Post on 06-Apr-2018
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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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CofferdamsTYPES OF IMPOSED LOADS
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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CofferdamsTYPES OF IMPOSED LOADS
Forces due to Soil Loads (Contd)
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CofferdamsTYPES OF IMPOSED LOADS
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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CofferdamsTYPES OF IMPOSED LOADS
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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CofferdamsTYPES OF IMPOSED LOADS
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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Cofferdams Construction Sequence
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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Cofferdams Construction Sequence
7. Excavate inside the grade or slightlybelow grade, while leaving the cofferdam
full of water.
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Cofferdams Construction Sequence
8. Drive bearing piles.9. Place rock fill as a leveling and support
course.
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Cofferdams Construction Sequence
10. Place tremie concrete seal.
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Cofferdams Construction Sequence
Tremie concrete seal.
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Cofferdams Construction Sequence
Tremie concrete seal.
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Cofferdams Construction Sequence
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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Cofferdams Construction Sequence
13. Construct new structure.14. Flood cofferdam.
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Cofferdams Construction Sequence
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!!