Pore Scale Model of Wet Granular Material

Landslides are one of the main natural threats for both people and infrastructure in mountainousregions. In most cases, landslides are triggered by the loss of cohesion in a soil due to the increasingliquid saturation level after long or intense rainfalls. Only little is known on the triggering of failuresat the microscopic scale due to the limited experimental accessibility and the lack ofmicromechanical models that combine the granular mechanics and the fluid transport in the porespace for the relevant saturation levels simultaneously.We study this problem by a discrete model approach based on contact dynamics [1], where forcesexcreted from a liquid phase add to the motion of spherical particles. The model can deal witharbitrary saturation levels, ranging from the capillary bridge regime to the fully saturated state. Toaccount for liquid in the pore space we develop a geometrical pore-scale model similar to the modelsproposed recently by Gladkikh [2] and Motealleh [3]. In our model, we allow the formation ofisolated arbitrary-sized liquid clusters which can have different Laplace pressures and exchangeliquid through liquid films on the grain surface. These clusters can grow in size, shrink, merge andsplit, depending on local conditions and changes of accessible liquid and the pore space. In the firststep the model is applied to an immobile dense packing of spheres. We demonstrate the validity ofthe proposed model on benchmark examples, including evolution of a small single cluster (trimer), anequilibrated packing at different saturation levels and cluster growth by liquid injection [4, 5]. Next,we calculate the failure envelope for the dilatant material under multi-axial loading, consideringpossible rearrangements of particles and fluid interfaces at changing saturation states. REFERENCES[1] Mani, R., Kadau, D., Or, D. and Herrmann, H.J.: Fluid depletion in shear bands, PhysicalReview Letters 109 (2012), 248001. [2] Gladkikh, M. and Bryant, S.: Prediction of imbibition in unconsolidated granular materials,Journal of Colloid and Interface Science 288 (2005), 526-539. [3] Motealleh, S., DiCarlo, D. and Bryant, S.: Unified Model of Drainage and Imbibition in 3DFractionally Wet Porous Media, Transport in Porous Media 99 (2013), 581-611. [4] Scheel, M., Seemann, R., Brinkmann, M., Di Michiel, M., Sheppard, A., Breidenbach, B. andHerminghaus, S.: Morphological clues to wet granular pile stability, Nature Materials 7 (2008),pp. 189-193. [5] Scheel, M.: Experimental investigations of the mechanical properties of wet granular matter,PhD thesis (2009), Georg-August-Universität Göttingen.