Design of Cofferdam

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Temporary StructuresCM 420CM 420CM 420CM 420CM 420CM 420

Cofferdams

Lecture 9


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Cofferdams

A cofferdam is a temporarystructure designed to keep waterand/or soil out of the excavationin which a bridge pier or otherstructure is built.

- Standard Handbook of HeavyConstruction


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Cofferdams Cofferdams are temporary enclosures to keep out

water and soil so as to permit dewatering andconstruction of the permanent facility (structure) inthe dry.

A cofferdam involves the interaction of thestructure, soil, and water. The loads imposedinclude the hydrostatic forces of the water, as wellas the dynamic forces due to currents and waves.

Because cofferdams are typically constructedunder adverse conditions in a marine environment,

and because significant deformations of elementsmay occur at various stages of construction, it isdifficult to maintain close tolerances. Ampleprovisions must be made for deviations indimensions so that the finished structure may be

constructed according to plan.


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Cofferdams

The loads imposed on the cofferdam structureby construction equipment and operationsmust be considered, both during installation ofthe cofferdam and during construction of thestructure itself.

Removal of the cofferdam must be plannedand executed with the same degree of care asits installation, on a stage-by-stage basis. Theeffect of the removal on the permanent

structure must also be considered. For thisreason, sheet piles extending below thepermanent structure are often cut off and leftin place, since their removal may damage the

foundation soils adjacent to the structure.


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Cofferdams

In cofferdam construction, safety is aparamount concern, since workers will beexposed to the hazard of flooding and collapse.

Safety requires:

good design

proper construction

verification that the structure is being constructed asplanned

monitoring the behavior of the cofferdam and

surrounding area

provision of adequate access

light and ventilation, and

attention to safe practices on the part of all workers

and supervisors.


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Cofferdams

Types of cofferdam:

Braced

Earth-Type

Timber Crib

Double-Walled Sheet Pile

Cellular


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Cofferdams

Braced CofferdamsFormed from a single wall of sheetpiling

Driven into the ground to form abox around the excavation site

The "box" is then braced on theinside

Interior is dewatered

Primarily used for bridge piers inshallow water (30 - 35 ft depth)


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Cofferdams

Cofferdam Design ConsiderationsScouring or undermining by rapidlyflowing water

Stability against overturning or tiltingUpward forces on outside edge due totilting

Stability against vertical shearEffects of forces resulting from:

Ice, Wave, Water, Active Earth and Passive

Earth Pressures


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Cofferdams

Advantages of CofferdamAllow excavation and construction of

structures in otherwise poorenvironment

Provides safe environment to work

Contractors typically have designresponsibility

Steel sheet piles are easily installedand removed

Materials can typically be reused on

other projects


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Cofferdams

Items needed for installationPile driving hammer

Vibratory orImpact

Crane of sufficient size

Steel sheet piles are typicallyused

H-piles and/or wide-flange beamsfor wales and stringers

Barges may be required


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CofferdamsTYPES OF IMPOSED LOADS

Hydrostatic pressure The maximum probable height outside the

cofferdam during construction and the waterheight inside the cofferdam during various stagesof construction need to be considered. These

result in the net design pressure shown below:

Hydrostatic forces on

partially dewatered cofferdam


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Forces due to Soil LoadsThe soils impose forces, both locally

on the wall of the cofferdam and

globally upon the structure as awhole. These forces are additive tothe hydrostatic forces.

Local forces are a major component

of the lateral force on sheet-pile walls,causing bending in the sheets,bending in the wales, and axialcompression in the struts


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Forces due to Soil Loads (Contd)


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Wave forces

Waves acting on a cofferdam are

usually the result of local windsacting over a restricted fetch andhence are of short wavelength andlimited to height.

Waves can also be produced bypassing boats and ships,especially in a restricted waterway.


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Ice forces These are of two types:

the force exerted by the expansion of a closed-in solidly frozen-over area of water surface

(static ice force) and the forces exerted by the moving ice on

breakup (dynamic ice force).

As an example, for static ice force, a value

of 4000 lb/ft2

has been used on cofferdamsand structures on the great Lakes, whereasthe value due to dynamic ice force on acofferdam-type structure are often taken at12,000 to 14,000 lb/ft2 of contact area.


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Seismic Loads These have not been normally considered

in design of temporary structures in thepast. For very large, important, and deep

cofferdams in highly seismically activeareas, seismic evaluation should beperformed.

Accidental loads

These are the loads usually caused byconstruction equipment working alongsidethe cofferdam and impacting on it under theaction of waves.


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Cofferdams Scour

Scour of the river bottom or seafloor alongthe cofferdam may take place owing to rivercurrents, tidal currents, or wave-inducedcurrents. Some of the most serious and

disastrous cases have occurred when thesecurrents have acted concurrently.

A very practical method of preventing scouris to deposit a blanket of crushed rock or

heavy gravel around the cofferdam, eitherbefore or immediately after the cofferdamsheet piles are set. A more sophisticatedmethod is to lay a mattress of filter fabric,

covering it with rock to hold it in place.


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Cofferdams

Cofferdam Components

Sheet piling

Bracing frame

Concrete seal

Bearing piles


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Cofferdams

The structure inside may be foundeddirectly on rock or firm soil or mayrequire pile foundations. In thelatter case, these generally extendwell below the cofferdam.

In order to dewater the cofferdam,the bottom must be stable and able

to resist hydrostatic uplift.Placement of an underwaterconcrete seal course is the fastestand most common method.


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Cofferdams An underwater concrete seal course may

then be placed prior to dewatering in orderto seal off the water, resist its pressure, andalso to act as a slab to brace against theinward movement of the sheet piles in order

to mobilize their resistance to uplift underthe hydrostatic pressure.


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Cofferdams Construction Sequence For a typical cofferdam, such as for a

bridge pier, the constructionprocedure follow the listed pattern.

1. Pre-dredge to remove soil or soft

sediments and level the area of thecofferdam.


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Cofferdams Construction Sequence

2. Drive temporary support piles3. Temporarily erect bracing frame on the

support piles.


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4. Set steel sheet piles, starting at all fourcorners and meeting at the center of each side

5. Drive sheet piles to grade.

6. Block between bracing frame and sheets, and

provide ties for sheet piles at the top asnecessary.


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7. Excavate inside the grade or slightlybelow grade, while leaving the cofferdam

full of water.


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8. Drive bearing piles.9. Place rock fill as a leveling and support

course.


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10. Place tremie concrete seal.


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Tremie concrete seal.


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Tremie concrete seal.


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11. Check blocking between bracing and sheets.12. Dewater.

13. Construct new structure.


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Cofferdams

Cofferdam for the Sidney Lanier Bridge, Oregon


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13. Construct new structure.14. Flood cofferdam.


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15. Remove sheet piles.16. Remove bracing.

17. Backfill.


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Cofferdams Traditional Sheet Pile Shapes

Z-Type (Z)Used for intermediate to deep

wall construction

Larson / U Type (U)Used for applicationssimilar

to Z - Type

Flat / Straight Type (SA), (S)

Used for filled cell constructionArch shaped & lightweight

Used forshallower wall

construction


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Cofferdams Typical types of interlocks

Ball & Socket (BS) Single Jaw (SJ) Double Jaw (DJ)

Hook & Grip (HG) Thumb & Finger

one point contact (TFX)

Double Hook (DH) Thumb & Finger

three point contact (TF)


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Cofferdams Braced Cofferdam Construction

Install Wale and Strut System forFramework /Template


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Cofferdams Braced Cofferdam Construction

Install Wale and Strut System for Framework /Template (Contd)


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Cofferdams Tips for installing Sheet piles:

Always set-up a template system

Rule of thumb: Crane Boom length shouldbe twice that of the sheets

Drive the Sheets with the "male" interlockleading in order to avoid soil plugs

If the "female" interlock must lead, place abolt or other object at the bottom to avoid

debris filling the slot Align and plumb the first two sheets and

drive carefully and accurately

Drive sheets in pairs when possible placing

the hammer in the center of the pair


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Cofferdams

Tips for installing Sheet piles (Contd): Some contractors recommend not driving a

sheet more than 1/3 its length beforedriving the adjacent pile

Letting the sheets "freefall" and drop inorder to aid in penetration will generallycause the sheets to fall "out of plumb"

Cellular cofferdams require that all sheets

are set and "closed" before any driving isdone

Finally never rush the Pile Foreman!!

Design of Cofferdam

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