i TN NO: k1627

TITLE: INTERACTION OF ANCHORS WITHTITLE. SOIL AND ANCHOR DESIGN

] AUTHOR: RobertJ. Taylor

DATE: April 1982

SPONSOR: Naval Facilities Engineering Command

[ J PROGRAM NO: YF59.556.091.01.205

H0TI iNAVAL CIVIL ENGINEERING LABORATORY DTICPORT HUENEME, CALIFORNIA 93043 ELECTE

LU Approved for public release; distribution unlimited. J 7i.. _j JUL 7 1982'SM82 0 D82 07 07 019

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UnclassifiedSECURITY CLASSIFICATION 01 TIS PAGE (WI,., Oslo F..I-d.)

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IREPORT NUMBER 2 OTACCESSION NO. 3 nEciPirmrVs CATALOG NUMBER

TN-1627 1N987W05a TirLi (-d SoA(,II.J TYPE OF REPORT A PERIOD COVERED

INTERACTION OF ANCHORS WITH SOIL Fnl a 7-Jn8AND ANCHOR DESIGN A PERFORMINiG ORG REPORT NUMBER

7AUTHOR(.; & CONTRACT OR GRANT NUMBER(.)

Robert J. Taylor

9 PERFORMING ORGANIZATION NAME AND ADDRESS I0 PROGRAM ELEMENT PROJECT. TASK(AREA A &WORK U NOT NUMBERSNAVAL CIVIL ENGINEERING LABORATORY 62759N;Port Hueneme, California 934 YF59. 556.091.01.205

11 CONTROLLING OFFICE NAME AND ADDRESS 12 REPORT DATE

Naval Facilities Engineering Command April 1982Washington, DC 22332 1 NUMBER OF PATE

14 MONITORING AGENCY NAME & ADDRESS( IIlive-n110 M.M(n~,~ille01) IS SECUIRITY, CLASS (.1 Ph- .cp.1)

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17 DISTRIBUTION STATEMENT (,(.3 I.1 ..r,.d II. 91-1, 20. d dill-(R. IN- PR.r,)

IS SUPPL EMEN TARY NO0TES

19 'fEY BONDS fC-1n, no..P-.1.1. If -..,, d Id..flfy, 530k .b'

Anchors, site selection, ocean soils, deadweight anchors, direct-embedment anchors,drag-embedment anchors, pile anchors.

20 AS S IC*RI,nlO ,* * 0 1 dO *- If -y.d Idln',Iy 11, block ,1Mb,r)

he report provides a practical up-to-date guide that enables the practicingengineer to select and size common anchor types, including direct -embedment anchors,deadweight anchors, drag-embedment anchors, and pile anchors. For each anchor type, thereport includes site survey recommcndations, a brief description of various anchors withineach anchor category, methods for determining anchor performance, and, in certain cases,

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20. Continued

suggestions for improving poor anchor behavior. Sources for additional info[ mation aresuggested where the treatment of a broad topic is necessarily limited.

Library Card

Naval Civil Engineering LaboratoryINTERACTION OF ANCHORS WITH SOIL AND ANCHOR

DESIGN (Final), by Robert J. TaylorTN-1627 44 pp illus April 1982 Unclassified

1. Anchors 2. Site selection 1. YF59.556.091.01.205

The report provides a practical up-to-date guide that enables the practicing engineerto select and size common anchor types. including direct-embedment anchors, deadweightanchors, drag-embedment anchors, and pile anchors. For each anchor typt, the reportincludes site survey recommendations, a brief description of various anchors within eachanchor category, methods for determining anchor performance, and, in certain cases,suggestions for improving poor anchor behavior. Sources for additional information aresuggested where the treatment of a broad topic is necessarily limited.

UnclassifiedSECUPITY CL ASSIPICATION OF THIS PAGE'WS-', b- F0P1r0d)

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FOREWORD

This report was prepared for presentation at "Recent Developmentsin Ocean Engineering," sponsored by the University of California atBerkeley in January 1981. It was written in outline format to provide apractical up-to-date guide for the practicing engineer to enable selec-tion and sizing of common anchor types including direct embedment anchors,deadweight anchors, drag embedment anchors, and pile anchors.

For each anchor type, the report includes site survey recommendations,briefly describes various anchors within each anchor category, presentsmethods for determining anchor performance and, in certain cases, suggestspractical options for improving poor anchor behavior.

The topic of anchor design is broad, and this report does notpretend to provide complete solutions for all anchor selection anddesign problems. However, it does provide state-of-practice solutionsto most general anchoring problems and makes the designer more aware ofhis options and the limitations of each anchor type. For complex orcritical anchoring applications, the reader is referred to sources ofinformation and references that are provided throughout the report.

A majority of the information presented in this report was takenfrom published and unpublished reports by the Foundation EngineeringDivision of the Naval Civil Engineering Laboratory under the sponsorshipof the Naval Facilities Engineering Command and the Department of Energy.

Accession ForNTIS GRA&rDTIC TAB 5Unennounced 0-Justification

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Distribution/ cp.

_Avnilability CodeslAvtil and/orDist i Social

V

CONTENTS

Page

SITE SURVEY. .. ............................

GENERAL. FEATURES OF VARIOUS ANCHORS .. ................. 7

PLATE ANCHORS. ... ......................... 8

DRAG EMBEDMENT ANCHORS .... .................... 17

DEADWEIGHT ANCHORS. .................. ...... 25

PILE ANCHORS. ................. .......... 33

ACKNOWLEDGMENTS .. ................ ......... 40

REFERENCES. ................ ............ 41

vi

SITE SURVEY

A. Site Survey Requirements

-Requirements differ according to:

Anchor type [Pile, Deadweight, Drag, Embedment]Loading condition [Static, Dynamic]Soil type [Sand (cohesionless), Clay (cohesive)]Mooring use [Manned, Unmanned]

Minimum Recommended Site Survey Requirements

Required Site Information*

Anchor TypeNon-Critical Mooring Critical Mooring

Deadweight General seafloor type Seafloor type, depth of sedi-(mud, clay, sand, ment, areal variability, esti-rock). mate of soil cohesion, fric-

tion angle, scour potential.

Drag Embedment Seafloor type. Seafloor type and strength,(approximate) deptb to rock,stratification in upper 10'to 30' (depending on soiltype), areal variability.

Plate Anchor Seafloor type; Engineering soil data to ex-depth to rock; pected embedment depth (soilo Use estimated strength, sensitivity, density,properties provid- grain size, origin, depth toed or other avail- rock), additional data requiredable info. for dynamic analysis.

Pile Anchor Sediment type, Engineering soil properties todepth of sediment, full embedment deptho Use estimated (soil strength, sensitivity,properties provid- grain size, origin, density),ed or other avail- soil modulus of subgrade reac-able info. tion for laterally loaded piles.

*Geologic literature survey suggested for all situations to help definesoil type and existence of seafloor anomalies.

B. Sediment Property Determination

- Variety of tools exist to acquire quantitative or qualitative data.

Static, dynamic penetrometers, in-situ TM Sub-bottom profiling-vane shear device, corers, grab samplers side scan sonar

[Refer to Lee and Clausner (1979) - Soil Sampling Techniquesl

- Information/Data-Sources

Lamont-Doherty Geological Observatory of Columbia University, Palisades,N. Y. 10964

National Geophysical and Solar-Terrestrial Data Center, Environmental DataService, National Oceanic and Atmospheric Administration, Boulder, Colo. 80302

Chief of Operations Division, National Ocean Survey, NOAA, 1801 FairviewAvenue, East Seattle, Wash. 98102

Chief of Operations Division, National Ocean Survey, NOAA 1439 W. YorkStreet, Norfolk, Va. 23510

Naval Oceanographic Office, Code 3100, National Space Technology Laboratories,NSTL Station, Miss. 39522

Scripps Institution of Oceanography, La Jolla, Calif. 92093

Chief Atlantic Branch of Marine Geology, United States Geological Survey,Bldg. 13, Quissett Campus, Woods Hole, Mass. 02543

Chief Pacific Arctic Branch of Marine Geology, United States GeologicalSurvey, 345 Middle Road, Menlo Park, Calif. 94025

Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543

C. Sediment Property Estimation

(When detailed physical survey not practical)

- Determine whether sediments are:

Terrigenous (land-derived) sediments or pelagic(ocean-derived) sediments (e.g., pelagic clay, oozes).

2

, S

Ocean sediment distribution

1. Terrigenous Sediment Properties

- Assume all continental shelves and slopes are terrigenous.- Typically complex and varied sediment type particularly,

near-shore, glaciated areas, high current areas.- Refer to National Ocean Survey charts to determine whether

sand or mud (cohesive).

Sand

If nearshore and a "grab"sample is available for grainsize determination, safe values .. ~. .

of * and Ybare: ...

F - j 81-0n U-180 h

d. 6)A tlb/ft') _1-

I -g ded .. 35 bO 60)

For-locations classed as "'*" L"

Abyssal Plains properties forturbidites are appropriate.

Proxinal -< 30 miles from shoreDistal -> 30 miles from shore

Typical strength pr-fie -- turbidites.3

Mud

If sediment is mud (cohesive) .. . ...this provides a lower bound fora normally consolidatedsediment.

If site is near river mouth,Miss., Nile, Amazon, etc., mudprobably underconsolidated(Young - not yet in equilibriumwith wt overlying soil, may belimited strength buildup with .depth. Consult an expert for ,design advice. .,

Much of the nearshore isoverconsolidated (greater past ..,o ......... .,overburden than presently exist-ing) usually a desirable anchor-iag situation. Locations (e.g.,glaciated areas, high currentareas, tops of rises, passages).

Unsually strong overconsoli-dated sediment could lead toless conservative design (long-term loading).

2. Deep Ocean (Pelagic) Sediment Properties

If deep ocean site is not an abyssal plain, determine if depth isabove or below Calcite Compensation Depth (CCD).

Topography of the calcite compensation depth (CCD). Calcareous sediments arc foundonly in those locations where actual water depth is less than the CCD; numbers oncontours denote kilometers below sea surface

4

If above the CCD - sediment probably calcareous.If below the CCD - sediment probably pelagic clay.

IT"

\ .f

Typical profiles calcareous ooze. Typical profile pelagic clay.

If location is classed assiliceous ooze ..........

Whenever possible consult expertsat a nearby oceanographic institu- ",.tion for property data.

Typical profile -siliceous ooze.

A .

D. Hazardous/Unusual Seafloor Conditions

- If these coiditions are encountered or anticipated, caution isnecessary

- Design possible, but requires more detailed procedures thanpresented

Examples of Hazardous/Unusual Seafloor Conditions

* Submarine lava flows occupying a relatively small and irregular area.

o Small sediment channels, local extreme bottom slopes, cliff-liketopography, or giant seafloor ripples.

* Erratics from ice-deposited glacial detritus.

o Metallic nodules or "pavement" formations above soft sediments.

* Sloping seafloor greater than 10 degrees.

o Deep ocean siliceaous ooze (>30% biogenic and siliceous).

o Clean calcareous ooze (>60% biogenic and calcareous).

o Sensivity >6 in a cohesive soil.

o Cohesive soil strength varying by more than 50% or + 100% fromtypical profiles presented.

* Unconsolidated or very high void ratio clays with c/p values near0.1-0.15.

o Thin sediment layer above rock.

o Layered seafloors - soft sediment over stiff/dense sediment or viceversa.

6

GENERAL FEATURES OF VARIOUS ANCHORS

Deadweight Anchor Drag Embedment Anchor

Broad range of anchor types and sizes availableLarge vertical reaction component, permitting shorter mooringline scope High rapacity (greater than 100,000 lb) achievable

No setting distance

Reliable holding force, because most holding force due to Sanriard off the shelf Equipment

anchor mass Broad use experience

Simple, on-site constructions feasible, tailored to task Can provide continuous resistance even though maximum capacity

Size limited only by load-handling equipment exceeded

Economical; weighting material readily availableAnchor is recoverable

Reliable on thin sediment cover over rock

Mooring line connection easy to inspect and service Usable with wire or chain mooring lines

Good energy absorber when used in conjunction with j Anchor does not function in lithified seafloors

"non-yielding" anchors (i.e., piles, embedded plate anchors)

Good reaction to vertical load components; works well in Anchor behavior erratic in layered seafloorscombination with drag embedment anchors permitting shortmooring line scopes Low resistance to uplift, therefore, large line scopes required

to cause near horizontal loading at seafloor

Lateral load resistance low compared to other anchor types

Usable water depth reduced; deadweight can be undesirable Penetrating/Dragging anchor can damage pipelines, cables, etc.

obstruction

Plate Anchor

Sigb capacity (greater than 100,000 fb) achievable

Resists uplift as sell as lateral loads enabling short scopemoorings

Anchor dragging eliminated Pile Anchor

Higher holding capacity to weight ratio than any other typeof anchor High capacity (greater than 100,000 lb) achievable

Resists uplift as well as lateral loads permitting use withHandling is simplified due to relatively light weight short mooring line scopes

Anchors can function on moderate slopes and in lithified Anchor setting not required

seafloors Anchor dragging eliminated

Installation is simplified due to possibility of instan- *Short mooring line scopes permit use sn areas of limited sea

taneous embedment or seafloor contact room or where minimum vessel excursions are required

Drilled and grouted Piles especially suitable for hard coral crAccurate anchor placement possible rock seafloor

Does not protrude above seafloor Does not protrude above seafloor

Driven piles cost competitive with other high capacity anchors2'3'4* Can accomodate layered seafloors or seafloors with when driving equipment is availablevariable resistance because of continuous power Drilled and gruted piles incur high installation costs andenpenditure during penetration Dildai rue ie nu ihzsalto ut r2 x3d* drequire special skills and installation equipment

Penetration is controlled and can be monitored .idc range o sizes and shapes are possible (pipe, structuralshapes )

Susceptible to cyclic load strength reduction when used in ield h ap o otaut moorings in loose sand, coarse silt seafloors F e odfiartions permita piles o he tailored to suit require

meuts of particular applicationsFor critical moorings, soil engineering properties required >Taot .znua gs nay aggravate ship response to waves (lo

resilience)Anchor plate typically not recoverable

0Taut lines and fittings must continually withstand high

1.0 stress levelsSpecial consideration needed for ordnance

Costs increase rapidly in deeper water or exposed locatinosAnchor cable susceptable to abrasion/fatigue where special installation vessels are required

1.G Special equipment (pile extractor) requred to retrieve orGun system not generally retrievable in deep water (l,000 ft) refurbish the mooring

2,3,4* Surface vessel must maintain position during installation More extensive site data is required than for other anchor tpes

2,3* Operation limited to sediment seafloors *True for any taut mooring.

*1. Propellent-embedded anchor

•2. Screw-in anchor

*3. Vibrated-do anchor

*4. Driven Anchor

7

PLATE ANCHORS

A. Plate Anchors - Summary of Types (Refer to Taylor et al. (1975))

Propellant-Embedded Anchor

1/~q 2 3 4

.foeln i - oroje n

- P en dan

couchduwn Penetrotion Kiyinq Anchor estoblished

A.c-o ,,.e.b,, Current Developments

Primarily U. S. Navy developed CEL10k, 20k, 100k, SUPSALV 100k, 300k -Refers to normal long term capacityin soft seafloor.

o 100k anchor commercially available

Driven Anchor Menard Rotating Plate Anchor

IA.d I..- O

-4%

mandrel ~ ~

Oe enI In-wierce _o off tit Enlifed Perspecive

p estwn sition

UN-got l ost

Navy Umbrella PileAnchor (current workin U. K.)

8

Screw (Auger) Anchor Vibrated Anchor

-One or more Anchor at base of longhelices screwed slender shaft, vibratedinto the ground into the seafloor; platefrom surface or is "keyed" to operatingat seafloor, position.

b-V.

S..r P,. 6SETTLED SAND

-WATER INLETJetted-In Anchors LUG FOR

SAND

PHAA .6/[NTRE

WATER LUG FCA bINLET

AIP -I.Jt CNON 'CEME NT4 OINS/ I S A [ D ! ' J L tr' ". !

A pLATE 'UNDISTURBED

BOLTED TO A ' CLYANCHOR -- C

t lItI IRoyal Dutch shell jetted, ~.- ~ NOTZLE

PHR JETS N. hE anchor (Netherlands)

Hydropin Anchor (NationalEng. Lab. U.K.)

B. Plate Anchor Failure Definitions

--------- -- -

C. Plate Anchor Design Loading Conditions

Short-Term Loading - An increasing load to failure such that

Static in fine-grained soils drainage does not occur.Long-Term Loading - Uniform static load where full drainage

occurs.

9

Impulse Loading - Non-rhythmic loads > static capacity, < 10seconds in duration - sands; < 10 minutes duration - clays.

Dynami Cyclic Loading - Repetitive loading with double amplitudemagnitude > 5% static capacity.

Earthquake Loading - Cyclic loading induced to the entiresoil mass by earthquake energy.

D. Plate Anchor Design Process (Refer to Beard, 1980)

1. Site Survey: Determination of hazardous/unusual condition,soil property selection, soil type determination.

2. Determine Anchor Embedment Depth

a. Control embedded anchors (e.g., driven, jetted, vibrated,screwed).Depth = f/soil type, strength, plate, size, equipment

limitations.

b. Dynamically embedded anchors (propellant-embedded)

Cohesive soilCalculate by method of True (1976)

Cohesionless soil - Penetration prediction schemesare poor.

Calculated Penetrations for Estimated Penetrations forCEL Clay Flukes CEL Sand Flukes

Anchor Penetration, i (ft) for-- Anhor .enetraton, ii (ft) in --

30r1 1 2 9 I K Anchor LoOa f k( ) "I p SandsSoftasitrb dt 39.5 (64) 15.9 (52) 0.7 (35) 7.6 (25) Cal 1 S ,

(3I-*) r7.4 (57) 1331 (43) 8.2 (27) 5.8 (19) .a.,dlcor.c fluk, e 8 (12 3.4 (11( 3.3 (30)i I CEL 203K .

(1a. (49) 31.9 (39) 7.9 (26) 5.8 (19) nd/coral fluke .46

PCors I rorh, 3oZ 32.5 (41) 30.3 (33) 7.0 (23)5 2 7 ) a r/

Cootie'a eareona ooze 35.2 (50) (2.8 (427 8.2 (27)I

5.8 ((9) * 60 degre ; t 2 080 kg/a. ( (30 ./6t'5

Stk~reooo ooe 26.3 (79) I839 (65) 13.9 (43) 9.2 (307)

a cl 24 7 (f1) 20 7 (68) 1403 (3) b 0.7 (33)

Pla c clay 19 2 (63) 15.9 (52) 11. (2.)i 812 (27]

10

c. Anchor Keying

Plates embedded edgewise are "keyed" to assume horizontalorientation.

CEL propellant anchors key according to:

D(cohesive) = p - 2L (L = fluke length)

D(cohesionless) = Dp - 1.5L

3. Determine loading condition, calculate capacity.

a. Short-term static holding capacity (no drainage).

Shape factor(Skempton, 1951)

Fst A(c N c f + 'yb D N q)(0.84 + 0.16 B/L)

After Vesic (1969)with disturbancecorrection factor(f) by Valent (1978)

where Fst = Short-term holding capacity

A = Projected fluke area

c = Soil cohesion

f = Disturbance correction factor= 0.8 - terrigenous silty-clays, clayey-silts= 0.7 - pelagic clays= 0.25 - calcareous ooze (validity of this factor in doubt)

y = Buoyant unit weight of soil

D = Plate embedment depth

B = Plate width

L = Plate length

c = Short-term holding capacity factor-cohesive soilcN = Holding capacity factor for drained or frictionalq condition

11

2 4 i u z 1

t (Cohesive soill"_ = I

Neglect yb D N q e

Fsat (Cohesive soil) = A (s u ]c f ) (0.84 + 0.16 B/L)

F st (Cohesionless soil) c = s 0u

F (Cohesionless soil) = A y b D N q (0.84 + 0.16 B/L)

Short-Term Capacity Sloping Seafloors

Refer to Kulhawy et. al., (1978).

Short-Term Capacity Laterally Loaded Plates

Refer to Neely et. al., (1973).

- Plate anchor capacity is enhanced with lateral loading.- For propellant anchors, keying distance is minimized.

b. Long-Term Static Holding Capacity (full drainage)

- Time to full drainage = f (permeability load, drainagepath, anchor size, shape, etc.)

Cohesionless soil - drainage almost immediateF t (cohesionless soil) = F st

Cohesive soil - long-term capacity governed by drained strengthparameters: friction angle, #,and cohesion intercept c

12

M

F (cohesive soil) A(c'N + Y D N )(0.84 + 0.16 B/L)It 1

Fit Long-term holdingcapacity

C' = Soil cohesionintercept

A1 Yb' B1 L = Refer to short-termsection

N Holding capacityq factor drained/

frictional condition(Refer to short-term

_ _ _ _ _ _ section)N' = Long-term holding

c capacity factor for

cohesive soil

Loose/soft seafloors - failure associated with relativelylarge displacements; reduce c', 4 by 1/3. c = 2/3c',

= tan- (tan 2/3 0

Creep rupture - cohesive soil - increasing rate of shear untilfailure occurs (poorly understood phenomenon)

- Problem appears minimal for calcareous ooze, pelagic clay.- F x S = 2 adequate to prevent creep rupture.

c. Dynamic Holding Capacity

1) Impulse Loading - refer to Douglas (1978), or Beard(1980), for details of prediction procedure.

- Consider only if large infrequent loads may beunexpectedly applied to a plate anchor mooring.

- Can have a positive effect on anchor holding capa-city for loads of up to:

9 500 sec duration - cohesive soile 10 sec duration - cohesionless soil

- For load durations < .01 sec impulse holdingcapacity can be:

2-5 times short-term capacity for a normallyconsolidated clay.

2-6 times short-term capacity for a mid-densitysand

- Impulse loads near or somewhat above Fat can betolerated.

13

2) Cyclic Loading - Refer to Beard (1980) for detailsof cyclic capacity prediction scheme developed byHerrmann (1980).

- Caused by wave induced forces and cable strumming.

1

- Cyclic loads < 5% static capacity of no concern,therefore, cable strumming can be ignored.

- Cyclically loaded anchors designed to preclude failurefrom liquefaction or cyclic creep.

Characterized by Accumulation ofstrength loss and small movementssudden anchor that reduce anchorinstability depth until pull

out occurs.

Strength Loss During Cyclic Loading

The following procedure excludes soils such as uniform finesand, coarse silts, and some clean oozes which are susceptible totrue liquefaction failure. Use of plate anchors in these soilsunder cyclic loading is not recommended at this time.

14

Procedure ....

Determine tcd from the soil "'permeability. "I"

For the assumed sea conditions, 'determine the number of loadingcycles during t found from thesoils permeabily. Enter thefigure below to find the loadingbounds as a function of soiltype. This table can also beused directly to find thelimiting number of cycles fora given loading.

to I to x , It0I REIO TW BVt0) ot I0'I ON

Ott I P" u'ldt.OoO J..O. .' rIiOXIMsTl Ifitstios [It-oTt ...

701 ,. COEIFYtII T'1 ()F ItlMLIOILIT)7..fli AND oots% SIZE Ifn.I;L

Sllimit. of 200 tt~O f l

A ~~o* -. . mm l, os. ty

lo° o I tO

o too t o I., too Io SAMi Ilk 10-2 00.06 006 I 10

- 4 102

Silt 0.008 to I

- ) 0.002 ,

Cyclic Creep During Cyclic Clay 0001 0-1 1tp2

Loading

- Poorly understood phenomenathat does occur in the 'laboratory.

- Number and magnitude ofsignificant loading cyclesoccuring during the life of 'lan anchor control cyclic creep.

- For cases where static loadexceeds 20% static capacity,add portion above 20% to cyclic-.component and proceed.

,,L,

15

3) Earthquake Loading (Refer to Wilson, 1969)

- Cohesive soils not susceptible to significantstrength loss during earthquake loading.

- Granular soils can liquefy during earthquakeloading.

- Granular liquefaction is a function of soil rela-tive density. Potential for liquefaction isillustrated below.

Liquefaction potential profiles forearthquake loading of granular soils

(from Seed and Idriss, 1971).

Maximum ground accelerations are afunction of earthquake magnitudeand distance from the quake epicenter.5 . . . . .1 ..•

Maximum acceleration associatedwith earthquakes of variousmagnitudes (from Seed et al., 1969).

If analysis of the site and its expected earthquake indicates high

probability of soil liquefaction, the site is hazardous. Use of plateanchors which are loaded a significant percentage of time should be

avoided.

16

I' Si

. . . ... , . . . . . • .. . ', .,, .. -

DRAG EMBEDMENT ANCHORS

A. Anchor Descriptions (Refer toOgg, 1969; Valent et al.., 1976)

1. Standard Drag Anchor

- Significant portion ofanchor capacity generatedby anchor wt.

- Full embedment-rare.- Develops peak capacity

with minimal drag. (W STOCK (ADMIRALTY) ANCHOR 10) MUSH4ROOM ANCH.P

KEDGE TYPE REINFORCED CON~kETE

F J J Average Lateral Load

Aovhor Weigt(i eght (Wet

so 5 100 1SO

e~dge 10,550 6,000

aa:d 2.00 27,300 27,500 _T--:d_ 9.300 11.900 11,700 * .- -

... d 1050 600 21 300 23.000 21,600 101 MUSHROOM ANCHOR (d) PEARL HARBOR CONCRETE ANCHOF.d - 9.100 10,000 13.300 Saad[a a

2. Standard Burial AnchorSTMA- Achieves most of capacity as PAL GUSSE, FLUKEO,.rENJAS

result of soil shearKstrength. Lcapacity through draggingto cause embedment to deeper, 000

stronger soil. II- Most anchors in this category

fabricated according to rules MONIMIN

of geometric similarity wheredimensions ySproportional 3 O"o~aot"fto (weight)

Standard burial anchor per- AL~~

formance is idealized below. 11-fluke angle PA4

0 ~lak angle V +

Cro7wn fluke 79 ank f LK

j3 a STATO Mooring Ancoer

a. placed on seafloor b. flukes keying C. in dense/stiff $@afloor d. in soft seafloorinto seafloor 01o4+00 to 150 a - -200 to -450

17

Idealized anchor remains stable and holds stably even though dragged

(achieves equilibrium).

Pick Type Burial Anchor

- Anchor designed to turn topenetrating position even ifdropped on its side.

Cast Bruce Anchor

Twin-Shank Bruce Anchor.

Mud Type Burial Anchor

- Permanent mooringanchor.

- Designed to be controllowered to seafloor.

- Designed for very

soft seafloors.

Doris Mud Anchor

18

B. Anchor Performance

1. General Behavior (Refer to Saurwalt, 1971, 1972a, 1972b, 1973,1974 a, 1974b)

Seafloor Type - [Performance as defined by broad seafloor categories.]

Mud or silt - Wide range in anchor performance; "mud" strengthvaries considerably.

Sand - Performance reasonably consistent provided anchor penetrates;dense sand can be difficult.

Clay - good holding capacity.Coral - Function if anchors snag an outcrop, fall in crevice,

blasted in.Rock - Unsatisfactory.Layered (sand/clay/mud) - Performance erratic for high efficiency

anchors.

Roll Stability

- Anchors improperly stabi-lized will roll limitingpeak capacity.

- If an anchor rolls in amud or clay, the anchor

112 will come out with a "mudclod" fixing the fluke

preventing re-embedment.

0 - "- Erratic/poor performancecan sometimes be corrected

... -........... , ... .... ....... :by extending stabilizers.0 0 20 35 (5 122 )6A IFO 0 0 C. 3 1

40 60 80 100 120

achor travel - feetPerfo ne of 11,000-1b Nvy anchor asvn ad Staho% e bilizer.

SECTION A.A

viTIE FlA19

0 0

AJ

Navy anchor.

19

Mooring Line Angle Fluke/Shank Angle

Effect of line angle on mooring - Figure shows significance ofperformance can be significant. fluke angle on anchor performance.

- Optimum angle for mud (=500)- Optimum angle for sand (30-35")

IF 1. k, It d,

I

Majority of decrease probably -Figure illustrates problemattributed to reduction in chain with excessive fluke anglecapacity. in sand.

II / I

Mooring Line Type (Wire Versus Chain)

- Overall mooring capacity - similar assuming sufficient sedimentfor complete burial.

- Anchor penetration in mud significantly less with chain mooring-less sediment required.

- Anchor drag distance to peak load less with chain mooring - asmuch as 50 versus 250 ft.

- Anchor stability requirements greater for wire than chain mooring.

20

F 1

F2

LI T D Wir

Anchor Size

- Small anchors (<3,000 ib) often exhibit higher efficiencies (by asmuch as a factor of 1-1/2 to 2) than anchors 10,000 to 30,000 lb.

- Manufacturers' claims of constant efficiency with siz /qased ongeometrically similar designs (dimensions - anchor wt ). Datado not support this as a general rule. Refer to section onAnchor Capacity.

2. Recent Anchor Performance Data (Refer to Taylor, 1980a, 1980b,1980c)

STATO Anchor in Sand

0 W b*h

20 3

OAO Dw M,00, fi

O'2O370 fluke

a,4, "

32° h' nlil1

AA

--~ ~ -: - .m a l i o l .b

1 10 20 30 40

Aho, Dsq D*$t .DW

TtS e t 1. 3G0 Ib STATO w ,31 d- -4. n 0

21

0 0'0 . 0---

STATO Anchor in Mud

- Graph shows trajectory of anchor embedment in soft mud (PugetSound).

- Extended stabilizers (about 30% increase) needed to maintainstability (6,000 lb STATO with standard stabilizers rolled duringembedment).

- Majority of load carried by chain.

U/

so,210 1

aI - *0..- "

ao 5 0 5 20 25 J A 4W 45 bO 66 so 86 10

T.t No 23

.0*00..oo.. A-oW W.."0 XW-.

Stockless Anchor in Mud

A 1. 0 ,i0. o - o - '

W0a 10bW 14 80c)A Ii ~ I

An, 00to.. t

3. Chain Capacity (Refer to Cole and Beck, 1964; and Taylor,1980a, 1980b, 1980c)

- Chain efficiency varies considerably for "similar" soil types.sand - efficiency of I to >3 depending on densitymud - efficiency of 0.4 to 1.1 depending on strength and

clay content.

22

4. Tandem Anchors (Refer to Taylor, 1980a)

Option A - Shank to shackle technique; good tandem capacity; chainshould be lightly lashed to inbound anchor crown duringdeployment.

Option B - Crown to shackle technique; slightly less efficient than"A" but easier to install.

Option C - Ground ring to shackle technique; less efficient than "A"or "B" - relatively easy to install in shallow water;Anchor B installed first.

A- -

5. Options to Improve Poor Anchor Performance

Problem Possible Reason Solution

Poor mud performance - Flukes not tripping - Increase size oftripping palms

- Weld flukes inopen position

- Anchor unstable - Increase stabilizerlength/addstabilizers

- Unknown - Add chain- Use backup anchor

Poor sand performance - Flukes not penetrating - Check fluke angle;reduce if > 30-321

- Sharpen flukes

- Anchor unstable - Extend stabilizers- Add stabilizers

- Unknown - Add chain- Use backup anchor

23

C. Methods to Determine Drag Embedment Anchor Capacity

1. Method of Cole and Beck (1964)

- Verfied procedures relating anchor capacity to soil engineeringproperties not available.

- Available procedure dated to Leahy and Farrin (1955) is reasonableprovided anchor test data available

Anchor capacity relates to anchor wt as follows:b

F = CW a

Where F Short-term holding capacity (lbs)

W = Anchor wt in air (ibs)aC,b = Empirical soil constants, dimensionless

- Relationship plots as astraight line on log-logplot, C is the intercept,b is the slope.

500,000-- Results valid for thatanchor, mooring line type, - - - - -soil type.

100,-OPTIONAL-PROCEDURE 50.000 ...

- Perform single test, use C- 110

b = 0 .7 5 t o c a l cu la t e C . 10,00 b -.7.. . .. . ..-

- Extent of extrapolation ofthis procedure questionable.

- Theoretical limit for b is 100.2/3, where steel stress is o Moo o0oo 30.0ocontrolling factor. Refer to AN04OR WknM -LB

Valent et al., 1979.

- Use verified manufacturerdata to calculate C forb - 0.75.

2. Prototype Data

- Refer to manufacturers for data; data often based on smallanchor tests at unlimited drag (request details of tests).

- Data valid for specific test conditions; anchor performancevery sensitive to conditions (use data with caution for otherconditions).

3. Full-Scale Pull Test

- Most accurate/costly.24

DEADWEIGHT ANCHORS

A. Anchor Types

Vary from: sophisticated (concrete/steel anchors with cuttingedges) to engine blocks, concrete clumps, etc.

Added capacity from sophiscation must be balanced against cost.

i) nker (b) Squat clump (c) kadroad rails or (d) Concrete slab with open frame withscrap irnserkeyswegtdcmr

" efficient uplitr e low overturning * low bulk, high & high lateral capacity v high lateral capcty" easy t. handle * more urea con- weigh a scour conrrol * reduced owerng line

trring sod * ow cs dynam tensions

* shallow burial

If) Mushroom (g) Wedge (h) Slanted skirt 6 if 1gh lateral capacity, (l) Free fallfree fall (DELCO)

shallow burial . shallow burial * deeper bunal free fall installation . free fail jnstallarion" low overturning e uni-directional . high lateral capacity * efficient uplift" uni-directional

Several variations on the basic deadweight anchor.

B. Design Procedures

1. Simple Form (anchors w/oshear keys)

Idealized deadweightresists lateral loadcomponent by staticfriction; vertical loadresisted by portion ofanchor wt.-- "-

Net normal force, R ,pcontributes to lateral loadresistance, R1, according to:

RI = p Rn

Where V is the coefficient of friction between anchor block andseafloor; p varies w/seafloor type/strength.

25

fll i, , .... ... i ii i ''J

a. Cohesionless Seafloor Coffic ot. of Friction Bete. Coh.s.,Il. Soil. ndSomKarin Construction Kateriolo (Valent, 1979)

- Trapped water dissi-pates rapidly. pInternl Surface Frictio Coefficient for --

paes rapio up soi an Rouh oth Ro. Smoothto 0.8 possible (4 ... .tSteel Steel Concrete Concrete PYC=380); simple fric- -.

t 380)bsipehv ior c - Qort 0.67 0.27 0.60 0.60 0.69 0.33

tional behavior con- SandCorolline 0.67 0.20 0.63 0.63 0.66 0.20trols. Sand

-Friction coeffi- OolticSan c 0.79 0.23 0.56 0,58 0.74 0.26

cient dependent on o,-m 0.66 0.40 0.66 0.67 -- 0.40

surface smoothness, Is"-I

anchor material,sand type.

b. Cohesive Seafloor

p (immediate) can be < 0.1 (attributed to thin filmtrapped water between anchor and seafloor).

p (short term - normally consolidated seafloor) can be0.15-0.2

R Rn where 5.7 - Anchor bearing capacity1 5.7 Anchor-soil shear resistance

Value (5.7) assumes adhesion between anchor base and soil equals

soil undrained shear strength.

p (long term) - up to 0.7 for Odrained = 350

p (short term - over consolidated seafloor) depends upon soilstrength, anchor roughness.

c. Effect of SlopingSeafloor

- Low initial pcan cause in- u' cc

stability onsloping sea- o.4floors.

- Deadweights onslopes - 10° ,have slid underown weight.

- Avoid use onsloping sea-floors.

- Sloping clayseafloors 0.0

likely overconsolidated; 0 0 ..

down slop creep .*.. ... ,.. ...possible. 26

26

2. Detailed Procedure - (Refer to Valent, et al 1979)

Used when peak deadweight lateral capacity is desired.

a. Considerations

Bearing Capacity

- Not considered problem when resultant normal soilreaction lies within middle one-third of anchor base.

Vertical Load

- Resisted directly by a portion of the submerged anchorwt.; wt in excess of that required to develop lateralcapacity.

- Discount suction effect.

Horizontal Load

- Components composinghorizontal loadresistance F

13

1. shear along anchor base2. shear along base of passive wedge

at anchor front3. uplift (weight) of passive wedge

Items can be neglected 4. shear along anchor sides5. suction at rear of anchor

Calculated capacities assume displacement only to mobilize baseshear (< 10% anchor width; capacity will typically increase with drag)and assumes no auxiliary embedment means (jetting).

_ L L __ _ .. '

- Deadweight capacity enhanced by roughened surface oraddition of skirts.

- Skirts in cohesive soil move sliding surface to deeper,stronger soil; optimum length * 0.1B

27

- Skirts in cohesionless soil - marginal increase incapacity; optimum length - 0.05 B; interior skirts notneeded; exterior skirt helps reduce scour and under-cutting.

Overturning - Anchor must be designed to prevent overturning.Anchor center of mass should be low; mooringattachment points should be low.

- Stabilizing moment > overturning moment

Cyclic Loading

- Effect depends on magnitude of cyclic component relativeto the quasi-static load as well as the absolute loadlevel.

- "Porous" deadweight may be less susceptible to mooringline transmitted cyclic loads because drainage path isshortened (pore pressure dissipation occurs more rapidly).

- Refer to section on plate anchor design for added details;also, see Foss, et al, (1978).

Other Design Considerations

- Scour, slumping, wave induced instabilities of theseabed, earthquakes, wave forces on anchors.

- Degree of attention to these depends on location, waterdepth, soil type, soil degree of consolidation, seafloorslope.

b. Anchor Design - Cohesionless Soil

- Anchor designed to realize lateral capacity (R1)according to:

R ( - Fv) tan (-5 0) + 1/2 Kp Yb zs2 B

where: W = submerged anchor wt (F); F = uplift force (F);v

S= effective angle of internal friction (degrees);

K = coefficient of lateral earth pressure.P

c. Anchor Design Cohesive Soil

- Anchor designed to yield lateral capacity (R1) according to:

R1 = 5uz A +2 Su z B

1, u ua

28

where s = soil undrained shear strength at depth z (F/12 )

a = average soil strength between surface and z (F/L2 )ua

A = anchor base area

z = anchor or shear key penetration into seafloor (L)

B = anchor base dimension (L)

29

=- ~~~~::Emmm -- ... ...

.... -

.... .. .. 17..-...

DEADWEIGHT ANCHOR DESIGN PROCEDURES

Cohesionleas Seafloor: Deadweight Anchor Design Procedure Cohesive Seafloor: Deadweight Anchor Design Procedure

Step Item Equation Step Item EquationLoads Fh Fv (ien) a Loads Fh, Fv

b Soil yb, j (given) b Soil au' St yb versus z

Weight (in water) required Fh c Anchor width:torss ldigW=.F v (F 1/to sla .din v tan(i -5° ) without shear keys a b o

d Anchor width (sin): 1/3

with shear keysa

R u B2( . Swith shear keys = (W - Fv - 0.3 Fh)

d Shear keys'[6 Wh,11/3 200 a swithout shear keys B = number, n n = o au +

Shear keys* a /40 s +Yb /2

200(W - Fv ) tan(t-5° ) thickness, t t = - u Sfnumber n =rsp

0)22.4 1 bKp Ybt 0 Yb 2 weight per shear key, W k = 0.1 Yk

B2 t

thickness t =0.042[ "b ) B2

a/

\'h /embedment force for one qe = 9

ouz t B2 Bu - WWih tx00 k82tshear key, q, 5 S t k

e Submerged weight

embedent force for one Yb B2shear key e - OO 120 t N + B tan($ - 50)) (1) to resist overturning.

40 q(a) for H 0.2 B,

I Maximu pull height N = B1.2 h + FvB(W - F) z2 = OlBWith or without H 6 hshear keys (b) for H minimized, W 0.6 Fh + F06 F,

SAss imes shear key penetration 0.05 B. Z 0.1 B I - 6 -

Coefitcat ot P asiveAteral trt I E-sMre,sNo D s.,bt $beer Ker (,fte Tsrbeoi,-rIfP :62) 5 . (c) for )I minimizej, F

___ as = 0 6 F-ides ______ i B A Y .

i !Submerged weight (continued)

17,5 2.25 (2) to embed shear keys:20 2.59 (a) aonni-directional anchor W o 2 a qe

L 3o 4.78 (b) uni-directional anchor W n q

40 _0.3 where

5Aptas . soil key foss is vrots- 2 ,:''tE ub, aslis 5550he51 i s 0oiot• , . ~. ... ,.. q e. ss|e of sail ir1tc,,5

6 0.15@ e +., .. ,.. ox --S --- kSr... be .r ... Wa

W1, o &iso(s

aAssumes cutting edge penetration = 0.1 B, anchor square in plan.

LIST OF SYMOLS

h' F Horizontal and vertical BI Anchor lateral load Yk Submerged unit weight ofload components resistance shear key

su Soil undrained shear suz Soil undrained shear ya Buoyant unit Weight ofstrength strength at depth z deadweight

St Soil sensivity S Average soil strength * Effective asnle orbetween surface and s Internal friction

Yb Submerged unit weight ofsoil z Anchor shear key W Subsierged Weight of the

penetration into seafloor anchora Anchor width

b Allowable steel stress I oefticieot of lster.;ad Undrained shear strength b earth pressure

at aesfloor surface 2, Shear key height30 Coeffailent o passve30 q lateral esrth pressure

.... . .. . . . ... ,. - .. . . .. ... . * .. . . . . .. .. . . . ... . . . ./ .. ._ . .-. - . -r. . . .:

ii _________________________________________________

0. di

o 1~ Ut 0 0 0-.11.0 0 CS 0. -t 00

di *fl .00.tIn - flU .4 0.4 Ct 0

1 00 4 .4 adi .0 Mt

CO . U an 4 CU4t U -t - 1

4U .0*.411 I.. 4 . Cdi 0.

1. 4 0. 4.4 4.n I.o . - .~ r .,,~ - =C C 0 .. Sn K 4 0

40. .4 46 Ot U 04* 1 ... 0 U i. -41. --. 0 4.o.* 0 C.a N 4.4 4 KU di C. UC 0 N di - 0. 4 didi di Udi UU S C tI. CU di. di -0 .04 C 5* .. - - 0a . S Ofl C to CC

0 ON U 4~ V di 0 .-. di 44 00 C' N'di ii N di difl .0 U U 4 Ut Ca

di N U di -. iei U .00 0 C' - ). UC.0 .5 '41 4. U ... to di 4U 4. 44O C - 4%. 4 414. N 4 to! 0 di CC tU 0 101

U .4 .4 4 4 Otto a NIN C- tO tO" o0 0 U - 4 di .4.40 0 04 Ut di- C Ct 4 . a a S 0

* di 0 - .4 - .. a-c.. .~. t a! 4.41 .0 a . -- = alt..- A -.. iJ ~0 *4 N 0 I

0 di V . 0. C -444 .0 00 too 0di U a-. a -. .1 ......n .4 di i-.......4 tU U-a . .0.4 110 414

U U di N 0 UdiC 0. U di Cl tOt 0 0. Ct

di .4 .. U~ 4 CVC t a V N a CC C. .40 4.5di ... C~. Ii' ..... 4 0 0~ 4 1. 4 UC U di. to CU ~0.- S US C 4 di di

C... dit

.4*40.4 05U - C S N! UCCUA *.~ 3.4 U 0 M ae di tO C 4 4.4 to 40 nO C * l~1aN o n 00.0 di .. n U! CIt. 1111 3- U .~j di

di, Ut t .0 :~ ~ 000C0 4 di a 00 a U U 'ot ~t 0 .0 Cdi CA N UA 0 C C 4.10 0. U 4 Leo din

.-.

a. ... a a -Cdi U 4 a t '-~ 42.4 UC - 0 U 40 U! .0fr C di. 000 .0 a 410 - '-... A di V 0 CU tO to.4 tO ON . di 4 A 4- U 0. 4 Na on a N di di. UC U .0 ItO U.!C U4 N CU U IA -

to tO a N 0 - UC di0 C a .4 0 .00.4 C 0 di Ua C C Ii a

OUg di 0 0 to .4 r- U 4 I-nU to tOt -. 4 0" 5U5 .0 0 01 0 Ii It 00011 di tI

- 200 n

0.

0 n C C

C 04.40

0.4 .4 .4at .0 3

UtOl di

00 CC0.41 0. toll .4 U

* di.iC t cc . tt Cl.a dia.4t>U C tUA UtU di .4tdi.-lon - nUt-. U

Ut-C 40 diU ~0~0 .40di!..l UO C 4.4tU0 a a to UU4C di 44 U Cdi.00C di.4..i .4 OOC'4.43 .4.0. a- Ut U tCdi.0.4 ... 054U di 0 toI- 4.440 4 U 4.4 0.00 C 01t 04 OCOtO Ut 5 di 4 4 di .0U0di didiU 40.diOU .4 disH U 0.0 Cto diSU 40.440 . .4StolCdi . 44 4 4

tn-st I..I.AUUO00C 0.444Ut Mt -

AVata a 0 di 00.0 ... U Udill OttO U

.4 di .4di.0 U 04 C 4 UColto t4 .4

00diCU todiC di*40 U14C.n U 40 44 not

to .1' O'di. 0 044 5UU CAO.4t.0 Un 4 0 dia4wNC C .40.440 di U.404 4. UC-. U 0U4 di-. U C di .0t04 4 0.04di C a.iU0 St .. llUCt...l -. 444 di diC SCan4 0.0 4t0-a040 dillt Ct diU.O.4 .14.10 C A4C~C~ UCdi.4.4 ~ U

to 0 COast aU UdioC C U. 04.4 4 440 OU4CU , Itt di .. .4di. 5~.0 00 .40 .0 404 Udi...0.UOO di -l UO U .0 di

4 .44t-OC 0 Cl 0 44dia4 Ut.4dit4 Oto 4 -ia oo to -- tOO 4~..- .-. noo.a t..0t C 4.

tdi .4 U oaC.4 40. .04.45 .044 4 . -.C tt tCC 44.400 rOUt IUI.4C.4UU 0.14 0.4 tUZ 0 -

Mt U di4U a di4..04 0-4 UtU taOat OtUOS 4U4 0 40.0.0- t.4UU4 4 .0*4~94 .t - U .t-.40.4~ UCU4

0 40 4 diC 040 C4....aU t tO 410U0.O4....l di U 0.11 0 SOdiUt.t4 .4 Sdi.4 0*~ U U.04.0 3 40 di... A U * tUdit 400.4 0ton .40 - 4tU4 0.0 U Ca C U too.- . C

dii tO44 4 UC4C .40 tt*Ua C a US 4 II UON 5.1 04.4 di14 0 4.00 tO . 0* a 4 0

~ .0U~Ot "UdiC.. di II.

44U U ditOet 4. .0040 - U -

tO 04.0 4.444 ~ ~ -I ~CU Cdiv 40.045 didi*CtO. 5 4.- .4 0 4 U

s.l~ too anot OUSU4 *..tC a 4Otto 0 00 OUUUW4O4SUO .00" I 4 0 to -

.4.40 CdiOO C. 450. .4 44l.diU 4 . 00 4 .11 n.-...C UU t'40U4U S 4a f U ~ N -U3 U 40 U44'0.40 .41.1.40 0 4U a 0 I C -di .odi.44 dm4.0 4005.4 .04* -- 044 U II II 4

- .~. jt~~..no .0.0 tO-C OtoUxto C a

~ aa di 4.0 t-.t, 00.04 t~t- .4000 Cfl0di0 C0-4 .04

O 04 U di -0004,4~.4413 4:

CO 044 ~di 35

1 0.

di

U JI.) 11.1

.IU 44 U .4

31

f. I .

G o

10 v

-~~C 0 0-a a i V 0

-~ ~~ ~ A q *

Is 44 0

32A .... -

PILE ANCHORS

A. Operation

Definition. Pile anchorsachieve holding capacity bymobilizing shear strength ofsurrounding seafloor material. I/I, ,.

Bearing pressure and/or skinfriction/adhesion are used toachieve capacity. 41

Cost. High installation costs -- ,A.usually dictate pile anchor useas last resort.

Construction. Basic steelshapes usually modified to actas anchor piles.

Installation. By driving, often in partially predrilled holes; inhard strata, by grouting in fully predrilled holes. Screw-in pileanchor (considered under plate anchors) (Refer to Chellis, 1961,Havers and Stubbs, 1971, for detailed discussions of pile systems.]

B. Pile Types/Methods to Improve Performance

1. Mooring Line Connection

Surface attachment - Inspection and maintenance possible- Swivel/U-joint desirable to reduce

connection torsion (Ref Doris, 1977).

Subsurface attachment - Inspection not practical- Applicable to unidirectional loading- Enhances pile lateral load resistance;

pile bending stress reduced- Changes direction of pile load; higher

vertical, less lateral load.

2. Pile Head Burial

- Places pile in deeper-stronger soil- Used for offshore moorings when drillship is available for

drilling and grouting- Load at pile can be reduced significantly, by mooring line

resistance (see drag anchor section)- In sand, pile anchors buried few ft to allow for scour.

3. Near Surface Fins/Collars

- Used to limit pilehead deflection/bending moment

33

4. Built-up Sections

- Fabricated to produce section modulus to resist high bendingforces/limit bending

- Sections symmetrical or asymmetrical depending on loadingdirections.

Variations of the Basic Pile Anchor

(?) b) (W)

0e .A v7,ee7

Z. v

-f7-17141

C. Installation (Refer to Chellis, 1961, 1962, and 1979; Compton, 1977)

1. Driving

- Most piles in soil/soft rock installed by driving- Many hammer types easily modified for use to 80 ft- Pile hammers developed for underwater operation by Raymond

(1979), and Hydroblock (1979), (Hydroblock to 1,600 ft)- Can use follower in shallow water- Deep water - refer to Anon 1979 for discussion of a self

stabilizing "puppet" system for pile installation.

2. Drivability

- Best method for evaluation of pile drivability/hammer effi-ciency is the wave equation. Refer to Smith (1962).

34

3. Drilling and Grouting

- Used when predicted driving resistance exceeds hammer capacity- Typically used in hard coral, rock- Recommended for use in calcareous sands and soft silt wheredeveloped frictional capacities are low.

Pile Installation Methods

ii / II ,

- Grouted piles can be ,underreamed to greatlyincrease verticalcapacity.

- Underreams of more than - -

5m dia have been con- /- . -structed @S40m depth in " _the North Sea.

D. Pile Capacity ,

1. Lateral and upliftforce at the anchor

- Forces on buriedpile are altered.-in magnitude anddirection. / ' ,,;

a. SimplifiedAnalysis ' Vt I-

- Assumes nofriction N,,e *?c4

along buried (a) Deb. *'7-chain

35

- Results in over estimation of F and under estimationof Fh by up to 25%.

v

Sand2

Fcb = c db Yb Nq

Soil Frictiondb characteristic chain

or wire diameter (for Angle Nchain use 3 x chain size) q

20 3Clay 25 5

30 8Fcb 1 u d bz 35 12

u b c 40 22

s = soil undrained shearu strength

Force components at the anchor given by:

Fh = Ph -Fcb

F = P 2 - Fh2

b. Refined Analysis

- Refer to Reese 1973 and Gault and Cox 1974.

2. Lateral Pile Capacity

- Depends on soil strength, stiffness, load type, piledimensions and stiffness

- Rigid and long (semi-rigid) pile analyses are possible

Rigid Pile Analysis. Assumes soil failure occurs as an infinitelyrigid pile rotates about a point on its length.Procedure is very conservative; results in pile with minimum deflec-tion at head; can be used for preliminary pile selection for longpile analysis, (Refer to Czerniak 1957).

Long Pile Analysis

- Many procedures available (Refer to Gill 1970, Matlock 1970,Reese 1974, Broms 1964)

- Procedures are labor intensive; generally have been computerized- Procedures rely on a pile/soil interaction analysis where pile/

soil deflection characteristics are needed- Procedures rely on establishment of load-deflection (P-Y)

curves for soil, typically based on test experience.

3. Pile Axial Capacity

Capacity treated as function of shear along the pile/soil inter-face. Both cohesive and cohesionless soils can be treated asfrictional materials.

36

Y .... -. .... .. . . . . . t -

a. Cohesive Soil

Refer to semi-empirical methodof Vijayvergiyaand Focht (1972).

Pile frictional Aresistance (Ra)expressed asfunction ofmean undrained -

shear strength(s_) and meanefective over-burden stressa(_) over pilelength.

R =A(o + 2s )A

= empiricalcoefficient(below); A= lateralarea of em- 7-

bedded pile(use area of e1A/ CoZ/,e, ,enclosed rec- ,,,a 'Pezez7tangle for 1/Efff & /971-."H" pile inclay.

A Method Simplification

- Above equation is rearranged to simplify process. R a/As = f =average, pile side friction

- Iterative selection process required; Determine axial capacitythen increase length if needed.

37

~' . ~ L A' 21,' Z

'~ LT

~ ~2 0

1.1'

b. Cohesionless Soil

Unit skin friction f 1 at any depth is f =K a tan6

Assume K (coefficient of lateral earth pressure) = 0.5

a = effective overburden pressure

6 = angle of friction between pile and soil

(assume 6 = -.50)

- Pile capacity R a f A

-Average skin friction has been found to peak at pile embedment-20 diameter.

-Recommnended value of f, for long piles compiled from Ehlers (1977),Angemeer (1975).

38

Soil lnst alI 'I...

Sand driven or dri1 led .Q :

SIltyV sand driven or drill I-,

I Sandy silt driven cr .1i : Lied 1:1 1 ,-t, A -

Silt driven ,r dtillv3 a:il , - Q 4b -

Calcareous sand drilie and gr uted i c-1 0. ' 1 R8driven 0- 5 O0s 1.7 12

Depends on installation techni ,ae: may be as low as 3 ka (05 ps).

E. Anchor Pile Loading

- Effects of combined axial and lateral loading are poorly understood,currently treated separately.

- Repetitive loading can cause large increase in lateral pile deflec-tion. Methods to dampen/avoid repetitive loading should be consid-ered for piles in loose sand/soft silt seafloors.

- Chellis (1969), suggests "a rough assumption" coefficient ofhorizontal subgrade reaction for soils of high relative densitymight be reduced by 1/2, for soils of low relative density -reduced to 1/4 initial value (data provided for plate anchorsmay be useful as a guide in evaluating effects of repetitiveloading).

- Effects of repetitive loading on vertical piles are speculative,(research projects underway in United Kingdom and Norway).

39

ACKNOWLEDGIIENTS

Published and unpublished NCEL reports on anchors and soils providethe basis for this summary report. The contributions of Phil Valent,Hike Atturio, Rick Beard, and Homa Lee of the Foundation EngineeringDivision at NCEL are acknowledged.

40

REFERENCES

Angemeer, J. et al. (1975). "Pile load tests in calcareous soils con-ducted in 400 feet of water from a semi-submersible exploration rig," in1975 Offshore Technology Preprints, Houston, Tex., May 1975, p. 664.

Anon (1979). "Unsupported underwater pile driving: The soil-independentsystem," Petroleum Engineer International, Mar 1979, pp 10-15.

Beard, R. M. (1980). Holding capacity of plate anchors, Civil EngineeringLaboratory, Technical Report R-882. Port Hueneme, Calif., Oct 1980.

Broms, B. B. (1964). "Lateral resistance of piles in cohesionlesssoils," Proceedings ASCE I, Soil Mechanics and Foundation Engineering,vol 90, SM3, 1964.

Chellis, R. D. (1961). Pile foundations. New York, N.Y., McGraw-HillBook Company, Inc., 1961.

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