Heartsaver First Aid Part I Progress Indicator: Question 4 of 22 4.  Calculate the barrier pillar that should exist between two mines when the bottom elevation is 1,570 feet and the surface elevation is 1,971 feet with the coal bed being 6 feet, 2 inches in height. Click here for the answer! Choices: 98 ft. total thickness of barrier pillar 108 ft. total thickness of barrier pillar 118 ft. total thickness of barrier pillar 128 ft. total thickness of barrier pillar