We have had a busy summer, particularly in our Auckland office, undertaking, what I call “forensic engineering” assessments for houses, at the request of owners who have noticed damage to their properties.

It is not uncommon to occasionally notice minor damage to our houses, particularly after long dry periods. I have covered the dangers of “expansive soils” in my previous articles. Shallow foundations, supported on expansive soils, which are not appropriately designed, can be adversely affected by the seasonal swelling and shrinkage of the supporting soils, which can result in foundation damage.

Foundation Movement

However, there are times when observed damage to houses can be indicative of a different damage mechanism which is causing ongoing differential foundation movement. The damage associated with differential foundation movement can be extensive and can undermine the value of our assets. Tell-tale signs of foundation/floor movement may include the following:

1 stepped cracking through brick veneer
2 tapered cracking through concrete foundations
3 cracks in concrete floor slabs
4 significant cracking in ceiling and wall linings
5 noticeable floor out-of-levelness
6 noticeable roof truss displacement/damage
7 windows and doors jamming.

I would advise you to get an appropriately qualified and experienced engineer to undertake a forensic engineering investigation of the structure, in order to determine the likely mechanism of the damage. There are often several possible causes of the damage, so reliably determining the mechanism is critical, in order to come up with the most appropriate remedial solution.

Vertical Or Lateral Ground Displacement

Typically differential foundation movement is the result of vertical or lateral displacement of the ground supporting the foundations, and could be caused by any (or a combination) of the following:

a settlement of non-engineered fill
b localised settlement of trench backfill material
c slope instability
d displacement of retaining walls close to shallow foundations
e collapse of subterranean erosion features
f earthquake induced ground deformation (including liquefaction effects)
g soil swell/shrink.

'Forensic Engineering'

The “forensic engineering” investigation, as the name suggests, is a process by which engineers gather the site-specific evidence and, using their knowledge and experience, provide an objective opinion as to the likely cause of the observed damage. The investigation/assessment is an iterative process, and normally involves the following:

i determining the pattern of damage and foundation/floor out-of-levelness
ii determining the out-of-levelness of the superstructure and fixed features, including window sills, kitchen benches and door heads
iii a visual appraisal of the site and site features
iv undertaking a site-specific geotechnical investigation
v collating site information, in particular reconciling observed damage with the results of the foundation/floor survey
vi further site investigation works (following the initial assessment)
vii eliminating the unlikely causes (based= on results of assessment works)
viii based on the evidence (and elimination process) determine the most likely damage mechanism
ix determine appropriate remedial works

The nature, and cost, of any remedial works is very much dependent on the nature/cause of the damage. If you are going to the trouble of fixing damaged foundations, make sure your remedial solution is going to appropriately address the damage mechanism.

You need to ensure that the problem does not persist in the future. If you are concerned about ongoing damage/cracking to your house, engage an appropriately qualified and experienced engineer to undertake a forensic engineering assessment, in order to determine the likely cause of the damage, before you engage a builder to “fix” the problem. If the damage mechanism is not appropriately identified, the “fix” may only serve as a patch-up job, and the damage will likely manifest again in the future.