Stopes mined along plunging folds of Precambrian rock range between depths of 869 m (2,850 ft) and 1,174 m (3,850 ft) below surface. The stopes are up to 30.5 m (100 ft) wide. Fold plunge is 12◦. Fold dip varies, but stope walls are vertical. A vertical, rectangular shaft 4.6 × 6.4 m (15 × 21 ft) provides access. How close to the shaft should stopping be allowed? Explain.