases were consulted through this study to pair climate and soil conditions to ob-served mounds. The most relevant climate factors considered were temperature and rainfall. The most significant soil property considered was texture, espe-cially the proportions of sand and clay present, as the size of soil particles has a substantial impact on the soil’s response to water and loads.

The study found that termite mounds are affected quite differently by climate and soil conditions. De-spite common assertions in the literature that imply a heavy correlation between mound structure and cli-mate (e.g. Harris 1956, Korb 2003), this study did not find much evidence to support the claimwith respect to external mound archetypes. It is more likely that environmental forcings play a larger role in the inter-nal mound structure or wall thickness, physical attrib-utes that impact permeability and airflow. There did not appear to be any significant correlation between primary archetype for a mound region and tempera-ture or rainfall. It is worth mentioning that in regions with rainforest climates or monsoon seasons, mound features may erode substantively suggesting a differ-ent archetype in different seasons.

Soil conditions have an impact on the termite’s construction habits. Results of the meta-analysis demonstrate a higher incidence of sand in cathedral, meridian and conical mounds, whereas mushroom mounds were higher in clays. Dome shapes can be found which have highly variable sand and clay con-tents. When used in the construction of the mounds, these particles have a clear impact on the size and structure of the mounds. The high bearing strength and volumetric stability of sand allows cathedral, me-ridian, and cone mounds to develop into some of the largest and most massive of mounds, and the cohe-siveness of clay provides mushroom mounds the abil-ity to form their distinctive shape and thin walled in-terior structure. Figures 2 and 3 show correlations between clay content and mound shape and cone shape (in conical mounds), respectively.

Clayey regions more commonly resulted in domes and mushrooms, and areas with low clay contents more commonly resulted in cathedrals, cones, and meridian mounds. When separated based on size and degree of conicality, cone shaped mounds showed a significant correlation between sand and clay con-tents, with large, pointed mounds found in regions with high sand and low clay, and small and round mounds found in regions with the opposite.

While some scientists have placed significant em-phasis on termites’ ability to selectively choose parti-cles to create mounds which have high sand and clay contents (e.g. Jouquet et al. 2002), this study found that it may be possible that the termites have adapted to construct mound archetypes that are more stable in the inherently more suited to the available soil. As a test of termite responsiveness to stability, angled-build tests were performed with colonies of the ter-mite Odontotermes redemanni in Bangalore.

Figure 2. Clay content versus archetype Figure 3 Clay content versus cone shape

A clinometer was used to place tubes at angles of

0, 10, 15 and 45 degrees. The tubes were observed for three days; build height appeared to level off on day

two and no further expansion into the plastic bag was observed. Height of build into each of the plastic bag extensions was measured at the maximum and mini-mum heights on the 45 degree build or at three sample locations for the other tubes. (see Figure 4.)

Figure 4. 15 degree angled tube over Odontotermes redemanni termite nest The relationship between average build height in the plastic bag and the angle of build is highly propor-tional, with an R2 value of 0.99 (see Figure 5). For maximum build height versus angle, the R2 was 0.95. While this was a single, non-replicated experi-ment, if these results can be replicated, it may indi-cate suggest a high sensitivity of the termites to de-tect a stable build geometry.

Figure 5. Average build height into plastic bag versus angle of build tube

Based on these results, engineers may be able to consider new channels by which they may gain inspi-ration from termites. By noting that the structural form of a termite mound is indeed affected by certain environmental factors, it is clear that termites are true structural engineers, and not simply building accord-ing to random chance and differences in species. From this, engineers have reason to launch into more detailed observations of the mounds, more targeted research, using direct observation of mounds, may re-veal more detailed variation of structural characteris-tics than a meta-analysis of published works was ca-pable of providing. Doing so could show relationships between wall thickness, internal struc-ture, openings, and other features which are not com-monly documented in the literature.

3 BIOTURBATION FOR CONSTRUCTION In addition to preliminary examination of mound to-pology, the initial project study includes investigation of the geotechnical properties of constructed mounds. As an example, mounds vary in height from near ground level to approximately 6 m (depending on species and soil profile) in a 20 km stretch of road in Namibia. These heights are not possible without the modification of the soils by the termites in a manner that adheres the soils. Termite mounds are typically constructed through the selection, mastication and deposition of soil particles by individual termites leading to a hardened, durable, mortar-like material. As an example, one Australian species builds pave-ment-like mounds that survive over 200 years (Abensperg-Traun & Perry 1998). In essence, ter-mites create the equivalent of concrete skyscrapers, and the exact mechanism by which they bio-adhere the materials is a matter of debate. However, it is pos-sible the collaboration of biologists and engineers can use principals from material science and geotechnical engineering to establish strong, locally sources mate-rials that require minimal energy input.

Additionally, soil modification including physical, chemical and mechanical stabilization is a widely ac-cepted method to improve geotechnical performance of soils. Chemical stabilization with addition of chemical additives such as cement, fly ash, lime, and cement kiln dust, is the most frequently used method since they provide fast, efficient, repeatable, and reli-able improvements to soil properties. However, these methods may alter the subsurface conditions by in-creasing the pH level and enabling toxic metals to leach to the surrounding soils, groundwater and sur-face water. Applications of mound construction meth-ods to soil stabilization is another potential applica-tion of termite behavior to provide natural, non-toxic solutions.

Currently, there is no consensus among biologists on the manner in which termites select specific soil particles for deposition and whether they chemically

alter the soils prior to deposition. And while there are numerous studies investigating the physicochemical properties of termite mounds in soil ecology and ag-riculture (e.g. Wood 1988, Kaschuk et al. 2006, and Abe et al. 2009), there are almost no published studies on the resulting mechanical properties of constructed mound soils. One report (Kandasami et. al 2016) sug-gests that in-situ mound material of certain cathedral mounds in India has a compressive strength of up to ten times that of reconstructed soil from the same mounds.

In order to develop an initial material model for analysis of a constructed mound, a preliminary soil investigation of the mechanical characteristics of mound soils in Bangalore, India was performed. Soil samples were collected from tubes placed over active mounds. This method is used to observe active ter-mite construction. (see Figure 6.) Figure 6. Using tubes to develop samples of fresh built termite

structures in Bangalore, India.

Using fresh built soils and samples from estab-lished mounds, Tests were performed at the Soil Me-chanics Lab at the Indian Institute of Science cam-pus (IISc). Tests (including moisture content, liquid limit, plastic limit, unconfined compression, unit weight, and specific gravity) were performed in the lab according to the procedures outlined in ASTM D2216, D4318, D2166, D7263, and D854. Moisture content of the termite mounds was tested from the tube construction at each mound (emerging build) and from the exterior of the mound, (established build). These tests were performed on the same day as sampling to reduce the effects of moisture loss during transport. The remaining tests were per-formed on the disturbed soil samples from the mound and from the construction site.

The two primary differences between emerging build and established build were a) moisture content and b) density of build material. Established build typically included thick walls of material with large openings. Emerging build, often referred to as spongy build, is highly latticed, moist construction that is later backfilled. (see Figure 7.)

As anticipated, samples from the exterior of the

mound and from new build (see Figure 7) varied greatly in moisture content. Samples from multiple locations of emerging build had very similar mois-ture content; this may indicate termite regulation of their environment to produce that specific moisture content. This observation may be of particular note because it is at the value of the plastic limit of the soils, approximately 18.5 – 19.0%. By building mounds at a moisture condition near the plastic limit, termites can utilize the full strength of the ma-terial while maintaining workability. A recent publi-cation validates these initial findings that termites manipulate the soil moisture to at or near the plastic limit of the clay components of the soils (Kandasami et. al 2016).

Figure 7. Emerging “spongy” build at the base of the build tube shown in Figure 6. Comparison of the termite treated soils and red clay from a nearby construction site might suggest that termites are manipulating their environment even further. The plastic limit of the control sample was notably higher than that of the mound soil, which may indicate that the soil’s mechanical properties are being altered by termite treatment, or alterna-tively, termites are particle selecting based on pre-ferred construction properties of the soils. If so, it would enable the termites to build mounds at lower moisture contents in treated soil than in non-treated soil while retaining cohesion and workability.

Specific gravity (SG) of the termite-treated sam-ples from the mound was determined to be 2.48; since SG for inorganic clays typically ranges from 2.6 – 2.8 the lower specific gravity likely indicates the presence of organic components. The addition of organic components by the termites is hypothesized to be a component in the bio-adhesion process.

Unconfined compression tests were performed on the disturbed and reconstituted soils from the mound and the construction site soil. Standard proc-tor compaction was used on the three samples, and the water content was held consistent at 20% for all samples. The construction sample was tested in its in-situ composition, and a second sample was tested that included only the fines (75μm or smaller). The mound soil was found to be capable of holding

significantly more load than either other sample, having nearly twice the capacity of the fines from the construction site, and over four times the capac-ity of the in-situ soil suggesting that the termite treatment has a significant impact on the structural capacity and stability of the mounds. (See Table 1.)

Table 1 Comparison of Unconfined compression strengths of termite mound and control soils

As previously mentioned, Kandasami et al (2016)

reported unconfined compressive strengths of 1200 to 1800 kPa in undisturbed mound samples; these were ten times higher than reconstituted control soils which ranged up to 150 kPa in strength. As a comparison, unconfined compression tests were performed as part of the larger geotechnical study at SDSM&T on intact soil from the mound of Macrotermes michaelseni in Otjiwarongo, Namibia showed values of approx. 350 kPa. This sample was highly weathered with signifi-cantly lower clay content than the Indian samples. However, even this highly friable mound sample demonstrated a more than four times the compressive strength than the reconstituted soils tested in India.

CONCLUSIONS This paper represents the early results in an ongoing research project to examine the applications of mound building termites to the development of sustainable materials and integrated, energy efficient, stable structural topologies. Based on these results, engi-neers may be able to consider new paradigms for structural design and material selection by gaining in-spiration from termites. By noting that the structural form of a termite mound is indeed affected by certain environmental factors, it is clear that termites are true structural engineers, and not simply building accord-ing to random chance and differences in species. Their specific manipulation of the soil moisture and selection of soil types also indicates an innate sense of structural stability in their construction methods. The next evolution in structural engineering may not arise from advanced technologies, but rather from ac-knowledging that nature has developed sustainable solutions that have lasted hundreds of years.

ACKNOWLEDGEMENTS The authors wish to express their appreciation to Prof. T.J Sitharam and Dr. Naveen James of the Indian Institute of Science for their generous assistance in soils testing and access to testing equipment. They also thank Dr. J. Scott Turner, Ms. Pallavi Sharma, and Dr. Rupert Soar for their valued assistance with this ongoing research.

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