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The Challenge

Following the tragedy at Grenfell Tower in London surveys were carried out across the country, the three Mountwise Towers were identified as having similar cladding materials on the external facade. A comprehensive programme of removal and replacement was started by Plymouth City Council, but as the removal process was completed with the imminent start of the new cladding installation approaching, concerns were raised by the project engineers about the condition of the existing wall ties. We were asked to carry out a wall tie condition survey, prepare a report and cost for any remedial works based on our findings with a fast turnaround.

The Solution

The cavity walls were constructed using galvanised fish tail ties and as expected the Southerly facing higher level ties were at the most advanced stage of corrosion but most were not deemed to be an action level that needed immediate remedial works. As per the BRE 401 Digest a further inspection within 5 years would have been recommended but this would not be possible due to access issues once the project was completed and the access scaffolding removed. On checking the density of the existing ties it was obvious that they were not spaced at the correct intervals for a high rise building in a coastal position and the possibilty of wind suction causing structural failure meant that additional ties would be required. The Dri Flex helical wall tie was selected as it performed above the calculated loadings required to resist the loads.

Installation

Each tie was drilled with a pilot hole through the outer leaf carried out with a percussion drill to eliminate the potential for the back of the brick blowing out. This can reduce its structural area and its effectiveness to hold the tie and put debris into the cavity. A second pilot hole was then drilled using a hammer drill into the inner leaf (block) before a Dri Flex tie could be inserted under hammer action. With each tower requiring circa 12,000 ties the processes had the potential to cause Hand Arm Vibration damage to our operatives but with investment in the lowest impact equipment and HAVI monitoring systems we were able to complete the works quickly and safely. During the install process a strict regime of in-situ testing was carried out at each floor which had distinct panels of different types of brick and concrete panels.

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