In recent decades, the geotechnical research community has studied well the soil liquefaction problem, considered a significant threat to the entire built environment. Most investigations in the past were concentrated on homogenous soils (i.e., either clean sands or clays otherwise referred to as textbook soils), while studies executed on mixed soils for the evaluation of liquefaction susceptibility are still few. A literature review of past works on mixed soils (e.g., silty sands, clayey sands) indicated the existence of some elements of controversies regarding the trends of liquefaction susceptibility of the bracketed especially clayey sands. In reality, in-situ soils often exist as mixed soil composites and some case histories have indicated the occurrence of liquefaction of mixed soils in the past. Hence, the justification of the requirement of additional investigations on mixed soils is considered to comprehend their strength characteristic and resistance to natural disasters such as static and earthquake-induced soil liquefaction. In the current study, the static liquefaction mechanisms of east coast sand (ECS) and other derived mixed soil specimens by mixing ECS with 10%, 15%, 20%, and 30% by weight of an industrial kaolinite-clay are presented. ECS is a commercially available mined sand from the shores of Pakiri beach, Pakiri, and is mostly utilized in geotechnical earthworks construction works around Auckland and the Northlands of New Zealand. The statically-induced liquefaction cases were examined under the undrained triaxial compression conditions to provide experimental evidence as well as the numerical models for the observed extreme cases. In addition, the generation of excess porewater pressure (PWP) as a result of the application of synthetic dynamic/earthquake loads was studied with the aid of a 600N-capacity shaking table for the remolded soil specimens. The engineering characteristics of soils are well-known to exhibit nonlinear stress-strain relationships and overall complex behavior. The Norsand model framework was tested to numerically validate five aspects of the soil's undrained behavior under triaxial compression conditions. The studied aspects include the complex stress-strain relationships, effective stress paths, excess porewater pressures, stress-state dilatancies, and the critical state characterized by the state parameter. It is particularly useful to understand the soil liquefaction behavior of varieties of soils in practice as this would assist in the selection of soils that are suitable for soil replacement mitigation measures applicable in hydraulically placed fills, earth dams, tailing dams, and other earthwork applications in geotechnical engineering that are susceptible to any type of liquefaction-related failures. The obtained basic soil index properties of the control study sample (ECS) were predominantly of a poorly graded sand (SP) according to the unified soil classification system (USCS). The USCS classification of the utilized kaolinite clay indicated a lean or low plasticity clay (CL). The other created sand matrix soils indicated the soil classification of clayey sand (SC). Scanning electron microscopy of the ECS indicated that its grain shape is angular to subangular and fine in texture. The ECS has a specific gravity of 2.60, minimum and maximum density in the range of 1.43g/cm3 to 1.67g/cm3, corresponding minimum and maximum void ratios of 0.561 to 0.820, respectively, permeability in the range of 4.7610-4 to 6.6610-4cm/s, compression index C_c of 0.000232 and swelling index C_s of 0.0000597. The obtained liquid limit of the applied kaolinite was 45%, and the plastic limit of 35%, with a plasticity index of 10. The monotonic triaxial compression test results were conducted for the initial testing mean effective stress levels of 50kPa, 100kPa, and 200kPa for all the studied remolded soil specimens. These stress levels are considered realistic for soils that may be susceptible to static liquefaction failure. The undrained shear strength of the ECS decreased with the initial introduction of the kaolinite in its fabric and then increased at some marginal optimal percentage by weight content of the kaolinite. The resulting experimental evidence as per the executed triaxial monotonic compression tests suggests that an optimal static liquefaction resistance is achievable for the ECS mixed with 15% to 20% by weight of the kaolinite clay with a reduced excess porewater pressure. The analyses of the static liquefaction cases were interpreted based on the steady-state/critical state frameworks. Numerical modeling and data validation of the soil's advanced geomechanical properties was carried out for observed extreme cases of the studied samples; specifically, the clean sand (ECS00) and clayey sand with 30% kaolinite (ECS30), which experienced typical flow failures at all testing stress levels. The applied software for numerical modeling is the Itasca geomechanics software (numerical modeling tools), Fast Lagrangian Analysis of Continua (FLAC). The executed shaking table further showed that the generated excess PWP decreased as the kaolinite content increased within the fabrics of the ECS. Overall, the outcome of the current study contributes to the benefits of alternative soil additives as sustainable replacements of some conventional soil improvements. Inclusions are those that apply cement, lime, and other poisonous chemicals that may impair the sustainability of the built environment and impede the achievement of the global goal of zero-emission of greenhouse gases, notably (carbon dioxide, i.e., CO2). In addition, insights are provided on suitable soil parameter calibration and numerical modeling processes for mixed soils which have received little attention to date. The significance of the current thesis further highlights the need to reconsider either the application of finite element or finite difference methods to inform better design decisions of mixed soils’ global stability issues rather than the current applied conventional limit equilibrium method coupled with the well-known Mohr-Coulomb model in practice which are not capable of capturing important soil properties like plastic flow, hardening, softening, and excess porewater pressures.