Ovoid digesters shape Fukuyama skyline

The first phase of the Ashidagawa treatment plant in Fukuyama city, Japan, consists of one prestressed concrete ESD with a capacity of 5.000 metres³. The digester is 30,55 metres tall and has a maximum diameter at the mid-section of 20,45 metres.

The digester construction was divided into 6 sections. Starting with the bottom

cone section (S1) with integrated pile support and ring foundation at the perimeter

(S2), the remaining sections include the contruction with a double-curved geometry

making the egg-shape, plus a final cone section (S6). The wall thickness throughout

the bottom cone structure was 0,60 metres. At the beginning of the double-curved

section the wall thickness started at 0,60 metres and decreases to 0,4 metres

at the top cone section.

Precut trapezoidal formwork board elements are inserted into the assembled,

self-supporting steel skeleton. These form boards provide the surface upon which

concrete will be cast.

The most important goal in connection with a liquid retaining structure is

the bottle tightness of the concrete of the tank walls. Since through wall ties

and cold joints are weak spots, a custom-made formwork system provided by RSB

reduces the number of cold joints to an absolute minimum and eliminates the

need for form-ties by designing the inner and outer formwork for the tank walls

as two independent, self supporting systems.

Construction of the digesters on site is supervised by an experienced RSB formwork

specialist to assure exact erection of the formwork. Optimum placement of the

concrete can be guaranteed for the height of the form boards is only 0,5 metre.

The whole conical section can be cast in one step by continuously inserting

the form boards immediately before the concrete is placed and vibrated. This

way a complete section is poured in a continuous helix and cold joints in the

structure are reduced to an absolute minimum.

For Ashidagawa, specialty contractors were under the lead of Shimizu Construction

Corporation, bringing together years of experience in the design and construction

of axi-symmetric double-curved structures. The result is a low maintenance,

long life anaerobic digester that will serve for many decades in the future.

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