Tunnel engineering involves theoretical considerations of designing and constructing underground passages for transportation, utilities, and mining purposes. Construct a detailed theoretical framework delineating principles of tunneling methods, including tunnel excavation techniques, ground support systems, and tunnel ventilation design. Explore theoretical models for assessing tunnel stability, groundwater inflow, and geotechnical risks, and discuss theoretical approaches to optimizing tunnel alignment, cross$-$section, and construction sequence, aiming to ensure safety, durability, and cost$-$effectiveness in tunnel projects.