Assessment of the Impact of Degree of soil compactness Indicator on Soil Hard-Setting Index and Soil Water Retention Curve in Gypsum-Containing Soils

Laboratory experiments were conducted to analyze the reflection of the soil compactness degree index (DC%) which represents the ratio between the apparent soil density (BD natural) and the critical apparent soil density (Proctor density) (BD ref.) in the soil stiffness index (H-index) and the Soil Moisture Retention Curve (SMRC) for seven soil samples with different gypsum content (60.12-414 g kg⁻¹). Soil specimens with gypsum contents of (G2) 111, (G3) 155, (G4) 219, (G5) 263, and (G6) 362 g kg⁻¹ They were prepared through mixing a low-gypsum surface soil sample (G1) containing 60.12 g kg⁻¹ gypsum with a high-gypsum subsurface soil sample (G7) containing 414 g kg⁻¹ gypsum. The soil moisture retention curve was determined for each gypsum soil sample at suction pressures of 0, 2, 7, 33, 100, 200, 500, 1000, and 1500 kPa after compacting the gypsum soil samples to densities of 1.4, 1.5, 1.6, and 1.8 Mg m⁻³. The Soil Hard-Setting Index (H-index) was calculated using the van Genuchten-Mualem equation and applying the RETC program. The results showed that the values of the soil stiffness index (H-index) increased with the increase of the compaction degree index (DC%), while the soil moisture retention curve decreased with the increase of the compacted soil bulk density, i.e., with the increase of the compaction degree index. Additionally, both the soil stiffness index and the compaction degree index decreased With the rising gypsum content in the soil.