Between 15 September and 15 November 1942, groups under Herbert L. Anderson and Walter Zinn constructed sixteen experimental reactors (known at the time as "piles") under the Stagg Field stands. Fermi designed a new uranium and graphite pile that could be brought to criticality in a controlled, self-sustaining nuclear reaction. Construction at Argonne fell behind schedule due to Stone & Webster's difficulty recruiting enough skilled workers and obtaining the required building materials. This led to an industrial dispute, with union workers taking action over the recruitment of non-union labor. When it became clear that the materials for Fermi's new pile would be on hand before the new structure was completed, Compton approved a proposal from Fermi to build the pile under the stands at Stagg Field.
