How do varying soil composition and moisture content impact the stability and performance of deep foundation systems in civil engineering projects? Consider the interplay between soil properties such as cohesion, friction angle, and permeability, alongside factors like groundwater level fluctuations and seismic activity. Explore how changes in these variables can influence the bearing capacity, settlement behavior, and overall structural integrity of piled foundations, caissons, and drilled shafts. Discuss the role of geotechnical investigations, including soil testing methods like Standard Penetration Test $($SPT$)$ and Cone Penetration Test $($CPT$),$ in assessing soil behavior and informing foundation design decisions. Additionally, delve into advanced analytical techniques such as finite element analysis $($FEA$)$ and numerical modeling to predict and mitigate potential failure modes, including bearing capacity failure, excessive settlement, and liquefaction$-$induced instability, in deep foundation systems subjected to complex loading conditions and environmental factors.