Cohesionless soils exist across the globe, under various geological settings and climatic regimes, in two distinct natural states. Disintegrated sediments, where soil particles are loosely held together through particle interlocking such as freshly deposited sediments, and intact deposits, in which soil particles are primarily held together through interparticle suction bonding. The geotechnical behavior of such soils is governed by field conditions of applied stress and atmospheric conditions. This research focused on developing a clear understanding of the saturated-unsaturated behavior of cohesionless soils by mimicking the two natural soil states for laboratory investigations (flow-through, volume change, and shear strength) and by using the test results for numerical modeling (two-dimensional and transient seepage-thermal and stability analysis). The main contributions of this research are summarized below. A simple test method was developed, by utilizing a single sensor and a digital camera, to determine the unsaturated hydraulic conductivity over the entire suction range. The soil exhibited a marginal water holding capacity with air entry value of 8 kPa and a residual suction value of 21 kPa. Following a newly developed sigmoidal function, the soil exhibited a low hydraulic conductivity of 10-7 m/s (saturated value) that gradually decreased with increasing suction (desaturation). Likewise, the difference between the fitted unsaturated hydraulic conductivity values based on upper (10-5 m/s) and lower (10-7 m/s) limits of saturated hydraulic conductivity decreased with suction and converged at vapor conductivity (10-14 m/s). The conventional oedometer test was improved, by adding a controlled water inflow and a digital data recording, to determine collapse and consolidation. Results showed that with the increase in pre-collapse stress from 25 to 600 kPa, unsaturated compression increased from 0.5 to 5.3% in disintegrated soil and remained close to 0.5% in intact soil. The wetting collapse decreased from 1.1 to 0.1% in disintegrated soil and increased from 6 to 9% in intact soils whereas the total collapse increased from 2 to 5.6% (disintegrated) and from 6 to 9% (intact). The transient volume change during wetting collapse followed a curvilinear trend for both soil states. The conventional direct shear test was used to determine the shear strength parameters of disintegrated and intact soils under saturated and dried conditions. The disintegrated soil exhibited identical behavior under both saturation states with no clear peak at failure. Apparent cohesion was not observed and friction increased from 44.5° (saturated) to 48°(unsaturated). The intact soil behaved similar to the disintegrated soil in saturated state due to the absence of suction and had a clear peak and residual similar to dense soils. Apparent cohesion and friction angle respectively increased from 0 kPa and 42° (saturated) to 91 kPa and 36° (unsaturated). A transient and two-dimensional seepage-thermal model was developed to determine the stability of typical embankments with a low slope (18 m) and high slope (26 m). These slopes were analyzed under mean, extreme wet, and extreme dry climatic conditions along with four ponding conditions (none, upstream, downstream, both) with and without vehicular loading. The laboratory protocols and the numerical model are crucial for shallow and young geological deposits that are in direct contact with the atmosphere and where most civil infrastructure resides. The findings of this research are useful for the near design, construction, and rehabilitation of urban facilities exposed to climatic change impacts.