These theories encompass a broad spectrum, including structural analysis, fluid mechanics, soil mechanics, and transportation engineering. Structural theories, like Euler$-$Bernoulli beam theory or Timoshenko beam theory, elucidate the response of structures to loads. Fluid mechanics theories, such as Bernoulli's principle or the Navier$-$Stokes equations, underpin hydraulic design and fluid flow analysis. Soil mechanics theories like Terzaghi's principle inform foundation design and slope stability analysis. Transportation theories like traffic flow theory aid in optimizing traffic management and infrastructure planning. These theories form the bedrock of civil engineering practice, enabling safe, efficient, and sustainable infrastructure development.