Identification of Quasi-Solid Bound Water in High-Liquid-Limit Soils Based on Water Density Evolution

High-liquid-limit soils may exhibit relatively low compressibility despite their high water contents and void ratios, suggesting that conventional descriptors alone may not fully capture the effects of soil–water interactions on their engineering behavior. This study proposes the concept of quasi-solid bound water (QSBW) and develops a density-evolution-based method to identify it. Seven soils with different liquid limits were investigated using helium pycnometry, one-dimensional consolidation tests, falling-head permeability tests, and bound-water characterization by NMR, thermogravimetric analysis, and the volumetric flask method. Water with a density exceeding that of free water (0.997 g/cm3) was operationally identified as QSBW and treated as an equivalent solid to define an effective void ratio. The measured QSBW contents ranged from 12.1% to 28.1% by dry soil mass, with an average density of approximately 1.25 g/cm3. At the same conventional void ratio, soils exhibited substantially different compressibility and hydraulic conductivity, which generally decreased with increasing QSBW content. When compared at the same effective void ratio, their consolidation curves and hydraulic conductivities showed marked convergence. The novelty lies in linking water density evolution with an effective-void-ratio framework for interpreting the compression and seepage behavior of high-liquid-limit soils.