What is the thickness required for undersea or river mine workings on fractured igneous rock?

The required thickness for undersea or river mine workings on fractured igneous rock is 50 meters. This measurement is established based on various geological factors that ensure stability and safety in mining operations in such challenging environments. Fractured igneous rock can have inherent geological instability due to the presence of fractures and potential water ingress, which can undermine the structure if not adequately supported.

A thickness of 50 meters provides a substantial buffer to mitigate the risks associated with water pressure and the dynamic forces that may act upon the mine workings. This level of thickness helps to ensure that the surrounding rock mass can adequately support the overlying weight and pressure, reducing the likelihood of collapse or flooding. It also aids in controlling the ventilation of the mine, which is critical for the safety of miners and the effective management of hazardous gases.

In contrast, the other thickness options are less suitable. Those values would not provide the necessary support and protection in such challenging conditions, increasing the risks associated with mining operations in fractured igneous rock under water bodies. Therefore, 50 meters is the most appropriate requirement for safety and stability in these scenarios.