"How do soil properties influence foundation design in civil engineering? Explain the theoretical principles governing soil mechanics and their application in ensuring structural stability. Discuss the role of soil composition, density, and moisture content in determining soil behavior under load. Additionally, explore the theoretical concepts behind various foundation types, such as shallow and deep foundations, and their suitability based on soil conditions. Highlight the importance of site investigation techniques, including soil testing and analysis, in assessing soil properties for informed design decisions. Ultimately, elucidate how theoretical understanding of soil mechanics informs resilient and sustainable infrastructure development."