Laboratory study on the effects of hydraulic and granulometric parameters on the response of granular soil to internal erosion

Erosion is a major environmental problem to agricultural land as well as to civil engineering infrastructures. Rainwater infiltration into granular soils can lead to the migration of fine particles by suffusion. This experimental study is conducted to evaluate the susceptibility to erosion of cohesionless soils. The soil under investigation was collected from the coastal region of Mostaganem (West of Algeria) where erosion has recently caused several damages. To assess soil instability to erosion, two approaches have been proposed in the literature: the geometric approach and the hydraulic approach. Few studies have examined the combination of the two methods. The objective of our study is the combination of the two approaches by determining the critical hydraulic load responsible for triggering erosion as a function of soil characteristics. An experimental parametric study was conducted to determine the influence of initial amount of fines, hydraulic gradient and axial stress on the initiation and evolution of suffusion. A combination of the interactions between these parameters allowed us to express the critical hydraulic gradient and to identify the hydraulic behavior of the soil according to the studied parameters. This approach can better estimate the erodibility of cohesionless soils. It can be used in future development studies at this site to reduce the risk of erosion

Jamming and Stress Propagation in Particulate Matter

We present simple models of particulate materials whose mechanical integrity arises from a jamming process. We argue that such media are generically "fragile", that is, they are unable to support certain types of incremental loading without plastic rearrangement. In such models, fragility is naturally linked to the marginal stability of force chain networks (granular skeletons) within the material. Fragile matter exhibits novel mechanical responses that may be relevant to both jammed colloids and cohesionless assemblies of poured, rigid grains.Comment: LATEX, 3 Figures, elsart.cls style file, 11 page

Analysis of Scarp Profiles: Evaluation of Errors in Morphologic Dating

Morphologic analysis of scarp degradation can be used quantitatively to determine relative ages of different scarps formed in cohesionless materials, under the same climatic conditions. Scarps of tectonic origin as well as wavecut or rivercut terraces can be treated as topographic impulses that are attenuated by surface erosional processes. This morphological evolution can be modelled as the convolution of the initial shape with erosion (or degradation) function whose width increases with time. Such modeling applies well to scarps less than 10m high, formed in unconsolidated fanglomerates. To a good approximation, the degradation function is Gaussian with a variance measuring the degree of rounding of the initial shape. This geometric parameter can be called the degradation coefficient. A synthetic experiment shows that the degradation coefficient can be obtained by least squares fitting of profiles levelled perpendicular to the scarp. Gravitational collapse of the free face is accounted for by assuming initial scarp slopes at the angle of repose of the cohesionless materials (30°–35°). Uncertainties in the measured profiles result in an uncertainty in degradation coefficient that can be evaluated graphically. Because the degradation coefficient is sensitive to the regional slope and to three-dimensional processes (gullying, loess accumulation, stream incision, etc.), a reliable and accurate determination of degradation coefficient requires several long profiles across the same scarp. The linear diffusion model of scarp degradation is a Gaussian model in which the degradation coefficient is proportional to numerical age. In that case, absolute dating requires only determination of the propotionality constant, called the mass diffusivity constant. For Holocene scarps a few meters high, in loose alluvium under arid climatic conditions, mass diffusivity constants generally range between 1 and 6 m^2/kyr. Morphologic analysis is a reliable method to compare ages of different scarps in a given area, and it can provide approximate absolute ages of Holocene scarplike landforms

Pile-Soil Interaction in Unsaturated Soil Conditions

The degree of saturation has been proven to significantly affect geotechnical engineering designs for foundations. The changes in water content will influence the way the soil behaves, including its strength and stiffness parameters. These characteristics were analyzed for a uniform silty sand by developing P-Y curves, which relate lateral loading to lateral deformations. These P-Y curves were input into FB-Multipier, a software developed by the Bridge Software Institute. The software is capable of generating deformations as a result of user-defined loading cases. The results indicated that the middle range of degrees of saturation produced the least amount of deformation. This is in accordance with stiffer response in partially saturated soils due to the presence of inter-particle suction stresses

Shear Flows of Rapidly Flowing Granular Materials

Shear flows of granular materials are studied in an open channel. The wall shear is calculated from an open channel momentum equation which includes the density variations in the flow. An experimental technique was developed that allowed the measurement of the average density of the flow at different longitudinal locations in the channel. Two sizes of glass beads are examined and results show the variations in the wall shear as a function of various dimensionless parameters

Flow of wet granular materials: a numerical study

We simulate dense assemblies of frictional spherical grains in steady shear flow under controlled normal stress $P$ in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci, assumed isolated (pendular regime), and to attractive forces. The system behavior depends on two dimensionless control parameters: inertial number $I$ and reduced pressure $P^*=aP/(\pi\Gamma)$, comparing confining forces $\sim a^2P$ to meniscus tensile strength $F_0=\pi\Gamma a$, for grains of diameter $a$ joined by menisci with surface tension $\Gamma$. We pay special attention to the quasi-static limit of slow flow and observe systematic, enduring strain localization in some of the cohesion-dominated ($P^*\sim 0.1$) systems. Homogeneous steady flows are characterized by the dependence of internal friction coefficient $\mu^*$ and solid fraction $\Phi$ on $I$ and $P^*$. We record fairly small but not negligible normal stress differences and the moderate sensitivity of the system to saturation within the pendular regime. Capillary forces have a significant effect on the macroscopic behavior of the system, up to $P^*$ values of several units. The concept of effective pressure may be used to predict an order of magnitude for the strong increase of $\mu^*$ as $P^*$ decreases but such a crude approach is unable to account for the complex structural changes induced by capillary cohesion. Likewise, the Mohr-Coulomb criterion for pressure-dependent critical states is, at best, an approximation valid within a restricted range of pressures, with $P^*\ge 1$. At small enough $P^*$, large clusters of interacting grains form in slow flows, in which liquid bonds survive shear strains of several units. This affects the anisotropies associated to different interactions, and the shape of function $\mu^*(I)$, which departs more slowly from its quasistatic limit than in cohesionless systems.Comment: 20 pages, 29 figures with 39 subfigure

Loose packings of frictional spheres

We have produced loose packings of cohesionless, frictional spheres by sequential deposition of highly-spherical, monodisperse particles through a fluid. By varying the properties of the fluid and the particles, we have identified the Stokes number (St) - rather than the buoyancy of the particles in the fluid - as the parameter controlling the approach to the loose packing limit. The loose packing limit is attained at a threshold value of St at which the kinetic energy of a particle impinging on the packing is fully dissipated by the fluid. Thus, for cohesionless particles, the dynamics of the deposition process, rather than the stability of the static packing, defines the random loose packing limit. We have made direct measurements of the interparticle friction in the fluid, and present an experimental measurement of the loose packing volume fraction, \phi_{RLP}, as a function of the friction coefficient \mu_s.Comment: 6 pages, 5 figure