Three dimensional engineering geological modeling of the Megenagna underground terminal site in Addis Ababa

This study presents strength-driven 3D engineering geological modeling and characterization of the proposed Megenagna Underground Terminal, which is the first major underground public transport facility in Addis Ababa. The study area is characterized by a complex succession of volcanic units and is located few meters from the major NNE-SSW- and NNW-SSE-trending faults and lineaments. A total of 44 investigation boreholes, with a minimum and maximum depth of 15 m and 120 m, respectively, were selected for the modeling purpose, which is supported by laboratory testing of 56 intact rock core samples. Four engineering geological units were identified, including: 1) silty clay topsoil, 2) highly to completely weathered ignimbrite (weak to very weak rock), 3) moderately to slightly weathered ignimbrite [UCS ( \(\sigma_{ci}\) ): 9.3 to 68.4 MPa, with a mean value of 27.7 MPa], and 4) paleosol interbeds. Rock mass quality was quantified using the Hoek–Brown criterion, in which GSI ranges from 35 to 48, while the intact rock constant ( \(m_{i}\) ) ranges from 6 to 10. Derived rock mass properties include the uniaxial compressive strength of the rock mass ( \(\sigma_{c}\) ) that ranges from 0.22 to 3.56 MPa, tensile strength ( \(\sigma_{t}\) ) from 0.01 to 0.13 MPa, and deformation modulus ( \(E_{rm}\) ) from 0.5 to 9.0 GPa (with mean value of 2.1 GPa). Shallow groundwater, at a depth range from 3 to 5 m, was recorded in few boreholes but reaches 28–30 m in deeper groundwater boreholes. The developed 3D engineering geological model highlights variability in rock quality and the presence of potentially unstable, weak zones that require special treatment during construction. The findings provide site-specific analysis to optimize design and construction methodologies for safer, more effective underground infrastructure development in the complex volcanic rocks of Addis Ababa's urban setting.