Settlement of foundation after flood 1

[structian]

Hi All
I put this question in the structural section but probably need to ask it here as well.
I am looking at a brick veneer double storey house. Ground floor and surrounds flooded during a storm due to blocked outfall drains. There has been a severe drought causing some cracking to brickwork over several years. Since the flood cracks start to become more severe over a period of two years. Questions: Is the flood enough to saturate a clay soil (increase pore pressures) to cause permanent settlement of the foundation? Would a surface flood saturate the clay at foundation level assuming it was cleaned up after a day or two? What soil tests could be carried out to test that conclusion? Or do we just blame the continuing drought and consequently shrinkage of the soils at greater depths?
Thanks in anticipation of responses.

 

[Ron]

structian...against the forum policy to cross post. Pick a forum and ask the question there. I'll throw in my comments here, then red flag your post in structural.

Different clays react in different ways. Depending on the permeability, clay particle structure and classification of the material, the clay could have expanded during the flood, then proceeded to shrink during the drought. It is not a guarantee that expansion would have occurred, but it is very likely that shrinkage would occur, thus increasing settlement of the foundation.

Expansion wouldn't cause settlement...it would cause heaving, which could have a similar differential effect.

Get a local geotech involved. He/She can tell you how local soils would react to your noted conditions.

 

[oldestguy]

Ron is right in his explanation. However, if you would want general opinions from this group, if you indicate what general area of what country you are talking about, it is likely some local person from that area may give some useful information.

Also a brief description of the site, including trees nearby, it will help.

Still the cure, if there is one, should come from that local experienced geotech.

 

[fattdad]

As a general rule I don't often blame saturation as a root cause of foundation settlement or failure. I'd be more inclined to blame inproper subgrade preparation. For the case that the subgrade included soil with improper compaction (or dried mud, taken to an extream), at the point of initial construction there may have been fairly substantial dry strength. With the innundation, this dry strength may have been reduced to allow for settlement.

That said, initial construction should not have occurred on soil with improper density or descicated mud.

Just something to consider.

f-d

¡papá gordo ain’t no madre flaca!

 

[structian]

The house is in Lalor, North west suburb of Melbourne, Heavy, basaltic clays. There were some trees along beside the house but these were removed 10 years ago. The house is 30 years old.
Could the flood described above increase the pore pressures enough to reduce the shear stength of the clay thus producing permanent strain (settlement)?
Or how is this scenario: there was some settlement due to drought prior to the flood, cannot see the flood causing heave, saturated clay dries out over following two years causing further shrinkage.
Thanks for your comments.

 

[msquared48]

The removal of the trees could have exascerbated the problem, allowing the clay to become more saturated with time.

Mike McCann
MMC Engineering

 

[apsix]

My money is on the continuing drought causing continuing shrinkage. The effects would be especially noticeable if there are trees/bushes adjacent, or if water is introduced into the soil in localised spots, such as due to garden watering or leaking pipes etc.

I can't see the flood causing settlement due to saturation of the clay.

I have seen heave occur due to watering within a day or two. It's unlikely to be excessive but if it's differential movement it will cause problems.

 

[namron2]

I'm a geotech based in Melb

I would say that the damage is due to reactive clay movement, which the Newer Volcanics is famous for.

Although these types of clays do reduce in shear strength when they wet up are cannot see it leading bearing capacity failure / excessive settlement, particularly given the reasonably light loads involved.

The dry conditions are probably contributing, particularly if the house is on a thin strip footing and there is no paving round the exterior walls to assist in maintaining a stable moisture condition in the reactive clay founding soils. (Typically these stiff Class H raft slabs handle these clays better than isolated spread footings)

The cracking may be exacerbated in upper storeys as relatively small foundation movements will be multiplied at the top of the wall. If the masonry panels of the house havent been well articulated then cracking may be severe.

My advice in this situation is to engage a reputable geotech consultant to do a condition survey on the dwelling. Map/record the location and apeture of cracking and compare to the damage categories in AS2870. Undertake about 3 footing probes and record the footing type, founding depth and condition of founding soils. If its a Class H strip footing then going by the current standard its founding depth should be >1.0m.

Solutions for this type of problem are unfortunately limited. The best option are building on a stiff slab will well articulated masonry, but unfortunately in this case that aint going to fly.

Next cheapest option is to limit the damage by sealing the perimeter of the building with paving and making sure leaking services, etc are fixed. The only other option is to underpin, which is both expensive and not guaranteed to work unless it is deep.

Would suggest you look up Peter Ytrupp he runs his own small consultancy locally near geelong and specialises in this kind of thing.