What grade of concrete used for tunnel cushion

Design of lining concrete is C30, what should be used in the construction of concrete arch cushion concrete?

Course (bed) refers to the structural layer below the base level. Its main function is to separate water, drainage, antifreeze in order to improve the working conditions of the base and foundation, and its water stability requirements.

Now the actual project is basically C15, if the economy is less than C15.

The thickness of the cushion should not be less than 70mm, and the concrete strength grade should not be less than C10;