Cofferdams
· Provide a working area at foundation level from which ground and water is excluded sufficiently to permit safe working.
· Do not necessarily exclude all water, since it would be uneconomical to attempt to do so.
· The choice of material and type of cofferdam will depend upon the following conditions:
· Location of cofferdam, e.g. onshore, estuarine, offshore
· Depth of and size of excavation
· Volume of water, velocity of flow, tide levels
· Availability of materials
· Accessibility of site.
· If the head of water is low, then an earthfill cofferdam may suffice.
· A face of impervious material may be used to protect the earthfill.
· Site investigation should be carried out.
· Essential when constructing cofferdams to assess the insitu permeability and strength of the soil.
· In fast-flowing rivers the site investigation should assess the problems which may occur due to current-velocities and wave action.
Types of cofferdams and methods of construction
· Site investigation report
· Whether the cofferdam is required on land or in water
· The size of the working area required inside the cofferdam
· The nature of the permanent works to be built
· The amount of water or soil to be excluded whilst work proceeds in the excavation
· Soil conditions
· Water conditions, i.e. strength of flow, wave action, tide or groundwater range
· Availability of materials and plant
· Possible effect of the cofferdam construction on adjacent structures
· Possible methods of constructing the cofferdam
Cost in comparison with other solutions.
· Final choice may result in a combination of cofferdam types. e.g. sheet piling and ground stabilisation.
Gravity Cofferdams
· Earth and rockfill cofferdams are the most common gravity type.
· Very suitable for protecting large areas of excavation against flood waters.
· If the velocity or water is small and the head of pressure low, then earth fill is suitable.
· If the velocity is likely to be high, rock fill offers better resistance to scouring if founded on similar material.
· The water at high tide or flood level may be control led by sealing the dam with clay or other suitable fine material.
· Water that flows beneath the dam can be intercepted by a ditch and drained away.
Sheeted cofferdams
· Sheet piling has gradually replaced timber sheeting
· A combination of H-beam piles and timber sheeting is very suitable for supporting deep excavations in waterlogged ground but only if ground water lowering methods are also employed inside and outside the cofferdam.
· Crib cofferdams are formed by a framework of heavy timbers or precast concrete units which are laced together in a criss-cross fashion
· The water-face of the crib can be made water- tight by driving a single line of steel sheet piling along the face.
· A water cut-off is formed by driving the sheet piling to an impervious stratum.
· Concrete sheeting includes precast sheet piles, precast or cast in-situ panels or slabs, contiguous bored piles and conventional diaphragm walling.
· The most popular methods of concrete sheeting are contiguous piling and diaphragm walling
· Not necessarily an integral part of the permanent works but can be designed as such.
· Contiguous piling may be used where headroom or vibration prevents the driving of steel sheet piling
· Diaphragm walling is particularly suitable for large cofferdams in weak or waterlogged ground.
· Steel sheeting can be divided into two distinct types: single-skin, which has one or more vertical stages, and double-skin. which consists of two lines of sheet piles or circular cells of sheeting filled with rock or other material.
· Cellular cofferdams are constructed from straight-web steel sheet piling.
· Interlocked and driven to form cells.
· Cellular cofferdams are suitable for resisting the considerable head differences which are encountered in harbour and dock works.
· Cellular cofferdams can be used on irregular beds of rock when the sheet piles are cut to fit the rock profile.
· Double-wall cofferdams consist of two parallel lines of sheet piling connected by walings and tie rods at one or more levels: the space between the walls is filled with material to give stability.
· The inner line of piling is designed as a retaining wall suitably keyed into solid strata, and the outer line as anchorage. Ordinary steel pile sections are preferable.
Economic factors
· The design for a cofferdam may be executed in various ways and in different materials, but a cost ‘break-even’ point can be established for the various solutions.
· Limitations are size and depth of cofferdam.
· Above a certain size shoring becomes prohibitive with thin wall dams and thick diaphragm walling may well prove more economic.
· Earth and rockfill are even cheaper than steel sheet piling for very large shallow cofferdams.
· The nature of the permanent works and method and sequence of excavating may affect the amount of strutting and shoring needed.
· Safety is also importance, many accidents have occurred in cofferdams.
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