Extensions to existing buildings are always vulnerable to cracking where they meet the main structure, normally due to shrinkage or differential settlement. Such cracking is normally superficial and is not expected to deteriorate further.
Nowadays, movement joints are placed at specific locations in order to allow for such movement. Most movement joints in Ireland are in fact contraction joints as concrete blockwork will always shrink first. The degree to which shrinkage will occur will depend on a few factors, but particularly the time and conditions to which blocks are allowed to cure.
These movement joints, normally placed on the external leaf of a traditional cavity wall, are constructed with a proprietary joint bead to allow movement of ± 3mm. Homebond provide good details for the design and construction of movement joints in different forms of construction.
However, this extension in Capwell, Cork city has failed due to an escape of water from a vitreous clay drain at the rear. Services which are leaking over long periods gradually reduce the load bearing capacity of the soils by removal of the finer particles in granular soils.
Although sometimes diagnosing subsidence through a simple present condition survey may sometimes prove difficult without further investigation, the degree of cracking and evidence of a previous repair strongly suggests that movement is ongoing. Note the failure of the traditional ‘toothing in’ below eaves level. Movement of about 15mm was also noted internally.
This cracking is typical of subsidence in an extension where a vertical crack forms between both components; the crack being wider at the top as the extension rotates away from the original structure; the corner of the extension acting as the point to foundation movement.
The approach to this problem is to replace the drain and underpin the rear elevation through micropile supported reinforced concrete ground beams.