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(a) No hoisting or other work shall be permitted in the upper part of the shaft while men are in the lower part, unless the men are protected by one or more of the following means:
(1) By leaving a rock pentice at the bottom of the shaft that is to be deepened. The depth of such pentice shall be not less than one and one-half times the least dimension of the shaft.
(2) By installing a substantial bulkhead at the bottom of the shaft to be deepened.
Such bulkhead shall consist of not less than 2 layers of heavy timbers placed with the layers at right angles to each other, and shall be covered with loose rock to a depth of not less than 15 feet.
(b) Any opening through the pentice or bulkhead shall be not larger than is necessary to accommodate the sinking operations, and shall not be located under the hoisting compartments of the shaft being deepened.