The Geometric Necessity of Reverse Excavation: A Re-evaluation of Qanat Construction Methodology through Constructability Analysis

While historical consensus suggests that Qanat systems were constructed via downward vertical shaft sinking followed by horizontal gallery connection, this paper argues that such a methodology presents an insurmountable "Alignment Paradox" given the surveying limitations of antiquity. By applying modern civil engineering constructability analysis to ancient subsurface excavation, this study proposes the Reverse Excavation Model (REM). Under this model, excavation begins at the mazhar (outlet) and proceeds toward the mountain (the source), with vertical shafts "raised" from the tunnel floor to the surface. This methodology eliminates the high probability of lateral deviation error and ensures perfect gradient control. This paper challenges existing narratives and provides a new framework for understanding the sophisticated spatial intelligence of ancient hydraulic engineers.