Suddenly shots rang out from the side of the road. Within a few seconds, 23 bullets from a Russian-made submachine gun had struck the jeep; the driver, astonishingly, received only a knee wound. No trace of the ambushers could be found by the friendly patrol dispatched to investigate the incident. An activity of an entirely different nature that same month was the assignment of battalion LVTs to break up the heavily encrusted ice that had formed around and endangered supports of three bridges in the I Corps area. A rash of minor incidents involving would-be, but unsuccessful, enemy infiltrators also took place during the winter months in the amtrac sector. In November, three agents attempted to cross the Imjin on their way to the division area, but were engaged by a battalion patrol. After a brief fire fight, friendly artillery was called down on the retreating boat and it was believed destroyed. Enemy agents on foot were engaged by National Police or Marine listening posts again in January and March and deterred from their espionage missions. Commitment to an infantry role in the KANSAS line, meanwhile, had permitted little time for operation of the battalion tractors. In December, construction began on a storage park for those LVTs not in use. By placing the non-operating tracked vehicles in a single area, the battalion could handle routine maintenance with just a few men. This facility, located at Ascom City, was completed early in 1953. By March, a total of 34 tractors had been placed there in caretaker status. Implicit in this economy measure was the requirement that all stowed tractors could revert to combat status, if necessary, on a 48-hour alert. _Changes in the Concept of Ground Defense_[337] [337] The material in this section is derived from: _PacFlt EvalRpt_ No. 5, Chap. 8. During the winter months of 1952–1953, the 1st Marine Division modified the organization of its tactical defense, although it retained the basic concept of the combat outpost system as the backbone of MLR defense.[338] Development of much of the KANSAS line and parts of the Marine MLR during this period reflected several new ideas on how the ground defense could be better organized. Recent experience during Communist attacks had shown that defensive emplacements and positions could be dug deeper and below ground to withstand massed enemy fires. Contrary to traditional concepts, it had also been found that centering the defense on the military crest[339] of a hill was not always the best procedure. Emplacement of machine guns downslope or in low firing positions to cover draws or flat ground was not entirely suitable to the Korean terrain, enemy, or nature of positional warfare.