By Second Lieutenant Robert Dieter
Two AVLB teams from Charlie Company, 2d Engineer Battalion, provided mobility support for the III MEF. After conducting their own evaluation of the bridge site, the AVLB teams linked up with the Marine engineer officer in charge to discuss the cribbing requirements. They determined the rise of the existing bridge then consulted the operator's manual and estimated that the AVLB would deflect from 2 to 5 inches when an MlA1 reached the bridge's center of mass. They decided that the cribbing should rise 16 inches above the ground level to provide a 3-inch safety factor. Both ends of the AVLB required cribbing. The final cribbing was 3 feet wide and spanned the width of the bridge. Marine and Army engineers constructed two sets of cribbing by modifying 8x8x10, 8x8x16, and 8x8x20 dressed timbers. They banded both sets together to restrict movement when the MlAls entered and exited the bridge. Banding also allowed material-handling equipment to rapidly emplace and remove the cribbing material.
The next challenge was to get the Abrams tanks up to the level of the bridge, which was now elevated 16 inches. The AVLB operator's manual calls for a 3-foot gap between the beginning of the bridge and the cribbing. Using a straight ramp from the ground to the elevated bridge caused concern for two reasons:
The engineers constructed two ramps: one to bring the tank onto the approach platform and a second to bring the tank off the bridge at the same declination. Both ramps were modified 8x8x10 lumber planks stacked to an incline and decline, respectively. The exit ramp was 2 feet longer than the entrance ramp because there was no exit platform; the extra 2 feet were required to maintain the same declination slope. Figure 2 shows the final plan for the cribbing, platform, and en-trance ramp.
After the engineers completed the construction, they conducted a rehearsal to measure the actual deflection of the AVLB as the tanks crossed and to see how the MlAl crews handled entering and exiting the bridge. Front loaders moved and shifted the heavy cribbing bundles, approach platforms, and ramps into place. Once the cribbing was set, the AVLB launched the bridge on the cribbing and moved to a secure location so the platform and ramps could be emplaced. After the engineers completed the set-up, they ground-guided each tank across the bridge. They crossed seven tanks during the rehearsal with an average of 3.5 inches of deflection, measured with the tanks at the bridge's center of mass and the center pins inserted. The greatest deflection was 4 inches and the least was 2.75 inches. The rehearsal verified the planning factors and allowed the MlA1 crews to gain confidence and experience in crossing a gap on an AVLB. They detected no problems during the rehearsal, and the AVLB had no problems launching and releasing the bridge, in spite of having to launch onto the 16-inch cribbing.
The crossing for the exercise took place at midnight to minimize disruption to civilian traffic. Initial planning called for a 4-hour window of operation. Due to the close coordination of Army and Marine engineers and the full-scale rehearsal, it took only 55 minutes to emplace the cribbing, launch the bridge, cross the MlAls, and remove the bridge and cribbing. The time was reduced to 41 minutes during the return crossing.
Combat engineers of the 2d Engineer Battalion were continuously challenged by Korea's mountainous terrain and the lack of fording sites at streams and rivers. However, crossing the Marine detachment gave them valuable mobility experience. More importantly, Operation Freedom Banner clearly de-monstrated that the AVLB is still an effective tool for mobility operations.
Second Lieutenant Dieter is the Assault and Obstacle Platoon Leader of C Company, 2d Engineer Battalion, Republic of Korea. He is a graduate of the Engineer Officer Basic Course, Airborne School, and Ranger School. Second Lieutenant Dieter holds a bachelor's degree from Creighton University, Omaha, Nebraska.