Engineer

MAY-AUG 2013

Engineer presents professional information designed to keep U.S. military and civilian engineers informed of current and emerging developments within their areas of expertise for the purpose of enhancing their professional development.

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Thebes Reach Rock Removal T he Thebes Reach presents problems for maintaining a safe and reliable navigation channel in the Mississippi River. Sand and gravel at the bottom of the river, which can be dredged easily, are the usual hazards in the channel. However, natural limestone formations near Thebes, Illinois, (about 38 miles above the confuence of the Ohio River) proThe dredge Potter vacuums sediment from the channel and pumps it out of a trude from the bedrock and impede pipeline that extends as much as 1,200 feet. shipping traffc when water levels are low. Periods of low water are the optimum time to remove obstacles since they allow River Training Structures excavators and hydraulic hammers easier access to the river iver training structures are rock dikes used to direct bottom. Taking advantage of the low water caused by the the fow of the river. If used correctly, they can bene- drought, the St. Louis District contracted with two compaft the river industry and the environment. Below nies to remove rock at the bottom of the reach. The excavaGranite City, the channel is maintained primarily by using tion work began on 15 December 2012, with the frst blast river training structures and sediment dredging to keep occurring on 21 December. The contractors mechanically buildup from occurring in the channel. The effect maximizes excavated or blasted the most critical 890 cubic yards of rock the fow of water within the channel itself, carrying sedi- formations out of the way. The St. Louis District is evaluatment out of the channel. ing a second phase to widen the channel for additional safety The previous major drought in the Midwest was in 1988– in several nearby river bends. The work is a joint operation 1989. Many river training structures have been constructed between USACE, the U.S. Coast Guard, river industry represince then. This work is a permanent improvement to the sentatives, and local stakeholders with an interest in ensurchannel and has reduced the number of dredges needed in ing safe navigation during the rock removal. To help ensure safety, St. Louis District employees and qualifed contractors the middle Mississippi area from eight to two. routinely conduct surveys to verify the depth of the channel. R Dredging Operations T he dredge Potter was primarily used by the St. Louis District during the drought, operating around the clock with a rotating crew of 54 from July 2012 to January 2013. In operation since 1932, it is one of the oldest working pieces of equipment in the Army. Potter uses a dustpan style dredge that acts like a giant vacuum, pumping 4,500 cubic yards of water, sand, and gravel each hour from the bottom of the river out through a pipeline extended as much as 1,200 feet. During the drought, 8,400,000 cubic yards of sediment were dredged out of the channel, more than double the amount removed during a nondrought year. The dredge removes sand and rocks up to roughly basketball size. Any bedrock or large boulders require specialized removal techniques, such as those used near the Thebes Reach. 22 Engineer Contractors blast a limestone obstacle at the Thebes Reach. May–August 2013

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