Explore the interplay between soil mechanics and structural engineering, considering how varying soil types and conditions influence the design and stability of structures. Delve into the complexities of soil behavior under load, including factors like shear strength, consolidation, and settlement, and how engineers can mitigate potential risks through appropriate foundation design and construction techniques. Assess the critical role of geotechnical investigations in informing structural design decisions, examining methods such as soil sampling, laboratory testing, and in$-$situ measurements. Additionally, analyze the challenges posed by expansive soils, liquefaction, and other geotechnical hazards, and evaluate strategies for mitigating their impact on the performance and longevity of civil engineering projects.