Sudden Long-term Settlement of foundations 4

[CEFDESIGN]

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.

 

[paganel]

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.

 

[Ron]

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?

 

[TDAA]

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.

 

[Ron]

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.

 

[TDAA]

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.

 

[CEFDESIGN]

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.

 

[howardoark]

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.

 

[Ron]

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

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

 

[muuddfun]

"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?

 

[KVgeo]

@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