# Mechanism of DOT # 57 Stone settlement



## biswa (Apr 2, 2009)

Can anyone tell me how much # 57 stone settles over period of time?

I am working on a project for a utility company. They had replaced some sewer service lines on a street about two years ago. The trenches were about 15 feet deep. They backfilled the trenches with # 57 stone below the base (crusher run of approximately 12 inches). Now, the city came back about month ago and overlayed the road with 1 1/2 inch of topping. In the service lines area, you can see the settlement about 1 1/2 inches creating a draw. We cored the site and took some compaction testing on the base material. Moisture was about 4 percent above optimum and compaction ranged from 75 to 85 percent.

what can cause the settlement. Where can I find some information regarding #57 stone to see how much it settles over a period of time.

Any information would help me greatly. Thanks


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## MA_PE (Apr 2, 2009)

biswa said:


> Can anyone tell me how much # 57 stone settles over period of time?
> I am working on a project for a utility company. They had replaced some sewer service lines on a street about two years ago. The trenches were about 15 feet deep. They backfilled the trenches with # 57 stone below the base (crusher run of approximately 12 inches). Now, the city came back about month ago and overlayed the road with 1 1/2 inch of topping. In the service lines area, you can see the settlement about 1 1/2 inches creating a draw. We cored the site and took some compaction testing on the base material. Moisture was about 4 percent above optimum and compaction ranged from 75 to 85 percent.
> 
> what can cause the settlement. Where can I find some information regarding #57 stone to see how much it settles over a period of time.
> ...


No. 57 stone should be essentially self-compacting and have minimal "settlement" over time. Settlement observed where it was used as a base would likely come from migration of the materials (either below or above) into the No. 57 stone. If the surrounding material was granular then it will migrate in. A geotextile filter fabric should be used between the stone and the surrounding fill to prevent migration of fines. In an active pipeline if there is leakage from the joints, the flow of effluent into the backfill will cause continued migration of the fill into the stone.

Hope this helps.


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## biswa (Apr 2, 2009)

MA_PE said:


> No. 57 stone should be essentially self-compacting and have minimal "settlement" over time. Settlement observed where it was used as a base would likely come from migration of the materials (either below or above) into the No. 57 stone. If the surrounding material was granular then it will migrate in. A geotextile filter fabric should be used between the stone and the surrounding fill to prevent migration of fines. In an active pipeline if there is leakage from the joints, the flow of effluent into the backfill will cause continued migration of the fill into the stone.
> Hope this helps.



I don't understand why #57 stone is self-compacting...what about the different sizes of stones and voids between them and self weight of the material. Does not that influence the settlement over a period of time. My understanding is settlement is minimal if it was consolidated in lifts as oppose to just loose dump filling it. The pipe is about 15 feet below the existing surface. migration from the base (which is dense graded material) can be a factor but will that explain 1 1/2 inches of settlement in two years. Thank you for your help.


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## MA_PE (Apr 2, 2009)

Stone is essentially "self-compacting" because of the high compressive strength of the material its jagged surface and the internal friction between stones. think about it, in order to get stones to shift thier position you really need to violently shake thier controlled volume. You're not going to shake up the substrata like that and a vibratory compactor on the top isn't going to do a whole heck of a lot either. Rodding/raking (i.e. physically moving the stone around) is the most effective. Therefore once placed, there typically isn't much change. The high compressive strength of the stone layer means you're not going to get long-term "settlement" or compression from overburden, like in a disturbed loosely compacted soil.

Your talking about 15 ft of fill in the trench.

Where is your #57 stone? under the pavement or at the pipe invert?

What is the back fill material to the roadway surface?

How big a pipe and what is the trench width?

What kind of pipe?

1.5 in. settlement in 15 ft isn't unheard of especially if the backfill wasn't compacted properly.


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## biswa (Apr 2, 2009)

MA_PE said:


> Stone is essentially "self-compacting" because of the high compressive strength of the material its jagged surface and the internal friction between stones. think about it, in order to get stones to shift thier position you really need to violently shake thier controlled volume. You're not going to shake up the substrata like that and a vibratory compactor on the top isn't going to do a whole heck of a lot either. Rodding/raking (i.e. physically moving the stone around) is the most effective. Therefore once placed, there typically isn't much change. The high compressive strength of the stone layer means you're not going to get long-term "settlement" or compression from overburden, like in a disturbed loosely compacted soil.
> Your talking about 15 ft of fill in the trench.
> 
> Where is your #57 stone? under the pavement or at the pipe invert?
> ...


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## MA_PE (Apr 2, 2009)

> 8" pipe -pipe envelope was backfilled with #7 stone(app 12") and #57 stone (app. 13 feet) was backfilled directly on top of #7 stone. 12" dense graded material was placed on the top of #57 stone below the asphalt.


8 in. pipe. what was the trench width? I suspect is was no more than 16-24 in. wide. Dumped stone to fill trench. What was the sidefill material. If you have a 13 ft column of stone you could get lateral migration of the stone into the sidefill. Also if there is no fabric between the top of the stone and the 12 in. of graded material under the asphalt, then the graded material can migrate in to the stone too.

In the absence of any project specifics, that's my 0.02. I'd suggest doing a comparison of the grading of the "dense graded material" and of the #57 stone to see what the liklihood of migration of the graded material to the stone is.


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## biswa (Apr 2, 2009)

MA_PE said:


> 8 in. pipe. what was the trench width? I suspect is was no more than 16-24 in. wide. Dumped stone to fill trench. What was the sidefill material. If you have a 13 ft column of stone you could get lateral migration of the stone into the sidefill. Also if there is no fabric between the top of the stone and the 12 in. of graded material under the asphalt, then the graded material can migrate in to the stone too.
> In the absence of any project specifics, that's my 0.02. I'd suggest doing a comparison of the grading of the "dense graded material" and of the #57 stone to see what the liklihood of migration of the graded material to the stone is.



It was 10' L by 5' W and side material, I assume, is limestone derived clay. The area is in busy traffic street. There was no fabric place between dense graded material and #57 stone. Settlement had occured previously and they came back and overlayed the whole street recently and all the depressions/settlement showed up in the backfilled areas. My initial thought was when #57 stone settled, it probably created some "air pockets" between asphalt and stone and when they recently overlayed the street...big compaction machine caused that air pockets to close creating visible draws on the road.

Thank you for the input


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## AJK (Apr 3, 2009)

So you had a 5' by 10' hole that was 13' deep? If so the contractor must have used some type of shoring to get down there to install the pipe. Can you provide some details on the shoring and the procedure used? Depending on the type of shoring system and its use, it may have contributed to the settlement.

As MA_PE stated, normally I would expect very minimal settlement with No. 57's.


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## MA_PE (Apr 3, 2009)

did they juist replace one pipe length? hence the 10ft x 5ft hole? Also it doesn't make a lot of sens to me to backfill the entire hole 10x5x13 with imported no. 57 stone. Seems to me they would have used compacted soil for the bulk of the back fill. Maybe with a stone base under the asphalt.

Poorly compacted fill would settle.

five ft wide for an 8 in. pipe? Big hole. needed to accomodate a trench box?

I think where AJK was going is --&gt;extraction of the trench box with compacted fill inside the box = uncompacted fill left in the trench when the box is removed.


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## biswa (Apr 3, 2009)

AJK said:


> So you had a 5' by 10' hole that was 13' deep? If so the contractor must have used some type of shoring to get down there to install the pipe. Can you provide some details on the shoring and the procedure used? Depending on the type of shoring system and its use, it may have contributed to the settlement.
> As MA_PE stated, normally I would expect very minimal settlement with No. 57's.


It was a sewer service line. They used trench box to install the pipe. They removed the box once the pipe was in place. They backfilled the trench after removal of the trench box. It was all stone backfill, no dirt used.


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## MA_PE (Apr 3, 2009)

I vote for lateral migration of the the stone into the trench walls and vertical settlement of the stone in the bottom of the excavation.

24 yds of imported stone (50 tons) for one pipe excavation. wow!


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## AJK (Apr 3, 2009)

MA_PE said:


> I vote for lateral migration of the the stone into the trench walls and vertical settlement of the stone in the bottom of the excavation.


The only other issue I can think of is if the surrounding pavement was undermined, or if the walls of the trench were caving in as the 57's were placed/dumped.

Andrew


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