Soil report recommendation

[jrmbac]

I have this soil report that says "The structure to be constructed can be supported by shallow spread footings embedded at least 1.80 m below existing ground surface". It goes on to say that the foundation "can be designed with an allowable soil bearing capacity of 200 kPa for gravity loads". Yet, the soil report also says that "The site is prone to liquefaction".

Can I follow the soil report's recommendation?

 

[Ron]

It would be better if you discussed the report's recommendations directly with the author of the report and ask specific questions. In general, the report might be correct and the author is protecting his liability by putting in the report the higher liquefaction potential. As a structural engineer, you might be able to counteract the localized increase in liquefaction potential through your foundation design (rigid perimeter with reinforced grade beams connecting isolated column footings for example).

Ask the geot for a more clear assessment of liquefaction potential and whether it is isolated, localized, what depth it occurs, etc.

It appears from the recommended bearing depth that the geotech might be expecting a mat foundation, which would also be one way to bridge localized liquefaction zones.

 

[msquared48]

I don't consider 5.5 - 6 feet to be a "shallow spread footing".

I wonder if the Geotech read his report...

Something's not right in... well... wherever the project is.

Mike McCann, PE, SE (WA)

 

[hokie66]

I'm guessing that he thinks liquefaction on the site is not an issue at the 1800 depth...but that is just my guess.

Mike, I think spread footings founded at 1800 below the surface are quite common, and I would call that a shallow footing as opposed to deep, which usually means piles of some sort.

 

[msquared48]

To me, shallow means at or very the frost depth. Pile, of course that is deep. If you have serious over excavation, then deep is very likely. For this area anything over 4 feet is getting deep. Rarely have to go 9ver 2 feet. Just a judgment and experience I guess.

Mike McCann, PE, SE (WA)

 

[hokie66]

Maybe regional as well. Where I am, we have no frost. But 1800 is not considered deep.

 

[BigH]

msquared48 - I consider a 1.8 m excavation for a footing to be a shallow foundation - in northern parts of Ontario, Quebec, Alberta, etc. where frost penetration is in the order of 2 to 3 m or more, shallow foundations, unless treated by use of insulation protection, have to go that deep in frost susceptible soils. The usually accepted break point between shallow and deep foundations is in the order of 4 to 5. So for a 1 m wide foundation, founding 3 m below ground - I'd consider this a shallow foundation.

 

[Terratek]

We have no frost depth in my area and I consider 5.5 to 6 ft as a shallow depth for a foundation. Classification of shallow vs deep is related to slabs, mats and footings vs piers and piles. The frost depth has absolutely no influence on the classification for shallow vs deep.

As Ron pointed out, the liquefaction statement may be a general comment to protect liability. Sort of like groundwater comments where no groundwater was encountered in the borings. Maybe it's boiler plate that got left in by accident, maybe not. Probably better to speak the engineer who prepared the report rather than just assume it's wrong.

 

[BigH]

Terratek - I was simply pointing out that because of frost depths we have in Northern Canada - foundations do go down that deep or even more - and I also pointed out, I believe, that these would be considered shallow foundations.

 

[Ron]

Depending on loading and the needed bearing capacity, the depth of these footings is not necessarily unusual, even in non-frost areas (have had similar here in North Florida, USA). This is a shallow foundation system.

 

[Terratek]

BigH,

Understood. Mike stated that he considered the frost depth as an indicator of the definition between shallow and deep. I could be getting out of my depth here (no pun intended), but I believe the most absolutely technical difference between shallow and deep is whether the theoretical slip surface intersects the ground surface or not. So, a spread footing could be considered a deep foundation if it was deep and narrow enough. Likewise a pier can behave as a shallow foundation if it is short enough. In a full clay profile, as Nc approaches (arrives at) 9, the foundation becomes a "deep" foundation.

 

[killswitchengage]

Ask the geotech how he assessed the liquifability of the soil ? did he used SPT data or CPT or other correlations