Evaluation of capillary water retention effects on the development of the suction stress characteristic curve

Abstract

The suction stress characteristic framework is a practical approach for relating the suction and the water-filled pore volume to the stress state of unsaturated soils. It predicts the effective stress by developing the suction stress characteristic curve from the soil-water retention curve. In this framework, the effective degree of saturation is usually calculated by the empirical water retention model of van Genuchten (published in 1980). In this paper, the use of a generalized soil-water retention model proposed by Lu in 2016, which differentiates the role of capillary and adsorption mechanisms, in the suction stress characteristic framework is studied. A redefinition of the effective degree of saturation is suggested, by choosing the retention state where capillarity approaches zero instead of the residual retention state. The validity of this assumption is examined using experimental data obtained by unsaturated shear strength and retention tests and datasets collected from the literature. The proposed definition is applicable for a variety of soils where capillarity is the dominant mechanism in producing suction stress within the range of suction 0–1500 kPa. In addition, it is observed that the generalized soil-water retention model presents a more realistic prediction of unsaturated shear strength compared with empirical water retention models.

Investigation of the Water-Retention Characteristics and Mechanical Behavior of Fibre-Reinforced Unsaturated Sand

“…Consequently, the water-retention characteristics improve [52]. The main factors affecting the total soil suction are capillary action [53,54], short-range adsorption (interaction between particles and pore water) [55], and infiltration [56]. The foregoing analysis indicates that fibres affect the water-retention characteristics of soil.…”

Effect of Composition on Engineering Behavior of Clay Tills

“…This means that the numerous larger pores easily released most of the capillary water at low suction values thereby governing the initial portion of the curve from 0.1 kPa to 700 kPa. Conversely, the fewer smaller pores released the remainder of capillary water and part of the adsorbed water only with the application of high suction values, that is, in the range of 700 kPa to 10 6 kPa [68]. Irrespective of the index property on the y-axis, the WRC comprised two air entry values and two residual suction values associated with capillary water drainage [69,70]: from larger pores (AEVL = 2 kPa and RSVL = 20 kPa) and from smaller pores (AEVS = 700 kPa and RSVS = 5 × 10 4 kPa).…”

“…This means that the numerous larger pores easily released most of the capillary water at low suction values thereby governing the initial portion of the curve from 0.1 kPa to 700 kPa. Conversely, the fewer smaller pores released the remainder of capillary water and part of the adsorbed water only with the application of high suction values, that is, in the range of 700 kPa to 10 6 kPa [68].…”

“…The low swelling potential (2.5%) under the seating stress (5 kPa) is attributed to the presence of numerous larger pores, which consumed clay expansion in all directions between the lumps. Further decrease in e under applied stresses is attributed to the gradual reduction in the number of large pores and the expulsion of water and air from these pores [68,81]. The adjacent particles slipped due to the presence of water, thereby resulting in re-arrangement of soil particles and reduced pore space.…”

Discussion of “Unified Effective Stress Equation for Soil” by Chao Zhang and Ning Lu

“…In contrast, stresses in soil reflect the mechanical energy per unit volume of the soil-water-air mixture." That is presumably why many researchers have previously suggested that capillary water, solely, plays a role in the mechanical behavior of unsaturated soils (Konrad and Lebeau 2015;Zhou et al 2018;Maleksaeedi and Nuth 2019). In fact, Lu et al (2010) argued that "the residual water content is the amount of water that remains primarily in the form of thin films surrounding the soil particle surfaces at very high suctions but has very little effect on the interparticle suction stress."…”