How do varying soil properties affect the design and construction of deep foundation systems in civil engineering? Discuss the implications of soil types, such as cohesionless or cohesive soils, on the selection of appropriate foundation types, bearing capacities, and settlement considerations. Explore how factors like soil composition, moisture content, and compaction density influence the stability and performance of deep foundations, including pile foundations and drilled shafts. Additionally, analyze the role of site investigation techniques, such as geotechnical testing and soil sampling, in characterizing soil behavior and informing foundation design decisions. Evaluate the challenges posed by heterogeneous soil conditions and propose strategies for mitigating risks associated with differential settlement and soil$-$structure interaction. Furthermore, examine the influence of environmental factors, such as seismic activity and groundwater fluctuations, on the behavior of deep foundation systems, highlighting the importance of interdisciplinary approaches in addressing complex geotechnical challenges in civil engineering projects.