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.

 

[Ron]

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?

 

[hokie66]

Is subsidence of an old mine a possibility?

 

[CEFDESIGN]

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.

 

[kelowna]

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

 

[CEFDESIGN]

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.

 

[CEFDESIGN]

Aerial photos show the settlement of the old road.

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

 

[msucog]

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

 

[ishvaaag]

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.

 

[Ron]

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.

 

[oldestguy]

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.

 

[Ron]

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!!

 

[CEFDESIGN]

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.

 

[CEFDESIGN]

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.

 

[hokie66]

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.

 

[cvg]

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.

 

[FixedEarth]

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

 

[Patgeotech]

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?

 

[CEFDESIGN]

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.

 

[CEFDESIGN]

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.