Final answer:
Engineered protective systems are required for geothermal drilling projects that aim to reach depths of at least 1 km, as the safety and technical challenges increase significantly. The exact depth at which these systems are needed is determined by engineering assessments, with the context of conventional safety practices such as those by OSHA for trenches 5 feet deep or greater.
Step-by-step explanation:
Excavations deeper than a certain depth require specially designed and engineered protective systems to ensure the safety of the workers and the structural integrity of the excavation. In the context of geothermal heating, it is mentioned that sustainable extraction requires boreholes at least 1 km deep to reach temperatures hot enough for heating purposes.
However, the practical challenges of drilling to such depths on a large scale are significant. The deepest mine is less than 4 km deep, and the deepest drill hole, the Kola Superdeep Borehole, reached about 12 km before drilling was stopped due to technical challenges, such as increasing rock temperatures and equipment failure.
Considering these technical challenges and safety requirements, any excavation or drilling operation that aims to go significantly deep, potentially in the order of kilometers as discussed for geothermal applications, will necessitate an engineered protective system. The Occupational Safety and Health Administration (OSHA) typically requires protective systems for trenches that are 5 feet deep or greater, but geothermal or other deep Earth excavation projects would go well beyond this and require much more robust systems. The specifics of the depth at which these systems are required for such advanced projects would vary based on engineering assessments and the particular conditions of the project.