Cal. Code Regs. tit. 8, § 6991 – Bearing Sets.

(a) Bearing sets shall be installed in all vertical shafts and in all inclined shafts where it is necessary to support the shaft timbers.
(b) Bearing sets shall be installed as close to the shaft collar as is practical, and along the shaft at necessary intervals.
It is recommended that the distance between bearing sets be not more than 100 feet.