@article {Santos2011,
	title = {{Compression and shear-wave velocities in discrete particle simulations of quartz granular packings: Improved Hertz-Mindlin contact model}},
	journal = {GEOPHYSICS},
	volume = {76},
	number = {5},
	year = {2011},
	month = {sep},
	pages = {E165{\textendash}E174},
	abstract = {The Hertz-Mindlin (HM) contact model has been a cornerstone for the development of several effective medium theories (EMTs) aimed at describing the mesoscopic and macroscopic mechanical behavior of granular materials like unconsolidated sands. In addition, this model is at the core of most of the discrete particle method designs used to numerically solve for the responses of these heterogeneous materials to external perturbations, like acoustic and stress-strain experiments. However, this model has shown shortcomings in the description of the shear response characterization of granular materials, partly due to the non-affine motions experienced by the grains. We have developed a correction of the model based on a detailed calibration of our acoustic numerical results with previous empirical data. Using a microscopic approach to the grain-grain contact surfaces, the nature of the corrections found appear to be related to the shear resistant asperities and the smaller scale of the grain-grain contact areas compared to the total area assumed by the HM model. An improved HM model characterized by a tangential stiffness weakening is based on these surface corrections. Using this observation an enhanced EMT theory emerges based not only on the tangential stiffness modification but also on the velocity-pressure dependence obtained during the calibration of our numerical model. {\textcopyright} 2011 Society of Exploration Geophysicists.},
	keywords = {3D, Compression wave (P-wave), Rock physics, Shear-wave (S-wave), Wave propagation},
	issn = {0016-8033},
	doi = {10.1190/geo2010-0376.1},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84857461284\&partnerID=tZOtx3y1},
	author = {Santos, Carlos and Urdaneta, Vanessa and Garcia, Xavier and Medina, Ernesto}
}
@article {Garcia2008a,
	title = {{Strong-weak network anisotropy switching and hysteresis in three-dimensional granular materials}},
	journal = {Physical Review E},
	volume = {78},
	number = {2},
	year = {2008},
	pages = {021305},
	abstract = {We address hysteresis of three-dimensional polydisperse granular packs, comparing macro- and microscopic viewpoints, to reveal their elastic/inelastic mechanics and force network anisotropy. During the uniaxial loading-unloading cycle of an appropriately prepared pack, one can decompose the force network into weak and strong subnetworks. The first stages of loading exhibit arching, where all the fabric displays negative anisotropy. For later stages, the strong (weak) network shows positive (negative) anisotropy. On unloading, the force network progresses to a fabric wide hydrostatic point, where the anisotropies of the weak and strong subnetworks switch signs. During the loading stage, a Mohr circle analysis permits the identification of a well-defined macroscopic internal friction angle, whose value is larger than that of grain-grain interactions. To analyze unloading, a generalized local Coulomb-friction argument predicts a continuously changing friction angle, that vanishes at the hydrostatic point. A suggestive interplay between microscopic friction and fabric structure, at different loding stages, is proposed as the mechanism for the emergence of a macro internal friction angle. {\textcopyright} 2008 The American Physical Society.},
	issn = {1539-3755},
	doi = {10.1103/PhysRevE.78.021305},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-50849085166\&partnerID=tZOtx3y1},
	author = {Garcia, Xavier and Medina, Ernesto}
}
@article {Garcia2007,
	title = {{Acoustic response of cemented granular sedimentary rocks: Molecular dynamics modeling}},
	journal = {Physical Review E},
	volume = {75},
	number = {6},
	year = {2007},
	month = {jun},
	pages = {061308},
	abstract = {The effect of cementation processes on the acoustical properties of sands is studied via molecular dynamics simulation methods. We propose numerical methods where the initial uncemented sand is built by simulating the settling process of sediments. Uncemented samples of different porosity are considered by emulating natural mechanical compaction of sediments due to overburden. Cementation is considered through a particle-based model that captures the underlying physics behind the process. In our simulations, we consider samples with different degrees of compaction and cementing materials with distinct elastic properties. The microstructure of cemented sands is taken into account while adding cement at specific locations within the pores, such as grain-to-grain contacts. Results show that the acoustical properties of cemented sands are strongly dependent on the amount of cement, its stiffness relative to the hosting medium, and its location within the pores. Simulation results are in good correspondence with available experimental data and compare favorably with some theoretical predictions for the sound velocity within a range of cement saturation, porosity, and confining pressure. {\textcopyright} 2007 The American Physical Society.},
	issn = {1539-3755},
	doi = {10.1103/PhysRevE.75.061308},
	url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-34547305692\&partnerID=tZOtx3y1},
	author = {Garcia, Xavier and Medina, Ernesto}
}
