Feather Edges in Mid Cork

Crack at Gable 1 A
Crack at Gable 1 B
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Construction sites are not always level and the design of economic foundation solutions to the engineer can be quite challenging.

In gently sloping sites, strip foundations may be stepped. However, where there is a more appreciable change in gradient, a cut and fill process can be adopted where the higher level is cut away and the lower level is filled. In some cases, the cut area is used to fill the lower area if the soils are appropriate.

If the cut soils are not appropriate, engineered fill will be required. This is where selected fill is placed and compacted to an engineer’s specification and carefully monitored to ensure that it can act as a suitable support for the new foundation.

This dwelling, built about fifteen years ago, has a raft foundation constructed on a cut and fill site, with half the building founded on the cut side and the other half founded on the filled side – this ‘design’ is known as feather edging and carries risk. The drawings suggest that the whole footprint of the building was cleaned down to the rock and filled with ‘rolled broken rock’ to a depth of about two meters in the fill side and about 300mm on the cut side.

It is important to note that it is differential settlement in a building rather than uniform settlement than results in foundation failure. Raft foundations should be constructed fully on natural ground or fill of uniform thickness. Raft failure will occur in one of two ways: geotechnically, where it will tilt (as one unit) in one direction, or structurally, where the raft will break, and superstructure cracking will result.  The factors that effect this include the reinforcement of the raft and the geometry of the building.

In this case, the raft has structurally failed, due to differential settlement of the fill.