Overview

The customer had transferred all power generation processes to a new reactor installation, after the decommissioning of the previous reactor, to be replaced by a more efficient, EU-compliant reactor housed in an adjacent reactor hall. To facilitate construction of an interactive visitor centre and art gallery in the disused reactor hall, 30 supporting columns were to be removed, and 8 large openings formed in the ceiling. This would allow exhibition space to be constructed in the disused hall, and the installation of stairwells and elevator shafts in order to provide access to the visitor centre, to be subsequently implemented through an additional roof-top storey, resulting from a specially-commissioned architectural design competition.

 Objectives

 Procedure

From a mobile scaffolding tower, a wire saw was used to detach each of the designated columns from the ceiling. Each column was supported on all sides using a set of 4 adjustable steel Acro props.

Each column was then demolished, through progressive removal of the column in three sections. Each column section was secured by sinking four 25cm Liebig rock anchors into each section, which were then used to harness the section to a mobile crane using high-tensile weight-bearing cables.

The wire saw was then used to detach the bottom of the section. When the section was free, the crane was then used to pull the section out of its existing position, and transported to a refuse vehicle outside the building.

The section removal process was then repeated for each section of the column, and for each of the remaining columns to be removed, meaning that a total of 90 column sections were removed in this way.

The positions of the ceiling openings were determined, and the four corners for each opening were marked with the extraction of a diamond-drilled core. The cores were drilled from inside the reactor hall, up through the whole 0.5 metre thickness of the concrete ceiling, using the mobile scaffolding tower. 32 cores were removed in total, one for each corner. The sections of the ceiling to be removed were then supported from below using another series of Acro props.

The ceiling openings were marked from above, using the drilled core-holes for guidance, and the mobile crane and a floor saw were both transported up onto the roof.

For the first of the eight ceiling openings in turn, four 25cm Liebig rock anchors were sunk into the section to be removed, and harnessed to the mobile crane using high-tensile weight-bearing cables, to provide support from above.

The opening could then be formed, by cutting along the marked outlines from above using the diamond floor saw, using a blade with 50cm cutting depth. Once the concrete section was free, the mobile crane was used to extract the section, and transport it to the edge of the building, where it was lowered into a refuse vehicle.

The process was then repeated for each of the ceiling sections to be removed.

The roof-top installation to be subsequently developed would include an external steel marquee structure, harnessed to the perimeter of the existing reactor hall superstructure. The marquee harnessing points would need to be anchored to the existing roof structure using rock anchors.

The exact locations for the 76 anchoring points were determined, and marked, and the correct angle at which each anchor bolt should enter the roof was determined, working from architectural plans. Four different anchor angles were required, depending on the intended location for each harnessing point.

A pair of adjustable custom drilling rigs were deployed, each of which facilitated diamond core drilling at the precise angles required. The two rigs were deployed concurrently, each used to drill half of the required anchoring core holes. Each hole was drilled obliquely to a depth of 35 cm, at a diameter of 10 cm.

A 25cm Liebig rock anchor was installed into each hole, and fastened with fast setting industrial resin. The rock anchors would provide a secure anchoring point for the subsequent installation of the marquee structure.

 Additional information