Controlling role of suction stress in the Atterberg limits
Soil plasticity and consistency are indicators of a soil’s strength and deformation as a function of water content. They are often treated as secondary concepts in geotechnical analysis, likely due to the arbitrary and empirical nature of their definitions—such as those associated with plastic and liquid limit determinations. However, disregarding these concepts is tantamount to overlooking the fundamental role of water in soil behavior. Suction stress governs the Atterberg limits, and variations in suction stress behavior dictate transitions in soil consistency. Specifically, internal soil stress interactions that generate suction stress control the liquid and plastic limits, and ultimately govern soil plasticity. Consequently, the liquid limit should indicate the onset of these internal mechanisms effect in the soil. Additionally, internal stresses controlling soil behavior above the plastic limit must differ mechanically from those controlling soil behavior below this limit. Much research has been conducted on the Atterberg limits, yet few studies have investigated the internal stresses influencing the liquid and plastic limits. Therefore, this thesis leverages the concept of suction stress to establish a more rational, theoretical, and nuanced understanding of soil plasticity and consistency. An extensive testing protocol was performed on 34 soil samples exhibiting varied plasticity characteristics. A novel testing technique—the suction stress test—is introduced in this dissertation to measure the Modulus Characteristic Curve (MCC), the Soil Shrinkage Curve (SSC), and the Suction Stress Characteristic Curve (SSCC). Hypotheses and results presented herein were validated by comparisons with ASTM D4318 liquid and plastic limit tests conducted under rigorous quality control. The analysis is structured into two main parts: (1) assessing soil consistency at very high-water contents and (2) analyzing soil plasticity with respect to strength, deformation, and stiffness. The onset of the adsorptive component of suction stress controls the ASTM D4318 liquid limit. It was found that soil strength behavior is controlled by adsorptive forces at high water contents. These adsorptive stresses are primarily composed of van der Waals attraction forces. The point at which these forces begin acting within the soil skeleton is defined as the "onset of suction stress." This dissertation proposes determining the corresponding water content using the Soil Shrinkage Curve. Since inter-particle stress is activated by van der Waals forces, the onset of suction stress coincides with the initiation of effective stress, defining the water content boundary between slurry soil and slurry liquid. The primary distinction between these two states is the soil’s ability to mobilize effective stress, making the onset of suction stress closely related to the fundamental concept of the ASTM D4318 liquid limit. Indeed, water content at the onset of suction stress strongly correlates with the ASTM D4318 liquid limit (R² = 0.99). Furthermore, the average suction stress at the ASTM liquid limit was determined to be approximately –2 kPa, closely matching the average undrained shear strength at this limit (1.7 kPa). This connection bridges the theoretical framework presented here with conventional soil mechanics. Thus, the suction stress test offers two alternative methods for determining the ASTM liquid limit—one based on the SSC and another on the SSCC. Importantly, it demonstrates that adsorptive forces govern the brittle failure observed as a miniature landslide in the Casagrande percussion cup test at the liquid limit. Capillary stresses govern the soil behavior when the soil shows ductile characteristics, and adsorptive stresses are associated to the brittle nature of the soil at low and very low water levels. Hence, the transition between capillary and adsorptive control is fundamentally related to the ASTM D4318 plastic limit. Evidence gathered in this dissertation suggests that capillary mechanisms impart ductility to the soil, whereas adsorptive forces contribute to its brittleness. This dissertation proposes a new soil index, termed the “brittleness characteristic point,” which is related to—but distinct from—the ASTM plastic limit. The brittleness characteristic point can be associated with the maximum adsorptive water content, that is, the maximum volume of water adsorbed in the soil. Moreover, this characteristic point marks the onset of stiffening behavior of the sample, typically characterized by a marked increase in the soil’s modulus. Additionally, mathematical modeling of the Suction Stress Characteristic Curve suggests that, above the plastic limit, the water bridges that facilitate surface tension act as a glue, holding the particles together during the ASTM D4318 plastic limit test. A deficit in these water bridges translates into an increase in the soil’s modulus, causing regions lacking capillary water to bear greater forces. As more force concentrates in areas with adsorptive water, cracks begin to form in the sample. When these cracks coalesce, the soil crumbles—a phenomenon that is marked at the ASTM plastic limit. Thus, adsorptive internal mechanisms not only control the sudden failure observed in Casagrande’s cup test but also govern the brittle failure that occurs at the plastic limit.
effective stress, liquid limit, plastic limit, soil shrinkage curve, suction stress, suction stress characteristic curve
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Colorado School of Mines. Arthur Lakes Library