A circular tunnel of radius 3 m is constructed in a hydrostatic stress field of 15 MPa. The modulus of elasticity and Poisson’s ratio of the rock are 5 GPa and 0.25, respectively. A uniform support pressure $p_i$ is applied at tunnel boundary to restrict the radial deformation at the tunnel boundary to 4 mm. The value of $p_i$ in MPa is______. (round off to two decimal places)