Building & Construction Technology EAA 484 Cofferdams PDF

BUILDING & CONSTRUCTION TECHNOLOGY EAA 484 BY DR IZWAN JOHARI COFFERDAMS  Learning Outcomes Able to explain the role of cofferdams, advantages, limitations and their installation. Jetty Ferry Sultan Abdul Halim, Penang  Definition...

BUILDING & CONSTRUCTION TECHNOLOGY EAA 484 BY DR IZWAN JOHARI COFFERDAMS  Learning Outcomes Able to explain the role of cofferdams, advantages, limitations and their installation. Jetty Ferry Sultan Abdul Halim, Penang  Definition A temporary structure designed and constructed to keep water and /or soil out of an excavation area in which a bridge pier or other structure or structural component is constructed. Cofferdams ~ these are temporary enclosures installed in soil or water to prevent the ingress of soil and/or water into the working area with the cofferdam. They are usually constructed from interlocking steel sheet piles which are suitably braced or tied back with ground anchors. Alternatively a cofferdam can be installed using any structural material which will fulfil the required function. A cofferdam (also called a coffer) is a temporary enclosure built within, or in pairs across, a body of water and constructed to allow the enclosed area to be pumped out, creating a dry work environment for the major work to proceed.  Cofferdams Cofferdams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent facility (structure) in the dry. A cofferdam involves the interaction of the structure, soil, and water. The loads imposed include the hydrostatic forces of the water, as well as the dynamic forces due to currents and waves. Because cofferdams are typically constructed under adverse conditions, such as in a marine environment, and because significant deformations of elements may occur at various stages of construction, it is difficult to maintain close tolerances. Ample provisions must be made for deviations in dimensions so that the finished structure may be constructed according to plan.  Cofferdams The loads imposed on the cofferdam structure by construction equipment and operations must be considered, both during installation of the cofferdam and during construction of the structure itself. Removal of the cofferdam must be planned and executed with the same degree of care as its installation, on a stage-by-stage basis. The effect of the removal on the permanent structure must also be considered. For this reason, sheet piles extending below the permanent structure are often cut off and left in place, since their removal may damage the foundation soils adjacent to the structure.  Cofferdams In cofferdam construction, safety is a paramount concern, since workers will be exposed to the hazard of flooding and collapse. Safety requires: – good design – proper construction – verification that the structure is being constructed as planned – 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.  Types of Cofferdam Types – typically single skin-cofferdam and double skin coffer dam. Single skin cofferdams - Consist of single row of sheet piles forming an almost completely watertight box. Double skin cofferdams – are self supporting gravity structures constructed by using two parallel rows of sheet piles with a filling material placed in the void created. Single-skin cofferdam Double-skin cofferdam  Types of Cofferdam Braced cofferdams: – Formed from a single wall of sheet piling – Driven into the ground to form a box around the excavation area – The box is then braced on the inside – Interior is dewatered – Primarily used for construction of bridge pier in shallow water Braced cofferdam  Advantages of Cofferdam 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  Limitations of Cofferdam Special equipment required Relatively expensive Typically very time consuming and tedious Steel sheet piles sometimes driven out of lock  Installation requirements Pile driving hammer; vibratory or impact Crane of sufficient size Steel sheet piles H piles and /or wide flange beams for wales and stringers Barges may be required  Steel sheet pile properties Moderately watertight High shear and bending strength High interlock strength Easy to install and remove Reusable Can be cantilevered but typically require additional structural member (waling and cross bracing)  Typical Cofferdam Components Sheet piling Bracing frame Concrete seal Bearing piles Sheet piling  Cofferdams The typical cofferdam, such as a for a bridge pier construction, consists of sheet piles set around a bracing frame and driven into the soil sufficiently far to develop vertical and lateral support and to cut off the flow of soil and, in some cases the flow of water.  Cofferdams Install Wale and Strut System for Framework /Template  Cofferdams The structure inside may be founded directly on rock or firm soil or may require pile foundations. In the latter case, these generally extend well below the cofferdam. In order to dewater the cofferdam, the bottom must be stable and able to resist hydrostatic uplift. Placement of an underwater concrete seal course is the fastest and most common method.  Cofferdams An underwater concrete seal course may then be placed prior to dewatering in order to seal off the water, resist its pressure, and also to act as a slab to brace against the inward movement of the sheet piles in order to mobilize their resistance to uplift under the hydrostatic pressure.  Typical Cofferdams Construction Sequence For a typical cofferdam, such as for a bridge pier, the construction procedure follow the listed pattern. – Pre-dredge to remove soil or soft sediments and level the area of the cofferdam.  Typical Cofferdams Construction Sequence Drive temporary support piles Temporarily erect bracing frame on the support piles.  Typical Cofferdams Construction Sequence Set steel sheet piles, starting at all four corners and meeting at the center of each side Drive sheet piles to grade. Block between bracing frame and sheets, and provide ties for sheet piles at the top as necessary.  Typical Cofferdams Construction Sequence Excavate inside the grade or slightly below grade, while leaving the cofferdam full of water.  Typical Cofferdams Construction Sequence Drive bearing piles. Place rock fill as a levelling and support course.  Typical Cofferdams Construction Sequence Place tremie concrete seal. tremie — a pipe or tube through which concrete is deposited under water, having at its upper end a hopper for filling and a bail for moving the assemblage. tremie seal — the depth to which the discharge end of the tremie pipe is kept embedded in the fresh concrete that is being placed; a layer of tremie concrete placed in a cofferdam for the purpose of preventing the intrusion of water when the cofferdam is dewatered.  Typical Cofferdams Construction Sequence Tremie concrete seal placement Typical Cofferdams Construction Sequence Tremie concrete seal placement  Typical Cofferdams Construction Sequence Check blocking between bracing and sheets. Dewater. Construct new structure. Typical bridge pier construction  Typical Cofferdams Construction Sequence Construct new structure. Flood cofferdam. Typical Cofferdams Construction Sequence Remove sheet piles. Remove bracing. Backfill. Traditional Sheet Pile Shapes Typical types of interlocks Braced Cofferdam Construction Braced Cofferdam Construction