1.5" washed rock base for residential addition foundation 4

[TroyD]

I have a rural client constructing an addition to his mid-1980s residence. The homeowner used a backhoe to excavate for the perimeter foundations and dug 2 feet too deep in some areas. The building official is asking for a fix/sketch from a licensed engineer. (Frost depth is 42 inches). It is generally agreed to backfill with 1.5" washed rock, rather than re-use the excavated material. There is no soils investigation, but the material on-site is best described as Silt Loam. I have observed this 1.5" washed rock used as pipe bedding in soft/wet areas on municipal utility projects. Are there general guidelines for using this material as a base for a residential footing and foundation wall? Does it need to be placed/plate compacted in specific lifts? I think I will require the bedding width oversized 1 foot minimum each side of the footing. The addition is just a split level basement wall, with a floor and roof above...not a significant axial load.

And additional input is appreciated.

 

[cvg]

My preference would be a dense graded aggregate such as aggregate road base. However, I realize this is a local preference. either way it needs to be compacted.

 

[oldestguy]

Agree with cvg. If layers can be used rather than one lift, so much the better.

 

[PEinc]

I assume that the 1.5" washed rock has no fines in it. You should consider the effect (settlement?) of fines migrating from the virgin, silt loam soil into the clean 1.5" stone. I agree with cvg who said, "My preference would be a dense graded aggregate such as aggregate road base."

http://www.PeirceEngineering.com

 

[MTNClimber]

I was going to say that it sounds like filter fabric underlying the 1.5" washed rock is probably a good idea. Also make sure that the stone is angular and not rounded.

 

[oldestguy]

If there is no water running through the open graded stuff and it is compacted, no sweat.

 

[cvg]

fines migration is why I would prefer a well graded mixture of crushed rock over an open graded one. open graded rock has a lot of void space allowing water and soil to migrate through, promoting settlement - even if it is compacted. well graded material such as aggregate base material does not have this problem. and no need for filter fabric either.

 

[TroyD]

Thanks for the great responses. The excavated area is partially submerged due to heavy rain and lack of outlet. I instructed the owner to pump out the water, but it is unclear if it can be thoroughly dried out. That was my reason for suggesting the angular washed rock. I know the well-graded Class V aggregate provides a superior roadway base but it only works if the subgrade is dry. The ~10% fines in that mixture can really make things soupy. If the owner can pump it down and we can get a break from the rain, the Class V aggregate road base should work fine.

 

[oldestguy]

Your plan sounds right. It won't be totally dry and at least the first layers need to take up the mud in the voids. What you want is finally getting good resistance to the compactors. A common technique for testing is using a 1/2" rod with cross handle for pushing down full body weight. If it can't go more than a few inches, great. I say this assuming at least the lower layers are open graded, single sized stone to take up the mud when well tamped in...

 

[Ron]

Agree with cvg, OG and PEinc...the grading of the aggregate is key. Must include proper grading to prevent fines migration and resulting subsidence of overlying soils.


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[EireChch]

OG - "A common technique for testing is using a 1/2" rod with cross handle for pushing down full body weight. If it can't go more than a few inches, great". Some people are bigger than others, i would include a body weight coefficient.

1/2" rod penetration of a few inches (4 inches) = great. Note applicable to human of 13 stone

1/2" rod penetration of a few inches (4 inches x 1.5 body weight coefficient = 6 inches) = great. Note applicable to human of 19.5 stone

**Linearly interpolate penetration for body weights between 13 to 19.5 stone.







The above is of course a joke.....

 

[oldestguy]

EireChch:

As crude as it sounds, this Old Guy used this for years and never had a claim that compaction was insufficient. Your dimensions of penetration may be too generous for this site.

 

[Ron]

EireChch...same here as OG....useful technique for quick and dirty.


A Great Place For Engineers to Help Engineers

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[EireChch]

OG / Ron - it was tongue in cheek penetrations .

Indeed quick and dirty but can be valuable too. While undertaking shear vane tests in compacted cohesive soils we would run around the site with a gum spear to find the "soft spots". We would do our shear vanes at these locations and be confident the remainder of the site was stiffer than these values.

The gum spear was used for gum digging which has been undertaken in New Zealand for 1000s of years. Probe the ground then find the gum and dig it up, tough job!

https://teara.govt.nz/en/kauri-gum-and-gum-digging

 

[oldestguy]

Golly! What a surprise to see what likely are my distant relatives from centuries back. Those darn Vikings from Norway and Sweden sure must have had a rough time sailing way down there and plundering and of course leaving descendants.

 

[BigH]

EireChch - try to contact me (or linkedIn or if you know Ron or OG's email addy - through them) - I have something on gumdigging that you might be interested in . . .

 

[EireChch]

Hi Big H - I am afraid I dont have any email addresses so not sure how to contact you. Interested to see what you have on gumdigging though, If the powers to be could connect us somehow