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ENGINEERING AND DESIGN TEXTBOOK

DREDGING EQUIPMENT

(FORMERLY NAVFAC DM-38.2)

JULY 1981

NAVFAC DM-38.2JULY 1981

APPROVED FOR PUBLIC RELEASE

DREDGING EQUIPMENT

DESIGN MANUAL 38.2

DEPARTMENT OF THE NAVYNAVAL FACILITIES ENGINEERING COMMAND200 STOVALL STREETALEXANDRIA, VA. 22332

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ABSTRACT

Information is presented for use in procurement of dredging services andequipment by local OICC’s and Command engineers. Legal considerations, sitecharacteristics, environmental aspects, and basic economics of dredging opera-tions are included. Types of available dredging equipment, their capabilities,and their limitations are described.

38.2-iii

FOREWORD

This design manual is one of a series developed from an evaluation of facili-ties in the shore establishment, from surveys of the availability of newmaterials and construction methods, and from selection of the best designpractices of the Naval Facilities Engineering Command, other Governmentagencies, and the private sector. This manual uses, to the maximum extentfeasible, national professional society, association, and institute standardsin accordance with NAVFACENGCOM policy. Deviations from these criteria shouldnot be made without prior approval of NAVFACENGCOM Headquarters (Code 04).

Design cannot remain static any more than can the naval functions it serves orthe technologies it uses. Accordingly, recommendations for improvement areencouraged from within the Navy and from the private sector and should befurnished to NAVFACENGCOM Headquarters (Code 04). As the design manuals arerevised, they are being restructured. A chapter or a combination of chapterswill be issued as a separate design manual for ready reference to specificcriteria.

This publication is certified as an official publication of the Naval Facili-ties Engineering Command and has been reviewed and approved in accordance withSECNAVINST 5600.16.

CommanderNaval Facilities Engineering Command

38.2-v

DM Number

38.1

38.2

38.3

38.4

38.5

CIVIL ENGINEERING DESIGN MANUALS

Chapter supersededin Basic DM-38

1

2

3

4

5

Title

Weight Handling Equipment and Service Craft

Dredging Equipment

Yard Craft

Pile Driving Equipment

Towing Nonself-propelled Floating Structures

38.2-vi

CONTENTS

Page

Section 1. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . 38.2-l

1. SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . 38.2-l

2. CANCELLATION. . . . . . . . . . . . . . . . . . . . . . . 38.2-l

3. RELATED CRITERIA . . . . . . . . . . . . . . . . . . . . . 38.2-l

4. PLANNING . . . . . . . . . . . . . . . . . . . . . . . . . 38.2-1a. Jurisdiction . . . . . . . . . . . . . . . . . . . . . 38.2-1b. Work Accomplishment . . . . . . . . . . . . . . . . . 38.2-l

Section 2. DREDGING DESIGN . . . . . . . . . . . . . . . . . . . . . 38.2-2

1. PERMITS AND OTHER LEGAL CONSIDERATIONS . . . . . . . . . . 38.2-2a. Corps of Engineers, Department of the Army . . . . . . 38.2-2b. Disposal of Dredged Material . . . . . . . . . . . . . 38.2-2c. Environmental Considerations . . . . . . . . . . . . . 38.2-2

2. SITE INVESTIGATIONS . . . . . . . . . . . . . . . . . . . 38.2-2a. Hydrographic Surveys and Mapping . . . . . . . . . . . 38.2-2b. Map of Disposal Area . . . . . . . . . . . . . . . . . 38.2-3c. Characteristics of Area To Be Dredged . . . . . . . . 38.2-3d. Cross Sections . . . . . . . . . . . . . . . . . . . . 38.2-3e. Test Borings . . . . . . . . . . . . . . . . . . . . . 38.2-3f. Test Dredging . . . . . . . . . . . . . . . . . . . . 38.2-4

3. PROJECT DEPTH . . . . . . . . . . . . . . . . . . . . . . 38.2-4

4. OVERDEPTH DREDGING. . . . . . . . . . . . . . . . . . . . 38.2-4a. Overdepth Allowance . . . . . . . . . . . . . . . . . 38.2-4b. Designed Overdepth . . . . . . . . . . . . . . . . . . 38.2-4c. Mandatory Maximum Dredged Depths . . . . . . . . . . . 38.2-5

5. OVERWIDTH DREDGING . . . . . . . . . . . . . . . . . . . . 38.2-5

6. SPOIL AREAS AND UTILIZATION OF SPOIL . . . . . . . . . . . 38.2-6a. Utilization of Spoil . . . . . . . . . . . . . . . . . 38.2-6b. Environmental Considerations . . . . . . . . . . . . . 38.2-6c. Dikes . . . . . . . . . . . . . . . . . . . . . . . . 38.2-6d. Spoil Data . . . . . . . . . . . . . . . . . . . . . . 38.2-8

7. REMOVAL OF ROCK . . . . . . . . . . . . . . . . . . . . . 38.2-8

8. RECOVERY OF SUBSTRATUM (UNDERCUTTING) . . . . . . . . . . 38.2-8

Section 3. TYPES OF DREDGING EQUIPMENT . . . . . . . . . . . . . . . 38.2-9

Part 1. MECHANICAL DREDGES. . . . . . . . . . . . . . . . . . . . 38.2-9

1. BUCKET DREDGE . . . . . . . . . . . . . . . . . . . . . . 38.2-9a. Description . . . . . . . . . . . . . . . . . . . . . 38.2-9b. Capacity and Depth . . . . . . . . . . . . . . . . . . 38.2-9c. Uses, Advantages . . . . . . . . . . . . . . . . . . . 38.2-9d. Disadvantages . . . . . . . . . . . . . . . . . . . . 38.2-9

38.2-vii

2.

3.

4.

Part 2.

1.

2.

3.

4.

5.

6. NAVY HYDRAULIC DREDGE INVENTORY . . . . . . . . . . . . . 38.2-23

CONTENTS

Page

MOUNTING CONVENTIONAL, LAND-BASED EXCAVATIONEQUIPMENT ON A BARGE. . . . . . . . . . . . . . . . . .

DIPPER DREDGE . . . . . . . . . . . . . . . . . . . . . .a. Description . . . . . . . . . . . . . . . . . . . . .b. Capacity and Depth .. . . . . . . . . . . . . . . . .c. Advantage . . . . . . . . . . . . . . . . . . . . . . .d. Disadvantages . . . . . . . . . . . . . . . . . . . .

LADDERDREDGE . . . . . . . . . . . . . . . . . . . . . .a. Description . . . . . . . . . . . . . . . . . . . . .b. Capacity and Depth . . . . . . . . . . . . . . . . . .c. Use . . . . . . . . . . . . . . . . . . . . . . . . .

HYDRAULIC DREDGES . . . . . . . . . . . . . . . . . . . .

PLAIN SUCTIONTYPE. . . . . . . . . . . . . . . . . . . .a. Description . . . . . . . . . . . . . . . . . . . . .b. Capacity and Depth . . . . . . . . . . . . . . . . . .c. Uses, Advantages . . . . . . . . . . . . . . . . . . .d. Disadvantages . . . . . . . . . . . . . . . . . . . .e. Length of Discharge Pipe . . . . . . . . . . . . . . .

CUTTER HEAD TYPE . . . . . . . . . . . . . . . . . . . . .a. Description . . . . . . . . . . . . . . . . . . . . .b. Capacity and Depth . . . . . . . . . . . . . . . . . .c. Uses, Advantages . . . . . . . . . . . . . . . . . . . .d. Disadvantage . . . . . . . . . . . . . . . . . . . . .e. Length of Discharge Pipe . . . . . . . . . . . . . . .

38.2-9

38.2-1238.2-1238.2-1238.2-1238.2-12

38.2-1238.2-1238.2-1338.2-13

38.2-14

38.2-1438.2-1438.2-1438.2-1438.2-1638.2-16

38.2-1638.2-1638.2-1638.2-1938.2-1938.2-19

DUST PAN TYPE . . . . . . .. . . . . . . . . . . . . . . 38.2-19a. Description . . . . . . . . . . . . . . . . . . . . . 38.2-19b. Capacity and Depth . . . . . . . . . . . . . . . . . . 38.2-19c. Uses, Advantages . . . . . . . . . . . . . . . . . . . 38.2-19d. Disadvantage . . . . . . . . . . . . . . . . . . . . . 38.2-19

HOPPER DREDGE . . . . . . . . . . . . . . . . . . . . . . 38.2-19a. Description . . . . . . . . . . . . . . . . . . . . . 38.2-19b. Capacity and Depth . . . . . . . . . . . . . . . . . . 38.2-19C. Uses, Advantages . . . . . . . . . . . . . . . . . . . 38.2-22d. Disadvantages . . . . . . . . . . . . . . . . . . . . 38.2-22

SIDECAST DREDGE . . . . . . . . . . . . . . . . . . . . . 38.2-22a. Description . . . . . . . . . . . . . . . . . . . . . . 38.2-22b. Capacity and Depth . . . . . . . . . . . . . . . . . . 38.2-22C. Use . . . . . . . . . . . . . . . . . . . . . . . . . 38.2-23d. Disadvantage . . . . . . . . . . . . . . . . . . . . . 38.2-23

38.2-viii

CONTENTS

Page

Part 3. MISCELLANEOUS EQUIPMENT . . . . . . . . . . . . . . . . . 38.2-23

1. SECTIONALIZED DREDGES . . . . . . . . . . . . . . . . . . 38.2-23a. Description . . . . . . . . . . . . . . . . . . . . . 38.2-23b. Use . . . . . . . . . . . . . . . . . . . . . . . . . 38.2-23

2. DUMP SCOWS . . . . . . . . . . . . . . . . . . . . . . . .a. Side Dump ..........................................

38.2-23

b. Bottom Dump . . . . . . . . . . . . . . . . . . . . . 38.2-26

3. PIPELINES . . . . . . . . . . . . . . . . . . . . . . . . 38.2-26

Section 4. ECONOMICS OF DREDGING . . . . . . . . . . . . . . . . . . 38.2-26

1. BASIC PRINCIPLES . . . . . . . . . . . . . . . . . . . . . 38.2-26a. Depth . . . . . . . . . . . . . . . . . . . . . . . . 38.2-26b. Other . . . . . . . . . . . . . . . . . . . . . . . . 38.2-26

2. RELATIVE COSTS OF REMOVAL BY DIFFERENT TYPESOF EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . 38.2-26

3. CONCEPTUAL OR PLANNING COST ESTIMATES . . . . . . . . . . 38.2-27

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference-1

APPENDIX: Metric Conversion Factors . . . . . . . . . . . . . . . . . A-1

FIGURES

Figure

1.2.3.

4.5.6.7.8.9.

10.11.12.13.14.15.

Table

1.

Title

Sketch Illustrating Some Inaccuracies in Leadline Soundings .Typical Discharge Pipe Patterns in a Retention Basin . . . . .Use of Silt Curtain to Control Disturbed Sediment inDredgeArea . . . . . . . . . . . . . . . . . . . . . . . .

Open Water Extended Aeration Treatment/Disposal . . . . . . .Bucket Dredge . . . . . . . . . . . . . . . . . . . . . . . .Dipper Dredge . . . . . . . . . . . . . . . . . . . . . . . .Bucket-Ladder Dredge . . . . . . . . . . . . . . . . . . . . .Suction Dredge . . . . . . . . . . . . . . . . . . . . . . . .Typical Pump Barge Dredging Operation . . . . . . . . . . . .Hydraulic, Cutter Head, Pipeline Dredge . . . . . . . . . . .Typical Cutter Head Dredging Operation . . . . . . . . . . . . .Typical Dust Pan Dredging Operation . . . . . . . . . . . . .Hydraulic Seagoing Hopper Dredge . . . . . . . . . . . . . . .Sidecast Dredge in Operation . . . . . . . . . . . . . . . . .Typical Side Dump Scow . . . . . . . . . . . . . . . . . . . .

Page

38.2-538.2-7

38.2-738.2-838.2-1038.2-1338.2-1438.2-1538.2-1638.2-1738.2-1838.2-2038.2-2138.2-2238.2-25

TABLE

Title Page

Hydraulic Dredges Currently in Navy Service Craft Inventory . 38.2-24

38.2-ix

DREDGING EQUIPMENT

Section 1. INTRODUCTION

1. SCOPE. This manual contains the following: information on procurementof dredging; types of equipment available, their characteristics and capaci-ties; basic economics of dredging operations; and preparation of plans andspecifications for the procurement of dredging for harbors, anchorages, turn-ing basins and ship channels. A catalogue (description and characteristics)of dredges currently in the Navy inventory is included for guidance in thepotential procurement of dredging by assignment of Navy equipment.

2. CANCELLATION. This manual on dredging equipment, NAVFAC DM-38.2, cancelsand supersedes Chapter Two, NAVFAC DM-38, Weight Handling Equipment and Serv-ice Craft, of August 1975.

3. RELATED CRITERIA. Certain criteria related to the subject matter ofthis manual appear elsewhere in the design manual series. See the followingsources:

Subject Source

Civil Engineering . . . . . . . . . . . . . . . . . . . . NAVFAC DM-5 SeriesHydrographic Surveys

Plans, Specifications and Cost Estimates . . . . . . . . NAVFAC DM-6 SeriesSurvey-Based Plans and Maps

Soil Mechanics . . . . . . . . . . . . . . . . . . . . . NAVFAC DM-7 SeriesSoils Exploration

Harbor and Coastal Facilities . . . . . . . . . . . . . . NAVFAC DM-26 SeriesHarbors and Ship ChannelsTurning Basins

4. PLANNING.

a. Jurisdiction. Jurisdiction over navigable waters in the UnitedStates, its territories and possessions, is assigned to the Corps of Engi-neers, Department of the Army. Application forms and permits for dredging areobtained from them.

b. Work Accomplishment.

(1) With Navy-owned equipment. Use Navy-owned dredges to themaximum extent consistent with economy.

(2) By the Corps of Engineers. When Navy-owned dredges suitablefor the project are unavailable, the work may be accomplished by contractualagreement with the Corps of Engineers, U.S. Army.

38.2-l

(3) By contract with private firms. When the only suitablefloating equipment- (dredges, pipelines, scows, or drillboats) is in privateownership, or where the workload exceeds the capability of available govern-ment equipment, dredging may be done by private contractors.

Section 2. DREDGING DESIGN

1. PERMITS AND OTHER LEGAL CONSIDERATIONS.

a. Corps of Engineers, Department of the Army. Permit must be obtainedfor all work in navigable waters of the continental United States, its terri-tories, and possessions. This permit is obtained by the agency procuring thedredging.

b. Disposal of Dredged Material. The disposal site (unless a publicspoil area is to be used) should be selected and acquired by the agency pro-curing the dredging. The disposal site must be clearly indicated on the proj-ect plans or in project specifications. A permit is required from the Corpsof Engineers, Department of the Army when disposal is to be in one of the pub-lic spoil areas under control of procuring agency, or when the runoff from thespoil area will be returned to any navigable water, including spoiling on shoreor foreshore areas. Permits from various state or local agencies concernedwith pollution, stream control, fish and wildlife, and wetlands preservationalso may be necessary. It is necessary to make local inquiry and investiga-tion in each case and to do so thoroughly as such agencies are being commis-sioned at an increasing rate and failure to obtain a necessary permit canresult in a costly delay of a project. These permits may be obtained by theagency procuring the dredging or by a contractor. Normally, however, becauseof the time required to obtain said permits, they are obtained by the agencyprocuring the dredging.

Environmental Considerations. An environmental impact assessment(EIA) must be included with every program cost estimate (PCE). A candidateenvironmental impact statement (CEIS) may also be required.

2. SITE INVESTIGATIONS.

a. Hydrographic Surveys and Mapping. See also related criteria. Thesurvey should cover the entire area to be dredged, including slope areasand any areas of potential sloughing. If payment for dredging is to be on thebasis of price per unit volume, the survey (or a resurvey) should be made justprior to doing the dredging. The results of the survey should be presented inmap form. For preliminary layouts and similar purposes, consider availabilityof maps or charts promulgated by various governmental agencies, in particularthe Navy Oceanographic Office, and the National Ocean Survey under the NationalOceanic and Atmospheric Administration in the Department of Commerce. Drawingof contours is desirable, but not essential. The position of all known ob-structions (including bridges, jetties, piers, overhead and submarine cablesand pipelines, wrecks, and large items of debris) should be shown, togetherwith the nature of said obstructions. Soundings for preparation of hydro-graphic maps, preferably, should be made by echo sounding wherever there isadequate depth of water for the proper operation of such equipment. In shal-low depths, sounding with a leadline or pole (Figure 1) is permissible but hascertain inherent inaccuracies including sinkage of lead into bottom, drift of

38.2-2

sounding line, and variable slacking of sounding line. If a lead is used,type and weight should be specified. Ranges should not be spaced more than 50feet apart, with soundings at like intervals (or continuous). Where localdetail is required, ranges and soundings should be at closer intervals. Datumfor soundings shall be mean low water (MLW) or mean lower low water (MLLW),whichever is used as station datum.

b. Map of Disposal Area. Map of disposal area showing existing con-tours and final grading is required where finished grading of disposal area isdesired. Any desired planting of disposal area should be indicated. Gradingof disposal area should allow for any required diking, for inevitable losses(shrinkage) in conveying and depositing of spoil material, and for losses dueto runoff. If rock dredging is involved, the in-place fill will “bulk” com-pared to the predredged volume. A bulk factor for rock, in common use, is 30%.

c. Characteristics of Area To Be Dredged. Indicate hydrographic con-ditions such as direction and approximate velocities of currents and waves,Indicate tidal range. Meteorological data such as temperature, winds, fog andice conditions should be listed.

d. Cross Sections. If payment for dredging is to be on a price perunit volume basis, usually quantities are determined on the basis of averageend areas. Sometimes the computation is made on the basis of horizontalslices. Either method is applicable, but the method should be stipulated asthe results are not always identical. If the end area method is to be used,cross sections will be required. If more than one type of material will beencountered, this should be indicated on the cross sections.

e. Test Borings.

(1) Number and spacing. At the present state of the art, no gen-erally accepted rules have been formulated. Number and spacing of test bor-ings required to adequately define the work remain a matter of individualjudgement, based on the following principles:

(a) For maintenance dredging, in general, review of histor-ical records should be adequate and no borings are required.

(b) Spacing of borings should be determined by degree ofvariation in the material to be dredged. Spacing of 100 to 500 feet apart isreasonable if conditions are substantially uniform. Where conditions are var-iable, closer spacing is required.

(2) Depth. Borings should penetrate at least 2 feet below pro-posed final dredged level.

(3) Test data. For purposes of evaluating the necessary dredgingplant and the probable production, the principal element of interest is thedensity (or consistency) of the material to be removed and, for cohesivesoils, shear strength. Density (or consistency) may be estimated from theStandard Penetration Test. For firm (or stiff) cohesive soils, consider exe-cution of unconfined compression tests. For loose (or soft) to medium soils,strength tests usually are not required. For purposes of estimating stable

38.2-3

underwater side slopes, additional data may be required (see DM-7 Series forprinciples and sample calculations). Approximate values, for purposes of pre-liminary design are:

Soil consistency

Hardpan and gravelly clay . . . . . . . 1:1-1:2 1:1-1:2

Sand and gravel with some fines . . . . 1:2-1:5 2 1:2-1:3 2

Coarse sand and gravel withoutor with little fines . . . . . . . . 1:3-1:4 --

Mud and soft silt . . . . . . . . . . . 1:6-1:8 3 1:4-1:6

Firm cohesive soils . . . . . . . . . . 1:1-1:1[ --

f. Test Dredging. For very large projects, consider test dredging inrepresentative areas before the dredging plant is selected and/or prior toselection of side slopes.

3. PROJECT DEPTH. The project depth usually is determined by the sponsoringCommand or Agency. For harbor and channel depth requirements, see Harbors,NAVFAC DM-26.1.

4. OVERDEPTH DREDGING.

a. Overdepth Allowance. Usual overdepth allowance is 1 foot in landcuts or in cuts in protected waters, and 2 feet in exposed locations which aresubject to substantial wave action. Another rule often used is an overdepthallowance of 2 feet in soft bottoms and 1 foot for hard bottoms. This allow-ance is intended as a tolerance to accommodate the inaccuracies inherent inthe dredging process. Two common ways of treating the tolerance follow:

(1) Allow, for example, 1 foot of tolerance all over the area tobe dredged, which volume is to be paid for, whether dredged or not. However,the level everywhere must be below the minimum depth specified.

(2) Make a final survey and pay for everything dredged, down tothe limit of the tolerance, the bottom everywhere being below the minimumdepth specified.

Tolerance on side slopes also is necessary as side slopes normallyare excavated in benches, with some of the loose material inevitably fallingto the bottom of the slope or onto the benches.

b. Designed Overdepth. Provide for mandatory overdepth dredging as anallowance for advance maintenance to compensate for future silting. The depthto be allowed is determined by economic analysis, weighing the investment valueof the increased expenditure against the costs of successive future mobiliza-tions of equipment and other fixed costs involved in redredging.

1Slopes show ratio of rise to run.2Finer material will assume flatter slopes.3May be as flat as 1:15.

38.2-4

FIGURE 1Sketch Illustrating Some Inaccuracies in Leadline Soundings

c. Mandatory Maximum Dredged Depths. Specify any limitations imposedby considerations of stability of adjacent structures, cover over buried util-ities or structures, etcetera.

5. OVERWIDTH DREDGING. Consider need for dredging to excess width of chan-nel to provide advance maintenance to compensate for silting or sloughing ofside slopes.

38.2-5

6. SPOIL AREAS AND UTILIZATION OF SPOIL.

a. Utilization of Spoil. The key to economy in dredging is the profit-able use of the dredged material. Use as landfill (for commercial development,recreation, etcetera) is common. Recovery of aggregates or minerals should beconsidered. Every possible attention should be devoted to this aspect of thework. See paragraph 1.b for disposal area permits.

b. Environmental Considerations.

(1) Control of discharge. Provide settling basin for dischargeprior to its effluence into any water course. Design outlet and boundaryfacilities to permit pool level regulation and sectionalization of fill areas.Outlet facilities usually include a weir of adjustable height and are locatedso that overflow water must travel as great a distance as possible before pass-ing out of settling basin, thereby giving maximum settling time and minimumsolids content in the overflow water (see Figure 2). A limitation on thesolids content in the overflow water of 8 grams/liter is a common measure ofthe adequacy of retention in the settling basin.

(2) Control of disturbed sediments in dredged area. Consider useof silt curtains (Figures 3 and 4) to control the spread of disturbed sedi-ments near dredged area.

(3) During creation, and upon completion of above-water fills,take necessary precautions to eliminate undrained areas and to prevent erosion.

(4) Upon completion of below-water fills, consider seeding withshellfish larvae or a variety of organisms to produce a productive area.

(5) Polluted sediments. Under current (1973) standards of theDepartment of the Army, Corps of Engineers, if any one of the following pollu-tion parameters exceeds the limit expressed below, the sediment is consideredpolluted and unacceptable for disposal in open water.

Pollutant

Volatile solidsChemical oxygen demand (C.O.D.)Total Kjeldahl nitrogenOil-greaseMercuryLeadZinc

Concentration % (dry wt. basis)

6.05.00.100.150.0010.0050.005

c. Dikes. Artificial dikes or levees are often required for retentionboundaries of settling basins. For principles of design see hydraulic fillsin NAVFAC DM-7 Series. Consider need for slope protection for dike. Forprinciples of design of slope protection see NAVFAC DM-26 Series.

38.2-6

FIGURE 2Typical Discharge Pipe Patterns in a Retention Basin

FIGURE 3Use of Silt Curtain to Control Disturbed Sediment in Dredge Area

38.2-7

FIGURE 4Open Water Extended Aeration Treatment/Disposal

d. Spoil Data.

(1) Approximate Percentage of Solids in Hydraulic Spoil. Thepercentage of solids handled by hydraulic dredges depends on the type ofequipment being used, the character of the materials being pumped, the effec-tiveness of the operator, and the efficiency of the equipment. The percentageof solids also depends on the depth of the materials below water, the specificgravity of the solids and the slurry, and the suction velocity in the pipeline.The use of a proper cutter is important. The solids usually vary from 5 to20% in the pipeline.

(2) Typical Unit Weights of Spoil. Slurry weights in the dis-charge pipeline vary from 68 to 80 pounds per cubic foot.

7. REMOVAL OF ROCK. Removal of rock (except for soft rock) requires thatthe rock be broken into sections of a size which can be handled by availableequipment. This can be accomplished by drilling and blasting or by breakingwith impact tools. No special provisions of the contract are required, otherthan to use safe blasting techniques and to beware of blasting near existingstructures. Water transmits the blast shock wave which is, in effect, anoverpressure which decays with time and distance from the explosive source.

8. RECOVERY OF SUBSTRATUM (UNDERCUTTING). Consider a soil profile consist-ing of several feet, or tens of feet, of muck overlying sand. It is desiredto recover the sand without stripping the muck. This can be done by a methodcalled undercutting. It also is possible to increase the depth of water byundercutting and allowing the level of the muck to sink into the undercut.This is a little-known, but potentially important, technique. It consists ofusing a jet ejector, passed through the overlying muck to penetration in thesand, and switching to horizontal jetting which causes the surrounding sand tofunnel to the ejector, leaving wells in the sand. The procedure is repeatedwell after well. Eventually, the wells collapse and the bottom sinks afterthe ejector has been removed.

38.2-8

Section 3. TYPES OF DREDGING EQUIPMENT

Part 1. MECHANICAL DREDGES

1. BUCKET DREDGE.

a. Description. Essentially a crane or derrick mounted on a floatingplatform and operating a clamshell, orange peel, or dragline bucket. May bea specially built machine (Figure 51, or simply a land machine (usually acrawler) secured to a floating platform, or the turret and upper works of aland machine mounted on a floating platform.

b. Capacity and Depth. Capacity depends on the character of materialsto be handled, weight, and cohesion. The depth is an important factor. Thespeed of the bucket affects the ability of the material to stay in it. Thecapacity can only be determined by the size of the bucket, the percentage of abucket load one can get, the speed of the hoisting operation, and the locationand manner of discharge. Handling a sandy material from shallow water, onecan get a swing in 45 seconds, while handling clay from deep water might re-quire 120 seconds per swing. This type of equipment can be used in depths to100 feet or more. The depth is limited only by economical limits and drumcapacity.

c. Uses, Advantages.

(1) Small operating crew.

(2) Can dig anything other than rock or highly cemented soilmaterials. Can dredge rock when broken to pieces by blasting.

(3) Highly versatile machine.

d. Disadvantages.

(1) Slow moving. Relatively low production rate.

(2) Point of spoiling must be directly alongside the dredge.Otherwise requires use of barges and of tugs for moving barges.

2. MOUNTING CONVENTIONAL, LAND-BASED EXCAVATION EQUIPMENT ON A BARGE. Thisvariation of a bucket dredge has been useful in isolated and advanced baseenvironments where conventional equipment cannot be procured. The criteriafor floating cranes in DM-38.1 are also generally applicable to this conver-sion. The barge must provide the necessary buoyancy and satisfactory stabil-ity, and the overall cost of conversion should not exceed the cost of engaginga conventional dredge. Prepare the barge and excavating equipment as follows:

(1) Investigate the following loads on the barge and their effect onstability and structural strength.

(a) Dead load. Include vessel, excavating machinery, and auxili-ary equipment mounted in or on the barge.

(b) Live load. Include weight of payload, water and fuel tanks,and anticipated deck live load.

38.2-9

FIGURE 5Bucket Dredge

38.2-10

(c) Pulls on cables. After the operating cables for the excava-tion equipment have been selected, assume that their breaking strengths repre-sent the maximum forces which can be applied to the barge due to this phase ofdredging operations.

(d) Loads due to vertical and lateral acceleration of bucket, atmaximum reach.

(e) Vertical and horizontal components of the mooring cable pulls.

(f) Wind, current, ice, and wave loads. For special wind loadson cranes see DM-38.1. Where the equipment will be used only in shelteredwaters, use L/30 trochoidal wave; use L/10 to L/20 wave for seagoing hullsunless otherwise specified in the project criteria. (L is the length of thehull of the dredge, at the water level.)

(g) Load applied to spuds through spud hoist. Consider spudscapable of lifting the entire vessel, including live load.

(h) Damage control.

(i) Buoyant force of water.

(2) Provide necessary anchorages and/or tiedowns to secure the excavat-ing equipment to the barge.

(3) Install limit and overload switches to restrict operating forces tothe allowable operating loads of the various components.

(4) Provide reinforcement in or on the barge deck to distribute theconcentrated loads from the excavating equipment.

(5) Consider adding blisters to the hull for added stability.

(6) Check existing hull compartmentation for damage control. SeeNAVSHIPS Technical Manual, NAVSHIPS 0901-882-0002 Chapter 9880.

(7) When converting an existing hull, check for damaged or deterioratedframing and/or deck and hull plates. Repair or reinforce as required.

(8) It is desirable, for safety, that the beam of the barge be adequateto provide a walkway for personnel beyond the tailswing of the deck-mountedexcavating machinery. Where the original beam is inadequate, sponsons may beadded. Alternately, the base of the machine may be elevated to provide over-head clearance (6.5 to 7 feet) if the additional height does not have too mucheffect on the overall stability of the barge.

(9) If a shovel or backhoe is mounted on a barge, set the machine asclose to the deck edge as permitted by stability, to avoid the necessity fordigging in a vertical position from which the digging force is small. Movethe dipper stick far enough out on the boom so that the minimum clearancebetween the stick and the hull fender will be 6 inches for the close-in posi-tion. When the boom is in lowest position, the angle to the horizontal shouldnot be less than 25 degrees in order not to overstress the boom and/or itssupports.

38.2-11

(10) Where required by the material to be dredged, perforate the bucketsto provide additional drainage paths. Holes for silt and sand should not belarger than 0.5 inch; for gravel and clay, about 1 inch; and for rock, coral,and the like, 2 inches. Spacing should be chosen in accordance with the char-acter of the material to be dredged. If the type of material is expected tovary significantly while dredging, provide more than one bucket.

(11) Repaint the excavating equipment with protective coating resistantto a marine environment. If necessary, install enclosures or shielding toreduce the entry of spray into machinery areas.

(12) Where possible, used fixed ballast. If this cannot be done or isinadequate, provide pumps and watertight compartments (at the end of the ves-sel away from excavating equipment) to allow water to be used for counterweightor ballast. If water is used for ballast, the compartments must be full toeliminate free surface effects on stability.

(13) Add lighting facilities as required.

3. DIPPER DREDGE.

a. Description. Essentially, a power shovel, with the bucket at theend of a very long stick and mounted to accommodate the intended depth of dig-ging (see Figure 6).

b. Capacity and Depth. Capacity of a dipper dredge depends primarilyon the character of materials handled. In hard bottoms the bucket may be onequarter full, whereas in sand or clay, a heaped bucket might be obtained.Most dippers have been designed for 45-foot maximum channel depths, but somecan be used to 60-foot depths.

c. Advantage. Can dig virtually any material other than firm rock.

d. Disadvantages.

(1) Point of spoiling must be directly alongside the dredge.Otherwise requires use of barges and of tugs for moving barges.

(2) Relatively low output.

(3) Larger operating crew than for bucket dredge.

4. LADDER DREDGE.

a. Description. A series of buckets on an endless chain which rollsover a drum or sprocket at either end of a long frame (the ladder). One endof the ladder is lowered to the bottom, which brings the buckets into contactwith the material to be dredged. The buckets dump, by gravity, near the upperend of their travel. (See Figure 7.)

38.2-12

b. Capacity and Depth. Preliminary designs have been made for machinedigging to depths of 150 feet. Probably feasible to dig to depths of over 300feet. Digging volumes to 100 cubic yards per hour under average conditions.

c. Use.used for mining.

Not extensively used in general dredging practice. Mostly

FIGURE 6Dipper Dredge

38.2-13

FIGURE 7Bucket-Ladder Dredge

Part 2. HYDRAULIC DREDGES

1. PLAIN SUCTION TYPE.

a. Description. Operates on principle of a vacuum cleaner. A streamof water is caused to flow through the suction pipe by means of a centrifugalpump. Loose materials are sucked up into and carried along with the streamand discharged from the end of the conveyance pipe. (See Figure 8.) The pumpbarge (Figure 9) is a variation of this type.

b. Capacity and Depth. This type of dredge can bring up solids of 20%or more in loose materials as long as the suction is kept into the materialand the proper amount of water can flow to the suction mouth. The limit ofdepth is about 115 feet in sandy materials.

c. Uses, Advantages.

(1) Often used to recover sand from wet borrow pit and for exca-vating jobs in loose sand.

(2) No need for barges or for towing barges.

(3) Operates continuously and in darkness or fog.

(4) Relatively small crew.

38.2-14

OUTBOARD PROFILE

FIGURE 8Suction Dredge

38.2-15

FIGURE 9Typical Pump Barge Dredging Operation

d. Disadvantages.

(1) Limited to removal of loose material.(2) Discharge pipeline is an obstruction to passing traffic.

e. Length of Discharge Pipe. Length of discharge pipe depends on thepressure produced by the pump, the velocity, and percentage and character ofmaterials being pumped. Some dredges can pump up to 15,000 feet in sand orsilt. Beyond that, booster pumps are put in the line.

2. CUTTER HEAD TYPE.

a. Description. Same principle as plain suction type, but employs a"cutter" at the end of the intake pipe to loosen the material to be dredged.Cutter consists of a rotating basket frame of spiral knives surrounding thesuction nozzle (see Figure 10). The cutter head carves clay; breaks offchunks of softer rock, such as coral or shale; and stirs up gravel or sand sothat the stream of suction water is continually, and heavily, loaded with theexcavated material (see Figure 11).

b. Capacity and Depth. Capacity varies from 5 to 20% of solids.Depth limit is usually less than 50 feet, as these dredges are designed forchannel work, but with ladder extensions, they can be effective to at least100 feet.

38.2-16

FIGURE 10Hydraulic, Cutter Head, Pipeline Dredge

38.2-17

FIGURE 11Typical Cutter Head Dredging Operation

c. Uses, Advantages.

(1) Most widely used type of hydraulic dredge.(2) No need for barges or for towing of barges.(3) Operates continuously and in darkness or fog.(4) Relatively small crew.

d. Disadvantage. Discharge pipeline is an obstruction to passingtraffic.

e. Length of Discharge Pipe. (See Plain Suction Type.)

3. DUST PAN TYPE.

a. Description. Similar to cutter head type, except that horizontallyspread suction heads are employed instead of a single suction nozzle, andloosening of material to be dredged is accomplished by means of high velocitywater jets. (See Figure 12.)

b. Capacity and Depth. Capacity is similar to suction and cutter typedredges. Since these are used mostly on shallow shoals and soft material, thepercentage of solids should be on the minimal side, about 5 to 10%. This typeof equipment seldom is used in depths greater than 45 feet.

c. Uses, Advantages.

(1) Originally developed for cutting channels in loose material.

(2) Operates off lines running parallel to direction of travel ofdredge. Therefore, not necessary to have lines crossing, and interfering in,adjacent channels.

(3) High percentage of solids in discharge.

(4) Dredge and its equipment can be secured and moved to anotherlocation more readily than, for example, a cutter head dredge.

d. Disadvantage. Generally requires nearby spoil site.

4. HOPPER DREDGE.

a. Description. Operates as a self-propelled suction or cutter headdredge, but the dredged material is pumped into, and retained in, hoppers inthe hull of the vessel for transport to the spoil area, where it is dischargedfrom the hoppers into the spoil basin. (See Figure 13.)

b. Capacity and Depth. Capacity depends on the size of the hoppers.These vary from 1,200 cubic yards to 8,000 cubic yards. This type of equip-ment usually is used on channel maintenance work to 45-foot depths, but canload to 100-foot depth when at anchor with the proper suction extension.

38.2-19

FIGURE 12Typical Dust Pan Dredging Operation

OUTBOARD PROFILE

FIGURE 13Hydraulic Seagoing Hopper Dredge

c. Uses, Advantages.

(1) Used where distance from dredged area to spoil area-exceedsreasonable pipeline length.

(2) Operates without pipelines, scows, or tugs.Capable of operating in rough water.

Self-propelled.

d. Disadvantages.

(1) Time in transit often considerable.(2)(3)

Dredge not excavating while discharging.Limited operation in darkness or fog.

5. SIDECAST DREDGE.

Description. Discharge pipe is mounted on a rotating boom.is cast to side of dredged channel (see Figure 14).

Spoil

b. Capacity and Depth. Capacity depends on the size of pump. Solidsload depends on the amount of material available to the suction mouth. Mayvary from 5 to 20%. Usually used in depths to 45 feet.

FIGURE 14Sidecast Dredge in Operation

38.2-22

c. Use. Used where material is such that it will not re-enter thedredge channel, that is, not used for sandy materials or silts that can becarried away in flotation by currents.

d. Disadvantage. Material may re-enter the channel or float anddeposit into unwanted areas. Great discoloration of the surrounding waters.Environmental impact is serious.

6. NAVY HYDRAULIC DREDGE INVENTORY (1981). Hydraulic dredges currently inthe Navy inventory are listed. in Table 1.

Part 3. MISCELLANEOUS EQUIPMENT

1. SECTIONALIZED DREDGES.

a. Description. Conventional dredges, but specially detailed andmanufactured to be disassembled for transport and reassembled at the site ofthe work.

b. Use. Generally for inland work (lakes, muck removal for stabiliz-ing embankments, etcetera) and advanced base military operations.

2. DUMP SCOWS. Operation of bucket-type dredges ordinarily involves the useof dump scows. There are two general types of dump scow, side dump and bottomdump. Both are barges divided transversely into a series of open-top bins toreduce the size of the closing gates. Elsewhere, the barge is completelyclosed to provide buoyancy when the decks are awash, as is frequently the casewhen loaded.

Side-dump scows are used in shallow water of insufficient depth forbottom-dump gates to hang freely. If the depth of water is adequate, bottom-dump scows are preferable to side-dump scows because of their greater capacityand inherent stability.

Dump scows (Figure 15) usually are built of steel, with carrying capaci-ties of between 500 and 3,000 cubic yards. The usual preference is for 750-to 1,500-cubic-yard scows, because they can be handled easily with the dredge’swinch or windlass and are large enough for economical towing to distant dis-posal grounds. Scows of more than 1,000-cubic-yard capacity normally areequipped with mechanical/electrical gear for control of the hopper doors.Power, when required, is supplied to scow from the tug. Wire rope, ratherthan chain, has come into use on larger scows for holding hopper doors closedand has proved entirely satisfactory.

Dump scows with shaped or rounded ends are also coming into use. A shapedend offers much less resistance to towing than square ends and needs less tugpower for handling the tow. The same tug power can also handle more scowswith shaped ends.

Depressed, hopper-division bulkheads are used on some modern scows, al-lowing quicker loading by dipper dredge. They are of special advantage inloading scows by hydraulic dredge discharge, because the depressed bulkheadspermit the pumped mixture to flow to all hoppers at once.

38.2-23

TABLE 1Hydraulic Dredges Currently in Navy Service Craft Inventory

Hull Displacement Dredge

YM Pontoon Light Full Load PipeLocation

LOA Length Beam Depth Weight Draft WeightDraft

Suction Discharge Ladder Digging(ft) (ft) (ft) (ft) (tons) (tons) (in) (in) (ft) (ft) Year

17 195 134 38 10 482 5'-7½” 500 5’-10” 18 16 66.5 401 1934 Charleston, NSY

32 34 14 5 23 2’-6” 8 15 9 1969 Coronado, Ca.

33 43 15 6 43 2’-6” 12 30 20 1969 Davisville, R.I.

352 118 80 28 6 122 2'-4” 170 3’-3” 18 16 60 45 1970 Mare Is., NSY

38 151 111 38 8 288 4’-0” 379 5’-3” 24 20 72 503 19504 Subic Bay, P.I.

1Depth capability increased from 35’ to 40’ in 1964.2Dredge is of three sections bolted together.3Can be modified to 63 feet.4Acquired by USN in 1968.

FIGURE 15Typical Side Dump Scow

38.2-25

a. Side Dump. Side-dump scows have bins with a longitudinal hip run-ning through the center so that the bottom slopes down from the center to theside. The side consists of flap gates, hinged at the top, that may be closedtightly with chains and ratchet wheels. To dump the scow, the pawls are dis-engaged from the ratchets, allowing the load to force the gates open and slideout. After the bins are emptied, the gates are closed with ratchet wheels.

b. Bottom Dump. Bottom-dump scows have bins shaped like the invertedfrustum of a pyramid, the bottoms of which are closed by one or a pair of flapgates. To unload the barge, the flap gates are dropped open, downward. Afterthe bins are emptied, they are closed by ratchet wheels similar to those of aside-dump scow.

3. PIPELINES. The discharge line from the centrifugal pump of a standardhydraulic cutter-head dredge is divided into three parts: the pipe on thedredge, floating line, and shore line.

The dredge pipe is connected with the floating line by a flexible joint,allowing considerable play as the dredge swings about one of the rear spuds.The floating line is made up of 50- or 60-foot lengths of pipe, each supportedby a pontoon and successively connected by flexible ball joints. A walkway issupported above the pipe. Usual walkway width is 16 inches. The floatingline is swung in a wide arc between the dredge and the shore line to allow thedredge to maneuver in advance without frequent additions to the line.

The shore pipeline is made up of light sections of pipe, about 15 to 20feet long, to permit easy handling in the field, connected by tapered slipjoints of band-and-lug construction. The line may have two discharge endsseparated by a Y, one of which remains closed by a valve in the Y when addi-tional pipe is added. This arrangement saves stopping the dredge to addlengths of pipe.

Section 4. ECONOMICS OF DREDGING

1. BASIC PRINCIPLES.

a. Depth. As the dredge depth increases, the economy of removaldecreases.

b. Other.

increases.(1) As the hardness of the dredged material increases, the cost

(2) As the discharge pipeline length increases, the costincreases.

2. RELATIVE COSTS OF REMOVAL BY DIFFERENT TYPES OF EQUIPMENT.

Bucket dredgeDipper dredgeHopper dredge

1.251.251.00

38.2-26

Hydraulic dredge, cutterhead 0.75Hydraulic dredge, plain 0.60Hydraulic dredge, dust pan 0.30Sidecast dredge 0.25

3. CONCEPTUAL OR PLANNING COST ESTIMATES. Original dredging, such as chan-nel and turning basin dredging, not directly related to the specific construc-tion of an item, carries the category code 165. Dredging performed primarilyto provide fill, such as site improvement, shall carry the category code 932.Dredging directly related to the specific construction of an item shall bearthe same category code as the item to be constructed.

38.2-27

REFERENCES

(Publications containing criteria cited in this manual)

NAVFACENGCOM Design Manuals and P-Publications.

Government agencies may obtain Design Manuals and P-Publications from the U.S.Naval Publications and Forms Center, 5801 Tabor Ave., Philadelphia, PA 19120.TWX 710-670-1685, AUTOVON: 442-3321. The stock number is necessary forordering these documents and should be requested from the NAVFACENGCOM Divi-sion in your area.

Non-Government organizations may obtain Design Manuals and P-Publications fromthe Superintendent of Documents, U.S. Government Printing Office, Washington,DC 20402.

DM-5 series: Civil Engineering

DM-6 series: Mechanical Engineering

DM-7 series: Soil Mechanics, Foundations, and Earth Structures

DM-26 series: Harbor and Coastal Facilities

DM-38.1: Weight Handling Equipment and Service Craft

Reference-l

APPENDIX

METRIC CONVERSION FACTORS1

0.5 inch = 1.25 cm

1 inch = 2.5 cm

6 inches = 15 cm

1 foot = 30 cm

2 feet = 60 cm

15 feet = 4570 m

20 feet = 6 m

50 feet = 15 m

100 feet = 30 m

150 feet = 45 m

15,000 feet = 4.5 km

100 cu yd = 76 m3

500 cu yd = 380 m3

1,000 cu yd = 760 m3

1,500 cu yd = 1,140 m3

8,000 cu yd = 6,080 m3

68 pounds per cubic foot = 1,090 kg/m3

80 pounds per cubic foot = 1,280 kg/m3

lConversions are approximate.

*U.S. G O V E R N M E N T P R I N T I N G O F F I C E : 1 9 8 1 - 7 3 0 - 1 7 1A-1