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.