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Concrete Block Wall - The CONTRACTOR shall construct a solid <br />concre a oc wa against the backstop which completely seals <br />the mine opening. The constructed wall shall extend to contact <br />the roof and each side wall, and all voids remaining between <br />the roof, walls, floor and the concrete blocks shall be filled <br />with grout, A level base for the wall shall be created by <br />excavating a 16" wide partial or wedge shaped keyway into the <br />sloping concrete floor beginning at the line of the backstop <br />and extending the full length of the wall. Block courses shall <br />be alternated and laid in the configuration shown on <br />Exhibit 6F, Masonry units sha17 be laid plumb, level and true <br />to line in full beds of mortar, <br />Joints shall be made in block work uniform and not more than <br />3/4" wide. After becoming "thumb-print" hard, joints exposed <br />shall be tooled to a weather joint. Mortar placement shall be <br />discontinued when the air temperature reaches 40 degrees F and <br />is falling and shall not be resumed until the air temperature <br />reaches 35 degrees F and is rising. No mortar shall be place <br />on base course containing frost or other frozen material. <br />Mortar shall be machine mixed in an approved type of mixer in <br />which the quantity of water can be accurately and uniformly <br />controlled. The mixing time shall be not less than five (5) <br />minutes, approximately two (2) minutes of which shall be for <br />mixing the dry materials and not less than three (3) minutes <br />for continuing the mixture after the water has been added. <br />For work requiring on]y small batches of mortar or grout, it <br />may be mixed by hand in a water tight mortar mixing box. The <br />materials for each batch shall be well raked and turned over <br />together before the water is added, until the material is an <br />even color throughout the mass, after which the water shall be <br />gradually added until a thoroughly mixed mortar of the required <br />plasticity is obtained. All mixing boxes and equipment shall <br />be kept clean. Mortars shall be prepared in batches or volume <br />that will be used before initial set takes place and in no case <br />longer than 45 minutes before delivery to mason's mortar board <br />at point of use, Retempering will not be permitted. Mortar <br />shall be allowed to cure for a minimum of 48 hours after block <br />wall Completion and prior to placement of backfill material. <br />_17_ <br />