Estimating water retention curves and strength properties of unsaturated sandy soils from basic soil gradation parameters

Abstract

This study proposed two pedotransfer functions (PTFs) to estimate sandy soil water retention curves. It is based on the van Genuchten's water retention model and from a semiphysical and semistatistical approach. Basic gradation parameters of d60 as particle size at 60% passing and the coefficient of uniformity Cu are employed in the PTFs with two idealized conditions, the monosized scenario and the extremely polydisperse condition, satisfied. Water retention tests are carried out on eight granular materials with narrow particle size distributions as supplementary data of the UNSODA database. The air entry value is expressed as inversely proportional to d60 and the parameter n, which is related to slope of water retention curve, is a function of Cu. The proposed PTFs, although have fewer parameters, have better fitness than previous PTFs for sandy soils. Furthermore, by incorporating with the suction stress definition, the proposed pedotransfer functions are imbedded in shear strength equations which provide a way to estimate capillary induced tensile strength or cohesion at a certain suction or degree of saturation from basic soil gradation parameters. The estimation shows quantitative agreement with experimental data in literature, and it also explains that the capillary‐induced cohesion is generally higher for materials with finer mean particle size or higher polydispersity.

Estimation of Unsaturated Hydraulic Conductivity of Granular Soils from Particle Size Parameters

“…where ω and β are unitless constants. After a database consisting of 431 unlithified sediment samples [18] the two parameters are fitted as ω = 0.2 and β = 0.93 by Wang et al [12]. However, the model in Equation (7) didn't consider the porosity variation of a particular sample due to its micro-structure arrangement.…”

“…In literature, there are a number of mathematical models to predict the water retention behaviour of an unsaturated soil from its particle size distribution, which is usually called pedotransfer functions. One of the most recent models is proposed by [12] using a semi-physical (based on dimensional analysis) and semi-empirical approach after van Genuchten's closed-form equation. The authors have proved that for sandy soils it generally has better performance than other typical empirical models, especially when the grain size is more uniform.…”

“…Normally, unsaturated hydraulic conductivity is regarded as a parameter associated with its saturated hydraulic conductivity and the soil water retention curve (WRC, the relationship between suction and degree of saturation) [1][2][3]. For sandy soils, it is also widely accepted that using grain size distribution parameters can estimate its saturated hydraulic conductivity [4][5][6][7][8] and water retention curve [9][10][11][12]. Therefore, the unsaturated conductivity properties of sandy soils may also be primarily predicted from their particle size distributions.…”

“…Therefore, the unsaturated conductivity properties of sandy soils may also be primarily predicted from their particle size distributions. The recent work of Wang et al [8,12] can be extended for estimating unsaturated conductivity properties, for example, the relative permeability.…”

“…The method shows the best prediction accuracy among the classic methods. Moreover, they have also developed a method to estimate water retention behaviour of sandy soils after van Genuchten's closed-form equation [12], which is also related to the unsaturated permeability. In this paper, we will clarify the applicable soil type (range of particle size distributions) for Wang's estimation methods by assuming that the methods are more suitable for sandy soils with unimodal pore-size distributions, which has not been covered by the previous studies.…”