DREDGING FOR INDUSTRIAL PURPOSES 239 



These barges have a capacity of about 400 cu.yds. of sand 

 and draw about 6 ft. 9 in. in the loaded condition. They have 

 demonstrated their ability to withstand the severe service and 

 are much more economical in operation than the wooden type. 

 Of course the steel construction costs considerably more than the 

 wood, but when depreciation, cost of upkeep, and interest on invest- 

 ment are considered, it will be seen that the steel barge, with its 

 life triple that of the wood, is the better business proposition. 



In elevator or ladder dredging, the open-hold barge is used to 

 receive the sand or gravel. From the nature of this dredging, 

 practically no free water is carried into the barge, and it is only 

 necessary to provide a suitable open carrier. As a rule, these barges 

 do not have to meet the severe conditions incidental to Mississippi 

 River dredging. The distance they are towed is usually much 

 less and the unloading machinery is not of such heavy type. The 

 barges, however, are of substantial construction and are well adapted 

 to the service. The common carrier used in the Pittsburg district 

 is about 100 ft. long, 24 ft. in beam, and has a depth of 8 ft., and a 

 capacity of about 175 yds. 



Unloading Machinery. The usual arrangements provided for 

 the reception of the sand from the barge consist of elevated bins 

 situated conveniently on the river bank in connection with some 

 type of unloading device. These bins are usually so arranged that 

 cars or other means of conveyance may be loaded readily. In the 

 type of unloader much diversity exists, but the iUustrationsiifhow 

 some of the most modern Mississippi River stations with their 

 machinery. These structures are of steel. Steam and electric 

 power are used, and manual labor is reduced to the minimum. 

 In unloading the barges, no attempt is made to clean out the cargo 

 box beyond the action of the grab bucket. These buckets are very 

 heavy and at times much injury is done the deck by allowing them 

 to descend too rapidly. The capacities of the buckets vary from 

 2 to 4 yds. and the speed in operation from 75 to 150 yds. per hour. 

 At some of these plants the sand or gravel is carefully screened 

 into different grades for various purposes. In the upper Ohio district 

 the unloading stations are not built on such a large scale and the 

 unloading machinery is not of such large capacity nor of such elaborate 

 parts. Some of the unloaders consist of a boom derrick and bucket 

 which is swung into the barge and filled by hand, while others consist 

 of a medium-size grab bucket operated by boom derrick. Storage 



