Foundation Adjacent Grade Tolerance

[LincolnPE]

Our soils report recommends at least 10% for 10 feet perpendicular to the foundation walls. If compaction was done to at least 90% ASTM D 698, how much tolerance in grade differential from 10% is the standard of care when examining these grades 7 years after construction?

Potential movement of the soil up at the foundation wall interface location, movement of soil at the toe location of the 10% envelope, or movement at both locations can affect grade, or together effectively cancel out a net change in grade over the 10 feet.

The site soils are sandy clay.

 

[LincolnPE]

Please check that : The site soils are actually clayey sand.

 

[oldestguy]

What is your purpose in posting? Plese explain little of the site situation and perhaps a question then. It looks like you are replying to something somewhere.

 

[LincolnPE]

My second post is simply correcting the soil type I stated in the first post. Do you have any thoughts on grade tolerance for the situation in my first post?

 

[LincolnPE]

We are examining the grading around the foundations of a 6-7 year old townhome development for compliance with the original soils report.

 

[cvg]

10% in 10 feet is a total of 1 foot drop from the edge of the foundation. typical grading vertical accuracy would be about a tenth of a foot. assuming negligible settlement and no erosion, than you should be plus/minus a tenth, maximum.

 

[LincolnPE]

Thanks. Though, my question is about the perpendicular foundation grade tolerance due to soil settlement (if any) on a 6-7 year old townhome development assuming the compaction and original grading was done per the soils report as I state in the OP. Any thoughts?

Are you saying, by stating only the typical 0.1' survey measurement tolerance in your response, that the settlement should be negligible if grading and compaction was done per those soil report recommendations in the OP at original construction for a clayey sand material?

 

[cvg]

you said it was sand and compacted to 90%. I would not assume much if any long term settlement of any fill. you have not indicated what the subgrade is, but I assume it is not clay

 

[LincolnPE]

Interbedded claystone and sandstone bedrock from 2' to 25' below surface. The original site was mined for gravel.

 

[oldestguy]

Lincoln:

cvg has explained possible tolerance standards, but what is your purpose now? Has the site changed due to settlement or work since and are you trying to relate it to a 7 year old job? ? Is there some drainage problem? What does the local code require, if there is a local code? After 7 years is someone trying to claim damage due to something done 7 years ago? Has there been some possible site grading change after the original job, such as plants removed or planted? If so, good luck. 10% may be nice for drainage, but perhaps other factors make that not possible, such as paving or walks. That's darn steep for walk ways in some situations.

 

[LincolnPE]

That's my question. If original construction was done in strict conformance to the original geotech requirements (grades and compaction) I have listed, would you expect to see settlement within 10 feet of the foundations in these clayey sand soils 6-7 years later?

 

[cvg]

well compacted clayey sand does not tend to settle much with time, especially in an area that would not expect to see any significant loading, so no you should not expect it. This assuming that it does not have a large component of clay. You should be aware that clayey sand could be 51% sand and 49% clay which would behave quite differently...

Again, you have not indicated if this building is on fill or cut, which might make a difference. And have you verified that the material is actually clayey sand as indicated in the geotech report?

10% is steep and some erosion should be expected. I would normally expect to see no more than about 5% slope away from a foundation.

 

[LincolnPE]

Gradation results indicate a range of 93% to 95% sand.

 

[emmgjld]

Based on a lot of experience, assuming the ASTM D698 90% minimum compaction & usually very little more,I expect sandy soils to settle up to 3% of the total fill height. For Silty Clays, I expect up to 6% (!) and other clayey soils between 3% to 5% of the total fill height.

The amount of settlement around the foundations is variable, almost always worse at the front door porch (Murphy the Engineer). This influence fro Murphy, PE can have grave consequences for basement height backfills.