Mat foundation for a house 1

[tvar]

I have been asked to look at an alternate house foundation type. A soil boring and geotechnical report was done. An engineer recommended mini piles or helical piles. There is a good layer of sandy-silt ( 7' w/ N values between 20 and 32) over a weaker layer of silty - clay ( 19' w/ N values 4 and less). The owner doesn't want to use these piles because he said they only guarented them for 20 yrs. I looked at a mat foundation and if I can get uniform pressure is will be about what the insitu pressure is now at the bearing grade ( 500 psf @ -5'-0&quot. The question, will there be settlement problems and is there a reference to use to use to ensure the mat is stiff enough to distribute the load?
Thanks

 

[Focht3]

You want to design the foundation as a very stiff "waffle" slab. BRAB is one source; PTI is another. But they don't directly address your site conditions. Your judgment is your most important tool. And what does your geotechnical consultant say?

Sounds like 1+m deep beams on 4.5-5m centers is a starting point. And lots of steel - it's pretty cheap.

Perhaps you could develop some severe settlement scenarios and then do some computer modeling.



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[GeoPaveTraffic]

The general design method for a mat foundation is to do a beam on elastic subgrade analysis or a finite element analysis. Either one will require the expertise of the geotechnical engineer for the project along with the preliminary structural design for the mat.

By the way, you are correct that the stiffness of the mat is the key to whether the mat foundation and the house will experience differential settlement. With the information you provided, total settlement should be approximately zero.

Good luck.

 

[tonks]

I would be particularly worried about the soft silty clay. Even though the bearing capacity might not be exceeded by your raft foundation, this sort of material may be subject to settlement through consolidation by underdrainage. In other words, any reduction in ground water levels will reduce the pore water pressure and risk consolidation and settlement. I have seen settlements of over 300mm through lowering of the water table.

I may be wise to consult the original geotechnical engineer to ascertain whether this is a risk with this material.

Regards,

Tonks

 

[boo1]

Why are the piers only good for 20 years?

 

[Focht3]

The installer is limiting his future liability. The piles won't go poof! after 20 years!

SPT is an exceedingly poor way to evaluate clay strength. And I'll bet the driller used a hollow stem auger (HSA). The groundwater table (gwt) is pretty shallow (I'd guess 6.5m to 7.5m) and the samples below that depth were disturbed by "artesian" pressures when the plug was withdrawn to allow sampling. I've seen this even in fairly clean fine to medium sands. It's one of the reasons I won't use HSA for geotechnical projects where the gwt is shallower than the boring depth.

But [blue]Tonks[/blue] has a point - lowering the gwt could impact the foundation. However, this would lower the entire area - not just the house.

[blue]GeoPaveTraffic[/blue] gave good advice - my first post was a little hurried. FE analyses - or a good STRUDL-type analysis with Winkler spring supports to model the subgrade - is appropriate. Get the appropriate spring constants from your geotechnical consultant.



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[jimbo2]

Hi folks:
I searched the forums with the keywords soft, clay, consolidation, water table and lower.... and this thread was the only match. I wonder if anyone cares to do a reality check for me.

Residences and small commercial buildings are being built, and many are experiencing significant differential settlement and damage, in a region with a soft subsurface profile. Among the theories is that development of the infrastructure is lowering the water table in the vicinity of the buildings. I wanted to see if the dewatering of the upper soils might result in any significant consolidation settlement of the underlying soft clay.

The subsurface is characterized by a loose to compact silty sand or sand (N = 5 to 20) to about 10 to 12' below ground surface, which is underlain by a thick (typically 15 to 40 feet), expansive, very soft, normally to slightly overconsolidated silty clay (w = 40 to 56%, LL = 49, PL = 23, PI = 26, Shrinkage Limit = 18).

I have no consolidation test data, but assuming a Cc = 0.2, and an initial void ratio of 1, I computed a consolidation settlement of just under 4-inches for a 43-foot thick soft clay layer given the effective stress increase due to an 8-foot drop in the water table.

Does that sound within the reasonable range? How much higher than that could it be?

 

[Focht3]

You can develop a good estimate of the initial void ratio by measuring the water content of several submerged samples and assuming they are fully saturated. (That may be how you got the e[sub]0[/sub] you presented.) Otherwise, your values don't seem odd. But don't get too far ahead of yourself.

Is the settlement fairly uniform, or does a consistent pattern present itself (i.e. the house perimeter is always down)? This is key to identifying the cause...



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[Yvel]

Why are the piles only guaranteed up to 20 years?
Use concrete when in doubt or at least from above the groundwater line. lumber piles can last centuries when completely submerged under water as has been proven in Amsterdam where century old buildings still stand. Be carefull with downdrag due to consolidation settlement in the cohesive layer.
Another option is a balanced strip foundation,where the strips are sized for a uniform bearing pressure and connected with tie beams for load distribution.

 

[Burr]

I'm not a civil, but a recent course taken for non-soils engineers suggested the use of geo-tech fabric to mitigate the problem. There are lots of sources for this info. At the prices received for this "tech" fabric, you'll get a lot of support.

Not an expert, but it would seem that the use of the fabric over the silt, covered by 12" of #2 stone, then your stiffened slab wouuld work.