Cofferdam

Objectives:

Objectives:

••

Definition Of

Definition

Of Cofferdam

Cofferdam

••

Types Of

Types

Of Cofferdam

Cofferdam

••

General Design

General

Design Considerations

Considerations Of

Of

Cofferdams

Cofferdams

••

Advantages And

Advantages

And Disadvantages

Disadvantages Of

Of

Cofferdam

Cofferdam

••

Typical Items

Typical

Items Needed

Needed For

For Constructions

Constructions Of

Of

Cofferdam

Cofferdam

••

Overview Of

Overview

Of Steel

Steel Sheet

Sheet Piling

Piling

••

Pictures Of

Pictures

Of Basic

Basic Construction

Construction Procedures

Procedures

For a

For a

B r a c e d - C o f f e r d a m

B r a c e d - C o f f e r d a m

••

Pictorial Presentation

Pictorial

Presentation Of

Of Olmsted

Olmsted Lock

Lock

Project

What is a

What is a Cofferdam??

Cofferdam??

•• “A cofferdam is a

“A cofferdam is a temporary structure

temporary structure

d e s i g n

d e s i g n e

ed t o

d t o k e e

k e ep w

p w a

at e

t er a

r an d

n d //o r s

o r s o i l o u

o i l o u t  

t  

o f t h e e

o f t h e ex c a v a

x c a v at i o n in w

t i o n in w h i c h

h i c h a b r i

a b r id g

d g e  

e  

 pier or other

 pier or other structure is

structure is built.” 

built.” 

--S t a

S t an d

n d a

ar d H a

r d H an d

n d b o

b o o k o

o k o f  

f  

H e

Types of Cofferdams

Types of Cofferdams

•• BracedBraced •• Earth-TypeEarth-Type •• Timber CribTimber Crib

•• Double Walled Sheet PileDouble Walled Sheet Pile •• CellularCellular

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

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 • 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 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

Pictures of Pile Hammers

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

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 & lightweight

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

Typical Types of Interlocks

Ball & Socket (BS) Single Jaw (SJ)

Double Jaw (DJ) Hook & Grip (HG)

Double Hook (DH) Thumb & Finger

One Point Contact (TFX) Thumb & Finger

Braced Cofferdam Construction

•Install Wale and Strut System for Framework /

Template

Braced Cofferdam Construction

Braced Cofferdam Construction

Braced Cofferdam Construction

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

Olmstead Lock & Dam Project

•Project required massive cofferdam for Lock construction

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

-

Olmsted Cofferdam Project

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

Olmsted Cofferdam Project

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

Olmsted Cofferdam Project

Deflector and cell being constructed simultaneously

cells

Deflectors 11,July,1994

Olmsted Cofferdam Project

Olmsted Cofferdam Project

Typical Pile driving operation Using a vibratory hammer.

Olmsted Cofferdam Project

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

Olmsted Cofferdam Project

Olmsted Cofferdam Project

Typical Round Cell Construction

Olmsted Cofferdam Project

Olmsted Cofferdam Project

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

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

Olmsted Cofferdam Project

Completed Cofferdam , turned over to lock contractor 28,Dec,1995

Olmsted Lock Project

Typical day of operations within the Cofferdam Sept,1998