mooring bridge caissons during construction using driven plate anchors

7/27/2019 Mooring Bridge Caissons During Construction Using Driven Plate Anchors

1/8

MOORING BRIDGE CAISSONS DURING CONSTRUCTION USING JETTED-IN

DRIVEN PLATE ANCHORS

Osama A. Safaqah, Ph.D., Ben C. Gerwick, Inc. San Francisco, CAMarc Gerin, Ph.D., P.E. Ben C. Gerwick, Inc. San Francisco, CARobert Bittner, P.E., Ben C. Gerwick, Inc. San Francisco, CA

ABSTRACT

Driven plate anchors can be used to hold in place large bridge caissons duringfloating construction. Their small size and simple method of installation makethem a very cost effective alternative to other direct-embedment, gravity, anddrag anchors. The geotechnical design of plate anchors involves the anchorgeometry, embedment, drivability, and proof testing. This paper describes theexperience gained with plate anchors used during construction of the newTacoma Narrows Bridge. For this project, the anchors had to be driven in denseto very dense sands, and had to resist large forces from 7-9 knot currents.

INTRODUCTION

Driven plate anchors are conceptually verysimple: as shown in Fig. 2, plate anchors consistof a steel plate welded onto a section ofstructural steel beam to which a padeye isattached. As shown in Fig. 3, the anchor isconnected to a follower (H-Pile) by a hydraulicclamp and is driven vertically into the sea floorusing an under-water hammer. After driving theanchor to the required depth, the follower isretrieved and the anchor is proof-tested. This willcause the anchor to rotate to the horizontal orkey in thus developing maximum resistance topull out (Fig. 3).

Driven plate anchors are simple, inexpensive,quick to install and have a very high holding-capacity-to-weight ratio compared to otherunderwater anchor systems. This makes themvery suitable for the temporary mooring of bridgecaissons during their construction.

Figure 2: Driven Plate Anchor

Figure 1: Floating Caisson held in Position byAnchors.


2/8

The driven plate anchor design described in thispaper was developed for the construction of thenew Tacoma Narrows Bridge. The Narrows is adeep waterway located at the south end ofPuget Sound, on the West coast of WashingtonState. The deepest point along the bridgealignment is about 67 m (220 ft) elevation,and the tidal currents are very fast (7-9 knots)between tidal shifts. The new 1-mile longsuspension bridge is supported by two maintowers. These towers are founded on two 24x40m (80x130 ft) caissons embedded 23 m (75 ft)below the mudline in 40-46 m (130-150 ft) ofwater. The concrete caissons were constructedfrom a raft which was cast in a dry-dock, towedto the site and moored in place. The remainingconcrete was cast-in-place in 3 m (10 ft) lifts.Until the bottom of the caissons reached theseabed, the floating caissons had to be mooredin place.

The mooring system for the bridge caissons had

to resist drag forces from up to 43.6 m (143 ft) ofdraft in 7-9 knots maximum tidal currents. Eachcaisson was moored by 16 lower anchors on anominal 300-ft radius and 16 upper anchors on anominal 600-ft radius. At the caisson end, thelower lines are attached approximately 7.6 m (25ft) up from the bottom of each caisson andremain fixed during construction. The upperlines are initially installed approximately 18.3 m

(60 ft) up from the bottom of each caisson. Theyare attached to a sliding beam, which is movedupward as the caisson is constructed. The lowermooring lines were 100 mm (4 in) diameter Oil-Rig-Quality stud-link chain and the upper lineswere 95 mm (3.75 in) diameter bridge strandconnected to one shot of 100 mm (4 in) chain.Each of the 64 mooring lines was attached to adriven plate anchor. See Figure 3b and 3c forschematic of anchor attachment to caisson.

GEOTECHNICAL DESIGN OF PLATE

ANCHORS

Anchors can have either a shallow or a deepmode of failure depending on the embedmentratio (H/B, anchor depth to anchor smallestdimension), and the soil friction angle. In ashallow failure, the failure surfaces extend to

the ground surface at small anchordisplacements. This mode of failure results inthe lowest anchor capacity. In a deep failure,failure starts locally around the anchor likebearing failure around a footing. Global failuretakes place at large displacements with aninverted-pyramid-shaped rupture surface. Thisfailure mode results in a much higher anchorcapacity. Plate anchors are designed to developa deep type of failure.

Figure 3a: Plate Anchor Concept andInstallation Procedure

Plate anchor capacity in sands can betheoretically estimated using the followingequation:

*' qu HNq = (1)

where , H, and Nq*

are the effective unit weightof soil, embedment length, and anchor capacityfactor, respectively. Obtaining a reasonablyaccurate estimate of Nq

*is the major task in

designing plate anchors. This is because itdepends on many factors including anchororientation, embedment ratio, soil dilatancy,anchor roughness, soil angle of friction, andinitial stress state. Further, the anchorinstallation method has an impact on the angleof friction and the stress state, which must beconsidered.


3/8

Figure3b:PlanView

ofAnchorLayout


4/8

Figure3c:Elevatio

n-AnchorAttachmenttoCaisson


5/8

Many methods have been proposed to estimateNq

*. Some are based on assumed failure

mechanisms, some are based on small-scalecentrifuge tests, and some are based on large-scale pull out tests. Unfortunately, N

q

*values

estimated from these methods range from veryconservative to very optimistic. The Majer (1955)method is based on a vertical slip failure surfaceand usually represents a lower, veryconservative, limit on Nq

*. On the other hand, the

Meyerhof and Adams (1968) method is basedon a pyramid-shaped failure surface and resultsin upper estimates ofNq

*, however, these can be

unconservative in dense sands. Other methodsinclude Vesic (1972), based on the cavityexpansion concept, and Vermeer and Sutjiadi(1985), which uses soil dilatancy to control theshape of the failure surface. A reasonable

analytical method was proposed by Rowe andDavis (1982). They used two-dimensional finiteelement analyses with an elasto-plastic soilmodel to develop plate anchor design charts.Their model accounts for soil dilatancy, initialstress state and anchor roughness.

Perhaps the best approach to plate anchordesign is one based on an empirical methodcalibrated with full-scale tests, and augmentedwith analytical analysis. The Navy design charts(Forrest et al., 1995) are well calibrated with full-scale field tests, and represent a good staring

point. These charts relate the holding capacityfactor (Nq

*) to the anchor embedment ratio, and

the soil friction angle.

SITE CONDITIONS

At the West tower, the mudline elevation isabout 36.6 m (120 ft), and the upper 21 m (70ft) of soil consists of very dense, gravelly sandand sandy gravel. At the East tower, the mudlineelevation varies between 36.6 m and -48.8 m,and the upper 14 m (50 ft) of soil consists of

medium dense to very dense gray silty finesand.

Because of the dense soil conditions at the site,driving is aided by jetting ahead of the anchor.The jetting process will disturb the soil aroundthe anchor reducing the friction angle, however,part of this loss can be recovered by using thevibro-hammer to compact the soil duringretrieval of the follower. Table 1 shows the soil

properties used in the design of the plateanchors.TABLE No. 1 Soil properties used in the designof the plate anchors

Location(1)

Condition(2)

FrictionAngle, (degrees)(3)

Buoyant

UnitWeight,

b(kN/m

3)

(4)

WestTower

Undisturbed 38 to 40 10.4

WestTower

After Jettingand Vibro-compaction

35 9.4

EastTower

Undisturbed 36 to 38 10.4

East

Tower

After Jettingand Vibro-compaction

35 9.4

DESIGN AND INSTALLATION OF PLATE

ANCHORS

The plate anchor dimensions and embedmentdepth were selected to provide a holdingcapacity of 4450 kN (1000 kip) at the mudlinewith a factor of safety of two. The chainembedded in the sand typically contributes atleast 20% of the total capacity, therefore, theplate anchor itself was designed to provide 3560

kN (800 kip) capacity with a factor of safety oftwo. Using the Navy charts and the Rowe andDavis method, Nq

*was estimated at 20 (Other

methods gave values ranging from 7 to 40). Theanalysis showed that an embedment ratio of 7 isneeded to ensure a deep failure mode at thissite.

The anchors consist of a 75 mm (3 in) thick steelplate 2.4 m x 1.5 m (8 ft x 5 ft) welded to an 2.4m (8 ft) long piece of W18x311 structural steelbeam (Fig. 2). The driving end of the plate andbeam are beveled at 45 degrees. A 75 mm (3 in)

thick padeye welded onto the beam provides theattachment point for the 100 mm (4 in) stud-linkchain.

For a plate width of 1.5 m (5 ft), the requiredembedment depth is 10.5 m (35 ft) after keying-in. The distance the anchor travels to rotate andkey in is a function of the soil friction angle, proofload orientation, and the configuration and


6/8

length of the anchor. Denser soils and smalleranchors require smaller keying distance. As arule of thumb, plate anchors in sands require 1.5times their length to key in. Therefore, theminimum driving depth for the Tacoma Narrowsanchors is 14.3 m (47 ft) below the mudline.The anchors are installed using a 18.3 m (60 ft)follower (a W14x342 steel beam) with an APE400 vibratory hammer mounted at the top, and ahydraulic clamp mounted at the bottom.Drivability analyses were used to select thehammer and to optimize the design to four water

jets at 2400 kPa (350 psi) pressure. The waterjets help loosen the sand making vibration moreeffective. A hydraulic clamp was used to holdthe anchor during driving, instead of a passiveattachment, to maximize the transfer of vibrationfrom the follower to the anchor, See Figure 4.

After delivering the anchor to the required depth,the follower was retrieved under vibration in

stages of 1.5 3 m (5-10 ft) each lasting 2-3minutes to densify the sand above.

To validate the design, two full-scale testanchors were installed and tested using theequipment designed for the production anchors.The tests indicated the installation system iscapable of driving the anchors to the required

depth in 20 to 40 minutes, and confirmed the4450 kN (1000 kip) holding capacity. See Figure5.

Figure 5. Installation of Anchors using water

jets.

ANCHOR LOAD TESTING

Load testing the anchors is an integral part ofthe anchor installation. This causes the anchorto rotate and key in, thus positioning the anchorfor maximum pull-out resistance, and minimizingany further displacement during operation. Loadtesting also verifies the anchor holding capacity.Load testing in sands can be carried out eithervertically or horizontally since the anchor chainremains vertical throughout most of itsembedded length. For this project, horizontaltesting was used at first. Later on, verticaltesting was used because it was faster.

Figure 4. Hook up of Jet Hoses and VibratoryHammer to Anchor

With horizontal testing, two opposing anchorswere pulled against each other using a loadingmechanism on a barge located between the two.The chain from one anchor was fixed to thebarge (the dead end), while the chain from theother anchor was connected to the end of asliding block (the live end). The sliding block wasconnected to a winch via a 12-part block andtackle. First, tension was applied to remove anyslack in the anchor chains; 220 to 310 kN (50 to70 kips) was required. Then, tension was slowlyincreased until one anchor broke loose, causing

the tension in the anchor chain to fall off,indicating the anchor is keying. The drop intension on one side caused the barge to moveaway from the keying anchor. This helps identifywhich anchor is keying first. After that, thetension started to build up again until the secondanchor broke loose and the tension fell off again.

After both anchors keyed, the tension wasincreased until the proof load was reached. For


7/8

this project, the proof load was set at 4000 kN(900 kip), slightly less than 50% of the ultimatecapacity of the anchor.

To reasonably estimate the keying distance ofeach anchor, and therefore the final embedmentdepth, the following data was recorded duringthe tests. Line tension, indicates when theanchors slip, and when the proof load isreached.Amount of chain pulled in, indicates thetotal keying length (sum of both anchors), and ifan anchor is pulling out. Barge movement, is themovement along the line between the anchorsneeded to proportion the total amount of chainpulled in to each anchor. Tidal elevation change,needed to adjust the amount of chain pulled in todetermine the final embedment depth. Fig. 6shows the data collected from one of thesetests.

Vertical testing of one anchor at a time hasproven more efficient because of the reducedmaneuvering of the test barge, and simplerchain hook-up. With a vertical pull, there is less

resistance from the chain embedded in thesand, thus the proof load can be reduced byabout 20%. For this project, the vertical testswere conducted to a proof load of 3560 kN (800kip). For these tests, the anchor chain ran over aroller installed on the end of the barge, and wasthen connected to the sliding end of the blockand tackle. Counterweights were used to keepthe barge relatively level during testing.Determining the final anchor embedment depth

is simpler with vertical tests since all the chainpulled in comes from movement of the oneanchor. Fig. 7 shows the results from verticaltesting of two anchors, which represent therange of keying behavior encountered in thisproject.

All anchors were successfully tested to at least3560 kN (800 kip) with insignificant anchormovement at the final test load. Tests dataindicate that the keying distance ranged from 2.0to 7.3 m (6.5 to 24 ft), with most anchors keyingin 3.03.7 m (10-12 ft) as predicted. Line tensionat keying ranged from 890 to 1780 kPa (200 to400 kips). Some anchors keyed-in suddenlywhile others keyed in gradually with little releaseof tension at each step. Differences in the keyingbehavior are attributed to variations in siteconditions. Since the anchors are spread over alarge area, soil conditions such as soil density

and silt/clay content and the slope of the seabedvary from one location to another.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Tension(kN)

0

4

8

12

16

C

hainPulled-in,

Bar

Line Tension

Pulled-in Chain

Barge Movement

16

12

8

4

0

AnchorEmbe

dment(m)

0 1000 2000 3000 4000

Line Tension (kN)

Anchor O3

Anchor M3

)m

eMovement(

First Anchor

Keys in

Second Anchor

Keys in

10 15 20 25 30 35 40 45 50 55

Time (Minutes) Figure 6. Horizontal Testing for Two plate

Anchors

Figure 7. Vertical Testing of Plate Anchors

CONCLUSIONS

Driven plate anchors can be jetted in very densesands to obtain high-holding-capacity anchorswith significant cost savings over other anchortypes. The challenge to the geotechnicalengineer in such soil conditions is usually theprediction of anchor drivability, the effect ofanchor installation on the keying behavior, and


8/8

the final capacity of the anchor. The role of thegeotechnical engineer starts by characterizingthe site conditions and reasonably predicting thedesign parameters of the soil. Then he candetermine the anchor geometry and requiredembedment depth, assess the anchor drivability,and help design the installation and testingmethods and equipment.

Jetted-in driven plate anchors with a designcapacity of 4450 kN (1000 kip) weresuccessfully used in the construction of the NewTacoma Narrows Bridge. They held in place thelarge concrete foundation caissons in the deepand fast waters of the Narrows.

REFERENCES

Dicken, E.A. (1988). Uplift behavior of

horizontal anchor plates in sands. Journal ofGeotechnical Engineering Division, ASCE, 114(11), 1300-1317.Forrest, J., Taylor, R. and Bowman, L. (1995).Design Guide for Pile-Driven Anchors.Technical Report TR-2039-OCN, Naval FacilitiesEngineering Service Center, Port Hueneme,California.Majer, E.L. (1955). Zur Berechnung vonZugfundamenten. OsterreichischeBauzeitschrift, 10(5), 85-90 (in German).Meyerhof, G. G., and Adams, J. I. (1968). Theultimate uplift capacity of foundations. Canadian

Geotechnical Journal, 5(4), pp 225-244Row, P. W., and Davis, E. H. (1982). Thebehavior of anchor plates in sand.Geotechnique, 32(1), London, England, 25-41.Vermeer, P. A., and Sutjiadi, W. (1985). Theuplift resistance of shallow embedded anchors.Proceedings, 11

thInternational Conference on

Soil Mechanics and Foundation Engineering,San Francisco, California, Vol. 3, 1635-1638.Vesic, A. S. (1972). Expansion of cavities ininfinite soil mass. Journal of Soil Mechanics andFoundation Engineering Division, ASCE, 98(SM3), 265-290.

mooring bridge caissons during construction using driven plate anchors

Documents