Nanoscale soil-water retention mechanism of unsaturated clay via MD and machine learning - Preprint

Aug 4, 2026

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This article explores the nanoscale soil-water retention mechanism of unsaturated clay using molecular dynamics and machine learning. Pyrophyllite is used as a stable 2:1 clay precursor, and low-saturation systems are simulated via a set of MD runs. Soil water is modeled through a center-of-mass point cloud, while the water-air interfacial area is computed using the alpha-shape method. Adsorptive pressure and capillary pressure are separated at varying mass water contents, and adsorption is quantified through the water number density profile. A neural-network model links matric suction, mass water content, and apparent interfacial area, showing van der Waals and hydroxyl hydration dominate adsorption at the nanoscale, while capillarity becomes more prominent with increasing water content.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages33

Reading Time1 h 23 min

AccessFree to download

Introduction

Soil-water retention curve and matric suction in unsaturated soils

Importance of adsorption effects at low saturation

Role of water-air interface and interfacial forces

Computational and experimental approaches to interfacial area

Nanoscale soil-water retention mechanism of unsaturated clay via MD and machine learning Zhe Zhang, Xiaoyu Song ∗ Department of Civil and Coastal Engineering, University of Florida, Gainesville, Florida Abstract In this article, we investigate the nanoscale soil-water retention mechanism of unsaturated clay through molecular dynamics and machine learning. Pyrophyllite was chosen due to its stable structure and as the precursor of other 2:1 clay minerals. A series of molecular dynamics simulations of clay at low degrees of saturation were conducted. Soil water was represented by a point cloud through the center-of-mass method. Water-air interface area was measured numerically by the alpha-shape method. The soil-water retention mechanism at the nanoscale was analyzed by distinguishing adsorptive pressure and capillary pressure at different mass water contents and considering the apparent capillary interface area (i.e., water-air interface area per unit water volume) . The water number density profile was used to quantify the adsorption effect. A neural-network based machine learning technique was utilized to construct function relationships among matric suction, the mass water content, and the apparent water-air interface area. Our numerical results have demonstrated from a nanoscale perspective that the adsorption effect is dominated by the van der Waals force and hydroxyl hydration between the clay surface and water. As the mass water content increases, the adsorption pressure decreases, and capillarity plays a prominent role in the soil-water retention mechanism at the nanoscale. Keywords: soil-water retention, interfacial area, unsaturated clay, adsorption, machine learning 1. Introduction The physics and mechanics of unsaturated soils are important in geotechnical and geoenvironmental engineering (e.g., Terzaghi et al., 1996; Gens, 2010; Fredlund, 2006; Ng and Menzies, 2014; Song, 2017; Alonso, 2021; Menon and Song, 2022, 2023) . Soil-water retention/characteristic curve (SWRC) is a mathematical relationship between soil water content and matric suction (e.g., Brooks, 1965; Van Genuchten, 1980; Fredlund and Rahardjo, 1993; Fredlund and Xing, 1994; Niu et al., 2020; Cao et al., 2018; Chen et al., 2019) . It is a fundamental constitutive law for modeling the physics and mechanics of unsaturated soils. For instance, a soil water retention curve is required in modeling multiphase fluid flow, shear strength, deformation, and stress-strain relationships of unsaturated soils (e.g., Alonso et al., 1990; Wheeler et al., 2003; Macari et al., 2003; Hoyos and Arduino, ∗CorrEmaiespl aodnddinressg: authoxyso[r](rng@ufl.edu)[ng@ufl.edu](rng@ufl.edu) (Xiaoyu Song) Preprint submitted to Elsevier 2008; Alonso et al., 2010) . In unsaturated soil mechanics and continuum-based numerical methods for modeling unsaturated soils with no osmosis effect, matric suction is usually assumed to be the difference between pore air pressure and pore water pressure and the latter is usually assumed to be the capillary pressure due to water menisci (e.g., Fredlund and Rahardjo, 1993; Borja, 2004, 2006; Menon and Song, 2020, 2021; Song et al., 2017; Wang and Song, 2020) without considering adsorptive water pressure. The adsorptive water pressure might be ignored at a high degree of saturation. However, at a low degree of saturation, it should be considered to interpret high matric suction (e.g., in the order of hundred megapascals) (Fredlund and Rahardjo, 1993; Lu and Likos, 2006; Zhang and Lu, 2019) . It is noted that the pressure of capillary water is lower than air pressure due to the curve water-air interface (i.e., meniscus), and pressure of adsorptive water is higher than air pressure due to adsorptive force (Luo et al., 2022) . Furthermore, both experimental and theoretical studies have suggested that the water-air interface should be taken into account to better describe soil water retention curves of unsaturated soils (Fredlun

FAQs

Why is pyrophyllite selected for studying unsaturated clay water retention?

Pyrophyllite is chosen for its stable structure and as a precursor of other 2:1 clay minerals.

How are soil water and the water-air interfacial area represented in the simulations?

Soil water is represented by a point cloud using the center-of-mass method, and the water-air interface area is measured numerically using the alpha-shape method.

What roles do adsorption and capillarity play across different mass water contents?

From nanoscale analysis, adsorption is dominated by van der Waals force and hydroxyl hydration; as mass water content increases, adsorption pressure decreases and capillarity becomes more prominent.

Aug 4, 2026

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