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 of 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 you house is 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, I'm order to determine the likely mechanism of the damage. The reason for this is that there are often several causes of the damage, so reliably determining the mechanism is critical, in order to come up with the most appropriate 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, so if you are going to the trouble of fixing damaged foundations 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.

Nick and his team specialise in advising on property-related transactions, valuation and restructure services, and tax planning. Withers Tsang & Co Phone 09 376 8860, www.wt.co.nz YOUR ACCOUNTANT NICK ASHFORD ‘Is it worth changing your residential rental to short term accommodation?’ asks Nick Ashford. THE COUNCIL CONUNDRUM Fraser Thomas is a multi-discipline engineering and land surveying firm. We have offices in Auckland (Highbrook and Mt Eden) and in Christchurch. We have also recently opened offices in Hawke’s Bay and Cromwell. We offer a “one-stop shop” for engineering and land surveying services required for building and land developments. This year the company is celebrating 50 years in business. If you require any engineering or land surveying services, call Mason on 021 979296 to discuss your project.