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Continuous slip formed gravity base structure supports under construction in a Norwegian fiord. The visible jib cranes would each be delivering buckets of concrete to the support cylinders during the continuous pour of concrete creating seamless walls.

The continuous poured, continuously formed, or slip formed construction method is a concrete construction technology that enables cast-in-place "flawless" (i.e. seamless) concrete members which have superior performance characteristics to piecewise construction using discrete elements because of the polymerizing characteristics of concrete (All concretes utilize polymer chemistry, though polymer concrete is a special quick setting formulation which is particularly rapid at curing to high tensile strengths).

Continuous pouring antedates slip forming by several decades, being a new technique pioneered during the construction of the Hoover Dam works project during the Great Depression. Slip forming was initially devised and utilized in the Interstate Highway building binge of the Eisenhower administration during the 1950's. In the method, concrete is laid down, packed, worked, and settled in place while the form itself, normally attached and part of a motive power producing machine which slowly 'slips it' along, moving it slightly ahead of the newly packed material exiting its trailing end, the front of which forms a new box form ready for more poured concrete with the fresh laid concrete closing the rear end of the moving slip form.

In subsequent decades the benefits of seamless slip formed construction have been adapted to mining, tower construction, and high rise office buildings, including structures and buildings which would otherwise not be possible such as the separate legs of the Troll A deep sea oil drilling platform which stands on the sea floor in water about 1000 feet (300 m) deep, has an overall height of 472 meters (1,549 ft) and weighs 656,000 tons and has the distinction of being the tallest structure ever moved (towed) by mankind. The platform stands on the sea floor 303 meters (994 feet) below the surface of the sea and each of the continuous-slip-formed[1] concrete cylindrical legs has an elevator that takes over nine minutes to travel[1] from the platform above the waves to the sea floor. The walls of Troll A's legs are over 1 meter thick made of steel reinforced concrete formed in one continuous pour [1] and each is mathematically a joined composite of several conical cylinders that flares out smoothly to greater diameters at both the top and bottom, so each support is somewhat wasp-waisted viewed in profile and circular in any cross-section. (See picture at right)

The four legs are joined by a "chord shortener", a reinforced concrete box interconnecting the legs, which has the designed function of damping out unwanted potentially destructive wave-leg resonances by retuning the leg natural frequencies.[1](Not present in the picture at right.) Each leg is also sub-divided along its length into compartments a third of the way from each end which act as independent water-tight compartments.[1] The legs use groups of six (40 m (131 ft) tall[1] vacuum-anchors holding it fixed in the muck of the sea floor.

Notes

  1. ^ a b c d e f National Geographic Channel production, documentary 2007(?), rebroadcast 2009-10-02, 12-13:00 hrs EDST (Comcast Cable Television system)
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