The Shaft Tunnel of the Biar aqueduct of Jerusalem: Architecture, hydrology, and dating

The Biar aqueduct is the most sophisticated of the aqueducts supplying water to Jerusalem during classical periods. It includes the Biar spring, an underground Shaft Tunnel ∼3 km long harvesting groundwater (cuniculus), a dam, a surface channel, and a tunnel traversing a ridge. We conducted a survey of the Biar underground Shaft Tunnel, mapping all of its accessible parts (∼1200 m), facilitating a new understanding of the cutting-edge hydrogeologic and engineering skills used for this project. The last ∼536 m of the Biar Shaft Tunnel was constructed with a unique ashlar channel, divided into segments, designed to withstand different loads: (a) in a mechanically weak bedrock, a channel with arched gables and barrel vault specus was built within a hewn winding tunnel, (b) when dug as an open shallow trench, a channel roofed with complex gables of ashlars with drafted margins was built, and (c) to release hydraulic pressure, a channel roofed with alternations of barrel vaults and simple gables set perpendicular to the course of the tunnel was constructed. ¹⁴C dating of plaster samples suggests that the Biar aqueduct was built in the mid-first century CE and renovated in the days of Aelia Capitolina in the second century CE.