# Experimental and Empirical Studies to Evaluate Liquefaction Resistance of Partially Saturated Sands
## 1 Introduction  
Earthquake-induced liquefaction is an important phenomenon of natural hazards that causes significant deformation of soil deposits and failure of geotechnical structures. The catastrophic effects of liquefaction have been recurrently observed in many seismic events such as the 1964 Alaska (USA) and Niigata (Japan) earthquakes [1, 2], the 1999 Kocaeli and Duzce (Turkey) earthquakes [3], the 2010 Haiti and Maule (Chile) earthquakes [4, 5], the 2011 Tohoku (Japan) and Christchurch (New Zealand) earthquakes [6, 7], and the 2016 Ecuador earthquake [8, 9]. Liquefaction occurs in granular deposits when excess pore pressures develop under seismic loading and cause a large reduction in the effective stress and significant degradation of the soil strength and stiffness. In aftermath of the 1964 earthquakes, comprehensive research has been conducted to elucidate the important mechanisms and fundamental understanding of earthquake-induced liquefaction. These studies have mostly benefited from the analysis of case histories and/or laboratory tests on undisturbed or reconstituted samples and have produced several semi-empirical principles and frameworks that are widely used in current design practice for the evaluation of liquefaction susceptibility and its effects [10, 11, 12, 13]. Natural soil deposits in seismically active zones may be found in a nearly/fully saturated, unsaturated, or partially saturated state depending on the groundwater level, degree of saturation (\(S_{r}\)), and distribution of gas bubbles in the soil [14, 15]. Granular soil deposits tend to contract when subjected to cyclic shear strains, which causes therearrangement and densification of soil grains. For the nearly/fully saturated state, which generally occurs well below the groundwater level, interparticle pores are filled with practically incompressible pore fluid (e.g., water). During an earthquake loading in an undrained condition, the pore fluid cannot move or contract because the rate of loading and the bulk stiffness of the pore fluid are high, thereby causing the build-up of large excess pore pressures. For the unsaturated state, which usually occurs well above the groundwater level, interparticle pores are filled with a continuous air and water meniscus, and the effective stress and hence liquefaction resistance of the soil deposit increase due to the presence of a large matric suction. For the partially saturated state occurring below the groundwater level, pores are filled with water and gas that may exist in a continuous or an occluded bubble form depending on the level of saturation. The gas takes an occluded bubble form at high degrees of saturation (typically \(S_{r}\geq 80\%\)), whereas the continuous gas phase is prevalent below this level [16]. In the former case, the presence of entrapped gas bubbles in pore spaces of submerged soil significantly reduces the bulk modulus and increases the compressibility of pore fluid, namely, the gas-water mixture. The gas bubbles tend to reduce in volume and absorb the excess pore pressures during an earthquake loading, thereby enhancing the resistance of the soil to liquefaction [17]. Partially saturated soils can be formed by naturally occurring gas bubbles via biological activities [18; 19] or via human activities. In the latter case, the degree of saturation of soil deposits is reduced by using various induced partial saturation (IPS) methods to enhance the liquefaction resistance. Air injection [20; 21; 22], electrolysis and drainage-recharge [23], use of denittifying bacteria [24; 25], and use of chemical composition [26; 27] are some of the most widely used IPS techniques. Previous studies on IPS methods showed that gas bubbles created in partially saturated soils can stay in the pore spaces and that the partially saturated state can last for a sufficiently long time [23; 28; 29].