Water ingress in soils through infiltration can trigger instability leading to failures in slopes and embankments under drained conditions. Subsequent investigations on such failures have shown that the infiltration of water in soils can cause a reduction in effective stress thus triggering the instability. In addition, studies have indicated that the constant shear drained (CSD) triaxial test can depict an unstable phenomenon where effective stress is continuously reduced. In the current study, the first section of research focuses on the Discrete Element Method (DEM) modelling of the instability behaviour of granular materials during CSD conditions. The CSD condition was modelled by decreasing the mean effective stress on an assembly of particles under strain-controlled loading. The instability condition was predicted at the particle scale level using particle second-order work increment. The DEM contact parameters have been calibrated to capture the macroscopic responses and the instability behaviour consistently with the laboratory experimental observations. The effect of different range of initial stress states at the beginning of CSD condition such as different initial mean effective stress, void ratio and deviatoric stress on the instability behaviour were analysed. In addition, the micromechanical parameters such as coordination number, anisotropic coefficients (geometric, mechanical) were extracted to assist in characterising the instability behaviour during CSD conditions. The stress state of the soil (i.e., at the onset of CSD) condition has shown a significant influence on the evolution of anisotropic coefficients, an evident behaviour change was noted once the CSD condition was imposed. Regardless of initial stress states and densities before the onset of the CSD condition, all samples have experienced instability during the shearing stage. However, the initial density of soil and stress state influence the time occurrence of instability. The second section of the research focuses on the DEM modelling of cone penetration test (CPT) on dry granular material. The influence of state variables and different modelling parameters on CPT measurements were investigated. The analysis details the effect of isotropic confining stress, relative density, stress ratio of the soil, mean particle size, boundary conditions and particle shape during CPT testing. The representation of all the DEM - CPT results on the soil behaviour type (SBT) chart resulted as 'Clean Sands.' This response coincides with the particle size distribution considered in this study. The stress distribution within the vicinity of the cone is dominant at the cone tip position by a large magnitude. Contact force network plots