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August 27

Serious v Minor Cracks

There’s a big difference between minor cladding cracks and a serious foundational issue – Fraser Thomas’ Geotechnical Director, Mason Reed, explains how engineers uncover the true source of damage in July’s issue of NZ Property Magazine.

As property owners, it is not uncommon to occasionally notice minor damage to our houses, such as cracking to exterior cladding or internal wall and ceiling linings. Most of the time this damage is cosmetic and is no real concern, as a “lick of paint” can normally remedy the situation.

However, there are times when observed damage to houses can be indicative of a more serious problem, such as on-going 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.

If your house is being affected by differential foundation movement, 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.  The reason for this is that 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. In my career I have seen, on many occasions, houses that have had remedial repairs undertaken, in order to attempt to fix foundation damage, without first determining what is actually causing the damage.  Invariably the remedial attempts are not suitable, because the appropriate forensic engineering works have not been undertaken. In some cases the remedial attempts make the problem worse.

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)           Soil swell/shrink

(f)            Collapse of subterranean erosion features

(g)           Earthquake induced ground deformation (including liquefaction effects).

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 out-of-levelness

(ii)           Undertaking a site-specific geotechnical investigation

(iii)          Collating site information

(iv)          Further site investigation works (following the initial assessment)

(v)           Eliminating the unlikely causes (based on results of assessment works)

(vi)          Based on the evidence (and elimination process) determine the most likely damage mechanism

(vii)         Determine appropriate remedial works.

Remedying foundation damage is not a cheap exercise, which is why, if you are going to the trouble of fixing damaged foundations, that you make sure your remedial solution is going to appropriately address the damage mechanism.  This will ensure that the problem does not persist in the future.

Of course, all of these remedial works can be avoided by ensuring that new foundations are not going to settle and be subject to damage.  Which is why it is important, when building new structures, to always engage an appropriately qualified and experienced geotechnical engineer. This will ensure that the proposed foundation system is suitable for the site conditions and that the foundations will not be subject to differential movement.

Mason can be contacted at our Christchurch office 03 358 5936.