At least once in each two (2) year period, after the establishment of the retirement system, the board shall cause an actuarial investigation to be made of the mortality, service and salary experience of the members of the retirement system and an evaluation of the funds under its control. Pursuant to such investigations and evaluations the board shall, from time to time, revise the mortality and service tables and the rates of contributions prescribed pursuant thereto.
T.C.A. § 8-3428