The t-butyl group must have a methyl group pointed back over the ring. The steric strain from this 1,3-diaxial interaction is so large (4.9 kcal/mol) that the t-butyl group is always equatorial. Note how the bulky tert-butyl group has forced the cyclohexane ring to distort so that the bond from the tert-butyl group to the ring is no longer parallel to the other axial bonds.
