Laboratory permeability tests on sand: influence of the compaction method on anisotropy

Abstract

New laboratory results are presented on how densification influences the hydraulic anisotropy of a sand. Results are given for two compaction modes. Tests were performed at different densities in order to derive the curves kv(e) and kh(e). The samples were statically compacted in the first series and dynamically compacted in the second series. The ratio rk = kh/kv is different for the two series. It is higher than 1 for static compaction and lower than 1 for dynamic compaction. Thus, the anisotropy ratio of granular soils can take several values for the same void ratio, depending on the preparation mode. Even if the anisotropy ratios differ, the first invariant of the hydraulic conductivity tensor, 11k, is found to be a function of e only for this sand, regardless of the densification mode. The evolution of an adimensional anisotropy index, I1k/kv, versus a density index, Ie, is compared for the sand and a dispersed clay. The evolutions are nearly identical, which means that the hydraulic anisotropy of natural homogeneous soils may be similar for sediments having settled in still water and influenced subsequently by gravity only. From an engineering viewpoint, a reasonably good fit is obtained between experimental results and predictive charts. The results also demonstrate that the high apparent anisotropy often found either in natural or in compacted soils is not induced mainly by deposition or compaction but rather is due to nonuniformity or segregation in the soil mass. Key words: soil, permeability, anisotropy, laboratory, compaction.

Stress-induced anisotropy in granular materials: fabric, stiffness, and permeability

“…We do see, however, similarities between the anisotropies of permeability and those of the median free path and the median radial breadth of the void space (see Figs. 12,13). Anisotropy in the permeability K is negatively correlated with the median free path matrix L v and is positively correlated with the median radial breadth matrix R v .…”

“…The hydraulic properties of the granular assemblies, which depend on both the size and geometrical features of the void space, may also change due to rearrangement of the voids and deformation of the grain network. From a theoretical standpoint, an anisotropy of the permeability tensor means that it has eigenvalues of distinct magnitudes, and there exist three orthogonal principal directions corresponding to these eigenvalues [10,13]. Wong [75] proposed a simple model that expands the Kozeny-Carman equation to an anisotropic model by simply assuming that the strain and permeability tensors share the same principal directions.…”

Discussion of "Network model for hydraulic conductivity of sand-bentonite mixtures"

“…Afterwards, it would be interesting to extend its use to predict the k value of any soil defined by its gradation and its limits of consistency. It would also be interesting to consider the effects of gravity and vertical static compaction to try to reproduce available data on permeability anisotropy of natural soils (e.g., Chapuis et al 1989b;Chapuis and Gill 1989).…”

Numerical modelling of fluid flow in microscopic images of granular materials

“…Few reliable results are available for granular materials. Chapuis et al [24] presented laboratory results on the e!ect of densi"cation methods on the permeability of a cohesionless soil. Results were given for two compaction methods.…”