Tunneling Under Building with Thin Rock Cover

Construction of an inclined escalator shaft entrance to the Metropolitan Atlanta Rapid Transit Authority's (MARTA) Peachtree Center Station required tunneling under a 15‐story building over 60 yr old. Because of the age of the structure, foundation records were not available in the design phase, and surrounding streets and structures prevented access to the foundations. The final details of the tunneling procedures were determined during construction, based on information gathered as the work proceeded. The critical part of the construction occurred as the shaft excavation passed below two of the structure's footings carrying a combined load of over 1,200 kips (5,340 kN), with only 7 ft (2 m) of partially weathered rock cover over the 23‐ft (7‐m) wide sloping tunnel. The development of the tunneling methods provides an interesting case history of investigative engineering and application of geologic and geotechnical knowledge.