University of Qom Civil Infrastructure Researches 2783-140X Articles in Press 2025 12 28 The Effect of Humidity and Calcium Carbonate on the Shear Strength of Type D Alluvium in Southern Tehran The Effect of Humidity and Calcium Carbonate on the Shear Strength of Type D Alluvium in Southern Tehran 4044 10.22091/cer.2025.13865.1651 FA Mehdi Talkhablou Department of Applied Geology, Faculty of Earth Sciences, Kharazmi University, Tehran 0009-0007-7657-7008 Arash Mahmoudi Department of Civil Engineering, Faculty of Engineering, University of Bu-Ali Sina, Hamedan, Iran. 0000-0001-9686-0870 Dariush Fallah Department of Applied Geology, Faculty of Earth Sciences, Kharazmi University, Tehran, Journal Article 2025 09 13 This study investigates the simultaneous effects of moisture content and calcium carbonate (CaCO₃) percentage on the shear strength of Type D alluvium in southern Tehran. Fifty soil samples were systematically collected from districts 11 to 20 at depths of 1.5–4.0 m, with sampling locations precisely mapped on the alluvial zoning map of Tehran (Figure 1). Laboratory tests were conducted under three controlled moisture conditions: 8%, 14%, and 31% (representing dry, intermediate, and saturated states, respectively). Results show that increasing moisture to saturation causes a significant reduction in shear strength: cohesion (c) decreases by 38.8% (from 5.20 kPa to 3.18 kPa), and the internal friction angle (φ) drops by 8.4° (from 25.5° to 17.1°). Mixed-effects regression analysis confirms that moisture content (β=–1.78, p<0.001) is the dominant predictor of cohesion, while calcium carbonate content (mean=12.4%, range:7–26%) shows no statistically significant correlation with shear strength parameters (r= 0.12, p>0.05). Notably, even in the presence of naturally occurring carbonate nodules (caliche), moisture remains the overriding factor controlling mechanical behavior. This study concludes that for geotechnical design in southern Tehran, critical moisture scenarios (particularly saturation) must be treated as governing conditions, as the soil exhibits relatively uniform shear response under identical moisture states contrary to common assumptions about the strengthening role of carbonate cementation. This study investigates the simultaneous effects of moisture content and calcium carbonate (CaCO₃) percentage on the shear strength of Type D alluvium in southern Tehran. Fifty soil samples were systematically collected from districts 11 to 20 at depths of 1.5–4.0 m, with sampling locations precisely mapped on the alluvial zoning map of Tehran (Figure 1). Laboratory tests were conducted under three controlled moisture conditions: 8%, 14%, and 31% (representing dry, intermediate, and saturated states, respectively). Results show that increasing moisture to saturation causes a significant reduction in shear strength: cohesion (c) decreases by 38.8% (from 5.20 kPa to 3.18 kPa), and the internal friction angle (φ) drops by 8.4° (from 25.5° to 17.1°). Mixed-effects regression analysis confirms that moisture content (β=–1.78, p<0.001) is the dominant predictor of cohesion, while calcium carbonate content (mean=12.4%, range:7–26%) shows no statistically significant correlation with shear strength parameters (r= 0.12, p>0.05). Notably, even in the presence of naturally occurring carbonate nodules (caliche), moisture remains the overriding factor controlling mechanical behavior. This study concludes that for geotechnical design in southern Tehran, critical moisture scenarios (particularly saturation) must be treated as governing conditions, as the soil exhibits relatively uniform shear response under identical moisture states contrary to common assumptions about the strengthening role of carbonate cementation.