# Spongy spot in Dense Grade Aggregate (base for patio) - what to do?



## Panzer5 (Oct 21, 2008)

I'm installing a patio for a customer who also ordered a pergola. Haven't done any hardscaping for a while, and am in the middle of preparing the ground & need advice.

We excavated 4 inches down & are putting in 6 inches of compacted DGA. Got about half way through by the end of the 1st day - and while walking about with the HO we noticed a spongy / springy spot (it's not wet at the surface - it just flexes when you stand on it...).

The area is near where we had to trench out for the outflow pipe from the HO's sump pump. The sump pump discharged a couple of times while we were digging & the area got wet. After fitting the new pipe to the sump pump discharge point, we refilled the trench with the spoil from digging out the old pipe & graded the earth (Kentucky clay). I'll admit that the refilling of the trench was not packed in quite as hard as the rest of the ground.

So I put in @ 2 inches of DGA, tamped with a vibrating plate tamper & added another 2 inches & tamped again. About this time we ran out of DGA & called it a day until I can get in another couple of tons (tomorrow).

Just wondering if the sponginess will solve itself once the area drains - or if I need to dig it out & pack the earth better before refilling with DGA?

All advice appreciated.

TIA.


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## neolitic (Apr 20, 2006)

Would have been best to backfill 
the trench with gravel. SOP


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## griz (Nov 26, 2009)

Dig out the soft spot, backfill with good compactable material.


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## rustyjames (Aug 28, 2008)

neolitic said:


> Would have been best to backfill
> the trench with gravel. SOP


Sorry, but I'm going to have to disagree with using gravel as SOP. If it's clay, and filling with gravel, the water will be attracted to that spot and act as a reservoir. 

I'd dig down and bring it back up (tamped) with the DGA. Although that has to be done carefully or more water can be attracted to that spot. Sometimes DGA will be mushy but after it dries out, it's like concrete.


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## neolitic (Apr 20, 2006)

Under pavement, trenches get
filled with gravel or concrete
around here.


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## tgeb (Feb 9, 2006)

Dig it out and fill with stone.

I don't know what DGA* is, but when we do pavers we undercut and put 9" of stone as a subgrade.

Might be a little overkill to some of you, but stone is (relatively) cheap, and you can drive trucks over it when we're done.


*I'm gonna guess DGA is what we call "crushed run", or CR-6, or CR-8, but essentially stone mix from 1 1/2" down to dust that compacts well.


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## neolitic (Apr 20, 2006)

tgeb said:


> Dig it out and fill with stone.
> 
> I don't know what DGA* is, but when we do pavers we undercut and put 9" of stone as a subgrade.
> 
> ...


Glad I'm not the only one. :thumbsup:
My best guess was, 
*D*ense *G*rade *A *ggregate. :laughing:


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## Rockmonster (Nov 15, 2007)

So the sump discharged into where the pergola is now going, and was rerouted to go further out? I'm just trying to picture, most pergolas I see are attached, very rarely detached....so it seems close to the house for discharge...just wondering....ps...we pay about 85./yard for 3/4, so it's not even "relatively" cheap here....sheesh


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## Captainsls (Nov 8, 2007)

Rockmonster said:


> So the sump discharged into where the pergola is now going, and was rerouted to go further out? I'm just trying to picture, most pergolas I see are attached, very rarely detached....so it seems close to the house for discharge...just wondering....ps...we pay about 85./yard for 3/4, so it's not even "relatively" cheap here....sheesh


Not trying to steal the thread... but do you have your material trucked via the boat from the mainland Rockmonster? I dont recall ever seeing a materials yard on either island.


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## Panzer5 (Oct 21, 2008)

Thanks for the replies. Some clarification:

*DGA* is indeed "Dense Grade Aggregate" (From the title to the thread!):thumbsup: It is "An aggregate graded to produce low void content and maximum weight when compacted." It also has good drainage, is a good base for patios, etc. Yadda-yadda yada.

The patio is replacing a deck that attached to the house. The patio and pergola will be 3 feet away from the house (free-standing pergola). The old sump discharge was trenched under the deck & ran through 1 inch pipe through the yard to discharge away from the house. While excavating we dug up the old discharge line. When the pump cycled during the day, it dumped water from the pipe where I had cut it next to the house. That water got into my dig.


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## natural1 (Aug 30, 2007)

Adding more gravel will not eliminate the problem. The spongy effect will not go away. I speak from experience. When ever we run into a spongy clay area we have to dig it all out until it is gone. Dig out the gravel and spongy soil then add the gravel and compact in lifts.


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## neolitic (Apr 20, 2006)

If it is angular stone, you can
fill and roll it in 'til it locks, and 
it will even bridge quicksand eventually.


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## Panzer5 (Oct 21, 2008)

A little feedback:

I found another spongy spot the next morning & dug out both of them. We then hand tamped spoil from the first day (not fresh-dug from the holes) back up to the level of the surrounding soil base and then filled with DGA & tamped again.

The first one did perfectly; it packed down just as hard as everywhere else. If you hadn't known it was there in the first place, you'd never know anything was wrong.

The second one was different. It was still spongy, even after we filled & tamped it. As it was the end of the day, I decided to forget about it & get back to it later.

The next day - it was gone. Perfectly firm.

So - moral to the story? Sometimes digging & packing works... Sometimes it doesn't. Sometimes just saying fuggitaboutit works too.

Thanks again for the advice.


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