The ability of masonry to arch (or ‘self-arch’) relates to the transfer of axial loads above openings to either side of the wall and is considered during lintel design. The term comes from the principle of how an arch works although there is in effect no arch (if that make sense).
For arching to occur, the main criterion is the presence of sufficient masonry above the apex of a 45° isosceles triangle, formed above the opening. Other factors include the bond pattern, location of movement joints and the ability of the masonry on either side to carry the loads.
In underpinning works, the arching action of masonry is considered when specifying centres of needle beams in micropile supported raft foundations and traditional pile and beam systems where beams are to be installed through rising walls.
However, foundation movement to the order of about 50mm has rendered this garage wall in Co. Laois cantilevered along a combined length of about five metres and defying the laws of gravity!!!! The walls are constructed of cavity block in the normal overlapping bond pattern with no opes or movement joints to limit a uniform load distribution.
Ground investigation showed that the building was partially constructed on made ground to a depth of about two metres. According to the homeowner, this area was made up during the construction of the primary dwelling with little or no controls in place. The surface water system was also poorly installed and contributed to the movement.