It took a hole 12 feet square and 7 feet deep to house the Army, Lincoln-logs-type bunker the Marines first used in the spring of 1952. The fortification, using tree trunks up to eight inches in diameter, had a cover of seven to eight feet. This consisted of four feet of logs, and three-to-four more feet of rocks, sandbags, and earth fill. By the summer of 1952, the division developed its own style of bunker, a prefabricated timber structure designed to fit into a hole eight feet square and somewhat less than seven feet deep. This size fortification could accommodate a .50 caliber machine gun, crew members, or several riflemen. Provision was also made for the inclusion of a sleeping shelf in the rear of the bunker. Its construction required no saws, hammers, or nails, only shovels to excavate. The major drawback to erection of the prefab was the difficulty in manhandling the heavy roofing timbers, 11 feet long, 12 inches wide, and 4 inches thick. On top of this was placed a two-foot layer of sandbags, tarpaper covering, and a four feet high layer of earth that completed the structure and partly camouflaged it. Battlefield construction was carried out by the infantry regiments to the limit of unit capabilities. The division engineers, one company per frontline regiment, augmented at times by shore party units, supplied the technical know-how and engineering materials and equipment. These combat support troops processed the lumber for bunker construction and built fortifications for forward medical treatment and one bunker for observation of battle action by civilian and military dignitaries, irreverently called VIPs (Very Important Persons), who frequently visited the division. Engineers also erected some of the barbed wire barriers in the forward areas and, when necessary, cleared firing lanes for weapons housed in bunkers. The processing of timbers for easier and faster bunker-construction had begun on 28 July, but this was hardly in time for the most difficult fighting the division had faced thus far in western Korea. Given the name Bunker Hill,[170] this battle would take place in the center sector of the division line manned since 27 July by Colonel Walter F. Layer’s 1st Marines.[171] On that date Lieutenant Colonel Armitage’s battalion, 3/1, took over from the 3d Battalion, 7th Marines on the left, and 2/1 (Lieutenant Colonel Roy J. Batterton, Jr.) relieved the 2d Battalion of the 7th Marines on the right.[172] [170] Since bunkers were in everyone’s mind and frontline units were heavily involved in the bunker-construction program, it is felt likely “someone in G-2 arbitrarily assigned the name.” Col Gerald T. Armitage ltr to Hd, HistBr, G-3 Div, HQMC, dtd 6 July 67, hereafter _Armitage ltr._ [171] Two days earlier Colonel Layer had taken over the command from Colonel Flournoy.