1946. Work began on the main facility area on 20 October 1943. Although generally flat, some of soil and rock had to be excavated from areas up to high, and six major areas had to be filled in, to a maximum depth of . Normally buildings containing complicated heavy machinery would rest on concrete columns down to the bedrock, but this would have required thousands of columns of different length. To save time soil compaction was used instead. Layers were laid down and compacted with sheepsfoot rollers in the areas that had to be filled in, and the footings were laid over compacted soil in the low-lying areas and the undisturbed soil in the areas that had been excavated. Activities overlapped, so concrete pouring began while grading was still going on. Cranes started lifting the steel frames into place on 19 January 1944. Kellex's design for the main process building of K-25 called for a four-story U-shaped structure long containing 51 main process buildings and 3purge cascade buildings. These were divided into nine sections. Within these were cells of six stages. The cells could be operated independently, or consecutively, within a section. Similarly, the sections could be operated separately or as part of a single cascade. When completed, there were 2,892 stages. The basement housed the auxiliary equipment, such as the transformers, switch gears, and air conditioning systems. The ground floor contained the cells. The third level contained the piping. The fourth floor was the operating floor, which contained the control room, and the hundreds of instrument panels. From here, the operators monitored the process. The first section was ready for test runs on 17 April 1944, although the barriers were not yet ready to be installed. The main process building surpassed The Pentagon as the largest building in the world, with a floor area of , and an enclosed volume of . Construction required of concrete, and of gas pipes. Because uranium hexafluoride corrodes steel, and steel piping had to be coated in nickel, smaller pipes were made of copper or monel. The equipment operated under vacuum pressures, so plumbing had to be air tight. Special efforts were made to create as clean an environment as possible to areas where piping or fixtures were being installed. J. A. Jones established a special cleanliness unit on 18 April 1944. Buildings were completely sealed off, air was filtered, and all cleaning