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 B r a c e d - C o f f e r d a m Pictorial Presentation Of Olmsted Lock Project

What is a Cofferdam?? • “A cofferdam is a temporary structure d e s i g n e d t o k e ep e p w a t er e r an a n d /o r s o i l o u t   o f t h e ex e x c a v at a t i o n i n w h i c h a b r id id g e  

 pier or other structure is built.”  - S t an a n d a r d H an an d b o o k o f   H ea ea v y C o n s t r u c t i o n  

Types of Cofferdams •

Braced

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Earth-Type

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Timber Crib

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Double Walled Sheet Pile

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Cellular

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

Earth-Type Cofferdams •Simplest Type of Cofferdam •Consists of an earth bank w/ a clay core or vertical sheet piling 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 by diaphragms  –Circular cells connected with tie-rods or diaphragms

Cellular Cofferdams •

Two main types are circular and segmental

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Can be used on temporary or permanent basis

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Forces are resisted by the mass of the cofferdam

Cofferdam Design Considerations • Scouring or undermining by rapidly flowing water •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 Passive Earth Pressures

Advantages of Cofferdams • •Allow excavation and construction of  structures in otherwise poor environment •Provides safe environment to work •Contractors typically have design responsibility •Steel sheet piles are easily installed and removed •Materials can typically be reused on other  projects

Disadvantages of Cofferdams • •Special equipment required •Relatively expensive •Typically very time consuming & tedious •If rushed, sheets can be driven out of locks or out of plumb •When in flowing water “log jams” may occur 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 wales and stringers •Barges may be required

Pictures of Pile Hammers

Vibratory Pile Driver Impact hammer

Steel Sheet Piling Properties •Moderately watertight •High shear and bending strength •High interlock strength •Easy to install/remove •Reusable •Can be cantilevered but typically require additional structural member (i.e. wales and cross bracing)

Traditional Sheet Pile Shapes

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

 Arch shaped & lightweight Used for shallower wall construction

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

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

Steel Sheet Pile Interlocks •No industry Standard •Interlocks Should:  –provide relative water or earth-tight connections  –permit reasonable free sliding to to connect sheets during installation  –provide minimum guaranteed pull strength  –allow minimum swing between locks in order to form a circle

Typical Types of Interlocks

Single Jaw (SJ)

Thumb & Finger Three Point Contact (TF)

Ball & Socket (BS)

Thumb & Finger One Point Contact (TFX)

Hook & Grip (HG)

Double Jaw (DJ) Double Hook (DH)

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 Construction •

Install Sheet piles using the Framework as Template

Braced Cofferdam Construction Tips 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 •

•Project required massive cofferdam for Lock construction L o c k  c o n s t r  u c t i o n

Rendering of completed project

Olmsted Cofferdam Project • •Overview of Cofferdam Construction  –“U-shaped” cellular cofferdam  –Consisted of 50 round cells and 49 peanut shaped connecting arcs  –Cells have 63’ diameter and place 81’ on center   –Round cells consist of 120 sheets and arcs consist of 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 1994 Slowed the river work

4,Mar,1994

Olmsted Cofferdam Project Deflector and cell being constructed simultaneously

Deflectors 11,July,1994

cells

Olmsted Cofferdam Project

Typical Cell Framework and Construction

Olmsted Cofferdam Project Typical Pile driving operation Using a vibratory hammer. Impact hammer were also used.

Olmsted Cofferdam Project

1,Nov,1994

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

Olmsted Cofferdam Project

Typical Cell Filling Operation

Olmsted Cofferdam Project

Location of connecting arc

Typical Round Cell Construction

Olmsted Cofferdam Project

2,May,1995

Olmsted Cofferdam Project

28,May,1995

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

Olmsted Cofferdam Project

8,Aug,1995

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

Olmsted Cofferdam Project

28,Dec,1995

Completed Cofferdam , turned over to lock contractor

Olmsted Lock Project

Sept,1998 Typical day of operations within the Cofferdam