dredging engineer 00 simo rich

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DREDGING ENGINEERING


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PUBLISHERS OF BOOKS FOR^Coal Age * Electric Railway JournalElectrical World v Engineering News-RecordAmerican Machinist v Ingenieria InternacionalEngineering 8 Mining Journal *- Powe rChemical & Metallurgical Engineering

Electrical Merchandising


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DREDGINGENGINEERINGBY

F. LESTER SIMON, B. S. IN C. EAssoc* M. AM. Soc. C. E.

FIRST EDITION

McGRAW-HILL BOOK COMPANY, INC.NEW YORK: 239 WEST 39TH STREETLONDON: 6 & 8 BOUVERIE ST., E. C. 4

1920


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COPYRIGHT, 1920, BY THEMCGRAW-HILL BOOK COMPANY, INC.

THE MA1'I,K FRBSS Y f > K JC I>A


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PREFACEWith the reader's indulgence, we will explain just what

this small book is by first defining what it is not. It isnot a compendium of statistics. Neither is it a record ofthe actual performance of specific dredges of all kinds andunder all conditions; nor yet a compilation of dredging costdata. Such information is already minutely availablein the Annual Reports of the Chief of Engineers, U. S.Army, and in the records of the various departments ofcommonwealths and municipalities controlling dredgingoperations in their respective localities.

It has been the author's intention first to describe the prin-cipal types of dredge in such manner as to impart a fun-damental working knowledge of their construction andoperation, and then to consider, in concise form, the usualproblems confronting the engineer in the conception andaccomplishment of dredging projects.Because of the fact that most literature upon the subject,

having been presented in the form of papers and articlesin the technical periodicals, is not only not readily available,but incomplete, in that each as a rule treats only of oneparticular phase of the subject, it is thought that a needexists for a comprehensive treatise, which should be helpfulalike to the student and engineer. To fill this want has beenthe author's objective.The importance of the subject, while not always ap-parent to the layman, is obviously paramount, involvingthe expenditure of many millions of dollars annually forthe necessities of commercial life.

F. L. S.BALTIMOEE, MD.,

April, 1920.

Vll


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CONTENTSPAGE

PREFACE . ..- v ., .',.., , . , . viiCHAPTER I. DEFINITION AND CLASSIFICATION 1CHAPTER II. GRAPPLE DREDGES 4

General Description 4The Bucket 6The Common Grab 6Bucket Axioms '. HThe Sliding Cross-head Bucket 12Other Types 13Grab Bucket Details and Appurtenances 15

Boom, "A" Frame and Back Guys 16Spuds, Spud-wells and Gallows-frame 20The Machinery 22The House 24The Hull 24Operation 26

CHAPTER III. DIPPER DREDGES 29General Description 29The Bucket 29Boom, Dipper-stick, "A." Frame and Back Guys 31The Spuds ..*... 37The Machinery 39The Hull ...;,....... 40Operation ........*..... 41Application of the Type ^12High-powered Dipper Dredges 44

CHAPTER IV. LADDER DREDGES 46Historical 46General Description 50Stationary Type 52Sea-going Hopper Type 52

CHAPTER V. Scows 54CHAPTER VI. HYDRAULIC DREDGES OF THE RIVER TYPE 58

Radial Feeding Dredge with Spud-anchorage ......... 58General Description ..;... 58Cutter Head and Ladder . . .... '. . .... .... 61Feeding v ......... 64Boom, "A" Frame and Back Legs . . V . 66The Pipe Line . 67The Pump * . . 71

ix


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X CONTENTSPAGEPower 78Method of Design ........ * 79

The Machinery 81Operation .' * 83Booster or Relay Pumps 83

Forward-feeding or Mississippi River Type .......... 86General Description 86

CHAPTER Vll. HYDRAULIC DREDGES OF THE SEA-GOING HOPPERTYPE. , 89

Historical ';. . . . 89General Description . . v /"". . . 91Advantages of the Type 92Typical Examples 93

PART II DREDGINGCHAPTER VIII. OBJECTS AND PHASES OF THE SUBJECT 95CHAPTER IX. PRELIMINARY ENGINEERING 97

Exploration of Site 97Estimating the Quantities 100Choice of Plant and Method of Disposal of Dredgings 107Plans, Specifications and Contracts 118Scheduling 121Estimating the Cost 123

CHAPTER X. PRELIMINARY CONSTRUCTION 125Dikes for River Control 125Dikes for Impounding Basins 130

Dry Dikes 130Wet Dikes 132Spur Piles 136Pressures on Wet Dikes . . -. ...... 139Design 146

Sluiceways. . 150Pipe Lines .-..>. 152

CHAPTER XI. OPERATING .......'.. 154Organization ' . 154Cut and Range Layout 155Hydrography . , 157Inspection >'-,, 158Basin Regulation 164Progress Keeping . . . 167

CHAPTER XII. REMOVAL OF SUB-AQUEOUS ROCK 170Undermining and Blasting 170Drilling and Blasting from the Surface 171Rock Breaking Machines 172Dredging the Broken Rock 173

INDEX. 175


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DREDGING ENGINEERINGPART I

DREDGESCHAPTER IDEFINITION AND CLASSIFICATION

A dredge is a floating excavating machine, and the processof removing subaqueous material is termed dredging.There are many kinds of dredges and several classifica-tions are possible, differing in the choice of a basis ofdistinction. It is the author's intention to discuss onlythe more important types, the usual equipment of to-day,disregarding the older and practically obsolete plant suchas stirring and pneumatic dredges. With 'this limitation,dredges may be classified broadly under two generalheads :

I. Bucket DredgesII. Hydraulic DredgesBucket Dredges, obviously, are machines that remove

the material by means of buckets, which, after obtainingtheir loads by biting or digging into the bottom, are raisedclear of the water to dump either into waiting scows, self-contained hoppers, or upon spoil banks.

Hydraulic Dredges, known also as Suction or Pumpdredges, excavate by direct centrifugal pumping throughsuction pipe, pump and discharge pipe into hoppers con-tained in the dredge itself, into hopper barges, into adjacentdeep water, or into natural or prepared reservoirs, calledimpounding basins, from which the great volume of water,1


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2 DREDGING ENGINEERINGnecessarily pumped along with the dredged material,runs off, leaving a deposit of solid matter.The bucket class may be sub-divided into three types :

(a) Grapple Dredges(b) Dipper or Scoop Dredges(c) Ladder or Elevator DredgesBoth Grapple and Dipper machines have a single bucket

each. In the former, it is suspended from the end of aswinging boom and consists of two or more shells or jaws,by the closing and opening of which the bucket is loadedand discharged. In the latter, it is a scoop attached to along handle and "digs" in the same manner as the familiarsteam shovel on land.Ladder Dredges consist of a series of smaller buckets

travelling in endless chain succession upon an inclinedframe called a ladder, in passing under the lower end ofwhich they receive their loads by scraping along and intothe bottom, discharging into a chute while passing over theupper tumbler. Apparently it is simply an applicationof the old principle of the bucket elevator.Grapple or Grab Dredges, in turn, may be divided intotwo sub-types:(a) Clam-shell Dredges(b) Orange-peel DredgesA distinction only by virtue of the style of bucket carried.A clam-shell bucket has two quadri-cylindrical shells

arranged in a manner sufficiently analogous to those of themore humble clam to warrant the pilfered title. An orange-peel bucket has generally four shells, forming a hemispheri-cal bowl when closed, but spreading when open like thequadrants of an half orange.Ladder Dredges are susceptible of sub-division intothree classes:

(a) Stationary Dredges(6) Self-propelled, Barge-loading Dredges(c) Sea-going, Hopper DredgesThe first is the usual river or calm-water type, which is

fed laterally or radially by means of anchorages or spuds


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DEFINITION AND CLASSIFICATION 3and hauling cables, and discharges either into waitingbarges, or into deep water or spoil basins more remotefrom the dredge.Both the second and third types have moulded hulls andsea-going qualities, but the second, because of the accom-panying barge, is confined to the calmer waters of portsand estuary channels, while the third is a sea-going vessel,comprising both barge and dredge in one.

Hydraulic Dredges are of two general types:(a) The River Dredges(b) Sea-going Hopper DredgesThere are two principal River Types :(a) Radial Feeding with Spud Anchorage(b) Forward Feeding or Mississippi River TypeAgain, Radial Feeding Dredges may be either of the

Swinging Dredge Type or the Swinging Ladder Types.In the former, the dredge itself pivots about a stern spud,and in the latter, the ladder only pivots about the bow ofthe dredge, which is held stationary by four spuds.The subdivision of Hydraulic Dredges might be carriedstill further with self-propulsion as the distinguishingfeature, as both Radial and Forward Feeding Machineshave been built to navigate under their own motive power.They, however, are the exception rather than the rule andwould result in unwarranted complication of the above.The entire classification may be summarized as follows:

Dredges


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CHAPTER IIGRAPPLE DREDGES

General Description. A grapple dredge is, in principle, aderrick mounted on a float and swinging a grab bucket.Any derrick lighter may perform the operation of dredgingby simply attaching a grab bucket to its fall. This, how-ever, is merely a makeshift, and by no means constitutes adredge. The operation of the bucket is but one of threeprincipal functions of the grapple necessary to the completebusiness of " mud-digging." They are:

1. To operate the bucket2. To control the position and local movement of the

dredge itself3. To handle the scowsThe first requires a derrick, comprising boom, "'A"

frame and back legs and the requisite hoisting machinery,consisting of a double cylinder, double drum engine withboiler, auxiliaries and appurtenances. The bucket is a veryimportant machine in itself, and its proper design is mostessential to the efficiency of the dredge. It will be discussedin detail subsequently. It is hung from the end of the boomby two wires or chains by means of which it is raised, low-ered, opened and closed and by which also the boom isswung.The position and local movement of the dredge is con-trolled by spuds, by anchors, or a combination of both.Spuds are long timbers of heavy section suspended fromtall masts or so-called gallows-frames and running verti-cally through ope'nings in or pockets attached to the hull.They are raised by hoisting engines and, when released,penetrate into the river bottom by virtue of their ownweight, thus anchoring the hull and holding it in positionwhile digging.


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GRAPPLE DREDGES


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6 DREDGING ENGINEERINGThe third function, that of handling the scows, is accom-

plished by three wires attached to the scows and lead tocapstans or to hoisting engines on the deck of the dredge.The scow is thus moored to the starboard quarter of thedredge and, as each pocket is loaded, is hauled aft untilthe next pocket comes under the bucket.

Figure 1 is a first-class representative grapple, the Dredge" Camden, " a 7^ yard clam-shell, of the American DredgingCompany of Philadelphia, shown working for the AmericanInternational Shipbuilding Corporation in the constructionof the Hog Island Shipyard on the Delaware River.For convenience, in a more detailed exposition of the

grapple dredge, each component part will be discussedsuccessively as follows:

1. The Bucket2. Boom, "A" frame and Back Legs3. Spuds, Spud-wells and Gallows-frame4. The Machinery5. The House6. The HullThe Bucket. The grapple dredge bucket, or the grab

bucket as it is called, is of two kinds, the clam-shell andthe orange-peel, as briefly defined in the foregoing classifi-cation. Both have the same principle of operation, butdiffer in that the clam-shell has two jaws or shells while theorange-peel has three or four. There are two principaltypes of clam-shell bucket, the common type and the slidingcross-head bucket, each of which may be distinguishedfurther as a hard, medium or soft-digging bucket accordingto the class of material for which it is designed. Figure2 is a soft-digging clam-shell of the common type.A hard-digging clam-shell bucket of the sliding cross-headtype with a capacity of 7^ cubic yards, is shown in Fig. 1and the frontispiece.

Figure 3 is the extra heavy, standard orange-peel bucketof the Hayward Co.The Common Grab. A general knowledge of the con-struction and operation of grapple buckets may be had byan analysis of the common grab.


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GRAPPLE DREDGES 7The bucket has five main constituent parts as shown

in the outline drawing, Figure 4.1. The two shells a2. The spool and shaft b3. The four arms c4. The cross-head d5. The two closing chains e

FIG. 2. Soft-digging clam-shell bucket of the common type. (Courtesy ofVulcan Iron Works, Inc., Jersey City, N. /".)The shells rotate about the main shaft, to which

the spool is keyed. The spool consists of a large centralsheave to the perimeter of which the closing wire / is at-tached, and two cylinders of smaller diameter, (the spoolsproper) to which the two closing chains e are fastened.


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8 DREDGING ENGINEERINGWhether a single casting or independent parts, these threewheels are keyed to the main shaft. The arms are pinconnected to both cross-head and shells. The upper endsof the closing chains e are attached to the under side of the

FIG. 3. Extra heavy orange-peel bucket. (Courtesy of the Hayward Co.)cross-head d. The closing wire / is confined by a leadsheave g mounted on the side of the cross-head. Theopening wire h is bridled to the cross-head.

Obviously, therefore, if the bucket is suspended fromthe opening wire with the closing wire slack, the spool andshells fall by virtue of their weight until the spool has


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GRAPPLE DREDGES 9rotated to the point at which the closing chains are verticaland taut, preventing further motion. The bucket ishanging open. If, now, the closing wire is given the load,and the opening wire slacked, the central sheave is causedto rotate so that the spools proper are rolled up on the clos-ing chains toward the cross-head until the bucket's jawscome together. The bucket is closed. The limit of ten-sion in either opening or closing wire is the weight of theloaded bucket in air, plus an allowance for impact and

Side Elevation(Closed)

Front Elevation(Closed)

Forces Acting oneach Shell

Force Polygon(One Shell)

FIG. 4. The common grab bucket.

breaking bottom suction. With the bucket resting openupon the bottom, however, the closing wire tension at thebeginning of the "bite" is necessarily less than the weightof the submerged bucket empty since the bucket obviouslywould be raised clear of the mud by a pull greater than itsweight. As the bucket closes, the weight of the enclosedmaterial becomes effective and the wire tension increases.The total stress in the two closing chains is equal to theproduct of the closing wire tension by the ratio of the di-ameter of the central sheave to that of the chain spools.The total compression in the four arms is equal to the sumof the total closing chain tension and the weight of the


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10 DREDGING ENGINEERINGcross-head divided by the cosine of the angle 6 between thearms and chains.

Isolating one shell of the bucket, (Figure 4) the forcesacting upon it when closing are the following:mn the thrust of the two arms.no one-half the upward pull of the main shaft.op the horizontal pull of the opposite shell.pq one-half the weight of shaft and spool.qr the weight of the shell itself.rm the resistance offered by the bottom material to the

cutting lips of the bucket.Beginning at m and going clockwise, the force polygon

is as shown, mnopqr. The force no represents one half thesum of the closing wire and closing chain tensions. Theforce qr acts through the center of gravity of the shell.For any given position of the bucket, the force rm, or thecutting resistance at the lips, will be maximum when thetension in the closing wire is greatest (i.e., when equal to theweight of the submerged bucket) but when this conditionobtains, the bucket is on the verge of rising so that thevertical component of rm is zero, or, in other words, thecutting force rm at the lips becomes horizontal, and itsvalue may be determined for any position of the bucketby taking moments about the main shaft as a center, aftercomputing mn as above and approximating the weight qrand its point of application. The form of the shell, there-fore, in so far as it influences the magnitude of the mo-ments mn } qr and rm about the shaft, is an important factor,i.e., the shape of the inscribed triangle formed by linesjoining"the main shaft, arm-pin and edge of lip determinesthe relation between the lengths of the lever arms of theforces acting.From the above considerations, the following conclusionsmay be drawn in reference to the bucket's ability to dig,or, in other words, its closing power as measured by thecutting force at the lips :

1. It is a function of three co-related quantities: theweight of the bucket, the form of the shell and the ratioof diameters of central sheave and chain spools.


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GRAPPLE DREDGES 112. It varies inversely with the depth of bucket from shaft

to lip.3. It varies directly with the width and weight of thebucket and with the diameter ratio of sheave to spools.

4. Weight in or near the curved back of the shells is nomore effective than that concentrated about the shaft.Bucket Axioms. The bucket designed for soft-digging

need not have inordinate closing power. It is made,therefore, as light as consistent with strength and durabilityand, in shaping the shells, the consideration of maximumcapacity outweighs that of the adjustment of the inscribedtriangle to maximum closing moment. The ratio of thediameter of the closing wire sheave to that of the closingchain spools is less than in the hard-digging bucket.The hard-digging bucket requires great weight that itmay obtain a full load in refractory material and, sincethe maximum combined weight of bucket and contents isfixed by the power of the main engine, bucket capacitymust be sacrificed to bucket weight. A dredge capableof swinging a 10 yard soft-digging bucket will carry a some-what smaller hard-digging bucket, probably about 7K yards.The relatively large sheave to spool ratio, essential to thehard-digging bucket, results in a central sheave of con-siderable diameter, since the diameter of the closing chainspools cannot be less than enough to reel up with one revo-lution a sufficient length of chain to close the bucket.The designer should be mindful, however, that the peri-meter of the sheaves must not extend below the horizontalplane through the lips of the open bucket, for the- reasonthat, on a hard level bottom, the bucket so built in-stead of resting on its two lips in the wide open condition ,would be supported by one lip and the central sheave andwould have to close partially before being able to "bite."This fault is negatived in the soft-digging bucket by the non-resistance of the bottom. It is entirely possible, too,that the closing power be excessive in proportion to theweight, causing the bucket to close too quickly beforeattaining the penetration necessary to get a full load.


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12 DREDGING ENGINEERINGThe depth of any bucket from main shaft to lips must

not be so great with respect to the width that, when closed,the main shaft is so far above the horizontal plane throughthe arm-pins that the closing moment is materially reducedfor the last part of the closing. A bucket having this faultmay be difficult to keep closed when loaded and will openvery quickly.The curvature of the shells in front elevation should besharper than a full quadrant so that the bucket will notfit the "bite" so neatly -as to create a mud suction resistingthe lifting of the bucket.

In side elevation, the lips of soft-digging buckets maybe straight or nearly so without detriment, but in the hard-digging type, lips of considerable curvature are moreeffective.

All buckets should be so designed in regard to shell curva-ture and maximum spread of opening that, when wideopen, all parts of the shells may lie within the verticalsthrough the lips. Otherwise, the pressure of the waterupon the protruding shell surface has a tendency partlyto close the bucket.The Sliding Cross-Head Bucket. It is difficult to de-

sign a hard-digging bucket of the common type just de-scribed that will combine all the advantages and omit allthe faults mentioned. Better results can be obtained intough bottom by the use of what is known as the slidingcross-head type, Figure 1, page 5. It consists, in principle,of a common grab bucket supplemented by a rectangularframe, the two side members of which act as guides forthe travel of the cross-head and the lower member of whichis formed by the main shaft of the bucket. The shells,instead of hinging directly on the main shaft, are pin-connected to the lower corners of a pair of triangular links,which, in turn, are hung from the main shaft. Thusthe hinge centers of the shells are below the main shaft,with a consequent decrease in the length of the lever armof the force rm, Figure 4, resisting the closing of the bucket.All the desirable features of an efficient bucket may readily


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GRAPPLE DREDGES 13be embodied without the presence of any of the faults.In addition to the prime advantage of great closing power,this bucket has the further good points that the elevationof the spool above the shell hinges allows plenty of openspread, keeps the spool clear of the contents of the loadedbucket and raises the central sheave well above the planeof the lips when open; while the frame adds effectiveweight, stiffens the entire structure and provides conven-ient fastening for the bucket poles. On the whole the slid-ing cross-head bucket is an excellent, durable and efficienttool.For soft digging, however, the author prefers the common

grab type, as, in this case, greater closing power is notrequired nor are bucket poles, and the frame, by addingunnecessary weight, reduces the capacity of the bucket.Other Types. In addition to the above, there are manytypes of grab buckets, some differing in minor detail andseveral in closing principle.Many small buckets, i.e^ from about % to 1% yardscapacity are rigged to close by a three and four sheaveblock purchase through which the closing wire is reaved.This type is seldom used in dredging however.Buckets of larger size have been constructed with thearms pinned to lugs projecting from the back or convex

side of the shells, outside the bucket, in order to yieldgreater closing moment by increasing the lever arm lengthof the arm thrust.The Stockton bucket resembles, in principle, a huge

pair of tongs, to the ends of the long curved handles ofwhich, the bridled closing wire is attached.The Arnold bucket is closed by compressed air, whichdrives a piston in a cylinder contained in the bucket. Theobject is to correct the omnipresent fault of the commongrab consisting of the loss in effective closing weight dueto the lifting propensities of the closing wire.The Williams bucket, page 14, is a powerful, capabletool, unique in its closing power arm.There are several single-wire buckets on the market,


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14 DREDGING ENGINEERING

FIG. 5. The Williams bucket. (Courtesy of G. H. Williams Co.)


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GRAPPLE DREDGES 15but they are seldom used in dredging. They present theadvantage of ready attachment to any two-drum hoist.Grab Bucket Details and Appurtenances. Both thecross-head and the shells may be either monolithic steelcastings or built up. If castings, the shells, after wearingaway at the cutting edge, may be fitted with attachablelips. The built up shell is made of steel plates varying in

FIG. 6. Three blade orange-peel bucket in operation.Co.)

(Courtesy of the Hayword

thickness from about % to % inches according to the sizeof the bucket. They are reinforced at the corners and edgeswith bent plates and steel castings and usually at one or twointermediate points in the width of the curved face withplates and angles. The cutting edge or lip of each shell isgenerally strengthened by the addition of a steel casting or,preferably, manganese steel, the better to resist the severeabrasion. The arms, in section are variously round, square,


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16 DREDGING ENGINEERINGrectangular and "H" shaped. All parts of a bucket, moreparticularly the pins, shafts and pin and shaft bearingsshould be generously proportioned and provided withrenewable bushings.The capacity of a bucket may be increased temporarilyfor soft digging, i.e., in material that has sufficient body andcohesion to stand up, by the use of so-called " side-boards "which are bent plates bolted to the arms, one to each shell,and having the effect of raising the backs and sides of theshells.The tendency of the bucket to rotation must be pre-

vented, as otherwise the two bucket wires would becomecrossed and fouled. This is accomplished in one of twoways. A wire called a "dorsey wire" is attached to theside of one of the bucket shells and is lead by sheaves upthrough the boom, about midway of its length, to a smallpendant weight, rising and falling in the plane of thegallows frame. Or a pair of hardwood poles may befastened to or set in the uprights of the bucket frame,extending up through rings attached to the boom head.The poles have the additional function of maintaining thebucket in an upright position on the bottom, i.e.j -preventingit from " falling over" when landed upon sloping, hardmaterial. For this reason, they are considered by many" mud-diggers" to be an absolute necessity for efficientdredging in hard stuff.Teeth are rarely used on large buckets, but on the

smaller types, which are relatively light in weight, they areoften helpful.Boom, "A" Frame and Back Guys. The derrick or

crane element of the dredge, by which the bucket is handled,comprises boom, "A" frame and back legs with guys.The "A" Frame is usually vertical, or nearly so, and isset some distance back from the bow of the hull in orderbetter to trim the ship and to permit a sufficient length ofhull forward of the boom heel for holding the scow alongsidewhen the first pocket is being loaded. Back guys andusually also back legs, i.e., struts paralleling and supporting


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GRAPPLE DREDGES 17the tension rods, extend from the peak of the


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18 DREDGING ENGINEERINGsheaves" mounted on the " table" which is analagous tothe horizontal bar of the letter "A" in the "A" frame,and from there to the drums of the main engine. Thetwo sets of table sheaves are spread some distance apartathwartships for the purpose of giving the bucket wiresenough lead to the boom to swing it.The weight of the loaded bucket and the boom and thepull of the engine create stresses in the boom, topping fall,"A" frame and back guys, which are readily determined.The manner of rigging and the forces acting at the boomend are shown in Figure 7a and b, and the stress diagram inFigure 7c. For the analysis, it will be. assumed that theclosing wire lies in the same vertical plane with the boomand that it takes the entire load of bucket and contents.Since this wire passes over a sheave in the boom head, theforces ab and cd representing the tension in it must beequal to each other and to the weight in air of the loadedbucket. It is advisable to increase all stresses by an impactallowance of at least 50 per cent. The boom compressionad and the topping fall tension be may be scaled from thediagram, a study of which will reveal that both thesestresses increase with the decrease in the boom angle 6and also with the decrease in the angle 4> between boomand closing wire to engine.To the above stresses must be added those due to theweight of the boom, which may be found graphically bydrawing a second force polygon of the three forces, endreaction of weight of boom, boom compression and toppingfall tension, omitting the bucket and closing wire.The topping fall tension creates stresses in the legs ofthe "A" frame, alternately tension and compression as theboom swings and maximum when the boom is at the limitof its arc. The tension in a back leg will be greatest whenthe boom is in the plane of that leg. A graphical analysiswill easily yield these stresses.The boom is designed for combined compression andbending due to its own weight. The choice of section isinfluenced by the use of wire cable or chain for the bucket


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GRAPPLE DREDGES 19operation. Either may be employed with equal success,the preference being largely personal. If wire is used,the sheaves in the boom and on the table must be of largediameter in order to prevent undue bending stresses in thecable. The end of the boom, therefore, must be designedin such a manner as to provide a fork between the prongsof which, the two large sheaves are mounted on a shaft.The guide sheaves need not be so large. If chain is selected,all sheaves can be much smaller and those at the boomhead may be suspended beneath the boom. There aremany types of boom. In the smaller machines, it may con-sist simply of a single stick of timber with or withouttruss rods. In the larger dredges it may be built up of twotimbers braced together side by side with sufficient clear-ance between to contain the two end sheaves. The timbersare often reinforced with steel plates or channels. Or itmay be a lattice or truss boom of timber or of steel. Aconvenient form of the latter comprises four angles lacedboth ways and with deep plates at the ends and one inter-mediate point at least. Booms of this type require trans-verse or sway frames to resist racking strains.The "A" frame members, if built of wood, must besupplemented with steel stay rods because of the reversalof stress. Although the back legs are subject to compres-sion only in rare instances, yet it is advisable to combine inthem both strut and rod the better to hold the "A" framerigidly and truly in its intended plane without oscillationor vibration.

All sheaves, shafts, boxes and the derrick fittings shouldbe proportioned generously to withstand the exceptionallysevere wear and tear and to cut repairs and renewals to aminimum. The connections of the "A" frame and theback legs to the hull and the bearing and thrust of theboom heel casting upon the hull require care as to the propertransmission of the stresses to the hull and will influence thehull design to the extent of the provision of adequatestrength at those points.


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20 DREDGING ENGINEERINGSpuds, Spud Wells and Gallows-Frame. The grapple

dredge is held in position, oriented and advanced in cutby means of spuds alone, or by wires alone, or by a combi-nation of spuds and wires. Many machines are fullyequipped both with a full complement of spuds and spudhandling apparatus and with all the necessary machineryand appliances for operation by wires and anchors. Thisdual arrangement is obviously advantageous, since, whilethe spud method is more convenient through the saving oflost time in running lines and anchors and through the

Anchor Anchor

Anchor

AnchorFIG. 8. Grapple dredge.

AnchorPosition control by wires.

non-interference with traffic and adjacent structures, yetthere are times when the wires must be used, as in deepwater or in swift currents or in the event of the breakingof a spud. 1For position control by wires, five lines are generally

used, two quarter lines on each side and a stern wire. Theymay be arranged as in Fig. 8a, or as in Fig. 8b. The riggingof 8b presents advantages over the former in that the twoquarter lines forward are not as likely to foul the bucketin digging and dumping into the scow. In either case, thetwo wires on the scow side of the dredge, usually the star-board, must be elevated to clear the light scow. This isdone by leading them from the drums up through overheadsheaves, one attached to the end of a cross arm set in the


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GRAPPLE DREDGES 21"A" frame or gallows-frame for the bow wire and anothermounted on a post or column erected on the after deckfor the stern quarter line. Some machines have symmetri-cal wire equipment so that the wires may be elevated oneither or both sides. Others have but two spuds, used inconnection with the wires. It is readily seen that thehigh wires on one side and the low deck wires on the otherexert a force couple tending to resist the listing of the dredgein the direction of the low wires when the boom is swungto that side.Another expedient, which has been successfully used,

to prevent scow and other interference with the anchorwires is the complete submergence of the wires by runningthem through sheaves in the toes of the spuds. Thisarrangement has been patented by the Osgood DredgeCompany. The 10 yard clam-shell dredge "Finn MacCool" was so equipped.The spuds in the smaller dredges are single sticks ofround or square timber. In the larger machines they aresquare built-up members comprising four or nine piecesof heavy square timber bolted together. Some spuds areas large as four feet square. They are equipped withpointed metal shoes to facilitate penetration into the bot-tom and to add enough weight to overcome the buoyancyof the timber. All-steel and wrought iron spuds are some-times used, both round and square.The spud wells or openings at the deck through whichthe spuds travel may be either holes in the hull or housingsattached to the sides and stern of the hull and called " out-side" wells. The spuds must have sufficient clearance inthe wells to permit perfect freedom of operation.Three spuds are required for control entirely independ-ent of wires, two forward at the sides and one aft, in thecenter. The two forward spuds are suspended from a tall,vertical rectangular frame called the " gallows-frame "approximately in the plane of the "A" frame and oftenbraced to it. The stern spud is hung from a frame of morehumble proportions or from a single mast maintained plumb


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'2 "2 DREDGING ENGINEERINGby guys. The gallows-frame consists simply of two orfour columns with a top cross cap and the necessary trans-verse struts and diagonal braces or knees. It must beborne in mind that the load upon the gallows-frame may bemuch more than that due to the dead weight of the spudbecause of the difficulty frequently encountered in pullingthe spud out of the tenacious river bottom, i. e., " breakingthe suction. " It even may be greater than that developedby the maximum pull of which the spud hoisting engine iscapable due to the fact that the careening of the dredgewhen digging is resisted by the reaction of the spud on thehigh side, comprising both weight of spud and the gripof the mud. The spuds are raised by the spud engine,having a wire leading through a sheave at the gallows-frame cap thence down to a collar encircling the spud andfree thereof, but which grips the spud when the wire istaut. In rarer instances, the spud wire is made fast tothe spud well housing from which it is lead down through asheave in the toe of the spud itself, thence up through thegallows frame sheave and finally to the drum of the engine.Eigged so, the gallows frame need not be as high as the fulllength of the spud.

It is interesting to note here that in the case of a dredgeswinging a long boom, excessive listing is sometimes pre-vented by the use of boom logs attached to the sides of thehull by chains of such length that the logs are afloat whenthe machine is on an even keel, but are raised clear of thewater when the boom swings to the opposite side.The Machinery. The usual machinery of the clam shelldredge comprises the following units :

1. The Bailer: The Scotch Marine is a general favoritealthough Locomotive, Leg, Vertical and others are used.It is usually designed to carry from 125 to 150 poundspressure, and is located aft, where, with the appurtenantcoal bunkers and water tanks it acts as a counterweight tothe active loads forward.

2. The main engine for operating the bucket wires: gen-erally a two cylinder horizontal type driving two drums


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GRAPPLE DREDGES 23through pinions, spur gears and frictions. The drums arein line athwartship and spread to give a direct lead to thetable sheaves. The frictions may be either of the block ordisc type, the latter being preferable for large machines.The pinions are as a rule below the elevation of the drumshaft. In the older dredges, the drums were thrown intoclutch with the frictions by means of a long hand-operatedlever in the pilot house. Now, however, steam or com-pressed air is used, except in small machines. The fric-tions are a very important item of the unit and should begiven careful consideration. For efficient digging theirgrip must be positive and their release quick and complete.

3. The Secondary Engines: The operation. of the spudhoists, the anchor wire drums and the capstans or drums(as the case may be) for scow lines may be accomplishedthrough two engines if desired, one forward and one aft.If the dredge be fully equipped with a complete comple-ment of spuds and anchor wires and with symmetricalscow-handling equipment for right and left hand digging,a total of 14 drums or drums and capstans are required,3 for the spuds, 5 for the anchor lines, and 3 on each sidefor scow control. The secondary engines should be doublecylinder.

4. Pumps: A minimum of two pumps is essential, onefor the boiler feed and the other for pumping from the bilge,water tank or water scow and for fire purposes. If a sur-face condenser be used, circulating and air pumps will berequired.

5. Condenser: The main and secondary engines may berun as* condensing engines by piping the exhaust steamfrom all to a single condenser.

6. Miscellaneous equipment, such as electric light plant,air compressor and refrigeration plant.The runner's control in the pilot house comprises simplymain engine throttle and frictions. The secondary enginesare under local control upon signal from the pilot house.An idea of the relative engine sizes may be had fromthe following data: The dredge "ADMIRAL/' shown on


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24 DREDGING ENGINEERINGthe frontispiece, has a hull 110 ft. X 39 ft. X 11 ft. 10 in.,swings a 7>2 yd. bucket and has\ horizontal, two-cylindermain engine 20 in. X 24 in. The dredge " BALTIC,"American Dredging Company, has a hull 110 ft. X 39 ft.X 12 ft., a 5J^ yd. bucket and a horizontal two-cylindermain engine 16 in. X 24 in. The dredge " COLUMBIA,"of the same Company, has a hull 90 ft. X 35 ft. X 10 ft., a4 yd. bucket and a two-cylinder horizontal engine 14 in.X 20 in. The "PACIFIC" (same owners); hull 78 ft. X23 ft. 9 in. X 7 ft., bucket 2>4 yd., main engine, two-cylinder, horizontal 10 in. X 15 in. The dredge "FINNMAC CQOL;" hull 120 ft. X 40 ft. X 12 ft. 6 in., bucket10 yd. (soft digging) main engine, two-cylinder, 18 in. X24 in. The " ADMIRAL" mentioned above will swing a 10yd. soft-digging bucket. Dredges of this size usuallyhave secondary engines of from 8 X 10 to 10 X 12 doublecylinder. The engines of clam shell machines are seldomrun condensing.The House/ The usual grapple boasts a two-story frameor steel house, the first floor of which comprises boiler andengine housing, galley and mess room; and the secondfloor, pilot house or operators room, and sleeping quartersfor the crew and inspector. Deviations in detail from thisarrangement are not uncommon. The main engine andboilers are generally depressed below the main deck.The Hull/ The dredge hull must be of sufficient sizeto contain, with comfortable freeboard, all the above men-tioned superstructures and machinery with adequate fueland water storage. The beam and the length forward andaft of the "A" frame must be sufficient to provide adequatestability when dredging, i. e., to keep the ship in reasonabletrim in resistance to the listing moments of the swingingboom and bucket. The width of hull cannot be so great,on the other hand, as to necessitate an excessive length ofboom in order to reach beyond and clear the pocket coam-ing of the light scow. The freeboard should be such as toassure some reserve buoyancy when the machine is at thepoint of maximum inclination due to the limiting position


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GRAPPLE DREDGES 25of the loaded bucket in its arc. In brief, the problem isthe proper co-ordination of hull dimensions with the loca-tion and height of "A" frame, length of boom, bucketcapacity and disposition of machinery, fuel and water tanks.The solution is simplified by the commonly rectangularshape of the hull both in plan and section, with rake at thestern only to facilitate towing. Moulded hulls have beenconstructed for grapples but are quite rare. The totaldepth of hull is the sum of the draught of the empty hull,the displacement depth due to machinery, superstructures,fuel and water, and the desired freeboard. The firstquantity is first approximated by roughly estimating thetotal feet board measure of lumber in the hull (or the ton-nage, if steel) and checked subsequently from the accuratebill of material taken from the detailed design. Theratio of hull depth to the width is as a rule slightly lessthan 1 to 3 and that of beam to length a little more than1 to 3.The principal loads acting upon a dredge hull are the

normal water pressure on bottom sides and ends; theweight of the machinery, more or less concentrated underthe main engine and boilers ; the pull of the main and secon-dary engines; the weight of fuel, water tanks and super-structure; the thrust of the heel of the boom which may bein any vertical plane passing radially through the heelcasting; the alternate thrust and pull of the A frame; thepull of the back guys; the bearing of the gallows-frames;horizontal force couples due to spuds in their wells or toanchor wires; and finally wave action, causing both localimpact and bending moments in the structure as a whole.While the majority of the above loads are capable of reason-ably accurate determination, it is hazardous to design upona purely theoretical basis. The efficiency of a dredgedepends among other things upon the ratio of its workingtime to the total. The more the time lost for repairs andrenewals, the less valuable is the unit. Therefore, eithertemper the theory with the knowledge resulting from prac-tical experience or else use a large safety factor, to the end


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26 DRE])GING ENGINEERINGthat the members shall be proportioned generously towithstand the severe duty required of " mud-diggers. "Most hulls are built of wood. They are virtually heavilyconstructed boxes stiffened with bulkheads, trusses andknees to resist distortion. A hull that has become convexupward longitudinally is said to be " hogged" and whenconcave upward, "dished." Although varying widely indetail, the usual design is, in principle, as follows: Thebottom planking is laid transversely upon the under sideof longitudinal keelsons, heavy timbers spaced from about2 ft. 6 in. to 4 ft. c.c., extending the full length of the shipand spliced with long scarf joints from 4 to 6 ft. in length.The reactions of the keelsons are taken by heavy crosskeelsons, running athwartship at greater intervals on topof the keelsons and notched down and over them. Uponthe cross keelsons in turn bear the longitudinal bulkheadsor trusses and the stanchions. There are at least twotrusses, usually of the Howe type, or solid bulkheadsextending the full length of the hull which, with the keelsonsand side planking (acting also as deep longitudinal girders)furnish the requisite stiffness fore and aft. Upon thetrusses, bulkheads and stanchions are placed the deckbeams carrying the deck planking. More commonly thedeck beams are transverse members and the decking longi-tudinal. The deck is usually crowned about 3 inches.The side planks, called "strakes," are spiked to the sidestanchions, the thrust of which is transmitted to the crosskeelsons and the deck beams by fore and aft ribbon piecessometimes called side cleats. Frequently two or more sidestrakes are thicker than the others, extending beyond theside plane and acting as fenders. The inclined membersof the stern are called rake timbers. All the exposedplanking is dressed, outgaged and caulked with pitchand oakum. Deck spikes are covered with wood plugs.Transverse stiffness is provided by lateral bracing or byhackmatack knees.

Operation. The dredge is towed to the site of the workand placed in position at the starting point of the project.


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GRAPPLE DREDGES 27Her spuds are dropped or lines and anchors set as the casemay be. The cut to be dredged is indicated by range tar-gets stationed ahead of the dredge. A light scow is broughtalongside by the tug acting as tender and moored to themachine by the wires of the scow handling machinery,which are generally 3 in number located as follows: Abreast wire leading from a "U" bolt in the pocket coamingof the scow to the port bow of the dredge; a second wireattached to the port stern corner of the scow and runningforward to the starboard bow of the machine; and a thirdwire extending from the starboard stern of the scow tothe starboard stern of the dredge. The arrangement isshown in Figure 7, page 17. It is customary to interposea boom log between dredge and scow.The deck hands board the scow and, using bars as levers,wind up the door chains of each pocket about the shaftuntil the doors are raised to the closed position and heldso by ratchet and pawl. The end pocket nearest the dredgehaving been thus closed, digging is started.The operator in the pilot house releases the starboardfriction to slack the closing wire and the bucket openshanging by the port wire. He then slowly lowers the openbucket into the water by easing up the port friction untilthe bucket rests on the bottom, ready for the bite. Hereleases the port friction, grips with the starboard andpartly opens the throttle. The closing wire is thus stressedand the bucket closes upon its load and rises. Keepingthe load on the closing wire and controlling the resultingboom swing to starboard by a lesser backing strain on theport wire, the operator lifts the bucket up over the sideof the scow and pocket coaming until it is suspended abovethe pocket, when he closes the throttle, holds fast the portwire and releases the starboard, opening the bucket.Still gripping with his port friction, he opens the throttlepartially, swinging the boom to port and then lowers itopen into the water as before to take another bite. Whenthe pocket is fully loaded, he signals by blowing the whistleto the deck hands who haul the scow aft by operating the


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28 DREDGING ENGINEERINGscow-line drums until the next pocket is in position forloading. The scow, settles more and more deeply in thewater as the loading progresses, constantly decreasing thenecessary height of bucket lift.

If dredging in tide-water, the operator must know thestage of the tide so that he may dig the depth specified,referred to the datum, usually mean low water. A tideguage, therefore, is set where he can see and read it. Inaddition to actul lead-line soundings over the bow of thedredge, he is guided in his digging by his knowledge of theoverall height of the bucket or by graduations, or a singlemark upon one of the bucket wires or chains or upon thebucket poles, or by means of a wire leading from the bucketthrough sheaves in the boom end and "A" frame thencethrough reduction tackle to a weight sliding on a graduatedscale in the pilot house.When specified depth has been made over the area withinreach of the bucket, the dredge is moved ahead or " ad-vanced in the cut." If the machine is operating underwire control, this movement is accomplished simply bywinding in the forward quarter lines and slacking the sternquarters and stern line. If spuds alone are being used forposition control, the advance is achieved by so-called"walking on the spuds," as follows: The bucket isgrounded, i. e., lowered into the mud and the stern andone bow spud are raised clear of the bottom. The operatorthen stresses that bucket wire which tends to swing the boomtoward the side on which the bow spud is up, but the boomis anchored by the grounded bucket and the dredge is freeto pivot about the third spud, so that the boom retains itsposition and the dredge swings. When the free bowcorner has thus been pulled forward the desired distance,that spud is dropped and the operation repeated for theadvance of the other bow spud, after which the stern spudis dropped and digging is resumed. When all pocketsof the scow are full, the operator blows for the tug to bringa light scow and to remove the loaded one.


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CHAPTER III. DIPPER DREDGES

General Description. As has been said, the DipperDredge ' ' digs ' ' like the familiar steam shovel. The bucket,which is essentially a scoop with heavy teeth and a flapbottom, is attached to the end of the dipper stick, whichis carried by the boom at or near the centre of the latter.The boom is supported by "A" frame and back guys as inthe grapple, except that, frequently, the "A" frame istilted forward, when it is termed the " shear legs." Theboom is necessarily of heavier construction than that of thegrapple and, with the "A." frame, is set well forward atthe very bow of the hull.The bucket obtains its load by digging into the bottomunder the impetus of dipper stick and bucket wire, whichruns over a sheave in the boom head and thence to the mainengine. The boom is swing in one of two ways; either asin the grapple by means of two bucket wires, or by bullwheel and swinging engine, the latter being the morecommon. The hull is stiffer and the spuds heavier thanthose of the grapple, because of the greater strains. Thedredge is held in position fcy three spuds, and advances bygrounding her dipper and stressing the backing chain.As a rule, the crew is quartered on the dredge. The scowsare handled as on grapples.The Bucket. The bucket is an open steel cylinder,provided with a hinged flap bottom, a handle or bail anda reinforced cutting edge or lip, to which teeth are attached.Fig. 9 is a picture of a Bucyrus Dipper Dredge, swinging a6 cubic yard bucket.The bucket hoisting wire is made fast to the centre ofthe bail, and the backing chain, which draws bucket anddipper stick back toward the hull, is fastened either to the

29


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DIPPER DREDGES 31rear face of the bucket or to the dipper stick near thebucket. The bottom door or floor of the bucket is heldclosed by a latch, which locks automatically by virtue ofits bevelled end when the door is forced upward and closedby the water pressure, The latch is drawn back to openthe door by means of a line extending from it to a lever at apoint near the fulcrum, and a second line from the end ofthe lever to the cranesman stationed at the heel of theboom. In the larger buckets the latch rests on roller bear-ings to facilitate its movement and, in the recent, largecapacity, high powered machines, is steam operated. Theteeth are usually from three to five in number, with toolsteel points, and are detachable for sharpening and renewing.Boom, Dipper Stick, "A" Frame and Back Guys. The

boom, as in the grapple, is hung from the UA" frame bya fixed topping fall, but is usually at a flatter angle withthe horizontal than that of the grapple. The "A" frame,when vertical, is approximately in the plane of the heelof the boom, but, when inclined, the point of bearingon the deck may be some distance aft of the boom heel.For the same topping-fall stress, the stresses in "A"frame and back guys are greater in the case of the inclinedshear legs than in that of the vertical "A." frame. Theboom, necessarily, is set well forward in order that thedipper stick may clear the bow in all positions. Theback stays, more particularly in those dredges havinginclined "A" frames or shear legs, are frequently tensionmembers only, without back legs or struts. There is astructural economy in the vertical "A" frame, in that itand the gallows frame may be designed as a unit frame,whereas the inclined "A" frame necessitates a distinctand independent gallows frame.The boom is the most difficult part of the dredge todesign. Some of the very heavy loads to which it is sub-jected are indeterminate, principally those caused bystarting to swing the boom before the bucket is clear of thewater, or even while still in the mud, and by sudden stop-page and reversal of swing. The principal boom stresses


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32 DREDGING ENGINEERINGare again combined compression and bending, but, in thisinstance, the bending is due to live as well as dead loadand is very much greater. In addition there is, in thosebooms swung by a bull wheel at the heel, a horizontal bend-ing movement, caused by the rotation of the bull wheel

Fig. BFIG. 10. A, Forces acting at the dipper for two positions of the arm. B, stressdiagrams for the two positions.

and maximum at the boom heel casting. Furthermore,the side thrust of the dipper stick requires considerablelateral boom stiffness. ^To investigate understandinglythe vertical bending stress in the boom, a knowledge ofthe action and control of the dipper stick is necessary.


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DIPPER DREDGES 33The stiek has two movements, one of translation with

respect to and through the boom at or near its mid point,and the other of rotation in a vertical plane through an arccentred at the same point. Referring to Fig. 10, thedipper may be pulled either toward the end of the boom bythe main engine wire A, or back toward the hull by thebacking chain B. The motion of the stick through theboom is controlled by two band friction wheels C on theboom, keyed to a shaft carrying a pinion which meshes intoa rack on the under side of the dipper stick, so that thestick may be held fast at the boom at any point of itslength, at the same time being free to rotate about thatpoint. The stick is pulled up through the boom by thetension in hoisting wire or backing chain, or both, and dropsdown through the boom by gravity alone. Many dippers,however, are equipped with a so-called "crowding engine/ 7which is mounted on the boom and drives the pinionmeshing with the rack of the stick, so that the stick can bepushed or pulled through the boom. By this means, thedipper can be thrust out beyond the end of the boom to anincreased reach.

It is apparent from the stress diagrams (fig. 10-b) thatthe digging power, or the thrust at the bucket perpendicularto the dipper stick, varies inversely as the length L ofstick below the boom and the angle 6 between boom andstick; and that th.e compression in the dipper stick ismaximum when L and 6 are maximum. This, then, isthe critical loading of the stick and, knowing the greatestpull of which the main engine is capable, the maximumcomprssive stress in the stick follows. Its length willusually be such as to necessitate the use of long columnformula in its design.

It is entirely possible, too, that the dipper stick be sub-jected to tension, which, although insufficient to influencethe choice of section, is enough to require a test of the stickdetails for resistance to the tensile stress involved, whichwill be the weight of the loaded bucket in air. This condi-tion obtains when, through faulty operation, or the parting


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DIPPER DREDGES 35of the main hoisting wire, or slipping main engine frictions,the full load of the bucket and contents is suspendedfrom the boom by the dipper stick, held by its frictions atthe boom. If there be a crowding engine, the same ten-sion may easily exist through the agency of that engine.In addition, there is a certain amount of twisting to beresisted.The direct stress in boom and topping fall may be found

graphically as in fig. 12. The forces be and ac, fig. 12-B,representing the bucket wire tension, are obviously equalin amount, as are ac and de of fig. 12-C, because the wirepasses over a sheave in the boom head, and are measured bythe pull of the main engine. The boom compression ismaximum when ac is perpendicular to cd, and the toppingfall tension cd increases with the angle 0. It must be re-membered in the design of the latter, that the angle 9 is notlimited by the vertical position of the bucket wire, as thebucket may easily be thrust out beyond the end of the boom,more especially by the use of a crowding engine. Thedead load stress in the topping fall is obtained as describedunder grapples.

There are four values of bending moment to be con-sidered in the boom: a, -the positive vertical B.M. causedby the suspension of the loaded bucket and dipper stickas mentioned above; 6, the negative vertical B.M., causedby the thrust of the dipper stick when perpendicular tothe boom; c, the positive dead B.M. due to the weight ofthe boom itself; and d the horizontal B.M. as a result of thebull-wheel rotation. The critical condition of stress inthe boom, then, is the greatest possible combination ofdirect compression and compressive fibre stress due tovertical and horizontal bending moment, not omittingto investigate and provide for the fibre tension. Thesestresses, in conjunction with those due to the indetermi-nate loads previously referred to and with the physicalconsiderations influencing the shape and location of someof the members, result in a sort of compromise design, aand b are maximum at the point of intersection of dipper


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36 DREDGING ENGINEERINGstock and boom; c at the centre of the boom; and d at theheel casting. The horizontal B.M. d is equal to the pullof the swinging engine multiplied by the radius of the bullwheel. It has little bearing upon the choice of the boom

FIG. 12.- A, The purchase-rigged dipper. B, boom-end forces and stressdiagram for purchase-rigged dipper. C, the same for the direct-wire dipper.section, but is an important destructive agent acting uponthe heel casting and the boom structure immediatelyadjacent thereto, and one warranting complete investiga-tion and the provision of ample resistance.


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DIPPER DREDGES 37Some dippers are rigged with a single sheave purchase in

the bucket wire, fig. 12-A; i.e. the end of the wire isattached to the boom end, from whence it passes down toa single-sheave block fastened to the bail of the bucket,thence up over the boom-head sheave and to the mainengine. It is apparent that the result of such a rigging isto double the lifting power of the engine and to halve thelifting speed and that, in dippers of the same digging power,the boom stress in the machine which is purchase riggedwill be less than that in the direct wire dredge (Fig. 10).The boom is a relatively large member, having considera-ble depth at the dipper stick and tapering toward bothends, and is of plate, girder or latticed truss construction.The stress in "A" frame or shear legs and back legs orstays are found as for grapples in Chapter I.

Spuds. The two bow spuds are set either in "outside"or " through" wells. They are of heavy section and aresubject to considerable bending moment occasioned by thereaction of the forward thrust of the dipper. Some dippersare rigged to "pin up," a term applied to the process ofmaintaining the dredge upon an even keel while digging,by transferring part of the weight of the machine to thetwo bow spuds, so that it is not dependent upon the sta-bility of the hull to resist transverse oscillation. In pin-updredges, the fore spuds are provided with sheaves bothtop and bottom. A wire attached to the spud-well housing,running down around the toe sheave and thence up to thedrum of the spud hoist, raises the spud, and a second wireof larger diameter, fastened at the same place and leadingup over the top sheave and down to the drum, becomestaut when the dredge attempts to list to that side, tendingto force the spud more deeply into the bottom.The dredge advances by grounding her bucket wellahead of the bow, raising the two bow spuds clear of thebottom and stressing the backing chain, which pulls themachine toward the dipper. The stern spud remains in themud during the operation, and, having a slotted well,slowly inclines forward as the dredge progresses. It is


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DIPPER DREDGES 39called the " walking-spud " or " trailing-spud " on thisaccount. The spuds are raised by spud hoist and gallowsframe, or by a rack on the spud engaging a pinion at thedeck level, obviating the necessity of a gallows frame.The Machinery. In dipper dredges having two bucketwires to swing the boom and raise the bucket, the mainengine is a two cylinder horizontal type, with two drums

FIG. 14. Dipper dredge with bank spuds. (Courtesy the Marion Steam ShovelCo.)

as in the grapple. If the boom is controlled by swingingengine and bull wheel, the main engine may have but onehoisting drum. In some machines, the drum cylinderis of large diameter for part of the width, stepping down to asmaller diameter for the balance, the object being to in-crease the pull on the bucket wire during the early stageof digging when the bucket is loading, after which, the wirecoils upon the larger drum surface, accelerating the raisingof the dipper. Such a device is termed a differential drum,and is used to good advantage with the purchase rigged


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40 DREDGING ENGINEERINGdipper. In addition to the main and swinging engines,complete dipper control requires an engine for handlingthe backing chain which draws the bucket back towardthe hull. Some machines are equipped also with a crowd-ing engine mounted upon the boom, with the function ofdipper stick motion as previously described. The mach-inery for spud and scow handling is similar to that of thegrapple.

FIG. 15. The Tellico U. S. Army Engineers 1M yd. dipper, triple hitch.(Courtesy the Osgood Co.)

The dredge PRESIDENT, American Dredging Company,pictured on page 38, is a "pin-up" machine, with directwire rigging, bull wheel and crowding engine, hull 105 ft.X 39 ft. 8 in. X 10 ft. 7 in., 2 cyl. hor. main engine 14 X16, and a 4>^ yd. dipper.The Hull. In general dimensions and constructiondetails, the hull is very similar to that of the grapple.There is, howrever, a need for greater stiffness due to thethrust of the dipper and the extreme forward mountingof the boom. In some types, e.g., the Bucyrus builtmachines, the longitudinal trusses are steel and of great


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DIPPER DREDGES 41depth, reaching from the cross keelsons to the roof of thedeck house, and tied horizontally by a top lateral truss.There is, too, relatively less stability forward than ingrapples to resist listing when the bucket is swung out overthe scow, from which it is apparent that the same hull willcarry a grab bucket of larger capacity than a dipper.The bow structure of the hull must be well stiffened andthe truss and superstructure design there is dependent insome measure upon the locating of the bull wheel, whichmay be upon the deck or elevated to the plane of the roof

FIG. 16. 14" X 16" engines and hoisting machinery for a 6 yd. dipper dredge.(Courtesy the Bucyrus Co.)

of the deck house. In the latter position, it exerts a moredirect pull upon the boom, thereby reducing the lateralstresses in that member.

Operation. Dippers require two men to operate theboom, dipper stick and bucket, in addition to the usualcrew of engineer, firemen, oilers and deck hands. One,the " operator" or " runner," controls the bucket hoist,backing chain and boom motion through the throttlesand frictions of the main, backing and swinging engines.The other, the "cranesman" or " dipper tender," is sta-tioned at the boom heel and regulates the dipper stickfrictions on the boom and opens the bucket.

In the process of digging, the runner slacks the bucketwire or wires, permitting it to drop into the water, at the


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42 DREDGING ENGINEERINGsame time pulling it back toward the hull by stressing thebacking chain. The cranesman releases the dipper stickfrictions so that it falls through the boom until the dipperrests on the bottom. The runner releases the backingchain and stresses the hoisting wire, and the dipper tenderapplies his friction bands, gripping the dipper stick at theboom. The bucket, therefore, cuts its way forward in anarc of radius equal to that portion of the dipper stick belowthe boom until it is loaded and clear of the mud, whereupon

FIG. 17. Hoisting and Backing Drums 14" X 16" dipper dredge. (Courtesythe Bucyrus Co.)

the cranesman releases the stick, which shoots up throughthe boom. The runner swings the boom until the bucketis suspended over a pocket of the scow and the cranesmanpulls the latch string, dumping the bucket.

Application of the Type. To the need of the Americancontractor for a simple, inexpensive and versatile machine,the dipper dredge owes its rapid development in thiscountry. It is very efficient in hard material, providedthe depth is not excessive, and is used to advantage incanal work through solid ground containing stumps androots, in the dredging of previously blasted or loose rock


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DIPPER DREDGES 43


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44 DREDGING ENGINEERINGand in the removal of old filled cribs, old foundations,sunken wrecks and stone dikes. Thus it is a most capablemachine, with a wide application. The conditions ofuniformly hard materials and moderate requisite depth ofchannel on the Great Lakes have occasioned an extensiveuse of the dipper there.

High-powered Dipper Dredges. The earliest dipperdredge of large size was the 12 yard machine ONONDAGA,owned and operated in New York Harbor about 1904by the contractors Hughes Brothers & Bangs. Since then,and prior to 1904, the largest exponents of the type wereprobably the two 10 yard machines used on the Cape CodCanal, built by the Atlantic Equipment Co., and the 15yard dredge TOLEDO, built by the Bucyrus Company.At the time of the closing of the Panama Canal by thefirst large slide, the largest dippers in use there were 5yard. It was decided that the only type of machine suita-ble for the removal of this huge mass of broken rock andearth, containing pieces of rock of all sizes and lackingall uniformity of formation, was a large capacity and high-powered dipper dredge. The decision resulted in the con-struction by the Bucyrus Company of three 15 yard dippermachines, the GAMBOA; PARAISO and CASCADAS, the latterbeing the last built and arriving at the Isthmus in Octoberof 1915. Although open to criticism perhaps as to theabnormally high cost of maintenance, attributed by someengineers to defects in the main hoisting wires, the dipperarms and the spuds, both the builders and the managementwere justified by the splendid performance of the machinesand by the fact that the first two were in large part copiesof the TOLEDO because of the lack of time to prepare newdesigns. Each of the three dredges had a demonstratedcapacity of more than 3,000,000 cubic yards per year.They are all pin-up machines, with steel hulls. The princi-pal statistics of the above dredges are given in the Table,page 45.


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DIPPER DREDGES 45ONONDAGA GAMBOA & PARAISO

Hull length 140' 144'width 50' 44'depth 15' 13'-6"Main Engines Type Double cylinder Compound

condensingSize 20".5 X 24" 16 and 28 X 24

Forward Spuds Material . . Oregon Fir Structural SteelSection ... 5' X 5' 4' X 4'Length 80' 82'

Dipper Arm Material Oregon Fir Long Leaf Y. P.Section 3' X 3'Length 80' 72'

Capacity of Dipper 15 cu. yd. Rock 10 cu. yd.Mud 15 cu. yd.Max. Working Depth 50' 50'Bail Pull 235,000 Ibs.

N. B. The CASCADAS differed principally in the width of hull, whichwas 55 ft., in the depth, 15'-6", and in the use of a gallows frame to raisethe spuds without the use of sheaves in the spud toes, which provedobjectionable because of the cable abrasion due to the rock. Severalimprovements were also made in the machinery and in other details as dic-tated by the experience with the other two.


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CHAPTER IVLADDER DREDGES

Historical. The ladder or elevator dredge is distinctlya European product and, until recently, has found littlefavor in the United States for major dredging projects.The reason lies not so much in the qualities of the dredgeitself, but rather in the quite opposite methods of admin-istration of port development of the two continents. Mr.A. W. Robinson, in his paper on the " Review of GeneralPractice," printed in Vol. LIV of the Transactions of theAmerican Society of Civil Engineers, 1905, writes: "InAmerica, the opening up of a vast, virgin country, both onthe seaboard and in the interior, required the rapid execu-tion of a great number of works suited to immediate neces-sities, under the small contract system. This system hasgiven rise to a class of contractors of all grades, who buildand own their plant, and as their contracts are liable tobe varied both as to locality and conditions, they do notinvest largely in special plant. The small contractor

. . must employ a plant which is adapted to a varietyof work, and which does not represent more capital in-vested than his contract will warrant. ... On the otherhand, the large corporation, or Board of Harbor Trustees,under the European method of administration, is able tolay out a comprehensive plan of the works under its chargeand provide permanent plant adapted to it, which will beassured of employment through a series of years. Thishas developed the large and complete seaworthy dredgeof the ladder and bucket type, which is found so frequentlyin Europe and so rarely in America."The inexpensive and versatile dipper dredge, therefore,was early and rapidly developed in this country, provingan excellent tool especially for harbor and shallow work as

46


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LADDER DREDGES 47


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48 DREDGING ENGINEERINGfound in the Great Lakes, and the grapple dredge attainedequally rapid development on the softer tidal work of theCoast, and both to the almost absolute exclusion of theladder or elevator type.

In more recent years, however, the tendency has beento the design of machines of larger capacity, in the beliefthat their lower unit cost of output justifies the greaterinitial outlay. This trend, together with the attentiondemanded by the ladder type in its economic performanceon the Panama Canal, has led to a more general acceptancein this country of the advantages of the elevator dredge, asis evidenced by the purchase in Scotland of a large machineof this type for use on the Panama Canal. The capacity ofeach of the soft-digging buckets of this dredge is 2 cubicyards, and of the hard-digging, 35 cubic feet.

Moreover, in Canada, on the St. Lawrence River ShipChannel, the ladder dredge has been in active use for years,even antedating the development of the dipper. Thetype secured a foothold in Canada from early examplesimported from Scotland, and, because of its adaptabilityboth to the local dredging conditions and to the method ofgovernment operation through a number of years, itspopularity there has persisted.It is now generally conceded that the very hard, indu-rated clays, shales, soft rock formations and hard-pans,when excavated in their original condition without pre-vious blasting, are most economically dredged by laddermachines and that, in hard clays, the choice between theladder and the dipper is difficult, except as determinedby depth of water and depth of cut.The ladder dredge of the so-called "stationary" typehas found favor for some years in two American industries,viz. dredging for gold in the western states with the largeplacer, elevator machine, and for commercial sand andgravel. The peculiar virtues of the type, to which it owesits superiority for gold dredging, are the clean pick-up ofthe gold bearing gravel with a minimum of agitation andthe delivery to the screen of an almost continuous stream


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LADDER DREDGES 49


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50 DREDGING ENGINEERINGwithout leakage. The failure of other types of dredgein these two essentials, resulting in an excessive loss ofgold, has militated against their employment in theindustry.

In Chapter IX, under the caption "Choice of Plant,"American and Continental opinion upon the relative meritof the ladder dredge is further cited.

General Description/ The digging mechanism of theladder dredge consists of a long trussed ladder, incliningfrom the top of a tower through a slot or well in the hulldown to the mud line; a series of buckets traveling inendless chain succession up the top side and down the lowerside of the ladder, and carried by two tumblers or drumsat the two extremities thereof and by rollers upon its upperface; and the main engine actuating the upper tumbler,which engages the endless bucket chains. The ladder maybe raised and lowered, pivoting about the tower top. Thebuckets obtain their loads by scraping along the bottomin the act of revolving about the lower tumbler, and dis-charge when they invert at the upper or driving tumbler,which is driven by the main engine through chain or belt,or more commonly through gears and a friction clutch.This last is intended to safeguard the engine against pos-sible sudden shock due to abrupt resistance or impact of thebuckets. The velocity of the bucket travel is varied tomeet the conditions of material and depth, preferablythrough the gearing independent of the main engine. It isessential that the dredge be equipped with two sets ofbuckets, one for hard and the other for soft digging. Theyvary in size generally from 5 cu. ft. to 1 cu. yd. capacity.The soft digging buckets are larger for the same machinethan the hard digging, and are re-enforced on the cuttingedge with hard steel lips. The buckets designed for hardmaterial are heavily constructed and are provided withteeth at the lip. Both digging and discharging are facil-itated by tapering the buckets in longitudinal section bothvertically and horizontally.


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LADDER DREDGES 51


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52 DREDGING ENGINEERINGStationary Type. The hull of the stationary type is

generally rectangular both in plan and in cross section,pierced with a well or slot through which the ladderoperates. The feeding movement is either lateral throughthe agency of six mooring cables to as many anchorages,or radial, the dredge pivoting about a stern spud in themanner of the radial-feeding hydraulic machine. Americanpractice tends toward the latter method. The objectionto the former and an argument in favor of the self-propelledtype is the obstruction presented by the cables to rivertraffic. The St. Lawrence River dredges are of the lateralfeed type, making a cut as wide as 750 feet in one operation,the extensive movement being permitted by exceptionallylong head lines of wire rope carried on floats for a consider-able distance ahead of the dredge to obviate the resistantdragging on the bottom.The dredged material is conducted to scows moored tothe dredge through chutes leading from the receiving hopperat the top of the tower. Machines operating in a mixtureof sand and gravel for commercial usage, contain a cylin-drical rotating screen at the top which segregates thematerial so that sand is discharged into a scow on one sideof the dredge and gravel into a second scow on the otherside. Some dredges have abnormally high towers and longchutes supported from the dredge, depositing the dredgingsat some little distance. Flow in the chute pipes is some-times accelerated by pumping water through them. Whenit becomes necessary to transport the material to pointsmore remote from the dredge, floating pipe lines areemployed, through which the material is forced by waterfrom a discharge pump. In some instances, a series ofbelt conveyors mounted on lighters has been used.

Sea-going Hopper Type. The sea-going ladder dredgeis a self-propelling steamer, with moulded hull and self-contained hoppers. When loaded, it travels to the dump-ing ground under its own power and discharges throughdoors at the base of the hoppers. They are usually ofthe twin-screw type, driven either by the main dredging


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LADDER DREDGES 53engine or by an independent unit. It is customary toprovide for discharging both into the dredge hoppers andinto hopper barges moored alongside.

Dredges of this type vary through wide limits as to size,capacity and maximum depth of dredging; in length fromabout 130 to 275 feet; in hopper capacity from about 500to 2200 tons; and in greatest possible depth of dredging,from about 30 to 50 feet. The previously mentioned ma-chine, purchased in Scotland by the Isthmian Canal Com-mission, has a specified capacity of 1200 cubic yards of mudor sand per hour, and a maximum dredging depth of 50feet. The speed of sea-going ladder dredges is generallyfrom 6 to 8. miles per hour, when loaded.

While the stationary machine may have two ladders,one on each side, the sea-going dredge, for reasons of sta-bility, has but one on the longitudinal axis of the hull.The tower is erected amidships and the well may be eitherforward or aft. There are two types of well, the close-ended and the open-ended. The latter is more commonlyused, due to the advantage of being able to dig flotationfor the dredge. General practice employs the bow wellfor single-screw steamers and the stern well for twin-screw.The well is proportioned to the length of the ladder. Inthe bow-well dredge, the ladder, when raised, lies whollywithin the well, and a breakwater is installed at the afterend of the well to deflect the water under the keel of thevessel when steaming ahead.The length of the ladder should be such as to reach themaximum required depth when at an inclination of 45degrees, since the maximum amount of work is said to ob-tain at that angle. Some designers have even gone so faras to build the ladder in two parts, so that the lower sec-tion may always have a slope of 45 degrees.For the sea-going ladder dredge, the advantage is claimedof entire seaworthiness in ocean dredging and the absencein rough water of the pounding of attendant plant, so de-structive both to such plant and the dredge itself. It isopen to the criticism, however, of intermittent dredgingoperation due to the frequent trips to the dumps.


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CHAPTER VSCOWS

The most common conveyance for the transportation ofdredgings (pumpings excepted) is the bottom-dump mudscow. It is in effect a hopper barge, without means of selfpropulsion, consisting simply of a hull rectangular in planand cross section, containing a number of independenthoppers called pockets, and with both ends raked orrounded (in elevation) for towing in either direction.Roughly, it is somewhat more than three times as long asit is wide, and the depth of hull from deck to bottom plank-ing is about one-third of the beam. When light,, it hasconsiderable freeboard, but when fully loaded, it floatswith decks awash, the material being retained by the pocket" coamings ' ; a name given to the portions of the pocketwalls projecting above the deck. The coaming height isusually from two to three feet.The common form of cross section is shown in fig. 22.

Neglecting the fore and after holds, between the endsof the scow and the faces of the first and last pockets,the vessel is dependent for buoyancy upon the polyhedralspaces between the sides of the scow and the sloping sidewalls of the pockets, unless the continuity of the pocketsis broken by one or two transverse holds, as is sometimesdone, not merely to increase the displacement, but alsoto add to the transverse strength. Thus, the displacementtonnage per inch immersion decreases quite rapidly as thedraft increases, and the loaded scow has no reserve buoy-ancy. While these two features may appear objectionable,and while we do hear, occasionally of the capsizing of amud scow, yet the good points of the design are so pre-ponderant and the disaster of capsizing so trivial that thepopularity of the type lives on.54


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scows 55It will be remembered that mud scows are loaded, not

as the designer would have them for minimum stressesin the structure, but from one end to the other, and thatusually they are dumped in the same way. Obviouslythe resultant tendency is to "hog" the scow, i. e. by nega-tive bending moment in the structure as a whole, torender it convex upward. To provide adequate stiffnesslongitudinally, therefore, a pair of strong trusses are builtin the plane of the side coamings for the full length anddepth of the scow. In timber scows, they are generally ofthe Howe type. Supplementing them, the sides of the

FIG. 22. Cross section of the common, bottom-dump mud scow.vessel are detailed to further resist the "hogging" strainsin that the upper strakes are in long lengths and splicedwith long tension scarf joints. The lower strakes are as arule framed with long scarf joints without the tensionfeature. The intermediate courses may be butted atstanchions. The truss rods should be provided with turn-buckles, that they may be adjusted to timber shrinkageand local fibre crushing. The transverse strength of thescow is provided by the walls between pockets, which aresolid bulkheads extending the full width of the ship.The floor of each pocket consists of a pair of doors,hinged at the sides to open downward at the centre. Achain at each end holds them horizontal or closed, runningthrough a lead chock to a shaft just outside the coaming.


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56 DREDGING ENGINEERINGUpon this the chain is wound by lever bar, ratchet wheeland pawl. Each pocket has its own gear and is dumpedindependently of the others, by pounding the pawl out ofengagement with the ratchet wheel, whereat the chain isreleased, the doors fall open and the load drops through.Although not always feasible, it is desirable that the depthof well below the door hinges be equal to the width of asingle door, in order that the doors when open may notproject below the bottom of the scow, in which positionthey are liable to damage in shoal water.Whatever the material of which the scow is constructed,it should be strongly and durably designed, to withstandthe severe treatment necessarily meted out to it. Althoughsteel has been used in some instances, timber appears tobe best adapted to the purpose and is quite prevalent.Current practice builds the body of the scow of long leafyellow pine, of prime inspection, with corners and deckstrake of white oak. The corners are armored with steelplates. The scow is always moored to the dredge with theshaft and winding gear on the side remote from the dredge,so that the danger of damage to the gear shall be reducedto a minimum. The coamings on the port side, therefore,are fitted with eye-bolts at each pocket to receive thehookof the breast line from the dredge. Consequently, it isthe port coaming and the port side of the hull that wearmost rapidly, due to the destructive impacts of the bucket.For this reason, the port side is not uncommonly sheathedwith 2 or 3 inch oak or pine, preferably the former. Some-times the two ends are protected likewise, and in teredo-infested waters, such sheathing on all sides is of paramountimportance. The scow is equipped with towing bitts, andwith hatches for ventilation and siphoning out fore and aft.For river and harbor work, scows of 500 or 600 yards

capacity are the .most popular. Larger sizes present theobjectionable features of great freeboard when light,requiring an inordinately high bucket lift, and great draftwhen loaded, requiring deep water for dumping. Thelatter becomes an important factor in dumping to a hy-


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SCOWS 57draulic machine for pumping ashore. In this instance,the so-called rehandling basin, or rectangular hole exca-vated by the pumps to receive the dumped material, islocated preferably as close to the impounding basin as theconsideration of minimum depth for dumping scows willpermit, the idea being of course to reduce the length ofdischarge pipe line to a minimum. Hydraulic dredges en-gaged in rehandling work are required to maintain a depthin the rehandling basin at least as great as the surroundingdepth, in order that as little material as possible shall belost between dumping and pumping. The limits of the re-handling basin are defined byranges to control the operation.

Scows of types other than the bottom-dumps are in lesscommon use for handling dredgings. For rock and com-mercial sand and gravel, deck lighters are employed.Particular attention must be given here to the design of thedeck for the heavy loads and impacts. For rock, a layerof sheathing is usually laid on the deck planking. Con-crete deck scows have been successfully used for sand andgravel. Deck lighters may be converted into mud scowsby erecting on the deck a series of gable bottom bins, withthe ridge in the plane of the longitudinal axis of the hull,and discharging through vertical doors on both sides. Suchan arrangement is particularly adaptable to shoal water,although the centre of gravity of the loaded scow is abnor-mally high, and dumpin

dredging engineer 00 simo rich

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