Geotechnical engineering serves as the cornerstone of many civil engineering projects, providing crucial insights into soil behavior and foundation design. Expound upon the fundamental theories and methodologies employed in geotechnical engineering, including soil classification, compaction, and settlement analysis. Delve into the complexities of soil - structure interaction, examining how engineers assess and mitigate potential risks such as landslides, liquefaction, and differential settlement. Additionally, explore the advancements in geotechnical instrumentation and numerical odeling techniques that enhance our understanding of subsurface conditions and inform decision - making processes in civil engineering practice.