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Sudden Long-term Settlement of foundations
4

Sudden Long-term Settlement of foundations

Sudden Long-term Settlement of foundations

(OP)
We have a dwelling, which has experienced sudden settlement after 10-12 years of static performance of its foundations. In 2006 the dwelling (built in 95/96) and the surrounding ground (10-15 m in diameter) started to settle, the surface soils receded any where up to 100 mm and the foundations have continued to settle up to 200mm.

The brick veneer dwelling is on 600mm x 300mm concrete foundations with Y12 reinforcing complying with Australian Classification Medium to Highly reactive (40mm-70mm) soil types.

On conducting a range of investigations we have found the following possible affecting factors:

1909 -1986 dwelling was constructed on and/or adjacent 100 year old road (earth construction with table drain/culvert running along side, leveled most likely in the 70s-80s) – information not available at time of construction.

2006 Demolition of neighboring dwelling and concrete driveway, driveway pulverized with excavator bucket for 6-8 hours shaking the dwelling that has suffered the Settlement of its foundations.

Construction of a marina 1.5 km away, which could have led to the water table lowering (still investigating this)

Sewer works (Open Cut) some 5-7 meters from the dwelling that has Settlement of its foundations but could have extended further.

Leaking sewer for a minim 5 years a sewer line some 40 meters away leaked (12 M of sewer line was eventually replaced)

Discharging of storm water directly over the affected area for up to 18 months.

In the most investigations it was found that directly below the area of the subsidence about 5.2 meters down was a highly pliable and water reactive layer of sand/clay FINE GRAINED SEDIMENT this layer was 600-800mm deep.

Our thoughts are that the Causative factors are:

1. General shrinkage of road soils and sewer soils in excess of the surrounding soils exacerbated by the construction method of the sewer and impact from leaking/discharging water.

2. Change in the nature of and settlement of the sand/clay layer due to demolition vibration, influence of additional leaking/discharging water coupled with movement of the water table is possibly thixotropic in nature.

Does anyone have any references which would be of assistance in narrowing the cause or case study's of similar experiences that would help withe formulating conclusions.
 

RE: Sudden Long-term Settlement of foundations

Do you have any trees near the foundation that have grown significantly over the 10-12 years?  If so, they could be pulling water from that susceptible layer down a few feet.

Is the settlement uniform over the whole building area?

RE: Sudden Long-term Settlement of foundations

Is subsidence of an old mine a possibility?

RE: Sudden Long-term Settlement of foundations

(OP)
Thanks Ron,

We have ruled out trees as the major causative factor for the following reasons.

Treees around unaffected areas of the building ar of similar size and type.
 
Investigations have discounted vigorous tree root growth.
 
Surface area of settlement has remained considerably wetter than the surrounding areas during the period of settlement.

Worth note is that one tree that was actually planted in the affected area fell over the ground unable to support it.

This is one cause which we realize can have an impact, in this case we have ruled out through several investigations.

Settlement is not uniform, other areas demonstrate strong stability.

High levels of surface shrinkage are observable for the whole length of the old road on the property in question and neighboring ones.

RE: Sudden Long-term Settlement of foundations

Mapping the settlements (if not done already) might help determine their cause

RE: Sudden Long-term Settlement of foundations

(OP)
Unfortunately an old mine is not really a possibility we are 800ft from a major Bay.

But the site was once flood plain/reclaimed beach.

The Geology classification is Alluvial fan and Plain, with the soil types are a mix of Aquic Vertosols and Yellow and Grey Sodosols.

RE: Sudden Long-term Settlement of foundations

(OP)
Aerial photos show the settlement of the old road.

Further mapping of the immediate settlement is a good way of localizing cause and effect.

RE: Sudden Long-term Settlement of foundations

how much fill is there above the "natural" ground? i've seen projects where settlement monitoring points began moving after a long period of them remaining constant. the movement was attributed to the groundwater dropping due to severe drought conditions and substantial lowering of the reservoir within the drainage basin. it helped visualize what was going on by plotting reservoir levels against the settlement readings.

good luck

RE: Sudden Long-term Settlement of foundations

On what said it looks you have it in concept narrowed (at least somewhat) already. You have reclamation fill with washable fines and wetter (irrigation?) for the period ob subsidence. So the most likely cause is migration of the fines along the period in the poor irregular soil. Remediation should encompass likely a deeper new foundation, if on floating piles or whatever maybe also needing stiffening elements to keep the building to shape. And monitorization of movement should be kept at work all along to make sound decisions.

RE: Sudden Long-term Settlement of foundations

CEFDESIGN....I'm not understanding something here.  I read in your original post that the area is being surcharged with runoff.  If these are clays subject to shrinkage and swelling, then that would cause swelling, not shrinkage.  Is there any chance these are predominately calcareous clays?  If so, and you have sulfates in the other overlying soils, perhaps you are having a sulfate reaction causing a volume reduction...its a long shot, but considering the lack of other expedient causes, perhaps something to look at.

 

RE: Sudden Long-term Settlement of foundations

You would help some if you say the country and what part.

Also, I have noted in these posts that some of the best advice comes from Ron, so keep him posted.

RE: Sudden Long-term Settlement of foundations

OG...that's very kind of you.  I've been around a long time, so I get to see lots of different things.  As we've said before in one or other of the forums, you, oldfieldguy, and SlideRuleEra need to collaborate on a book for the rest of us!!

RE: Sudden Long-term Settlement of foundations

(OP)
Good Idea Oldestguy here is a URL to the soil types in our area of Australia.

Ron the period during which the initial major movement occurred was a period in which the area was surcharged with runoff.

Once this runoff was discontinued, settlement slowed but continued in a downward trend, with seasonal variation continuing in a normal manner [Summer to Winter expansion and contraction], though the affects due to the dwelling being distressed were more noticeable.

RE: Sudden Long-term Settlement of foundations

(OP)
Msucog thanks for that we are trying to obtain ground water level monitoring as a new Marina was built within 1.5 km of the dwelling in the period of settlement.

RE: Sudden Long-term Settlement of foundations

Was the old road in cut?  You noted that aerial photos showed settlement of the old road.  Perhaps this settlement was incomplete, the filling of the old road cut used granular materials, the flooding and vibration compacted the fill.  I doubt that the thin layer 5.2 metres down has much to do with it.

RE: Sudden Long-term Settlement of foundations

alluvial soils, especially younger Holocene soils can collapse when saturated with water. Withdrawal of groundwater can also cause subsidence in these soils however this effect is not generally limited to such a small area. Groundwater withdrawal in areas of deep alluvial soils can also create fissures. Beginning symptoms of fissuring can be localized subsidence, especially when inundated by water. Both collapsing soils and fissuring are a problem in areas of the southwest US on alluvial fans and plains and in areas with longterm groundwater pumping and perhaps areas in Australia may have similar problems.

RE: Sudden Long-term Settlement of foundations

This sounds a lot like a deep seated settlement. I am wondering if you drilled a borehole to 50 ft deep?

RE: Sudden Long-term Settlement of foundations

Problem sounds familiar to a collapsible soil problem we had a few years back - house had stood intact for 12 years or so, a water pipe burst, saturating the foundation soils and the house started to settle.  I have seen collapsible (pinholed) soil fabrics in alluvial fan profiles before, varying in depth.  Has the settlement subsided/stopped?

RE: Sudden Long-term Settlement of foundations

(OP)
At this point we have gone down 6m and found the water table there.

Interesting settlement movement escalated for about a week when we did the borehole over the approximate center of the affected area (2m from the dwelling), this was noticeable through movement indicators from with in the dwelling.

RE: Sudden Long-term Settlement of foundations

(OP)
The major subsidence has slowed but has never stopped totaly.

as you can see in the prior post this increased when further influences were introduced in to the affected area.  

RE: Sudden Long-term Settlement of foundations

I would consider the 1st factor you mention.
As FixedEarth described, it is a deep seated settlement.It could be caused by the loss of deep soil. From your reference,"The subsoil is characteristically high in exchangeable sodium - i.e. sodic." It is easy to be corroded.And as a fine grained soil,it could lose its stability under water flow.
 

RE: Sudden Long-term Settlement of foundations

CEFDESIGN...I'm not familiar with your soils, but from the descriptions, these might well be weakly cemented calcareous materials or low cohesion clays.  In either case, inundation after having been in a dry state for so long could certainly cause the instability.  We have similar acting soils here in the southeast US, though their geology is a bit different.

Is there a chance that you have a raveling cavity at a depth greater than the 6m boring?  Have you considered doing a ground penetrating radar profile?

RE: Sudden Long-term Settlement of foundations

A cavity may be a possibility, but I doubt that GPR will see it below 6m of soil.  

Most of the time 10ft (~3m) is the best you will get due to the voids in the overlying soil. Even the last guys I listened to pedaling their GPR wares said about the same.

RE: Sudden Long-term Settlement of foundations

TDAA...the Florida Sinkhole Research Institute and the Florida Geological Society have documented and validated depths much greater than that in the karst topography of Florida.

RE: Sudden Long-term Settlement of foundations

Noted. I did not know of that.  In many areas, the soils are not conducive to GPR being useful to such depths.

I guess I should have indicated that my intent was to get with someone knowledgeable of the equipment capabilities in your area.

RE: Sudden Long-term Settlement of foundations

(OP)
RON Good question and something I will raise today as a possibility.

It seams to be a consensus that the saturation of the soils is a major factor.

In line with that we spoke with a geo tech which had a similar event withe a water tower where a trench was dug, left open, filled withe water which leached to a point where the soils reacted allowing the tower to sink on one side.

The geo tec also indicated that vibration would have an impacting factor.

We seem to be getting some were with the soil types and the impact of water on a specific area.

We are arranging to map the shrinkage of the old road and the shrinkage of the specific area of subsidence affected by the water.

We hope that this will allow us to isolate causes of which there seam to be two one specific [the water & vibration]and general [shrinkage/collapse of the old road].

With the possibility of radar imaging to get a look at the event it self.

Your thoughts on this approach would be good.

RE: Sudden Long-term Settlement of foundations

If you're in an arid climate, it's probably a weakly cemented aeolian deposit. The give away is high blow counts and low density. The soil is metastable until it gets wet and then it collapses. I was looking at a shopping center in Arizona last year where they had to jack up the back wall more than a foot because of collapsing soils. It's a major problem in desert climates.

RE: Sudden Long-term Settlement of foundations

CEFDESIGN...good approach...keep going!

Howardoark...yes..similar to my thoughts and others.  Assuming the soils are similar to Loess deposits in the US.

RE: Sudden Long-term Settlement of foundations

"Calcarosols are soils that generally have a small, gradual increase in clay content with depth and are strongly alkaline and calcareous throughout. These soils have largely formed on calcareous aeolian sediments of varying texture throughout the Mallee and parts of the northern Wimmera regions. Calcium carbonate (lime) is often abundant in the deeper subsoil, either in soft form, in nodules, or sometimes as large blocks, and it may extend to the surface of medium to heavy textured soils. Sodicity and salinity levels are also usually high in the heavier textured subsoils.  


 (I) Hypercalcic Calcarosol

These Calcarosols have subsoil horizons containing more than 20% of mainly soft carbonate (lime) as well as harder nodules or gravel. They are common in the Mallee region of Victoria and are used for dryland cropping (cereals and pulses) and also for viticulture and horticulture where irrigated (e.g. from Mildura to Swan Hill). These soils are also common in South Australia and would be a leading contender for State Soil in that state. Further information is provided on this soil, including a case for why it should be considered as the State Soil for Victoria. "

From the link at Victoria State Soils.
 

The above description of the soils in the link previously posted for your area, are what I would typicaly think of as collapsible soils.  I would strongly suspect that is what is going on at this site.  Have you taken any soil samples?  You should run some collapse tests on them in the lab and see what you get, that would likely tell you what is going on.  Once water hits the soil and desolves the cementation they can loose a lot of strength and will not support the load even though they were holding the load for a long time.

Can you post the boring log for the 6 meter boring that you took?

RE: Sudden Long-term Settlement of foundations

@CEFDESIGN

What part of Victoria are you in exactly?

Please give us some idea of the subsurface conditions beneath the house. If you have engaged a geotech then you should have a borehole log with UCS classifications of the subsurface soils.

Without knowing where the site is located, here are the following comments:

1) I did an investigation in Torquay where a house was built on 5 m of uncompacted dry sandy soils. After several years the building began to crack due to settlement. The most likely reason for settlement was identified as collapse settlement of the sand fills due to seepage and infiltration.

2) West of Melbourne the residual basaltic soils are overlain by aeolian silts. The silts are dry and low density and are potentially susceptible to collapse settlement under wetting and load.

3) Further southwest of Melbourne the overburden sedimentary soils are underlain by limestone deposits. The limestone is susceptible to decomposition in a wet environment. Additionally, due to weathering, limestone forms peaks and troughs. Sometimes the trough are infilled with realtively loose deposits of sand or other materials which may be suscetpible to collapse settlement.

4) Natural soils in the Melbourne region are typically overconsolidated, with the exception of some young quaternary deposits (Coode Island Silt) near the edge of the bay (Docklands area). Based on your description of settlement time history, I think it unlikely that the settlement is due to any type of consolidation.

KV

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