dew ate ring and underground water control

8/2/2019 Dew Ate Ring and Underground Water Control

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UFC 3-220-0516 January 2004

UNIFIED FACILITIES CRITERIA (UFC)

DEWATERING AND GROUNDWATER

CONTROL

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED


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UFC 3-220-0516 January 2004

1

UNIFIED FACILITIES CRITERIA (UFC)

DEWATERING AND GROUNDWATER CONTROL

Any copyrighted material included in this UFC is identified at its point of use.Use of the copyrighted material apart from this UFC must have the permission of thecopyright holder.

U.S. ARMY CORPS OF ENGINEERS (Preparing Activity)

NAVAL FACILITIES ENGINEERING COMMAND

AIR FORCE CIVIL ENGINEER SUPPORT AGENCY

Record of Changes (changes are indicated by \1\ ... /1/)

Change No. Date Location

This UFC supersedes TM 5-818-5, dated 15 November 1983. The format of this UFC does notconform to UFC 1-300-01; however, the format will be adjusted to conform at the next revision.The body of this UFC is the previous TM 5-818-5, dated 15 November 1983.


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UFC 3-220-0516 January 2004

2

FOREWORD\1\The Unified Facilities Criteria (UFC) system is prescribed by MIL-STD 3007 and providesplanning, design, construction, sustainment, restoration, and modernization criteria, and appliesto the Military Departments, the Defense Agencies, and the DoD Field Activities in accordancewith USD(AT&L) Memorandum dated 29 May 2002. UFC will be used for all DoD projects and

work for other customers where appropriate. All construction outside of the United States isalso governed by Status of forces Agreements (SOFA), Host Nation Funded ConstructionAgreements (HNFA), and in some instances, Bilateral Infrastructure Agreements (BIA.)Therefore, the acquisition team must ensure compliance with the more stringent of the UFC, theSOFA, the HNFA, and the BIA, as applicable.

UFC are living documents and will be periodically reviewed, updated, and made available tousers as part of the Services responsibility for providing technical criteria for militaryconstruction. Headquarters, U.S. Army Corps of Engineers (HQUSACE), Naval FacilitiesEngineering Command (NAVFAC), and Air Force Civil Engineer Support Agency (AFCESA) areresponsible for administration of the UFC system. Defense agencies should contact thepreparing service for document interpretation and improvements. Technical content of UFC isthe responsibility of the cognizant DoD working group. Recommended changes with supportingrationale should be sent to the respective service proponent office by the following electronicform: Criteria Change Request (CCR). The form is also accessible from the Internet sites listedbelow.

UFC are effective upon issuance and are distributed only in electronic media from the followingsource:

Whole Building Design Guide web site http://dod.wbdg.org/.

Hard copies of UFC printed from electronic media should be checked against the currentelectronic version prior to use to ensure that they are current.

AUTHORIZED BY:

______________________________________DONALD L. BASHAM, P.E.Chief, Engineering and ConstructionU.S. Army Corps of Engineers

______________________________________DR. JAMES W WRIGHT, P.E.Chief EngineerNaval Facilities Engineering Command

______________________________________KATHLEEN I. FERGUSON, P.E.The Deputy Civil Engineer

DCS/Installations & LogisticsDepartment of the Air Force

______________________________________Dr. GET W. MOY, P.E.Director, Installations Requirements and

ManagementOffice of the Deputy Under Secretary of Defense

(Installations and Environment)
http://www.wbdg.org/pdfs/ufc_implementation.pdfhttp://www.wbdg.org/ccb/browse_cat.php?o=29&c=4http://dod.wbdg.org/http://dod.wbdg.org/http://www.wbdg.org/ccb/browse_cat.php?o=29&c=4http://www.wbdg.org/pdfs/ufc_implementation.pdf


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ARMY TM 5-818-5

NAVY NAVFAC P-418AIR FORCE AFM 88-5, Chap 6

DEWATERING

ANDGROUNDWATER CONTROL

DEPARTMENTS OF THE ARMY, THE NAVY, AND THE AIR FORCE

NOVEMBER 1983


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REPRODUCTION AUTHORIZATION/RESTRICTIONS

This manual has been prepared by or for the Government and, except to the ex-tent indicated below, is public property and not subject to copyright.

Copyrighted material included in the manual has been used with the knowledgeand permission of the pr oprietors an d is acknowledged as su ch at point of use.Anyone wishing to make further use of any copyrighted material, by i t se l f and a p a r t f rom t h i s t e xt , should seek necessary permission directly from the proprie-tors.

Reprints or republications of this manual should include a credit substantially asfollows: Joint Departments of the Army, the Air Force, and the Navy, USA,Technical Manual TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418, Dewateringand Groundwater Control.

If the r eprint or republicat ion includes copyrighted material, the credit shouldalso state: Anyone wishing to make further use of copyrighted material, by i tself and apar t f rom th i s t ex t , should seek necessary permission directly from the pro-prietors.


6/161

TM 5-AFM 88-5, Ch

NAVFAC C-1

Change

No. 1

HEADQUARTERSDEPARTMENTS OF THE ARM

AIR FORCE, AND NAVYWASHINGTON, DC 27 J une 198


TM 5-818-5/AFM 88-5, Chapter 6/NAVFAC P-418, 15 November 1983 is changed as follows:1. Remove old pages and insert new pages as indicated below. New or changed material is indicated by a vcal bar in the margin of the page.

Remov e pages Insert pages

i a nd i i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i and iii

A - 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 1

2. File this change sheet in front of the publication for reference purposes.

By Order of the Secretaries of the Army, the Air Force, and the Marine Corps:

JOHN A. WICKHAM, JR.General, United States Army

Official: Chief of Staff

DONALD J. DELANDROBrigadier General, Unit ed St at es Army

The Adjutant General

Official:

EARL T. OLOUGHLINGeneral, United States Air Force,

Comm ander, Air ForceLogist ics Com mand

CHARLES A. GABRIELGeneral, Unit ed Sta tes Air For

Chief of Staff

H.A. HATCHLieut enant General, Marine Corps

Deputy Chief of St aff, Inst allat ionand Logistics Command


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*TM 5-818-5/AFM 88-5, Chap 6/NAVFAC

TECHNICAL MANUALNo. 5-818-5

HEADQUARTERS

AIR F ORCE MANUALDEPARTMENT OF THE AR

NO. 88-5, Chapter 6THE AIR FORCE,AND THE NAVY

NAVY MAN UAL

No. P-418 WASHINGTON , DC 15 N ov em be


CHAPTER 1. INTRODUCTIONPurpose and scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Construction dewatering. . . . . . . . . . . . . . . . . . . . . . . . . .

Permanent groundwater control . . . . . . . . . . . . . . . . . . . .

2. METHODS FOR DEWATERING, PRESSURE RELIEF,AND SEEPAGE CUTOFF

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Types and source of seepage . . . . . . . . . . . . . . . . . . . . . . .

Sumps and ditches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Wellpoint systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Deep-well systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Vertical sand drains . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Electroosmosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Cutoffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Summary of groundwater control methods. . . . . . . . . . . .Selection of dewatering system. . . . . . . . . . . . . . . . . . . . .

3. GEOLOGIC, SOIL, AND GROUNDWATER INVESTI-GATIONS

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Geologic and soil conditions. . . . . . . . . . . . . . . . . . . . . . . .

Groundwater characteristics. . . . . . . . . . . . . . . . . . . . . . .

Permeability of pervious strata. . . . . . . . . . . . . . . . . . . . .

Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surfacewater.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 . DESIGN OF DEWATERING, PRESSURE RELIEF, AND

GROUNDWATER CONTROL SYSTEMSAnalysis of groundwater flow . . . . . . . . . . . . . . . . . . . . . .

Mathematical and model analyses. . . . . . . . . . . . . . . . . . .

Flow-net analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Electrical analogy seepage models. . . . . . . . . . . . . . . . . . .Numerical analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Wellpoints, wells, and filters. . . . . . . . . . . . . . . . . . . . . . .

Pumps, headers, and discharge pipes. . . . . . . . . . . . . . . . .

Factors of safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Dewatering open excavations . . . . . . . . . . . . . . . . . . . . . .

Dewatering shafts and tunnels . . . . . . . . . . . . . . . . . . . . .Permanent pressure relief systems . . . . . . . . . . . . . . . . . .

Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control of surface water . . . . . . . . . . . . . . . . . . . . . . . . . .

5. INSTALLATION OF DEWATERING AND GROUND-WATER CONTROL SYSTEMS

G e n e r a l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Deep-well systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Wellpoint systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Vertical sand drains . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Cutoffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Piezometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6. OPERATION AND PERFORMANCE CONTROLGeneral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Control and evaluation of performance. . . . . . . . . . . . . . .

7. CONTRACT SPECIFICATIONSGeneral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Paragraph Page

l-1l-2l-3

l -4

l -1l -1l- 1

l -2

2-1 2-12-2 2-12-3 2-12-4 2-22-5 2-4

2-6 2-5

2-7 2-5

2-8 2-5

2-9 2-82-10 2-8

3-1 3-13-2 3-13-3 3-4

3-4 3-4

3-5 3-8

3-6 3-8

4-1 4-1

4-2 4-1

4-3 4-29

4-4 4-294-5 4-34

4-6 4-34

4-7 4-36

4-8 4-40

4-9 4-42

4-10 4-47

4-11 4-48

4-12 4-48

4-13 4-49

5-1 5-1

5-2 5-1

5-3 5-2

5-4 5-5

5-5 5-55-6 5-6

6-1 6-1

6-2 6-1

6-3 6-2

7-1 7-1

*This m anua l super sedes TM 5-818-5 / AFM 88-5 , Chap 6 / NAVFAC P-418 , Apri l 1971 .


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TM 5-815-5/AFM 88-5, Chap 6/NAVFACP-418

A PPEN D IX A.B.

C.D.

E.

F.

G.

Figure1-1

l-22-1

2-22-32-4

2-52-62-72-8

2-92-10

2-112-12

2-13

3-1

3-2

3-3

3-4

3-5

3-64-1

4-24-3

4-4

4-5

4-6

4-7

4-8

4-9

4-10

Paragraph

Types of specifications . . . . . . . . . . 7-2Data to be included in specifications . . . . . 7-3Dewater ing requir ement s and specifications . . . 7-4Measurement and payment . . . . . . . . 7-5Exam ples of dewat ering specificat ions . . . . . . 7-4REFERENCES AND BIBLIOGRAPHY . . . . . . . . . .NOTATIONS . . . . . . . . . . . . . . . . . . . . .FIELD PUMPING TESTS . . . . . . . . . . . . . . . . . . .EXAMPLES OF DESIGN OF DEWATERING AND PRES-

SURE RELIEF SYSTEMS . . . . . . . . . . . . . . .

TRANSFORMATION OF ANISOTROPIC SOIL CONDITIONSTO ISOTROPIC SOIL CONDITIONS . . . . . . . . . . .

WELL AND TOTAL DISCHARGE MEASUREMENTS . .EXAMPLES OF DETAILED DEWATERING SPECIFICA-TIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF FIGURES

Installation of piezometers for determining water table and arte-sian hydrostatic pressure . . . . . . . . . . . . . . .

Permanent groundwater control system . . . . . . . . . . . .Dewate r ing open excava t ion by d i t ch and s um pS e l f - j e t t i n g w e l l p o i n t . . . . . . . . . . .

Use of wellpoints where submergence is small . . .Drainage of an open deep cut by means of a multistage wellpoint

sys tem . . . . . . . . . . . . . . . . . . . . . . . .V a c u u m w e l l p o i n t s y s t e m . . . . . . . . .

Jet-eductor wellpoint system for dewatering a shaft . . . . .Deep-well system for dewatering an excavation in sand . .Deep wells with auxiliary vacuum system for dewatering a shaft

i n s t r a t i f i e d m a t e r i a l s . . . . . . . . . . . . .Sand drains for dewatering a slope . . . . . . . . . .Electra-osmotic wellpoint system for stabilizing an excavation

slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Grout curtain or cutoff trench around an excavation. .Dewatering systems applicable to different soils . . .Recharge of groundwater to prevent settlement of a building as a

resul t of dewater ing operat ions . . . . . . . . .Geologic profile developed from geophysical explorations . . . . .

Permea meters: (a) constant head a nd (b) falling head . . .

Specific yield of water-bearing sands versus D 10 South CoastalB a s i n , C a l i f o r n i a . . . . . . . . . . . . . .

D10

versus in situ coefficient of permeability-Mississippi RiverValley and Arkansas River Valley . . . . . . . . . . .

Formulas for determining permeability from field falling headt e s t s . . . . . . . . . . . . . . . . . . . . . . .

Inches of rainfall during 10 and 30-minute and l-hour periods . .Flow and head for fully penetrating line slot; single-line source;

artesian, gravity, and combined Flows . . . . . . .Height of free discharge surface h,; gravity flow . . .Flow and head for partially penetrating line slot; single-line

source; artesian, gravity, and combined flows . . . . . . . .Flow and head for fully and partially penetrating line slot; two-

line source; artesian and gravity flows . . . . . . .Flow and head (midway) for two partially penetrating slots; two-

l ine source; ar tes ian and gravi ty f lows . . . . . .Flow and head for fully and partially penetrating circular slots;circular source; artesian flow . . . . . . . . . . . . .

Head at center of fully and partially penetrating circular slots; cir-c u l a r s o u r c e ; a r t e s i a n f l o w . . . . . . .

Flow and drawdown at slot for fully and partially penetrating rec-tangular slots; circular source; artesian flow . . . .

Head within a partially penetrating rectangular slot; circulars o u r c e ; a r t e s i a n f l o w . . . . . . . . . .

Flow and drawdown for fully and partially penetrating singlewells; circular source; artesian flow . . . . . . . . . .

Page7-1

7-2

7-27-37-3A-1B-1C-1

D-1

E -1F-1G-1

Page

l-2l-32-22-32-4

2-42-52-62-7

2-82-9

2-9

2-102-13

2-143-23-3

3-4

3-6

3-73-8

4-24-3

4-4

4-5

4-6

4-7

4-8

4-9

4-10

4-11

ii Change 1


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TM 5-815-5/AFM 88 -5, Chap 6/NAVFA

Figure

4-11

4-12

Flow and drawdown for fully and partially penetrating singlewells; circular source; gravity flow

Flow and drawdown for fully penetrating single well; circulars o u r c e ; c o m b i n e d a r t e s i a n a n d g r a v i t y f l o w s

Page

4-12

4-13

iii


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TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-4

CHAPTER 1

INTRODUCTION

1-1. Purpose and scope. This manual providesguidance for the planning, design, supervision, con-struction, and operation of dewatering and pressurerelief systems and of seepage cutoffs for deep excava-tions for structures. It presents: description of variousmethods of dewatering and pressure relief; techniquesfor determining groundwater conditions, characteris-tics of pervious aquifers, and dewatering require-ments; guidance for specifying requirements for de-watering and seepage control measures; guidance fordetermining the adequacy of designs and plans pre-pared by contractors; procedures for designing, install-ing, operating, and checking the performance of de-

watering systems for various types of excavations; anddescriptions and design of various types of cutoffs forcontrolling groundwater.

1 - 2 . G e n e r a l .

a. It will generally be the responsibility of the con-tractor to design, install, and operate dewatering andgroundwater control systems. The principal usefulnessof this manual to design personnel will be those por-tions devoted to selecting and specifying dewateringand groundwater control systems. The portions of themanual dealing with design considerations should fa-cilitate review of the contractors plans for achieving

the desired results.b. Most of the analytical procedures set forth in this

manual for groundwater flow are for steady-stateflow and not for unsteady-state flow, which occursduring the initial phase of dewatering.

c. Some subsurface construction may require de-watering and groundwater control procedures that arenot commonly encountered by construction contract-ors, or the dewatering may be sufficiently critical as toaffect the competency of the foundation and design ofthe substructure. In these cases, it may be desirable todesign and specify the equipment and procedures to be

used and to accept responsibility for results obtained.This manual should assist design personnel in thiswork.

1-3. Construct ion dewater ing.

a. Need for groundwater control. Pr oper cont rol ofgroundwater can greatly facilitate construction of sub-surface structures founded in, or underlain by, per-vious soil strata below the water table by:

(1) Intercepting seepage that would otherwemerge from the slopes or bottom of an excavation.

(2) Increasing the stability of excavated sloand preventing the loss of material from the slopesbottom of the excavation.

(3) Reducing lateral loads on cofferdams.(4) Eliminating the need for, or reducing, air p

sure in tunneling.(5) Improving the excavation and backfill ch

acteristics of sandy soils.Uncontrolled or improperly controlled groundwcan, by hydrostatic pressure and seepage, cause pipheave, or reduce the stability of excavation slopes

foundation soils so as to make them unsuitable for sporting the structure. For these reasons, subsurfconstruction should not be attempted or permitwithout appropriate control of the groundwater (subsurface) hydrostatic pressure.

b. Influence of excavation characteristics. The tion of an excavation, its size, depth, and type, suchopen cut, shaft, or tunnel, and the type of soil toexcavated are important considerations in the seltion and design of a dewatering system. For mgranular soils, the groundwater table during constrtion should be maintained at least 2 to 3 feet below slopes and bottom of an excavation in order to ens

dry working conditions. It may need to be matained at lower depths for silts (5 to 10 feet below sgrade) to prevent water pumping to the surface making the bottom of the excavation wet and sponWhere such deep dewatering provisions are necessthey should be explicitly required by the specificatias they greatly exceed normal requirements and wonot otherwise be anticipated by contractors.

(1) Where the bottom of an excavation is undlain by a clay, silt, or shale stratum that is underlby a pervious forma tion under art esian pressu re (1-1), the upward pressure or seepage may rupture

bottom of the excavation or keep it wet even thouthe slopes have been dewatered. Factor of safety csiderations with regard to artesian pressure are dcussed in paragraph 4-8.

(2) Special measures may be required for excations extending into weathered rock or shale whsubstantial water inflow can be accommodated wout severe erosion. If the groundwater has not bcontrolled by dewatering and there is appreciable f


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TM 5-818-5/AFM 88-5, Chap 6 /NAV FAC P-418

(Modified from "Foundation Engineering," G. A. Leonards, ed., 1962, McGraw -Hill

Book Com pany. Used w it h permission of McGraw -Hi ll Book Com pany .)

Figure 1-1. Ins tallat ion of piez om eters for det ermin ing w at er ta ble and art esian hydrostat ic pressu re.

or significant hydrostatic pressures within the rock orshale deposit, rock anchors, tiebacks, and lagging orbracing may be required to prevent heave or to supportexposed excavation slopes.

(3) An important facet of dewatering an excava-tion is the relative risk of damage that may occur tothe excavation, cofferdam, or foundation for a struc-ture in event of failure of the dewatering system. Themethod of excavation and reuse of the excavated soilmay also have a bearing on the need for dewatering.These factors, as well as the construction schedule,must be determined and evaluated before proceeding

with the design of a dewatering system.

c. Groundwater control methods. Methods for con-

trolling groundwater may be divided into three cate-gories:

(1) Interception and removal of groundwater fromthe site by pumping from sumps, wells, wellpoints, ordrains. This type of control must include considerationof a filter to prevent migration of fines and possibledevelopment of piping in the soil being drained.

(2) Reduction of artesian pressure beneath thebottom of an excavation.

(3) Isolation of the excavation from the inflow of

groundwater by a sheet-pile cutoff, grout curtain,slurry cutoff wall, or by freezing.

1- 4 . Pe r m anen t g r oundw a te r con t r o lMany factors relating to the design of a temporary dewatering or pressure relief system are equally applicable to the design of permanent groundwater controsystems. The principal differences are the requirements for permanency and the need for continuouoperation. The requirements for permanent drainagsystems depend largely on the structural design anoperational requirements of the facility. Since permanent groundwater control systems must operate continuously without interruption, they should be conservatively designed and mechanically simple to avoi

the need for complicated control equipment subject tfailure and the need for operating personnel. Permanent drainage systems should include provisions foinspection, maintenance, and monitoring the behavioof the system in more detail than is usually requiredfor construction dewatering systems. Permanent systems should be conservatively designed so that satisfactory results are achieved even if there is a rise inthe groundwater level in the surrounding area, whichmay occur if water supply wells are shut down or if theefficiency of the dewatering system decreases, as mahappen if bacteria growth develops in the filter system. An example of a permanent groundwater controsystem is shown in figure 1-2.

1-2


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TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-

U. S . Ar my Co r p s o f E n g i n e e r s(Fruco & A ssociat e

Figure 1-2. Permanent groundwater control syst em.


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TM 5-818 -5/AFM 88-5, Chap 6/NAVFAC P-

CHAPTER 2

METHODS FOR DEWATERING, PRESSURE RELIEF,AND SEEPAGE CUTOFF

2 - 1 . G e n e r a l .

a. Temporary dewatering systems. Dewatering andcontrol of groundwater during construction may be ac-complished by one or a combination of methods de-scribed in the following paragraphs. The applicabilityof different methods to various types of excavations,groundwater lowering, and soil conditions is also dis-cussed in these paragraphs. Analysis and design of de-watering pressure relief and groundwater control sys-tems are described in chapter 4.

b. Permanent drainage systems. The principles and

methods of groundwater control for permanent struc-tures are similar to those to be described for construc-tion projects. A method often used for permanentgroundwater control consists of relief wells (to be dis-cussed subsequently in detail) installed beneath andadjacent to the structure, with drainage blankets be-neath and surrounding the structure at locations belowthe water table as shown previously in figure 1-2. Thewater entering the wells and drainage blanket iscarried through collector pipes to sumps, pits, or man-holes, from which it is pumped or drained. Permanentgroundwater control may include a combination ofwells, cutoffs, and vertical sand drains. Additional in-

formation on the design of permanent drainage sys-tems for buildings may be found in TM 5-818-1/AFM88-3, Chapter 7; TM 5-818-4/AFM 88-5, Chapter 5;an d TM 5-818-6/AFM 88-32. (See a pp. A for ref-erences.)

2-2 . Types and sourc e o f seepage.

a. Types of seepage flow. Types of seepage flow aretabulated below:

Type of flow Flow characterist ics

Artesian Seepage through the previous aquifer is confinedbetween two or more impervious strata, andthe piezometric head within the previousaquifer is above the top of the pervious aqui-fer (fig. 1-2).

Gra vit y The surface of th e water ta ble is below th e top of

the pervious aquifer (fig. 1-2).

For some soil configurations and drawdowns, the flow

may be artesian in some areas and gravity in otherareas, such as near wells or sumps where drawdownoccurs. The type of seepage flow to a dewatering sys-tem can be determined from a study of the ground-

water table and soil formations in the area anddrawdown required to dewater the excavation.

b. Source of seepage flow. The source and distL* to the source of seepage or radius of influenmust be estimated or determined prior to designinevaluating a dewatering or drainage system.

(1) The source of seepage depends on the logical features of the area, the existence of adjastreams or bodies of water, the perviousness ofsand formation, recharge, amount of drawdown,duration of pumping. The source of seepage may nearby stream or lake, the aquifer being drained

both an adjacent body of water and storage inaquifer.

(2) Where the site is not adjacent to a rivelake, the source of seepage will be from storage information being drained and recharged from raiover the area. Where this condition exists, flow toarea being dewatered can be computed on the assution that the source of seepage is circular and at atance R. The radius of influence R is defined asradius of the circle beyond which pumping of awatering system has no significant effect on the onal groundwater level or piezometric surface (see 4-2a(3)).

(3) Where an excavation is located close to a ror shoreline in conta ct with th e aquifer to bewatered, the distance to the effective source of seeL, if less than R/2, may be considered as being appmately the near bank of the river; if the distance toriverbank or shoreline is equal to about R/2, or grethe source of seepage can be considered a circle wradius somewhat less tha n R.

(4) Where a line or two parallel lines of *wellinstalled in an area not close to a river, the sourcseepage may be considered as a line paralleling theof wells.

2-3. Sumps and d i tches.

a. Open excavations. An elementary dewateprocedure involves installation of ditches, Fredrains, and sumps within an excavation, from wwater entering the excavation can be pumped 2-1). This method of dewatering generally should

*For convenience, symbols and unusual abbreviations are

in the Notation (app B).


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(Modi fied f rom Foundat ion Engineering, G. A. Leonards. ed.. 1962, McGraw -Hill Book

Company. Used with permission ofMcGraw -Hil l Book Com pany .)

Figure 2-1. Dew atering open excava t ion by dit ch and sump.

be considered where the groundwater head must belowered more than a few feet, as seepage into the ex-cavation may impair the stability of excavation slopesor have a detrimental effect on the integrity of the

foundation soils. Filter blankets or drains may be in-cluded in a sump and ditch system to overcome minorraveling and facilitate collection of seepage. Dis-advantages of a sump dewatering system are slownessin drainage of the slopes; potentially wet conditionsduring excavation and backfilling, which may impedeconstruction and adversely affect the subgrade soil;space required in the bottom of the excavation fordrains, ditches, sumps, and pumps; and the frequentlack of workmen who are skilled in the proper con-

struction or operation of sumps.

b. Cofferdams. A common m eth od of excavatin gbelow the groundwater table in confined areas is to

drive wood or steel sheet piling below subgrade ele-vation, install bracing, excavate the earth, and pumpout any seepage that enters the cofferdammed area.

(1) Dewatering a sheeted excavation with sumpsand ditches is subject to the same limitations and seri-ous disadvantages as for open excavations. However,the danger of hydraulic heave in the bottom of an ex-cavation in sand may be reduced where the sheetingcan be driven into an underlying impermeable stra-tum, thereby reducing the seepage into the bottom ofthe excavation.

(2) Excavations below the water table can some-times be successfully made using sheeting and sumppumping. However, the sheeting and bracing must bedesigned for hydrostatic pressures and reduced toesupport caused by upward seepage forces. Coveringthe bottom of the excavation with an inverted sandand gravel filter blanket will facilitate construction

and pumping out seepage water.

2-4. Wellpoint systems. Wellpoint systems area commonly used dewatering method as they are appli-

2-2

cable to a wide range of excavations and groundwaterconditions.

a. Conventional wellpoint systems. A conventionalwellpoint system consists of one or more stages of

wellpoints having 1 or 2-inch-diameter riser pipes,installed in a line or ring at spacings between about 3and 10 feet, with the risers connected to a commonheader pumped with one or more wellpoint pumps.Wellpoints are small well screens composed of eitherbrass or stainless steel mesh, slotted brass or plasticpipe, or trapezoidal-shaped wire wrapped on rods toform a screen. They generally range in size from 2 to 4inches in diameter and 2 to 5 feet in length and areconstructed with either closed ends or self-jetting tipsas shown in figure 2-2. They may or may not be sur-rounded with a filter depending upon the type of soildrained. Wellpoint screens and riser pipes may be as

large as 6 inches and a s long as 25 feet in certa in situa-tions. A wellpoint pump uses a combined vacuum anda centrifugal pump connected to the header to produce

a vacuum in the system and to pump out the waterthat drains to the wellpoints. One or more sup-plementary vacuum pumps may be added to the mainpumps where additional air handling capacity is re-quired or desirable. Generally, a stage of wellpoints(wellpoints connected to a header at a common eleva-tion) is capable of lowering the groundwater tableabout 15 feet; lowering the groundwater more than 15feet generally requires a multistage installation ofwellpoints as shown in figures 2-3 and 2-4. A well-point system is usually the most practical method fordewatering where the site is accessible and where theexcavation and water-bearing strata to be drained arenot too deep. For large or deep excavations where thedepth of excavation is more than 30 or 40 feet, orwhere artesian pressure in a deep aquifer must be re-duced, it may be more practical to use eductor-typewellpoints or deep wells (discussed subsequently) withturbine or submersible pumps, using wellpoints as a


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TM 5-818-5/AFM 88-5, Chap 6/NAVFAC

Figure 2-2. Self-jetting w ellpoint.


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(From Foundation Engineering, G. A. Leonards, ed., 1962. McGraw-Hi llBook Com pany. Used w it h permission ofMcGraw -Hi ll Boo k Company.)

Figure 2-3. Use of wellpoints where submergence is small.

supplementary method of dewatering if needed. Well-points are more suitable than deep wells where thesubmergence available for the well screens is small(fig. 2-3) and close spacing is required to interceptseepage.

(D10 0.05 millimetre) with a low coefficient of per-

b. Vacuum wellpoint systems. Silts and sandy silts

meability (k = 0.1 x 10-4

to 10 x 10-4

centimetresper second) cannot be drained successfully by gravitymethods, but such soils can often be stabilized by avacuum wellpoint system. A vacuum wellpoint systemis essentially a conventional well system in which apartial vacuum is maintained in the sand filter aroundthe wellpoint and riser pipe (fig 2-5). This vacuum willincrease the hydraulic gradient producing flow to thewellpoints and will improve drainage and stabilizationof the surrounding soil. For a wellpoint system, the netvacuum at the wellpoint and in the filter is the vacuumin the header pipe minus the lift or length of the riserpipe. Therefore, relatively little vacuum effect can be

obtained with a wellpoint system if the lift is morethan about 15 feet. If there is much air loss, it may benecessary to provide additional vacuum pumps to en-sure maintaining the maximum vacuum in the filtercolumn. The required capacity of the water pump is, ofcourse, small.

c. Jet-eductor wellpoint systems. Another type ofdewatering system is the jet-eductor wellpoint system(fig. 2-6), which consists of an eductor installed in asmall diameter well or a wellpoint screen attached to a

jet -eductor in st a lled a t the en d of dou ble r iser pipes, apressure pipe to supply the jet-eductor and anotherpipe for the discharge from the eductor pump. Eductor

wellpoints may also be pumped with a pressure pipewithin a larger return pipe. This type of system hasthe advantage over a conventional wellpoint system ofbeing able to lower the water table as much as 100 feetfrom the top of the excavation. Jet-eductor wellpointsare installed in the same manner as conventional well-points, generally with a filter as required by the foun-dation soils. The two riser pipes are connected to sep-arate headers, one to supply water under pressure tothe eductors and the other for return of flow from thewellpoints and eductors (fig. 2-6). Jet-eductor well-point systems are most advantageously used to dewa-ter deep excavations where the volume of water to be

pumped is relatively small because of the low permea-bility of the aquifer.

2-5. Deep-wel l systems.

a. Deep wells can be used to dewater pervious sandor rock formations or to relieve artesian pressure be-

(From Soils Mechanics in Engineering Practice, by K. Terzaghi and R. B. Peck, 1948,

Wiley & Sons, Inc. Used with permission of Wiley & Sons, Inc.)

Figure 2-4. Drainage of an open deep cut by means of a multistage wellpoint system.

2-4


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Note: Vacuum in header = 25 ft; vacuumin filter and soil in vicinity of well-point = approximately 10 ft.

(From Foundation Engineering, G. A. Leonards, ed.,

1962, M cGraw-H ill Book Company. Used wit h permission

of McGraw-Hill Book Company.)

Figure 2-5. Vacuum wellpoint system.

neath an excavation. They are particularly suited fordewatering large excavations requiring high rates ofpumping, and for dewatering deep excavations fordams, tunnels, locks, powerhouses, and shafts. Excava-tions and shafts as deep as 300 feet can be dewateredby pumping from deep wells with turbine or submersi-ble pumps. The principal advantages of deep wells are

tha t t hey can be installed ar ound the periphery of anexcavation and thus leave the construction area unem-cumbered by dewatering equipment, as shown in fig-ure 2-7, and the excavation can be predrained for itsfull depth.

b. Deep wells for dewatering are similar in type andconstruction to commercial water wells. They com-monly have a screen with a diameter of 6 to 24 incheswith lengths up to 300 feet and are generally installedwith a filter around the screen to prevent the infiltra-tion of foundation materials into the well and to im-prove the yield of the well.

c. Deep wells may be used in conjunction with a vac-uum system to dewater small, deep excavations fortunnels, shafts, or caissons sunk in relatively fine-grained or stratified pervious soils or rock below thegroundwater table. The addition of a vacuum to thewell screen and filter will increase the hydraulic grad-ient to the well and will create a vacuum within thesurrounding soil that will prevent or minimize seepagefrom perched water into the excavation. Installationsof this type, as shown in figure 2-8, require adequate

TM 5-815-5/AFM 88-5, Chap 6/NAVFAC P

vacuum capacity to ensure efficient operations osystem.

2-6. Ver t i ca l sand dra ins . Where a strasemipervious stratum with a low vertical permeaoverlies a pervious stratum and the groundwaterhas to be lowered in both strata, the water table iupper stratum can be lowered by means of sand d

as shown in figures 2-9. If properly designed anstalled, sand drains will intercept seepage in the stratum and conduct it into the lower, more permstratum being dewatered with wells or wellpoSand drains consist of a column of pervious placed in a cased hole, either driven or drilled ththe soil, with the casing subsequently removed. Thpacity of sand drains can be significantly increasinstallation of a slotted 1% or 2-inch pipe insidsand drain to conduct the water down to the morevious stratum.

2-7. E lect ra-osmosis. Some soils, such as

clayey silts, and clayey silty sands, at times canndewatered by pumping from wellpoints or wells.

ever, such soils can be drained by wells or wellpcombined with a flow of direct electric cu

through the soil toward the wells. Creation of draulic gradient by pumping from the wells or points with the passage of direct electrical cuthrough the soil causes the water contained in thvoids to migrate from the positive electrode (anodthe negative electrode (cathode). By making the ode a wellpoint, the water that migrates to the cacan be removed by either vacuum or eductor pum(fig. 2-10).

2-8. Cuto f f s . Cutoff curtains can be used to stminimize seepage into an excavation where the can be installed down to an impervious formaSuch cutoffs can be const ru cted by driving steel piling, grouting existing soil with cement or chegrout, excavating by means of a slurry trenchbackfilling with a plastic mix of bentonite andsostalling a concrete wall, possibly consisting of ovping shafts, or freezing. However, groundwater wth e ar ea en closed by a cutoff cur ta in, or leathr ough or under such a curt ain, will have pumped out with a well or wellpoint system as s

in figure 2-11.a. Cement and chemical grout curtains. A c

around an excavation in coarse sand and gravel oous rock can be created by injecting cement or cical grout into th e voids of th e soil. For groutin g effective, the voids in the rock or soil must be

enough to accept the grout, and the holes must be enough together so that a continuous grout curtaobtained. The type of grout that can be used depupon the size of voids in the sand and gravel or ro


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U. S Ar my Co r p s o f E n g i n e e r s

Figure 2-6. Jet -eductor w ellpoin t sy st em for dew at ering a sha ft .

2-6


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U. S . Ar my C o r p s o f E n g i n e e r s

Figure 2-7Deep-w ell sy st em for dew at ering an excavat ion in sand.

be grouted. Grouts commonly used for this purpose areportland cement and water; cement, bentonite, an ad-mixture to reduce surface tension, and water; silicagels; or a commercial product. Generally, grouting offine or medium sand is not, very effective for blockingseepage. Single lines of grout holes are also generallyineffective as seepage cutoffs; three or more lines aregenerally required Detailed information on chemicalgrouting and grouting methods is contained in TM5-818-6/AFM 88-32 and NAVFAC DM 7.3.

b. Slurry walls. A cutoff to prevent or minimizeseepage into an excavation can also be formed by dig-ging a narrow trench around the area to be excavatedand backfilling it with an impervious soil. Such atrench can be constructed in almost any soil, eitherabove or below the water table, by keeping the trench

filled with a bentonite mud slurry and backfilling itwith a suitable impervious soil. Generally, the trenchis backfilled with a well-graded clayey sand gravelmixed with bentonite slurry. Details regarding designand construction of a slurry cutoff wall are given inparagraphs 4-9g(2) and 5-5b.

c. Concrete walls. Techniques have been developedfor constructing concrete cutoff walls by overlappingcylinders and also as continuous walls excavated and

concreted in sections. These walls can be reinfoand are sometimes incorporated as a permanentof a structure.

d. Steel sheet piling. The effectiveness of sheeting driven around an excavation to reduce seepagpends upon the perviousness of the soil, the tighof the interlocks, and the length of the seepage Some seepage through the interlocks should be exed. When constructing small structures in open wit may be desirable to drive steel sheet piling arthe structure, excavate the soil underwater, and tremie in a concrete seal. The concrete tremiemust withstand uplift pressures, or pressure rmeasures must be used. In restricted areas, it manecessary to use a combination of sheeting and brwith wells or wellpoints installed just inside or ou

of the sheeting. Sheet piling is not very effectivblocking seepage where boulders or other hardstructions may be encountered because of drivingof interlock.

e. Freezing. Seepage into a excavation or shaftbe prevented by freezing the surrounding soil. Hever, freezing is expensive and requires expert deinstallation, and operation. If the soil around the vation is not completely frozen, seepage can cause


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TM 5-818-5/AFM 88-5, Chap 6 /NAV FAC P-418


Figure 2-8. Deep w ells w it h auxi lia ry va cuum sy st em for dew at ering a sha ft in st rat ifi ed materials .

id enlargement of a fault (unfrozen zone) with conse-quent serious trouble, which is difficult to remedy.

2-9. Summary of groundwater contro lmethods. A brief summary of groundwater controlmethods discussed in this section is given in table 2-1.

2-10. Select ion of dew ater ing system .

a. General. The method most suitable for dewater-ing an excavation depends upon the location, type,size, and depth of the excavation; thickness, stratifica-tion, and permeability of the foundation soils belowthe water table into which the excavation extends or is

2-8


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Figure 2-9. Sand drains for dewatering a slope.

underlain; potential da mage r esulting from failure of (3) Labor requirements.th e dewatering system; and th e cost of insta llation and (4) Dura tion of required pumping.operation of the system. The cost of a dewatering The rapid development of slurry cutoff walls has mamethod or system will depend upon: this method of groundwater control, combined with

(1) Type, size, and pumping requirements of proj- certain amount of pumping, a practical and econo

ect. ical alternative for some projects, especially tho(2) Type and availability of power. where pumping costs would otherwise be great.

U. S . Ar my Co r p s o f E n g i n e e r s

Figure 2-10. Electra -osmot ic w ellpoin t sy st em for st abil iz ing an excavat ion slo pe.


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TMTM 5-815-5/AFM 88-5, Chap 6/NAVFAC P-418


Figure 2-11. Grout curtain or cutoff trench around an excavat ion.

b. Factors controlling selection. Where foundationsmust be constructed on soils below the groundwaterlevel, it will generally be necessary to dewater the ex-cavation by means of a deep-well or wellpoint systemrather than trenching and sump pumping. Dewateringis usually essential to prevent damage to foundationsoils caused by equipment operations and sloughing orsliding in of the side slopes. Conventional deep-welland wellpoint systems designed and installed by com-panies specializing in this work are generally satisfac-tory, and detailed designs need not be prepared by theengineer. However, where unusual pressure relief or

dewatering requirements must be achieved, the engi-neer should make detailed analyses and specify the de-watering system or detailed results to be achieved inthe contract documents. Where unusual equipmentand procedures are required to achieve desired results,they should be described in detail in the contract docu-ments. The user of this manual is referred toparagraphs 6b, 14b, and 2fof Appendix III, TM5-818-4/AFM 88-5, Chapter 5, for additional discus-sions of dewat ering requirement s a nd cont ract speci-fications. Major factors affecting selection of dewater-ing and groundwater control systems are discussed inthe following paragraphs.

(1) Type ofexcavation. Small open excavations, orexcavations where the depth of water table lowering issmall, can generally be dewatering most economicallyand safely by means of a conventional wellpoint sys-tem. If the excavation requires that the water table orartesian pressure be lowered more than 20 or 30 feet, asystem of jet-eductor type wellpoints or deep wellsmay be more suitable. Either wellpoints, deep wells, ora combination thereof can be used to dewater an exca-

2-10

vation surrounded by a cofferdam. Excavations fordeep shafts, caissons, or tunnels that penetrate stratified pervious soil or rock can generally best be dewatered with either a deep-well system (with or withoutan auxiliary vacuum) or a jet-eductor wellpoint systemdepending on the soil formation and required rate ofpumping, but slurry cutoff walls and freezing shouldbe evaluated as alternative procedures. Other factorsrelating to selection of a dewatering system are inter-ference of the system with construction operations,space available for the system, sequence of construc-tion operations, durations of dewatering, and cost of

the installation and its operation. Where groundwaterlowering is expensive and where cofferdams are re-quired, caisson construction may be more economical.Caissons are being used more frequently, even forsmall structures.

(2) Geologic and soil conditions. The geologic andsoil formations at a site may dictate the type of dewa-tering or drainage system. If the soil below the watertable is a deep, more or less homogeneous, free-drain-ing sand, it can be effectively dewatered with either aconventional well or wellpoint system. If, on the otherhand, the formation is highly stratified, or the saturat-ed soil to be dewatered is underlain by an imperviousstratum of clay, shale, or rock, wellpoints or wells onrelatively close centers may be required. Where soiland groundwater conditions require only the relief ofartesian pressure beneath an excavation, this pressurerelief can be accomplished by means of relatively fewdeep wells or jet-eductor wellpoints installed aroundand at the top of the excavation.

(a) If an aquifer is thick so that the penetrationof a system of wellpoints is small, the small ratio of


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2-11


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TM 5-818-5 /AFM 88 -5 , Cha p 6 /NAV FAC P-41 8

screen length to aquifer th ickness ma y result in rela-tively little drawdown within the excavation, eventhough the water table is lowered 15 to 20 feet at theline of wellpoints. For deep aquifers, a deep-well sys-tem will generally be more applicable, or the length ofthe wellpoints should be increased and the wellpointsset deep and surrounded with a high-capacity filter.On th e other h and, if the aqu ifer is relatively thin or

stratified wellpoints may be best suited to the situa-tion.

(b) The perviousness and drainability of a soil orrock may dictate the general type of a dewatering sys-tem to be used for a project. A guide for the selectionof a dewatering system related to the grain size of soilsis presented in figure 2-12. Some gravels and rock for-mations may be so permeable that a barrier to flow,such as a slurry trench, grout curtain, sheet pile cutoff,or freezing, may be necessary to reduce the quantity offlow to the dewatering system to reasonable propor-tions. Clean, free-draining sands can be effectively de-watered by wells or wellpoints. Drainage of sandy silts

and silts will usually require the application of addi-tional vacuum to well or wellpoint dewatering sys-tems, or possibly the use of the electroosmotic methodof dewatering where soils are silty or clayey. However,where thin sand layers are present, special require-ments may be unnecessary. Electroosmosis should nev-er be used until a test of a conventional system of well-points, wells with vacuum, or jet-eductor wellpointshas been attempted.

(3) Depth of groundwater lowering. The magni-tude of the drawdown required is an important con-sideration in selecting a dewatering system. If thedrawdown required is large, deep wells or jet-eductor

wellpoints may be the best because of their ability toachieve large drawdowns from the top of an excava-tion, whereas many stages of wellpoints would be re-quired to accomplish the same drawdown. Deep wellscan be used for a wide range of flows by selectingpumps of appropriate size, but jet-eductor wellpointsare not as flexible. Since jet-eductor pumps are rela-tively inefficient, they are most applicable where wellflows are small as in silty to fine sand formations.

(4) Reliability requirements. The reliability ofgroundwater control required for a project will have a

significant bearing on the design of the dewateringpumps, power supply, and standby power and equip-ment. If the dewatering problem is one involving therelief of artesian pressure to prevent a blowup of thebottom of an excavation, the rate of water table re-bound, in event of failure of the system, may be ex-tremely rapid. Such a situation may influence the typeof pressure relief system selected and require inclusion

of standby equipment with automatic power transferand starting equipment.

(5) Required rate of pumping. The rat e of pum p

2-12

ing required to dewater an excavation may vary from5 to 50,000 gallons per minute or more. Thus, flow to a

draina ge system will have a n importan t effect on t hedesign and selection of the wells, pumps, and pipingsystem. Turbine or submersible pumps for pumpingdeep wells are available in sizes from 3 to 14 inches

with capacities ranging from 5 to 5000 gallons perminute at heads up to 500 feet. Wellpoint pumps are

available in sizes from 6 to 12 inches with capacitiesranging from 500 to 5000 gallons per minute depend-ing upon vacuum and discharge heads. Jet-eductorpumps are a vailable tha t will pump from 3 to 20 gal-lons per minute for lifts up to 100 feet. Where soil con-ditions dictate the use of vacuum or electroosmoticwellpoint systems, the rate of pumpage will be verysmall. The rate of pumpage will depend largely on thedistance to the effective source of seepage, amount ofdrawdown or pressure relief required, and thicknessand perviousness of the aquifer through which theflow is occurring.

(6) Intermittent pumping. Pumping labor costs can

occasionally be materially reduced by pumping a dewa-tering system only one or two shifts per day. Whilethis operation is not generally possible, nor advan-tageous, it can be economical where the dewateredarea is large; subsoils below subgrade elevation aredeep, pervious, and homogeneous; and the pumpingplant is oversize. Where these conditions exist, thepumping system can be operated to produce an abnor-mally large drawdown during one or two shifts. The

recovery during nonpumping shifts raises the ground-water level, but not sufficiently to approach subgradeelevation. This type of pumping plant operation

should be permitted only where adequate piezometers

have been installed and are read frequently.(7) Effect of groundwater lowering on adjacent

structures and wells. Lowering the groundwater tableincreases the load on foundation soils below the ori-ginal groundwater table. As most soils consolidateupon application of additional load, structures locatedwithin the radius of influence of a dewatering systemmay settle. The possibility of such settlement shouldbe investigated before a dewatering system is de-signed. Establishing reference hubs on adjacent struc-

tures prior to the start of dewatering operations willpermit measuring any settlement that occurs during

dewatering, and provides a warning of possible dis-tress or failure of a structure that might be affected.Recharge of the groundwater, as illustrated in figure.2-13, may be necessary to reduce or eliminate distress

to adjacent structures, or it may be necessary to usepositive cutoffs to avoid lowering the groundwaterlevel outside of an excavation. Positive cutoffs includesoil freezing and slurry cutoff techniques. Observa-tions should be made of the water level in nearby wellsbefore and during dewatering to determine any effect


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2-1

3


26/161



Figure 2-13. Recharge of groundwater toprevent settlement of a buildin g as a result of dewatering operations

of dewatering. This information will provide a basisfor evaluating any claims that may be made.

(8) Dewatering versus cutoffs and other proce-dures. While dewatering is generally the most ex-peditious and economical procedure for controllingwater, it is sometimes possible to excavate more eco-nomically in the wet inside of a cofferdam or caissonand then seal the bottom of the excavation with atremie seal, or use a combination of slurry wall orother type of cutoff and dewatering. Where subsurfaceconstruction extends to a considerable depth or wherehigh uplift pressures or large flows are anticipated, itmay occasionally be advantageous to: substitute acaisson for a conventional foundation and sink it to the

design elevation without lowering the groundwaterlevel; use a combination of concrete cutoff walls con-structed in slurry-supported trenches, and a tremiedconcrete foundation slab, in which case the cutoffwalls may serve also as part of the completed struc-

ture; use large rotary drilling machines for excavatingpurposes, without lowering the groundwater level; oruse freezing techniques. Cofferdams, caissons, and cut-off walls may have difficulty penetrating formationscontaining numerous boulders. Foundation designs re-quiring compressed air will rarely be needed, although

compressed air may be economical or necessary forsome tunnel construction work.

2-14


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CHAPTER 3

GEOLOGIC, SOIL, AND GROUNDWATER INVESTIGATIONS

3-1. General. Before selecting or designing a sys- 3-2. Geologic and soi l condi t ions. An utem for dewatering an excavation, it is necessary to derstanding of the geology of the area is necessaryconsider or investigate subsurface soils, groundwater plan any investigation of subsurface soil conditioconditions, power availability, and other factors as Information obtained from the geologic and soil listed in table 3-1. The extent and detail of these investigations as outlined in TM 5-818-1/AFM 88vestigations will depend on the effect groundwater Chapter 7 or NAVFAC DM7.1, should be usedand hydrostat ic pressur e will have on the const ruction evaluating a dewatering or groundwat er control prof th e project an d th e complexity of th e dewat ering lem. Depending on the completeness of inform atproblem. available, it may be possible to postulate the gene

Table 3-1. Preliminary Inv estigations

Item I n v e s t i g a t e Reference

G e o l o g i c a n d s o i l T y p e , s t r a t i f i c a t i o n , a n d Para 3-2; TM 5-818-1/

condi t ions th ickness of so i l involved AFM 88-3, Chapter 7

in excava t ion and NAVFAC DM7.1

dewater ing

C r i t i c a l i t y R e l i a b i l i t y o f powe r sys t e m ,damage to excavation or

founda t ion in event of f a i l u r e , ra te of rebound,e t c .

Groundwater or Groundwater table or hydro- Para 2-3 and 3-3

p ie z om e t r i c s t a t i c p r e s su r e in a r e a a ndp r e ss u r e i t s s o u r c e . Var ia t ion wi thch a r a ct e r i s t i cs r i ve r s t a g e , season of year ,

e t c . Type of seepage (ar te-s i a n , g r a v i ty , c om bine d ) .C he m ic a l c ha r a c te r i s t i c sand tempera ture of groundwater .

Permeabi l i ty Determine permeability from

v i s u a l , f i e l d , o r l a b o r a -t o r y t e s t s , p r e f e r a b l y b y

f i e l d t e s t s .

Power A va i l a b i l i t y , r e l i a b i l i t y ,and capacity of power at

s i t e .

D e g r e e o f p o s s i b l e R a i n f a l l i n a r e a . Runoff

f lood ing c h a r a c t e r i s t i c s . High-

wate r leve ls in nea rbybodies of water .

Para 3-4; Appendix C

Para 3-5

Para 3-6


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characteristics and stratification of the soil and rockformations in the area. With this information and thesize of and depth of the excavation to be dewatered,the remainder of the geologic and soil investigationscan be planned. Seismic or resistivity surveys (as wellas logged core and soil borings) may be useful in deline-ating the thickness and boundaries of major geologicand soil formations and will often show irregularities

in the geologic profile that might otherwise go unde-tected (fig. 3-1).

a.Borings.(1) A thorough knowledge of the extent, thick-

ness, stratification, and seepage characteristics of thesubsurface soil or rock adjacent to and beneath an ex-cavation is required to analyze and design a dewater-ing system. These factors are generally determinedduring the normal field exploration that is required formost structures. Samples of the soil or rock formationobtained from these borings should be suitable forclassifying and testing for grain size and permeability,if the complexity of the project warrants. All of the in-

formation gathered in the investigation should be pre-sented on soil or geologic profiles of the site. For large,complex dewatering or drainage projects, it may be de-sirable to construct a three-dimensional model of col-ored pegs or transparent plastic to depict the differentgeologic or soil formations at the site.

(2) The depth and spacing of borings (and sam-ples) depend on the character of the materials and onthe type and configuration of the formations or depos-its as discussed in TM 5-818-1/AFM 88-3, Chapter 7.Care must be taken that the borings accomplish thefollowing:

(a) Completely penetrate and sample allaquifers that may have a bearing on dewatering an ex-cavation and controlling artesian pressures.

(b) Identify (and sample) all soils or rocks thatwould affect or be affected by seepage or hydrostaticpressure.

(c) Delineate the soil stratification.(d) Reveal any significant variation in soil and

rock conditions that would have a bearing on seepageflow, location and depth of wells, or depth of cutoff.Continuous wash or auger boring samples are not con-sidered satisfactory for dewatering exploration as thefines tend to be washed out, thereby changing thecharacter of the soil.

b. Rock coring. Rock samples, to be meaningful forgroundwater studies, should be intact samples ob-tained by core drilling. Although identification ofrocks can be made from drill cuttings, the determina-tion of characteristics of rock formations, such as fre-quency, orientation, and width of joints or fractures,that affect groundwater flow requires core samples.

The percent of core recovery and any voids or loss ofdrill water encountered while core drilling should berecorded. The approximate permeability of rock stratacan be measured by making pressure or pumping testsof the various strata encountered. Without pressure orpumping tests, important details of a rock formationcan remain undetected, even with extensive boringand sampling. For instance, open channels or joints ina rock formation can have a significant influence onthe permeability of the formation, yet core samplesmay not clearly indicate these features where the corerecovery is less than 100 percent.


Figure 3-1. Geologic profile developed from geophysical explorations.

3-2


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c. Soil testing.(1) All soil and rock samples should be carefully

classified, noting particularly those characteristicsthat have a bearing on the perviousness and stratifica-tion of the formation. Soil samples should be classifiedin accordance with the Unified Soil Classification Sys-tem described in MIL-STD-619B. Particular atten-tion should be given to the existence and amount of

fines (material passing the No. 200 sieve) in sand sam-ples, as such have a pronounced effect on the perme-ability of the sand. Sieve analyses should be made onrepresentative samples of the aquifer sands to deter-mine their gradation and effective grain size D

10. The

D10

size may be used to estimate the coefficient ofpermeabililty k . The grada tion is required t o designfilters for wells, wellpoints, or permanent drainagesystems to be installed in the formation. Correlationsbetween k and D

10are presented in paragraph 3-4.

(2) Laboratory tests depicted in figure 3-2 can beused to determine the approximate coefficient ofpermeability of a soil or rock sample; however, perme-

abilities obtained from such tests may have little rela-tion to field permeability even though conductedunder controlled conditions. When samples of sand aredistributed and repacked, the porosity and orientationof the grains are significantly changed, with resultingmodification of the permeability. Also, any air en-trapped in the sand sample during testing will signifi-cantly reduce its permeability. Laboratory tests on

samples of sand that have been segregated or taminated with drilling mud during sampling options do not give reliable results. In additionpermeability of remolded samples of sand is usuconsiderably less than the horizontal permeabilitof a formation, which is generally the more signifk factor pertaining to seepage flow to a drainaget em.

(3) Where a nonequilibrium type of pumping(described in app C) is to be conducted, it is neceto estimate the specific yieldS y of the formatwhich is t he volume of water tha t is free to dra inof a material under natural conditions, in percenof total volume. It can be determined in the laboraby:

(a) Saturating the sample and allowing drain. Care must be taken to assure that capilstresses on t he su rface of the sample do not causincorrect conclusion regarding the drainage.

(b) Estimating Syfrom the soil type and D

10

of the soil and empirical correlations based on

and laboratory tests. The specific yield can be puted from a drainage test as follows:

S y =100V

y

Vwhere

Vy= volume of water drained from sample

V = gross volume of sampleThe specific yield can be estimated from the soil

(From Ground Water Hydrology by D. K. Todd, 1959, Wiley & Sons,

Inc. Used with permission of Wiley & Sons, Inc.)

Figure 3-2. Permeameters: (a) constant head and(b) falling head.


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(or D10) and the relation given in figure 3-3 or table3-2.

3-3. Groundwater character is t ics .

a. An investigation of groundwater at a site shouldinclude a study of the source of groundwater thatwould flow to the dewatering or drainage system (para2-2) and determination of the elevation of the water

table and its variation with changes in river or tidestages, seasonal effects, and pumping from nearby wa-ter wells. Groundwater and artesian pressure levels ata construction site ar e best det ermined from piezom-eters installed in the stratum that may require dewa-tering. Piezometers in pervious soils may be commer-cial wellpoints, installed with or without a filter (para4-6c) as the gradation of foundation material requires.Piezometers in fine-grained soils with a low perme-ability, such as silt, generally consist of porous plasticor ceramic tips installed within a filter and attached toa relatively small diameter riser pipe.

b. The groundwater regime should be observed foran extended period of time to establish variations inlevel likely to occur during the construction or opera-tion of a project. General information regarding thegroundwater table and river or tide stages in the areais often available from public agencies and may serveas a basis of establishing general water levels. Specificconditions at a site can then be predicted by correlat-ing the long-term recorded observations in the areawith more detailed short-term observations at the site.

c. The chemical composition of the groundwater isof concern, because some groundwaters are highly cor-rosive to metal screens, pipes, and pumps, or may con-

tain dissolved meta ls or carbona tes t ha t will form in-

crustations in the wells or filters and, with time, cauclogging and -reduced efficiency of the dewatering odrainage system. Indicators of corrosive and incrusing waters are given in table 3-3. (Standard methodfor determining the chemical compositions of grounwater are available from th e American Pu blic HealAssociation, Washington, DC

d. Changes in the temperature of the groundwat

will result in minor variations of the quantity of wateflowing to a dewatering system. The change in viscosty associated with temperature changes will result in change in flow of about 1.5 percent for each 1Fahrenheit of temperature change in the water. Onlarge variations in temperature need be considered

design because the accuracy of determining otheparameters does not warrant excessive refinement.

3-4. Permeabi l i ty of perv ious s t rata. Thrate at which water can be pumped from a dewaterinsystem is directly proportional to the coefficient permeability of the formation being dewatered; thu

this parameter should be determined reasonably accrately prior to the design of any drainage systemMethods that can be used to estimate or determine thpermeability of a pervious aquifer are presented in thfollowing paragraphs.

a. V isual classification. The simplest approximamethod for estimating the permeability of sand is bvisual examination and classification, and comparisowith sands of known permeability. An approximatioof the permeability of clean sands can be obtainefrom table 3-4.

b. Empirical relation between D10 and k. The pe

meability of a clean sand can be estimated from em

(From Ground Water Hydrology by D. K. Todd, 1959, Wiley & Sons,

Inc. Used with permission of Wiley & Sons, Inc.)

Figure 3-3. Specific yield of wat er-bearing sands versus D10, South Coastal Basin, California.

3-4


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Table 3-2. Specific Yield of W ater-Bearing Deposits in Sacramento V alley, California

Mater ia l

S p e c i f i cYield

pe r c e n t

Gravel 25

Sand, including sand and gravel, and gravel and sand 20

F ine sa nd , ha r d sa nd , t i gh t s a nd , s a nds tone , a nd r e l a t e dde pos i t s 10

Clay and grave l , g rave l and c lay , cemented grave l , andr e l a t e d d e p o s i t s 5

C l a y , s i l t , s a n d y cl a y , l a va r o ck , a n d r e la t e d fi n e -gra ined depos i t s 3

(From Ground W ater Hy drology by D. K. Todd, 1959, Wiley & Sons,Inc. Used w it h perm ission of W iley & Sons, Inc.)

Table 3-3. Indicators of Corrosive and Incrusting Wat ers

I n d i c a t o r s o f Corrosive Water

I n d i c a t o r s o f Inc rus t ing Wate r

1. A pH less than 7 1. A pH greater than 7

2. Dissolved oxygen in excess of 2 ppm 2 . T o t a l i r o n ( F e ) i n e x c e s sof 2 ppm

3. H yd rogen s u lfid e (H 2S) in excess of 3. Total manganese (Mn) in

1 ppm, detected by a rotten eggexcess of 1 ppm in con-

odorju nct ion wit h a h igh pHand the presence of oxygen

4. Tota l d isso lved so l ids in excess of 4 . T o t a l c a r b o n a t e h a r d n e s s

1 ,000 ppm indica tes an ab i l i ty to in excess of 300 ppm

c onduc t e l e c t r i c c u r r e n t g r e a te nough to c a use s e r ious e l e c t r o -

l y t i c c o r r o s i o n

5. Carbon d ioxide (CO 2) in excess of

50 ppm

6. Chlorides (Cl) in excess of 500 ppm

(Courtesy of UOP Johnson Division)


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Table 3-4. Approximat e Coef ficient of Perm eabi li t y for Variou s Sand s

Type of Sand (UnifiedS o i l C la s s i f i c a t ion S ys te m )

Coef f ic ien t of Permeabi l i ty k

x 10-4

cm/sec x 10- 4

f t / m i n

S a ndy s i l t 5-20 10-40

Si l ty sa nd 20-50 40-100

Very fine sand 50-200 100-400

Fine sand 200-500 400-1,000Fine to medium sand 500-1,000 l , 0 0 0 - 2 , 0 0 0

Medium sand 1,000-1,500 2,000-3,000Medium to coarse sand 1,500 -2,000 3,000-4,000

Coarse sand and gravel 2,000-5,000 4,000-10 ,000

U. S. Army Corps of Engineers

pirical relations between D 10 and k (fig. 3-4), whichwere developed from laboratory and field pumping

tests for sands in the Mississippi and Arkansas Rivervalleys. An investigation of the permeability of filtersands revealed that the permeability of clean, rela-tively uniform, remolded sand could be estimated fromthe empirical relation:

wherek = C (D

10)

2

(3-2)

C

k = coefficient of permeability, centimetres persecond100 (may vary from 40 to 150)

D 10 = effective grain size, centimetresEmpirical relations between D

10and k are only approx-

imate and should be used with reservation unt il a cor-

relation based on local experience is available.c. Field pumping tests. Field pumping tests are the

most reliable procedure for determining the in situpermeability of a water-bearing formation. For largedewatering jobs, a pumping test on a well that fullypenetrates the san d strat um t o be dewatered is war-ranted; such tests should be made during the designphase so tha t r esults can be used for design purposesand will be available for bidders. However, for smalldewatering jobs, it may be more economical to select amore conservative value of k based on empirical rela-tions than to make a field pumping test. Pumping tests

are discussed in detail in appendix C.d. Simple field tests in wells or piezometers. Th e

permeability of a water-bearing formation can be esti-mated from constant or falling head tests made inwells or piezometers in a manner similar to laboratorypermeameter tests. Figure 3-5 presents formulas fordetermining the permeability using various types andinstallations of well screens. As these tests are sensi-tive to details of the installation and execution of thetest, exact dimensions of the well screen, casing, and

3-6

filter sur rounding the well screen, an d th e ra te of in-flow or fall in water level must be accurately meas-

ured. Disturbance of the soil adjacent to a borehole orfilter, leakage up the borehole around the casing, clog-ging or removal of the fine-grained particles of theaqu ifer, or th e accum ulat ion of gas bubbles in oraround the well screen can make the test completelyunreliable. Data from such tests must be evaluated

Figure 3-4. D10 versus in situ coefficient of horizontal permeability

Mi ssi ssi ppi Ri ver va lley and Ark ansas Ri ver va lley.


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A S S U M P T I O N S

S O I L A T I N T A K E , I N F I N I T E D E P T H A N D D I R E C T I O N A L I S O T R O P Y (k, A N D k h C O N S T A N T ) - N O

DISTURBANCE, SEGREGATION, SWELLING, OR CONSOLIDATION OF SOIL - NO SEDIMENTATION OR

LEAKAGE - NO AIR OR GAS IN SOIL, WELLPOINT, OR PIPE - HYDRAULIC LOSSES IN PIPES, WELL-

P O I N T , O R F I L T E R N E G L I G I B L E .


Figure 3-5. Formulas for determiningpermeability from field failing head tests.

3


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carefully before being used in the design of a major de-watering or drainage system.

3-5. Power. The availability, reliability, andcapacity of power at a site should be investigated priorto selecting or designing the pumping units for a dewa-tering system. Types of power used for dewatering sys-tems include electric, natural gas, butane, diesel, andgasoline engines, Electric motors and diesel engines

are most commonly used to power dewatering equip-ment.

3-6 . Sur face w ater . Investigations for the con-trol of surface water at a site should include a study ofprecipitation data for the locality of the project and de-termination of runoff conditions that will exist withinthe excavation. Precipitation data for various localitiesand the frequency of occurrence are available in pub

lications of the U.S. Weather Bureau or other reference data. Maps showing amounts of rainfall that canbe expected once every 2, 5, and 10 years in 10-, 30-and 60-minute duration of rainfall are shown in figur3-6. The coefficient of runoff c within the excavationwill depend on the character of soils present or thtreatment, if any, of the slopes. Except for excavationin clean sands, the coefficient of runoff c is generallfrom 0.8 to 1.0. The rate of runoff can be determineas follows:

whereQ = ciA (3-3

Q = rate of runoff, cubic feet per secondc = coefficient of run offi = intensity of rainfall, inches per hour

A = drainage area, acres

(U. S. Department of Agriculture Miscellaneous Publication No. 204)

Figure 3-6. Inches of rain fal l during 10- and 30-minu te and 1-hou r periods.

3- 8


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CHAPTER 4

DESIGN OF DEWATERING, PRESSURE RELIEF, AND

GROUNDWATER CONTROL SYSTEMS

4-1. Analys is of groundwater f low.

a. Design of a dewatering and pressure relief orgroundwater control system first requires determina-tion of the type of groundwater flow (artesian, gravity,,or combined) to be expected and of the type of systemthat will be required. Also, a complete picture of thegroundwater and the subsurface condition is neces-sary. Then the number, size, spacing, and penetrationof wellpoints or wells and the rate at which the watermust be removed to achieve the required groundwaterlowering or pressure relief must be determined.

b. In the analysis of any dewatering system, thesource of seepage must be determined and the bounda-ries and seepage flow characteristics of geologic andsoil formations at and adjacent to the site must be gen-eralized into a form that can be analyzed. In somecases, the dewatering system and soil and groundwa-ter flow conditions can be generalized into rather sim-ple configurations. For example, the source of seepagecan be reduced to a line or circle; the aquifer to a homo-geneous, isotropic formation of uniform thickness; andthe dewatering system to one or t wo parallel lines orcircle of wells or wellpoints. Analysis of these condi-tions can generally be made by means of mathematical

formulas for flow of groundwater. Complicated con-figurations of wells, sources of seepage, and soil forma-tions can, in most cases, be solved or at least approxi-mated by means of flow nets, electrical analogy mod-els, mathematical formulas, numerical techniques, or acombination of these methods.

c. Any analysis, either mathematical, flow net, orelectrical analogy, is not better than the validity of theformation boundaries and characteristics used in theanalysis. The solution obtained, regardless of the rigoror precision of the analysis, will be representative ofactual behavior only if the problem situation and

boundary conditions are adequately represented. Anapproximate solution to the right problem is far moredesirable than a precise solution to the wrong problem.The importance of formulating correct groundwaterflow and boundary conditions, as presented in chapter3, cannot be emphasized too strongly.

d. Methods for dewatering and pressure relief andtheir suitability for various types of excavations andsoil conditions were described in chapter 2. The inves-tigation of factors relating to groundwater flow and to

design of dewatering systems has been discussechapter 3. Mathematical, graphical, and electroangous methods of analyzing seepage flow through eralized soil conditions and boundaries to varitypes of dewatering or pressure relief systems are sented in paragraphs 4-2, 4-3, and 4-4.

e. Other factors that have a bearing on the acdesign of dewatering, permanent drainage, and face-water control systems are considered in this cter.

f. The formulas and flow net procedures presen

in paragraphs 4-2, 4-3, and 4-4 and figures through 4-23 are for a steady state of groundwflow. During initial stages of dewatering an exction, water is removed from storage and the ratflow is larger th an required t o mainta in th e specdrawdown. Therefore, initial pumping rates will pably be about 30 percent larger than computed valu

g. Examples of design for dewatering and presrelief systems are given in appendix D.

4-2. Mathemat ica l and model analyse

a. General.(1) Design. Design of a dewatering system

quires the determination of the number, size, spacand penetration of wells or wellpoints and the rawhich water must be removed from the pervious sto achieve the required groundwater lowering or psure relief. The size and capacity of pumps and cotors also depend on the required discharge and drdown. The fundamental relations between well wellpoint discharge and corresponding drawdownpresented in paragraphs 4-2, 4-3, and 4-4. The etions presented assume that the flow is laminar,pervious stratum is homogeneous and isotropic,water draining into the system is pumped out at a

stant rate, and flow conditions have stabilized. Prdures for tra nsferring an anisotropic aquifer, withspect to permeability, to an isotropic section are sented in appendix E.

(2) Equations for flow and drawdown to drain

slots andwells. The equations referenced in pgraphs 4-2, 4-3, and 4-4 are in two groups: flow drawdown to slots (b below and fig. 4-1 through and flow and drawdown to wells (c below and fig. 4through 4-22). Equations for slots are applicabl


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COMBINED ARTESIAN-GRAVITY FLOW

(Modifi ed from Foundat ion Engineering, G. A. Leonards, ed., 1962, McGraw-Hi ll Bo ok Compan

Used with permission of McGraw- Hill Book Compan

Figure 4-1. Flow and head for fully penetrating line slot ; single-line source; artesian, gravit y, and combined flow s.

flow to trenches, French drains, and similar drainagesystems. They may also be used where the drainagesystem consists of closely spaced wells or wellpoints.Assuming a well system equivalent to a slot usuallysimplifies the analysis; however, corrections must bemade to consider that the drainage system consists ofwells or wellpoints rather than the more efficient slot.These corrections are given with the well formulas dis-cussed in c below. When the well system cannot besimulated with a slot, well equations must be used.The figures in which equations for flow to slots andwells appear are indexed in table 4-1. The equations

for slots and wells do not consider the effects of hy-draulic head losses H

win wells or wellpoints; proce-

dures for accounting for these effects are presented

separately.(3) Radius of influence R. Equations for flow to

drainage systems from a circular seepage source arebased on the assumption that the system is centeredon an island of radius R. Generally, R is the radius ofinfluence that is defined as the radius of a circle be-yond which pumping of a dewatering system has nosignificant effect on the original groundwater level orpiezometric surface. The value of R can be estimated

4-2


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TM 5 -818-5/AFM 88-5, Chap 6/NAV FAC P

(Modified from Foundation Engineering, G. A. Leonards, ed., 1962. McGraw -Hill

Book Com pany. Used w it h permission of McGraw -Hi ll Book Com pany .)

Figure 4-2. Height of free discharge surface hs; gravit y flow .


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(Modified from Foundation Engineering, G. A . Leonards, ed., 1962. M cGraw-H ill

Book Company . Used wi th permission of M cGraw-Hill Book Company .)

Figure 4-3. Flow and head for partially penetrating li ne slot; single-line source; artesian, gravity, and combined flow s.

4- 4


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F U L L Y P E N E T R A T I N G S L O T

T H E F L O W T O A F U L L Y P E N E T R A T I N G S L O T F R O M T W O L I N E S O U R C E S , B O T H O F I N F I N I T E L E N G T H ( A N D

P A R A L L E L ) , I S T H E S U M O F T H E F L O W F R O M E A C H S O U R C E , W I T H R E G A R D T O T H E A P P R O P R I A T E F L O W

B O U N D A R Y C O N D I T I O N S , A S D E T E R M I N E D F R O M T H E F L O W E Q U A T I O N S I N F I G . 4 - 1 . L I K E W I S E . T H E

D R A W D O W N F R O M E A C H S O U R C E C A N B E C O M P U T E D F R O M T H E D R A W D O W N E Q U A T I O N S I N F I G . 4 - 1 A S I F

O N L Y O N E S O U R C E E X I S T E D .

P A R T I A L L Y P E N E T R A T I N G S L O T

A R T E S I A N F L O W

(Modified from Foundation Engineering, G. A. Leonards, ed., 1962, McGraw -Hill

Book Com pany. Used w it h permission of McGraw -Hi ll Boo k Com pany .)

Figure 4-4. Flow and head for fully and partially penetrating line slot; two-line source; artesian and gravity flows.


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A F R E Q U E N T L Y E N C O U N T E R E D DEWATERING S Y S T E M I S O N E W I T H T W O L I N E S O F P A R T I A L L Y

P E N E T R A T I N G W E L L P O I N T S A L O N G E A C H S I D E O F A L O N G E X C A V A T I O N , W H E R E T H E F L O W

C A N B E A S S U M E D T O O R I G I N A T E F R O M T W O E Q U I D I S T A N T L I N E S O U R C E S .

(Modified from Foundation Engineering, G. A. Leonards, ed., 1962, McGraw -Hill

Book Com pany. Used w it h permission of McGraw -Hi ll Book Com pany.)

Figure 4-5. Flow and head (midw ay) for tw opartially penetrat ing slots; two-l ine source; artesian and gravity flow s,

4-6


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Figure 4-6. Flow and head for fully and partially penetrating circular slots; circular source; artesian flow.

from the equation and plots in figure 4-23. Wherethere is little or no recharge to an aquifer, the radius ofinfluence will become greater with pumping time andwith increased drawdown in the area being dewatered.Generally, R is greater for coarse, very pervious sandsthan for finer soils. If the value of R is large relative tothe size of the excavation, a reasonably good approxi-mation of R will serve adequately for design becauseflow and drawdown for such a condition are not espe-cially sensitive to the actual value of R. As it is usuallyimpossible to determine R accurately, the value should

be selected conservatively from pumping test data

if necessary, from figure 4-23.(4) Wetted screen. There should always be sucient well and screen length below the required ddown in a well in the formation being dewatered so the design or required pumping rate does not proa gradient at the interface of the formation andwell filter (or screen) or at the screen and filter starts to cause the flow to become turbulent. Thfore, the design of a dewatering system should alwbe checked to see that the well or wellpoints have


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U.S.

Arm

y

Cor

ps

of

En

gineers

Figure

4-7.Headatcenteroffullyandpartiallypenetratingcircularslots;circularsource;artesianflow.

4-8


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U. S . A r my Co r p s o f E n g i n e e r s

Figure 4-8. Flow and drawdown at

dew ate ring and underground water control

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