Citation: Kocaman, K.; Ozocak, A.; Edil, T.B.; Bol, E.; Sert, S.; Onturk, K.; Ozsagir, M. Evaluation of Soil-Water Characteristic Curve and Pore-Size Distribution of Fine-Grained Soils. Water 2022, 14, 3445. https:// doi.org/10.3390/w14213445 Academic Editor: Yuanzheng Zhai Received: 30 September 2022 Accepted: 26 October 2022 Published: 29 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Evaluation of Soil-Water Characteristic Curve and Pore-Size Distribution of Fine-Grained Soils Kadir Kocaman 1, * , Askin Ozocak 1 , Tuncer B. Edil 2 , Ertan Bol 1 , Sedat Sert 1 , Kurban Onturk 3 and Mustafa Ozsagir 1 1 Department of Civil Engineering, Faculty of Engineering, Sakarya University, Serdivan 54050, Turkey 2 Department of Civil and Environmental Engineering, College of Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA 3 Department of Civil Engineering, Faculty of Technology, Sakarya University of Applied Sciences, Serdivan 54050, Turkey * Correspondence: kadirkocaman@sakarya.edu.tr Abstract: A soil’s physical properties, mineral types, and pore structure significantly influence the shape and properties of the soil-water characteristic curve (SWCC). This study investigated the effects of the soil’s physical properties and mineral types on the SWCC and pore-size distribution (PSD). Eight different soils from an alluvial deposit in Istanbul and Adapazarı/Türkiye were used in the study. The test samples were prepared by compaction at optimum water content (OWC) and wet side of optimum water content (wet of OWC). The samples were prepared by consolidation from the slurry. The PSDs of the samples were calculated using the SWCCs and evaluated with scanning electron microscope (SEM) analysis. In addition, the mineral types of all soils were determined by X-ray diffraction analysis. The soil which contains illite-type minerals has higher matric suction than containing kaolin-type. The effect of the clay percentage is more pronounced in silty soils than in plasticity and activity. Soil suction increased with decreasing compaction water content in clayey soils. The air entry water contents rose as the void ratio, liquid limit, clay content, and plasticity increased. The compaction conditions affected the macropore structure more than the micropore structure. In addition, the ratio of macro-micro pore sizes increased with the rise of the compaction water content. Keywords: unsaturated soil; soil suction; SWCC; PSD; fine-grained soils 1. Introduction and Background Permeability, shear strength, and volume change properties mainly depend on the water retention capacity of soils. The water-holding capacity of unsaturated soils is defined as the soil-water characteristic curve (SWCC) [1–4]. The SWCC is expressed as water content (alternatively degree of saturation) versus suction. It has two critical properties: air entry value (AEV) and residual water content (θ r ). The AEV is the suction value where the water drains starting from the large pores, and thus the air becomes continuous in the soil [5]. Suction values smaller than the AEV indicate the saturated capillary zone, whereas those greater than the residual value indicate the residual water zone [6]. The residual value is the suction value where the drainage in the liquid phase in the soil pores starts to decrease, and the water evaporates [2,7]. Many soil characteristics, such as soil structure, stress state, mineralogy, specific surface, chemical composition, initial water content, void ratio, surface activity, pore water properties, and pore-size distribution (PSD), affect the engineering properties of unsaturated soils [8,9]. The compaction energy, compaction method, and initial water content, through their control of the pore structure of the soil, influence the shape of SWCC in compacted soils [6,10–15]. Marinho (2005) determined that the slope of the SWCC increased with the increasing plasticity by examining 18 soil samples between 24% and 95% percent liquid limit and 6–60% plasticity index [16]. Tinjum et al. Water 2022, 14, 3445. https://doi.org/10.3390/w14213445 https://www.mdpi.com/journal/water