In this episode, Jason talks about his experience leading the construction of a 350-meter-long pontoon bridge across the Tigris River in Iraq near Tikrit during the Iraq war in 2003.
Audio podcast can be streamed below.
You can subscribe at: Apple iTunes, Spotify, Google Podcasts, Overcast, Anchor, and more.
Building the Tikrit Pontoon Bridge
The below article by Jason Toth was originally published by the Society of Military Engineers (SAME)
In late September 2003, while in support of Operation Iraqi Freedom, the 555th Combat Engineer Group (TF ABLE) received the mission to construct a bypass around the damaged Tikrit fixed bridge crossing the Tigris River. This bypass would enable the 4th Infantry Division’s maneuver units east-west mobility across the Tigris River. The crux of the bypass was the actual crossing of the Tigris. The solution was constructing a 350m bridge, the largest Mabey & Johnson float bridge ever built – capable of trafficking MLC 110 military vehicles.
During this historic mission, the engineers from the 555th Combat Engineers demonstrated the full spectrum of modern engineering capability combining combat engineering, horizontal construction, and bridging operations to overcome one of the world’s most famous rivers. The 14th Combat Engineer Battalion provided the overall command and control of all the TF ABLE engineers. Charlie Company, 14th Engineer Battalion out of Ft. Lewis, WA and the 74th Multi-role Bridge Company out of Ft. Hood, TX led the project. Charlie Company had First Platoon, Bravo Company, 5th Combat Engineer Battalion stationed at Ft. Leonard Wood and 3rd Platoon, 229th Construction Support Element (CSE), Wisconsin Army National Guard Company under its control.
The bridge and bypass route is dual use, facilitating both military and civilian traffic until the permanent Tikrit Bridge is fully repaired by Bechtel Corp. The expected duration of use of the bridge was between 6 and 12 months. A compressed construction timeline and wide river necessitated a floating bridge be used to complete the mission. The 14th identified possible sites for the float bridge through a series of engineer recons on land and water. Site selection criteria were trafficability of existing route, hydrology impacts, engineer work required, ability to contract work, force protection, bypass route constraints, river width, bypass length, and environmental and population impacts. The first five site selection criteria carried the most weight in selecting the site.
Once the site was chosen, the construction planning and design work began. Force protection was the first priority. Securing the site involved first establishing a perimeter berm complete with guard towers to encompass and protect the equipment-park, construction material storage yard, jobsite, and life support area (LSA). The LSA was complete with field sanitation facilities, force protection bunkers, and housing structures. The scope of engineer work on the bypass included upgrading and repairing road surfaces, widening and construction of new road segments, and strengthening civil structures along the entire 24km bypass route to facilitate MLC 110 traffic. The civil structures added were a new 1m-irrigation culvert, a 300m-road section, and a 200m causeway with dual 3m box culverts needed to span an estuary of the Tigris River. An approach for the west shore of the Tigris needed to be cut out of the existing concrete erosion wall and the approach for the east shore included the construction of a 100m causeway out into the river. Concrete abutments were constructed at the end of each approach to establish a solid foundation for the bridge landing piers. Key to this process was the integration of Mabey & Johnson technical experts who advised the military engineers on the bridge’s capabilities. 350 meters of Compact 200 model Mabey & Johnson float-bridge manufactured in England needed to be assembled in its entirety and floated into place. Development and construction of detour signs and other traffic control measures were also important to ensure efficient flow of traffic along the bypass route.
Using a precise mix of military engineers from the 555th Combat Engineer Group and Iraqi civilian contractors, the 14th EN BN established a command and control scheme to manage all tasks associated with the project. C/14 EN BN focused on the upgrade of the route to include supervision of contracted work and troop construction tasks while 74th MRBC focused on construction and emplacement of the 350m bridge. Tying the two units together, the Battalion S-3 section provided the planning and executing synergy. By identifying the tasks on the critical path to opening the bridge and completing them with troop labor, the 14th EN BN ensured a smooth and controlled construction process thus avoiding many of the unforeseen difficulties of contracted construction.
During the 3 weeks of planning and 2.5 months of construction, the 14th Engineer Battalion met with a variety of challenges that forced them to exhibit engineering problem solving skills, developing innovative solutions to complex problems. CPL Hughes, the battalion surveyor and previous EIT, stated, “The ingenuity displayed during this project was incredible. We overcame numerous civil engineering problems and resource limitations. In the end we completed in 2 months what would have taken a civilian company 2 years.”
The hydrology of the Tigris River played a dominant role in selecting the bridge site and planning the project. However, there was no accurate historical data on the Tigris or Tigris watershed that could be used in designing the approaches, abutments, and bridge anchors. To fill this void of critical information, construction recon and Civil Affairs teams used field investigations to determine high water levels, flood plane width, river cross section, river bed and bank soil composition, and river speed. The 14th EN BN also requested an Army Dive Detachment. They used sonar to map river depth at the bridge site and investigate the composition of the river bottom. Interviews with local Iraqis residing in the vicinity of the bridge site were also critical to charting the history and behavior of the Tigris. Simple observations of mud lines on structures and vegetation on certain areas of the riverbank provided the key historical data. Applying open channel flow principles to a natural watercourse cross-section, the 14th EN BN construction planners designed the bridge approaches and abutments to sustain a 4m rise in water level.
Throughout the project, all involved respected the Tigris River and its volatile nature. This said, when the river rose 8 ft overnight the 14th EN BN knew future extreme fluctuations in the river’s height must be incorporated in the bridge’s design. The Compact 200 Mabey & Johnson float bridge uses a fixed and rolling bearing for each shore respectively to allow the bridge to flex with the river. Hand-adjustable winches were used to attach anchor lines to the pontoons. 74th MRBC developed a rudimentary depth gauge as a means to measure river depth. Combined use of the winches and a depth gauge negated the effect drastic changes in the river had on the bridge. Additionally, recording the depth gauge measurements over time established a historical file for the river. With such a large change in river depth between the winter and summer (wet and dry seasons, respectively), the bridge-to-shore connection was a concern. Using a combination of geometric calculations and innovative engineer problem solving, the 14th EN BN determined the ideal height of the abutments to negate the steep banks yet still prevent the bridge bearings from being submerged during the high water season. During construction, the Battalion S-2 monitored weather in northern Iraq and Turkey, identifying trends that enabled the 14th to predict the effects of the weather on the watersheds that fed the Tigris River, including the upper and lower Zaab.
When building out into a moving body of water, especially one with as much volume as the Tigris, erosion is a serious concern. The Mabey & Johnson float bridge has pontoon floats that are not designed to ground out on the bottom of the river. Due to this restriction, the 14th EN BN built a 100m causeway out into the Tigris from the shallow east shore to prevent low summer water levels from damaging the bridge. Site selection played an important role here as well. Protected by a natural curve in the river and a large patch of year-round vegetation, the features of the river afforded the causeway protection. During the design phase several options where discussed for erosion protection. These included combinations of everything from 7ft Hesco baskets to various geo-textiles. The chosen design involved layers of plastic geo-grid layered in the causeway, which had gently angled sides with a slope of 1:5, to provide additional soil stability. At the base of the slope, a layer of geo-fiber served as a base for stacked rows of 2000lb 4x4x4ft concrete tetrahedrons; thousands of which were available from a local concrete plant. The tetrahedrons served to block and disrupt the water while the geo-fiber safeguarded the fines in the soil. Lastly, debris from the construction site and a destroyed irrigation canal added a final layer of riprap to further protect the causeway. The design suited both the function of protection and used readily available materials. The cut made for the west shore abutment and approach required erosion protection as well. Here, Theater Construction basics where used in the form of a 5ft thick sandbag retaining wall covered with a concrete slurry to protect the sandbags from deteriorating from water abrasion. This simple but effective method of erosion control also worked well because the low overhead clearance under the bridge limited the work to manpower only.
The Tigris, with an average flow rate of 9-11 fps, presented a challenge both during the construction for the bridge erection boats maneuvering sections of the bridge into place and in anchoring the sections to the riverbed. Despite seemingly ideal riverbed conditions, the standard flute anchors did not stick. Instead they drug along the riverbed even after doubling the number of anchors to two per line. The solution was a dead weight anchor, scuttling three extra 20ft pontoons (the same ones used for the floating piers) filled with 20 tons of soil and rocks, forming the perfect dead man anchor at the bottom of the Tigris. On each bank, buried bridge transoms served as dead man anchors. Due to the Tigris’ ability to undergo dynamic river level changes, the 74th MRBC added downstream anchors in addition to the upstream anchors to prevent the bridge from listing, causing dangerous torsion forces in the structure.
Material transport and availability are always difficult in a war zone. This project was no exception. All bridge parts were transported by rail from Kuwait to Bayji Rail station and then line hauled the 35km to the bridge site by flat bed trucks. Material delays due to sabotaged rail lines occurred frequently throughout the project and transporting the mammoth 30,000lb pontoons and the 40ft containers of bridge parts to the jobsite proved to be more difficult than expected. The 14th learned that line hauling was the most reliable and efficient method of transport. Also delivery of key parts must be sequenced properly in order to ensure uninterrupted construction. As for procurement, purchasing construction materials from local contractors was the quickest and most effective way to fill critical shortages of common items. Specialty supplies must be planned for and requested months in advance to ensure availability.
In the end, ingenuity combined with the combat engineer work ethic ultimately resulted in mission success. Fundamental engineering knowledge and problem solving skills were the cornerstones for the entire project. As for the references, the electronic manuals from the Engineer School House were helpful and a Civil Engineering reference manual, such as the one used for the PE Exam, would also have been helpful. The FEST team was available as well to assist with technical advice and experience in specialized engineering fields. Mr. Alan Pearson, Retired Sergeant Major in the British Royal Engineers, was on site for the entire project as the Mabey & Johnson bridge expert. His extensive experience and technical expertise were vital to the construction of the bridge.
On 2 January 2004, the 14th Engineer Battalion opened the now named Haight-Jordan Bridge by proofing it with an Abrams tank loaded on a HET, an MLC 110 load. The 14th named the bridge after its two fallen comrades during Operation Iraqi Freedom. With the opening of the bypass, the 14th Engineers established a trend of cooperation with Iraqi contractors in rebuilding Iraq’s infrastructure. As a team, the troop construction units and Iraqi contractors developed a sound and feasible plan that succeeded in besting the ferocity of the Tigris and the bleakness of the desert terrain. Although this is a temporary bridge, we as engineers hope it will serve as a symbol of a permanent bridge between US forces and the soon-to-be new Iraqi government.
-The above article by Jason Toth was originally published by the Society of Military Engineers (SAME)
Threads to pull for future episodes
Pontoon bridges in the U.S.A. such as Hood Canal Bridge.
Podcast Hosts
Jason Toth, P.E., PMP and Scott Snelling, P.E.
To provide comments, contact the hosts on Twitter at @jasontoth_pe and @snellingscott on LinkedIn at jason-a-toth and scottsnellingpe
To ask a question to be played on the show, leave a voice message at anchor.fm/buildbigideas/message.
If you enjoy the podcast, please leave a review on Apple iTunes to help new listeners find us.
Comments