Cofferdam

Objectives:• Definition Of Cofferdam• Types Of Cofferdam• General Design Considerations Of Cofferdams• Advantages And Disadvantages Of Cofferdam• Typical Items Needed For Constructions Of

Cofferdam• Overview Of Steel Sheet Piling• Pictures Of Basic Construction Procedures

For a Braced-Cofferdam• Pictorial Presentation Of Olmsted Lock Project

What is a Cofferdam??

• “A cofferdam is a temporary structuredesigned to keep water and/or soil outof the excavation in which a bridgepier or other structure is built.”

-Standard Handbook of Heavy Construction

Types of Cofferdams • Braced

• Earth-Type

• Timber Crib

• Double Walled Sheet Pile

• Cellular

Braced Cofferdams

• •Formed from a single wall of sheet piling•Driven into the ground to form a boxaround the excavation site•The “box” is then braced on the inside•Interior is dewatered•Primarily used for bridge piers in shallowwater (30 - 35 ft depth)

Earth-Type Cofferdams

•Simplest Type of Cofferdam•Consists of an earth bank w/ a

clay core or vertical sheetpiling enclosing the excavation•Used for low-level waters with

low velocity•Easily scoured by water rising

over the top

Timber Crib Cofferdam

•Cellular-Type Cofferdam•Constructed on land and floated into place•Lower portion of each cell matched with

contour of river bed•Uses rock ballast and soil to decrease

seepage and and sink into place•Also known as “Gravity Dam”

Timber Crib Cofferdam (cont.) • Usually consists of 12’ x 12’ cells• Used in rapid currents or on rocky river

beds• Must be properly designed to resist lateral

forces such as: -Tiping/Overturning -Sliding

Double-Walled Cofferdam *Two-parallel rows of steel sheet piles

driven into the ground*Tied together with anchors and wales,then filled with soil*Three principle types:–Box: Consists of straight flush walls–Semicircular cells connected bydiaphragms–Circular cells connected with tie-rods ordiaphragms

Cellular Cofferdams • Two main types are circular and segmental• Can be used on temporary or permanent basis• Forces are resisted by the mass of the cofferdam

Cofferdam Design Considerations

• Scouring or undermining by rapidly flowingwater•Stability against overturning or tilting•Upward forces on outside edge due to tilting•Stability against vertical shear•Effects of forces resulting from:–Ice, Wave, Water, Active Earth and PassiveEarth Pressures

Advantages of Cofferdams

• •Allow excavation and construction ofstructures in otherwise poor environment•Provides safe environment to work•Contractors typically have designresponsibility•Steel sheet piles are easily installed andremoved•Materials can typically be reused on otherprojects

Disadvantages of Cofferdams

• •Special equipment required•Relatively expensive•Typically very time consuming & tedious•If rushed, sheets can be driven out of locksor out of plumb•When in flowing water “log jams” mayoccur creating added stress on structure

Items needed for installation

•Pile driving hammer–Vibratory or Impact•Crane of sufficient size•Steel sheet piles are typically used•H-piles and/or wide-flange beams for walesand stringers•Barges may be required

Pictures of Pile Hammers

Impact hammer

Vibratory Pile Driver

Steel Sheet Piling Properties

•Moderately watertight•High shear and bending strength•High interlock strength•Easy to install/remove•Reusable•Can be cantilevered but typically requireadditional structural member (i.e. wales andcross bracing)

Traditional Sheet Pile Shapes

Z-Type (Z)Used for intermediate to deep wall construction

Larson / “U” Type (U)Used for applications Similar to Z-Type

Arch shaped & lightweightUsed for shallower wall construction

Flat / Straight Type (SA),(S)Used for filled cell construction

Steel Sheet Pile Interlocks •No industry Standard

•Interlocks Should:–provide relative water or earth-tightconnections–permit reasonable free sliding to to connectsheets during installation–provide minimum guaranteed pull strength–allow minimum swing between locks in orderto form a circle

Typical Types of Interlocks

Ball & Socket (BS)Single Jaw (SJ)

Double Jaw (DJ) Hook & Grip (HG)

Double Hook (DH)

Thumb & FingerOne Point Contact (TFX)

Thumb & FingerThree Point Contact (TF)

Braced Cofferdam Construction •Install Wale and Strut System for Framework /

Template

Braced Cofferdam Construction• Install Wale and Strut System for Framework/Template

Braced Cofferdam Construction• Install Sheet piles using the Framework as Template

Braced Cofferdam ConstructionTips for installing sheet piles: -Always set up a template system -Rule of thumb:Crane Boom length should be twice that of the sheets -Drive the sheets with the “male” interlock leading in order to avoid soil plugs -If the “female” interlock must lead,place a bolt or other object at the bottom to avoid

debris filling the slot -Align the plumb the first two sheets and drive carefully and accurately -Drive sheet in pairs when possible placing the hammer in the center of the pair -Some contractors recommend not driving a sheet more than 1/3 its length before

driving the adjacent pile -letting the sheets “freefall” and drop in order to aid in penetration will generally

cause the sheet to fall ”out of plumb” -Cellular cofferdam require that all sheets are set and “closed” before any driving is

done -Finally…never rush the pile Foreman!!

Olmstead Lock & Dam Project •

Lock construction

•Project required massive cofferdam for Lock construction

Rendering of completed project

Olmsted Cofferdam Project • •Overview of Cofferdam Construction

–“U-shaped” cellular cofferdam–Consisted of 50 round cells and 49 peanut shapedconnecting arcs–Cells have 63’ diameter and place 81’ on center–Round cells consist of 120 sheets and arcs consistof 50 sheets (25 inside & out)–Sheet piling ranged from 50’ to 109’ in length–All cells filled with approx. 675,000 CY of sand

Olmsted Cofferdam Project -Pictorial Presentation of Cofferdam Construction

Site before construction

Olmsted Cofferdam Project

First six months of construction. Excavation of terrain performed to avoid “sliding” While lock foundation was being excavated.

1 Nov 1993

Olmsted Cofferdam Project Several high water events during the fall of1993 and spring of 1994Slowed the river work

4,Mar,1994

Olmsted Cofferdam Project Deflector and cell being constructed simultaneously

cells

Deflectors

11,July,1994


Typical Cell Framework and Construction


Typical Pile driving operationUsing a vibratory hammer.Impact hammer were also used.


Construction status improved significantly due during Summer and fall of 1994 due to extremely good river conditions

1,Nov,1994


Typical Cell Filling Operation


Typical Round Cell Construction

Location of connecting arc


2,May,1995


Dewatering of cofferdam was delayed due to a flood,in this photoThe water is within 1 foot of the top of cofferdam

28,May,1995


Dewatering of cofferdam took approx. 40 days,beginning in July of 1995

8,Aug,1995


Completed Cofferdam , turned over to lock contractor

28,Dec,1995

Olmsted Lock Project

Typical day of operations within the Cofferdam

Sept,1998

Olmsted Lock Project

Overview shot of project as of Dec,9,1998

Cofferdam

Documents

Transcript of Cofferdam