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Landslides are an important type of natural hazard that causes catastrophic consequences in terms of human causalities and economic losses. In Hong Kong, more than 100,000 visible natural terrain landslides have been identified. In the Three-Gorge Reservoir Zone in China, 684 large slopes with volumes larger than 100,000 m3 and numerous smaller slopes have been identified.

In this chapter, the hydraulic properties of the five design soils listed in Table 2.2 are investigated. The SWCCs were measured in the laboratory using the newly developed low suction SWCC device (Fig. 4.2) and three types’ commercial available devices (i.e., Fredlund SWCC device, Model 1600 pressure plate extractor, and WP4 dewpoint potentiometer device). The soil hydraulic conductivity functions (SHCFs) were measured using the wetting front advancing column tests as described in Chap. 4.

The soil water characteristic curve (SWCC) and the soil hydraulic conductivity function (SHCF) are two important soil parameter functions required when performing seepage analysis for saturated/unsaturated soil systems. Examples of engineering applications where a soil hydraulic conductivity function is involved include the triggering of landslides due to rainfall infiltration, and the modeling of flow and volume change in collapsing soils, compacted soils, and expansive clays. The SHCF is also required when modeling contaminant migration in vadose zones and the design of capillary barriers and cover systems. In this chapter, the techniques for measuring the hydraulic properties of unsaturated coarse granular soils (i.e., SWCCs and SHCFs) were introduced.

Seepage in unsaturated media is a common phenomenon in geotechnical and geoenvironmental engineering, such as rainfall infiltration into slopes (Ng and Shi, Comput Geotech 22:1–28, 1998; Wu et al., Comput Geotech 117, 2020a), water flow through capillary barrier covers (Chen et al., Comput Geotech 118, 2020; Zhan et al., Sci Total Environ 718, 2020), and contamination migration in unsaturated soils (Bai et al., Int J Heat Mass Transf 153, 2020; Wei et al., Migration and transformation of chromium in unsaturated soil during groundwater table fluctuations induced by rainfall. J Hazard Mater, 126,229, 2021.).

The microporosity structure of coarse granular soils with various coarse/fines contents has been introduced in Chap. 3. The skeletons of five types of compacted and saturated specimens change from clay aggregates to coarse particles gradually. The soils with coarse contents greater than 70% have a coarse-controlled microstructure and the soils with coarse contents lower than 70% have a fines-controlled microstructure. The coarse-controlled soils, such as GW-GM with sand and SM with gravel, dual-porosity structure are formed during the soil compaction and show limit changes after wetting and drying. As presented in Chap. 5, Figs. 5.6, 5.7, and 5.32, the SWCCs and SHCFs for colluvial soils with high coarse fractions show a bimodal feature with two air entry values and is essentially relates to a dual-porosity structure.

Failure of soil slopes is often associated with instability of soils. Instability refers to a behavior in which large plastic strains are generated rapidly when a soil element sustains a given load or stress. Currently, the research related to instability of soils is primarily conducted at saturated conditions through undrained triaxial tests on loose saturated soils (e.g., Lade, J Geotech Eng 118:51–72, 1992; Leong et al., Geotech Test J 23:178–192, 2000; Yang, Geotechnique 52:757–760, 2002) and drained constant shear tests on saturated medium and dense sands (Chu et al., Can Geotech J 40:873–885, 2003). However, many natural soil deposits encountered in engineering practice are often unsaturated. During rainfall infiltration, a reduction in soil suction causes a decrease in the shear strength, which leads to the development of plastic strains and ultimately to the instability of the soil. This process can be idealized as a wetting path along which the shear stress and net mean stress keep constant, but the suction decreases over time. However, the instability behavior of unsaturated granular soils along the wetting path has seldom been investigated.

Widely-graded soil deposits are very common in nature, including colluvial soil landslides, residual soil slopes, the natural loose-packing dams of barrier lakes, glacial till deposits, etc. For example, quite a number of the natural slopes in the Three-Gorges Reservoir Zone (TGTZ), landslides in Hong Kong, and colluvial soil deposits formed after the Wenchuan earthquake are composed of colluvial soils. The grain-size analysis of colluvial soils reveals that such soils are often widely-graded with a small sand fraction and show a bimodal feature of grain-size distribution.

Colluvial soils are widely distributed on natural terrains. Shallow-seated failures with sliding depths varying between 0.5 and 3.0 m are the main failure mode in colluvial soil deposits during rainstorms. The corresponding confining stresses are in a low range between 5 and 25 kPa. It is pertinent to study the behavior of loose, coarse, widely-graded colluvial soils under very low confining pressures to provide better understanding of shallow-seated failures in coarse-grained soils.

The microporosity structure of a soil provides important information of hydraulic conductivity and soil–water characteristics of the soil, as described in Chaps. 3 and 5. The microporosity structures of fine soils may change significantly during wetting and drying (Simms and Yanful, Can Geotech J 38:741–754, 2001; Cuisinier and Laloui, Int J Numer Anal Meth Geomech 28:483–499, 2004; Boivin et al., Soil Sci Soc Am J 68:1145–1153, 2004; Braudeau et al., Soil Sci Soc Am J 68:359–370, 2004). Such variations of soil microporosity structures are inherently related to the drying and swell/shrinkage behavior of the soil (Ferber et al., Eng Geol 104:200–210, 2009; Aung et al., Geotech Geol Eng 19:401–416, 2001) and are the main difficulty in establishing quantitative relations between soil fabric and soil shrinkage/swell behavior.

This book focuses on the engineering properties of unsaturated widely graded soils and their influence on slope stability. This book characterizes the natural widely graded colluvial soils from macroscale, mesoscale, and microscale viewpoints, introduces the techniques for measuring hydro-mechanical properties for unsaturated widely graded colluvial soils, and clarifies the hydro-mechanical behavior and the failure mechanism of widely graded colluvial soils subjected to environmental loads. This book improves the understanding of rainfall-induced landslides on natural slopes. Researchers and engineers in the field of civil engineering can benefit from the book. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023.

The microporosity structure of soil provides important information on the shear strength, compressibility, hydraulic conductivity, and soil–water characteristics of the soil. The soil microporosity structure changes with stress state, transfer of water and air, temperature fluctuations, long-term gravimetric actions, and weathering. To investigate the formation of a microporosity structure during compaction, the evolution of microporosity structure after saturation, and the variation of microporosity structure during wetting–drying, scanning electron microscopy (SEM) was used to characterize the soil surface structure directly, and mercury intrusion porosimetry (MIP) was used to quantify the soil pore-size distribution (PSD).