COFFERDAMCoffer – large strong box for valuables; sunken panel or box1. A temporary watertight enclosure that is pumped dry to expose the bottom of a body of water so that construction, as of piers, may be undertaken.2. A watertight chamber attached to the side of a ship to facilitate repairs below the water line.3. A water tight enclosure pumped dry to permit work below the water line eg for the construction of underwater foundations for bridges, piers (Oxford Dictionary) What is a Cofferdam?A cofferdam is a type of watertight construction designed to facilitateconstruction projects in areas which are normally submerged, such asbridges and piers.A cofferdam is installed in the work area and water is pumped out to expose thebed of the body of water so that workers can construct structural supports, enactrepairs, or perform other types of work in a dry environment. In some regions ofthe world, a cofferdam is better known as a caisson.Working inside a cofferdam can be hazardous if it is installed improperly or notsafely pressurized, but advances in engineering have led to increased safety forworkers using this unique work environment.A variety of materials can be used to construct a cofferdam, which is truly a featof engineering. Although a cofferdam is a temporary structure, it must reliablyhold water back from the work area and also withstand very high pressures inorder to be safe, and the construction of cofferdams is often used as a project forengineers learning their craft.The most basic type of cofferdam uses sheet metal, which is pounded into thebed of the body of water to create a watertight wall. Next, pumps are used to pullwater out of the enclosure so that it will be dry.
Some cofferdams are built from wood or concrete, while others use a doublewalled mechanism, with filler made from aggregate materials in between the twowalls.The walls of a cofferdam can extend all the way to the surface of the water,leaving it open at the top, or it can be built as an enclosed structure.In very deep water, enclosed and pressurized cofferdams are used for workersafety, while in shallower bodies of water, an open cofferdam can be used.Workers access a closed cofferdam through hatches and tubes, and care istaken to make sure that the air supply is consistent and the pressure is kept at anormal level.Shipwrights and repair yards also use a form of portable cofferdam, which can beattached to the side of a ship to enact repairs below the waterline. At sea, thiscan be a useful way to quickly address potential problems until the ship is takeninto dry dock for more long-term repair. Minor repairs can be undertaken with aportable cofferdam in a shipyard to avoid the expense of hauling the ship into drydock for the work to be completed.COFFERDAM AND DEWATERING2.04.01--Description: Work under this item shall consist of the design andconstruction of cofferdams as and where shown and specifically designated assuch on the plans; necessary dewatering, adjustments, repair or reconstruction;and the removal of temporary cofferdams and related facilities.2.04.03--Construction Methods:1--Cofferdams : Cofferdams shall be carried to adequate depths and heights,shall comply with Section 1.10, and shall be safe and watertight as necessary forthe proper performance of the work which must be done inside them. Cofferdamsshall be constructed so that the work can be safely carried to an elevation 600mm lower than the elevation shown on the plans for the bottom of the structurefooting, or, if a granular fill foundation is shown on the plans, to an elevation 600mm lower than the bottom of the granular fill foundation. The interior dimensionsof the cofferdams shall be sufficient for the unobstructed and satisfactoryompletion of all necessary substructure work, such as pile driving, form building,inspection and pumping. Cofferdams which become tilted or displaced prior tothe completion of all work to be done within them, shall be righted, reset, orenlarged as may be necessary to provide the clearance for the unobstructedperformance of all necessary work, and such corrections and adjustments ofcofferdams shall be at the sole expense of the Contractor. Cofferdams shall becompletely dewatered as required to complete the work entirely in the dry, exceptas specified below.
When conditions are encountered that render it impractical to dewater thecofferdam, the Engineer may require the placing of underwater concrete of suchdimensions as will be necessary to allow the Contractor to complete thesubstructure in the dry. The placement of underwater concrete shall comply with6.01.03-10.Cofferdams must be constructed to protect uncured masonry and concreteagainst damage from a sudden rising of the water and prevent damage tostructure foundations by erosion. No part of the cofferdam which extends into thesubstructure may be left in place without written permission from the Engineer.At least 30 calendar days prior to the start of constructing or installing acofferdam, the Contractor shall submit to the Engineer, for his information,detailed plans and computations of its proposal prepared by a professionalEngineer licensed in the State. The furnishing of such plans and methods shallnot serve to relieve the Contractor of its responsibility for the safety of the workand the successful completion of the Project. The Contractors proposal mustmeet all requirements established in regulatory permits for the Project and mustalso conform to the requirements of Section 1.10.2--Dewatering: Pumping from the interior of any cofferdam shall be done in sucha manner as to preclude the possibility of water moving through uncuredmasonry or concrete. During the placement of concrete or masonry, and for atleast 24 hours thereafter, any pumping shall be done from a suitable sumplocated outside the horizontal limits and below the elevation of the work beingplaced or as directed by the Engineer.The pumped water must be discharged in accordance with the requirements ofSection 1.10.Pumping to dewater a cofferdam shall not start until any underwater concrete hassufficiently set to withstand the hydrostatic pressure created by pumping.3--Removal of Cofferdams : Unless the Engineer directs otherwise, theContractor shall remove all parts of the cofferdam after completion of therequired work. This shall be done in such a way as not to disturb or otherwisedamage any permanent construction.Sheet piling used in constructing the cofferdam may be left in place with theapproval of the Engineer, provided the piling is cut off at elevations approved inadvance by the Engineer, and the cut off portions are removed from the site.2.04.04--Method of Measurement: Work under this item will be measured forpayment by the number of meters of cofferdam designated numerically on theplans.
2.04.05--Basis of Payment: Payment for this work will be made at the Contractunit price per meter for "Cofferdam and Dewatering," measured as describedabove, which price shall include all costs of design, materials, equipment, labor,work, and any related environmental controls used in dewatering operations,which are required for the construction of cofferdams shown in the plans; of anyrepair, correction, adjustment or reconstruction of such cofferdams required bythe plans; removal of obstructions; pumping and dewatering; removal of suchcofferdams and related environmental controls used in dewatering operations.If the Engineer requires the Contractor to construct an additional cofferdam notshown on the plans, or to enlarge a cofferdam beyond the dimensions of sameas designated on the plans, or if the Engineer accepts theContractors proposal to do so as being essential for the purposes of theContract, the Department will revise the Contract to indicate those changes andto designate the revised dimensions of cofferdam deemed necessary by theEngineer. If the total number of meters of any given cofferdam as designated inthe revised Contract is greater than the number of meters designated on theoriginal Contract plans, the Department will pay the Contractor for the revisednumber of such meters at the Contract unit price, subject to the provisions ofArticles 1.04.02 and 1.04.03.To the extent that the Engineer allows the addition or enlargement of a cofferdamfor the convenience or other benefit of the Contractor, but does not deem thataddition or enlargement essential for the performance of the Contract work, theDepartment will make no additional payment for the cofferdam or portion of thecofferdam which the Engineer does not so deem essential. The Department shallnot in any event pay the Contractor for fewer meters of a cofferdam than weredesignated on the original Contract plans unless the Department eliminates thatcofferdam in its entirety from the Contract.Even if, however, the Contractors plan for an additional cofferdam orenlargement of a cofferdam deemed essential by the Engineer includes apreviously-existing structure, in no case will a previously-existing natural or builtstructure, such as an abutment or an embankment, be measured for payment incalculating the revised number of meters of cofferdam on the Project.Any common cofferdam wall required for staged construction will be measuredfor payment only once. In no case will a given length or portion of cofferdam bemeasured for payment purposes more than once. Pay Item Pay Unit Cofferdamand Dewatering m
DIAPHRAGM WALLS TECHNOLOGY OVERVIEWTABLE OF CONTENTS 1. INTRODUCTION 2. METHODS 3. APPLICATIONS 4. CONCLUSIONINTRODUCTIONDiaphragm walls are underground structural elements commonly used forretention systems and permanent foundation walls. They can also be used asdeep groundwater barriers.Diaphragm walls are constructed using the slurry trench technique, which wasdeveloped in Europe and has been used in the United States since the 1940s.The technique involves excavating a narrow trench that is kept full of anengineered fluid or slurry. The slurry exerts hydraulic pressure against the trenchwalls and acts as shoring to prevent collapse. Slurry trench excavations can beconstructed in all types of soil, even below the ground water table. Back to TopMETHODSCast-in-place diaphragm walls are usually excavated under bentonite slurry.Various types of excavation equipment can be used depending on projectconditions, including hydraulic excavators and kelly-mounted or cable-hung clambuckets. Depths in excess 150 feet are possible. (The Hydrofraise, a highlyspecialized excavation tool, can reach depths of 500 feet.)Diaphragm wall construction begins with the trench being excavated indiscontinuous sections or "panels". Stop-end pipes are placed vertically at eachend of the primary panel to form joints for adjacent secondary panels. Panels areusually 8 to 20 feet long, with widths varying from 2 to 5 feet.Once the excavation of a panel is complete, a steel reinforcement cage is placedin the center of the panel. Concrete is poured in one continuous operationthrough one or more tremie pipes that extend to the bottom of the trench. Thetremie pipes are extracted as the concrete rises; however, the discharge end ofthe tremie pipe always remains embedded in the fresh concrete.The slurry that is displaced by the concrete is saved and reused for subsequentpanel excavations. As the concrete sets, the end pipes are withdrawn. Similarly,
secondary panels are constructed between the primary panels to create acontinuous wall. The finished wall may be cantilever or require anchors or propsfor lateral support.A variation of the technique is the precast diaphragm wall. With this method, acontinuous trench, or longer panel is excavated under self-hardening cement-bentonite slurry. The slurry is retarded to remain fluid during construction. After asufficent length of excavation is complete, a crane lifts the precast concrete wallsection into the trench. The cement bentonite slurry sets to form the finalcomposite wall. Alternately, the trench is excavated under bentonite slurry, whichis then displaced with cement bentonite slurry. Back to TopAPPLICATIONSDiaphragm walls are commonly used in congested areas. They can be installedin close proximity to existing structures with minimal loss of support to existingfoundations. In addition, construction dewatering is not required, so there is noassociated subsidence.The cut and cover method is used to construct tunnels. Two parallel diaphragmwalls are installed and the area between the walls is excavated. Floor and roofslabs are poured and area above the roof is backfilled. Back to TopCONCLUSIONThe diaphragm wall technique is a tried-and-true construction method, whichprovides unequaled support of existing foundations during adjacent constructionoperations. The method has many civil applications.