Moisture gradients, form a vapor cycle within the viscous boundary layer as an organizing principle to worker termites

Studies of termite mound building have considered the mud they prepare, its properties and its composition. Here we consider the behaviors of the mound building termites Macrotermes michaelseni, (Sjostedt), in the presence of the viscous boundary layer (VBL), which spontaneously forms over any surface that air passes over. We looked how soil moisture and air vapor are coupled to form a feedback loop and a spatiotemporal precursor to worker termites in the presence of mound material. We explored residency and activities of workers when presented with a VBL and either varying substrate temperature gradients or a soil moisture transition within the soil substrate. We report the emergence of a ‘vapor conveyor’, which forms around a neutral evaporative equilibrium point (NEEP) at the soil/air interface, where the soil-borne moisture temperature (along the gradient) and the 100% saturated air-borne vapor temperature coincide within the VBL, forming a bubble of neutral mass transfer which, we propose, worker termites are sensitive to as viscosity changes within. We found, on average, that 67% (std. dev 27%) of behavioral events (clustering, excavation, and deposition) occurred within 10C either side of the NEEP. We found negative correlation (-0.78) between the substrate temperature gradient (0.1-0.9 0C mm-1) and the extents of behavioral activity, suggesting coupling between soil-borne moisture and air-borne vapor advection within the VBL. We recorded unique behaviors relating to interaction with the viscosity of vapor saturated air at this scale. We speculate that workers may exploit the VBL to overcome a classic trade-off, i.e. how to push activities forward into potentially desiccating environments, while conserving moisture in both the termites and the soil they build with

Experimental Studies on the Mechanics of Cohesive Frictional Granular Media

The mechanical behaviour of cohesive-frictional granular materials is a combination of the strength pervading as intergranular friction (represented as an angle of internal friction - Phi), and the cohesion (C) between these particles. Most behavioral or constitutive models of this class of granular materials comprise of a cohesion and frictional component with no regard to the length scale i.e. from the micro structural models through the continuum models. An experimental study has been made on a model granular material, viz. angular sand with different weights of binding agents (varying degrees of cohesion) at multiple length scales to physically map this phenomenon. Cylindrical specimen of various diameters - 10, 20, 38, 100, 150 mm (and with an aspect ratio of 2) are reconstituted with 2, 4 and 8% by weight of a binding agent. The magnitude of this cohesion is analyzed using uniaxial compression tests and it is assumed to correspond to the peak in the normalized stress-strain plot. Increase in the cohesive strength of the material is seen with increasing size of the specimen. A possibility of ``entanglement'' occurring in larger specimens is proposed as a possible reason for deviation from a continuum framework

Manifestation of particle morphology on the mechanical behaviour of granular ensembles

This paper presents the effect of particle morphology (grain shape) on the mechanical response of granular materials. Two model systems with extreme differences in morphology were selected (spherical glass ballotini and angular sand) for this experimental programme. A series of hollow cylinder torsion tests were conducted in this programme under monotonic drained conditions on specimens reconstituted to the same relative density. Tests were conducted under different intermediate principal stress ratio (b) on both the model materials. The glass ballotini shows increased dilation at the outset of the test, however, at large strains, the particle rearrangement in the sand and the increased interlocking leads to higher strength at the critical state. The effect of individual particle morphology is manifested in both the increased friction angle and a larger sized failure locus in stress space with increase in angularity. The stresses developed in these two model materials are also accompanied by intriguing volume change behaviour. The glass ballotini despite a lower strength presents a predominantly dilative response immaterial of the ‘b’ value, while the angular sand shows increased strength at large strains, while showing a contractive response. These results allow incorporation of particle morphology effects at the ensemble level in plasticity based constitutive models

Effect of particle shape on the mechanical response of a granular ensemble

The mechanical behaviour of a granular ensemble at a macro scale is an integration of the interparticle interactions. Even though manifold parameters at multiple length scales govern the behaviour of granular materials, particle shape and size are considered paramount in governing the ensemble level mechanical response such as the strength, stiffness and packing fraction (emax , emin ), friction angle etc. In this research programme, an experimental study using hollow cylinder apparatus is undertaken in order to study the effect of particle shape on the ensemble material behaviour. Two model materials are chosen for these series of experiments - angular sand and spherical glass ballotini. The mean grain size of both sand and glass ballotini used in this study is 0.5 mm. Also the specimens are prepared at a relative density of 30% using the same preparation technique, ensuring almost similar initial fabric. These specimens are subjected to a gamut of stresses by varying the intermediate principal stress ratio (b) such that the critical state surface (or yield locus) could be traced on an octahedral plane (in the principal stress space). The angular particles showed increased critical state strength when compared to the glass ballotini at different intermediate principal stress ratio. The volume change in the angular sand particles are seen to be mostly contractive, while at the same relative density, the glass ballotini showed a dilative response. Finally, the particle shape controls the size of the critical state locus on the octahedral plane, in that, the angular particles show an increased size when compared to the spherical particles. © 2015 Taylor & Francis Group

Experimental studies on the influence of intermediate principal stress and inclination on the mechanical behaviour of angular sands

A comprehensive experimental study has been made on angular sand to investigate various aspects of mechanical behavior. A hollow cylinder torsion testing apparatus is used in this program to apply a range of stress conditions on this angular quartzitic fine sand under monotonic drained shear. The effect of the magnitude and inclination of the principal stresses on an element of sand is studied through these experiments. This magnitude and inclination of the principal stresses are presented as an “ensemble measure of fabric in sands”. This ensemble measure of fabric in the sands evolves through the shearing process, and reaches the final state, which indeed has a unique fabric. The sand shows significant variation in strength with changing inclination of the principal stresses. The locus of the final stress state in principal stress space is also mapped from these series of experiments. Additional aspects of non-coaxiality, a benchmarking exercise with a few constitutive models is presented here. This experimental approach albeit indirect shows that a unique state which is dependent on the fabric, density and confining stress exists. This suite of experiments provides a well-controlled data set for a clear understanding on the mechanical behavior of sands

Experimental studies on the mechanics of cohesive frictional granular media

The mechanical behaviour of cohesive-frictional granular materials is a combination of the strength pervading as intergranular friction (represented as an angle of internal friction - ), and the cohesion (C) between these particles. Most behavioral or constitutive models of this class of granular materials comprise of a cohesion and frictional component with no regard to the length scale i.e. from the micro structural models through the continuum models. An experimental study has been made on a model granular material, viz. angular sand with different weights of binding agents (varying degrees of cohesion) at multiple length scales to physically map this phenomenon. Cylindrical specimen of various diameters-10, 20, 38, 100, 150 mm (and with an aspect ratio of 2) are reconstituted with 2, 4 and 8% by weight of a binding agent. The magnitude of this cohesion is analyzed using uniaxial compression tests and it is assumed to correspond to the peak in the normalized stress-strain plot. Increase in the cohesive strength of the material is seen with increasing size of the specimen. A possibility of "entanglement" occurring in larger specimens is proposed as a possible reason for deviation from a continuum framework. © 2013 AIP Publishing LLC

Experimental investigations on cemented sands with varying intermediate principal stress ratio

Soils are subjected to complex boundary stresses such as the combination of compression, extension and simple shear etc. in the engineering of geotechnical structures. Recreating these complex stresses in the laboratory is paramount in understanding the material constitutivity. Hollow cylinder torsion (HCT) testing apparatus is one such device which not only controls the magnitude but also the direction of the principal stresses independently thereby simulating the complex stress conditions in the laboratory. Additionally, geomaterials found insitu are rarely composed on a unitary materials such as sands or clays. Sands have small amounts of cohesion between the grains often caused due to argillaceous or silicious cementing agents. In order to study the mechanical behaviour of cohesive-frictional granular materials, i.e. sands with small amount of cohesion (cementation) are reconstituted and studied in the laboratory using HCT. A slew of experiments are performed on loose cemented sands (4% cement) consolidated isotropically and sheared under different intermediate principal stress ratio. These experiments are analysed in a framework of plasticity theory, and a "failure surface" in stress space is drawn from this data. A compilation of the effects of this small amount of cohesion on the sand matrix is brought forth by understanding the stressstrain response, volume change and friction angle

Effect of intermediate principal stress on the mechanical behavior of angular sand

Sand is a naturally occurring, cohesionless, granular material with varying morphology. It has been well studied as a model frictional material, and the strength of the sand ensemble is derived from the inter-granular friction. Facets of sand behaviour, such as the inherent anisotropy and effect of intermediate principal stress, are extremely important to model and predict the constitutivity. A complex stress field is required in order to investigate the effect of intermediate principal stress on the failure behaviour of angular sand. Such stress fields can be replicated in the laboratory using a hollow cylinder torsional (HCT) testing apparatus. A slew of experiments are carried out at a particular density under drained conditions by varying intermediate principal stress ratio 'b'. The experimental observations are studied in the framework of classical critical state soil mechanics and the results are analyzed using plasticity theory. The effect of intermediate principal stress ratio on the non-coaxial and failure behaviour is highlighted. © 2014 American Society of Civil Engineers

Effect of biocementation on the strength and stability of termite mounds

Termite mounds are bioengineered granular ensembles that remain stable over decades, a vital requirement for termite societies that house millions of individual termites. An experimental study on the mechanobiology of mounds and mound soil of the fungus-growing termite Odontotermes obesus (Rambur) demonstrated that termites are capable engineers. Mound soil was significantly different in its physical and mechanical properties compared to the surrounding or ‘control’ soil. However, mound and control soils did not differ in clay mineralogy. Utilising the finer soil fraction, termites altered the soil significantly by cohering grains through their secretions into units called boluses, in the presence of water. Termites modulated the amount of water close to the plastic limit of the soil while preparing these boluses such that the soil could be effectively moulded. The cementation effected by termites using their secretions and/or excretions enhanced the strength of the soil tenfold, which may not be achievable otherwise. The soil modification achieved by the termites decreased mound susceptibility to erosion and collapse. Termites successfully cemented foreign materials, suggesting a wide range of cementation abilities. Slope stability analysis with intact mound soil revealed a significant increase in the safety factor of the mound compared to that of reconstituted soil

Mechanics and modeling of cohesive frictional granular materials

In nature, weakly cemented granular materials are encountered in the form of soft rocks such as limestone, sandstone, mudstone, shale, etc. The mechanical behaviour of these materials is quite different from the purely frictional granular materials. The presence of cementation between the grains causes a significant variation in mechanical response under complex boundary conditions. In order to understand the manifestation of this interparticle cohesion at the ensemble level, we have used a hollow cylinder torsional testing apparatus which is capable of independently controlling the magnitude and the direction of the three principal stresses. From this experimental programme, the small strain response, peak strength and post peak behaviour with changing intermediate principle stress ratio (b) and initial mean effective stress (I1) is studied. In addition to the analysis of stress strain behaviour at different b and I1, stress-dilatancy characteristics of these cohesive frictional material are also discussed. This experimental study is followed by calibration and validation of a single hardening constitutive model which considers cementation as additional confinement. Observations from validation exercises suggest that this consideration works well for stress-strain response whereas it fails to predict the volumetric behaviour