Sand and gravels ground replacement 4

[Jomando]

Hi,
sorry for such a long post!

We have been replacing 20000m3 of peat and soft silt below of the footprint of a 8 m high embankment with well graded sands and gravels, (we replaced down to a layer of sand and gravel also)
The depth of replacement varies 1 - 6m, and the level of ground water is 2-3 m below OGL made it impossible to compact with roller. (quick water ingress when we reached the gravelly layers). The area will flood when the rains come.
The good grading and cleaness of the gravels give very good appearance to the job, and the loaded dumpers cause no rutting at all.

We are unsure whether the lack of compaction during the laying of the gravels (just pushed them into the hole with a dozer) may mean that we need to vibrocompact or use other technique to consolidate the sands and gravels or we should just build up the embankment and check the settlements. This we have to do it anyway, but we would hate to find out that this settlement is very slow..
Somebody suggested Dyamic penetrometer testing to check the whole 1 to 6 m sands and gravels we layed.
What do you think?

 

[msucog]

what's going on top of all this (what kind of settlement tolerances you dealing with)?

 

[Jomando]

a motorway. the embankment approaches a structure at that point, so very little settlement.

 

[dirtsqueezer]

Last I checked, the DCP is a hand operated device. It would not be suited for gravelly material as it just doesn't have the drop weight required to get past that kind of material. You might just have an SPT come in and do the work. I would say just dig down and do a couple compaction tests, but 6m is too deep.

Did you say you were using a roller? And trucks were not rutting? If you weren't, then you could definitely expect settlement, as fill goes in (under dozer pressure) at about 88% compaction, way below what you want for a roadway certainly. If you did just dump it in the hole, I would forget our advice and just hire a geotechnical engineer. He/she could give you settlement numbers from drill rig logs and would be able to tell you just how long it's going to take to settle. Good luck.

 

[BigH]

jomando: What is the underlying base material? You are removing peat and soft silt (the former with very low density) and replacing with engineered, presumably high density fill. The additional fill loads just from the difference in densities might have a long term settlement effect of your embankment.
Further, I would presume that the abutments are piled? The fill might have some downdrag loading on the piles that you want to consider as well.
I would suggest that if you have the time, when you reach the top layer of the embankment fill to add another 2 m or so of fill for, say, 20 m behind the abutment and let it sit for as long as possible. This would effectively preload to some extent the materials so that on the preload fill removal, any settlement that might occur would have had at least a reasonable portion of it already induced. Highway construction - if you time things right, should be able to give you several months of preload before you have to remove it. Just a few thoughts that you should consider.

 

[Jomando]

We were not using a roller, impossible because the we were excavating below water level, so the filling was pushed over and the first one or 2 meters just sank below the water.

We do have geotechnical engineers. Their advice is to start building the embankment and monitor the settlement. They expect very little settlement because the material used to backfill has very good continuos grading.
But this kind of ground replacement is usually done with rock over here, so they dont have experience doing this with sands and gravels.

I was thinking in doing what BigH suggest, not spend money in vibrocompaction and surcharge the embankment and monitor settlement.
My fear is that the settlement being slow, we cant afford waiting too long...

The DPM testing they recommended me is not exactly DCP, is similar but it is done with a 30 kg cone, I suspect that will not be handheld!!
I was hoping someone here had some experience with this DPM or in vibroflotation/vibrocompaction.

 

[Jomando]

sorry, the underlying base material is sand and gravels also, with rock below.
So now the profile is
an avg of 3/4/5 meters of uncompacted gravels
--- 2/3 of naturally compacted sands and gravels
-- limestone rock.

The abutments are piled, probably because designers having rock only 8-9 meters deep they wont allow us direct foundation on the gravels. (the design is not closed).

Thanks for your input.

 

[msucog]

i may be getting out of my comfort zone here since we typically deal with silts here not sands. however, i would expect the some settlement out of less than well to moderately compacted crushed aggregate put in in a similar manner (pushed off several feet thick with no vibratory compaction). but here's my thoughts: it sounds like there needs to be some vibratory mechanism introduced to densify the sands. preferably, i probably would have tried to pump out the water via sump pump or drains out to daylight. just trying to visualize the whole thing, i picture less than well or maybe moderately compacted sands/gravels sitting down there and they settle out to some degree over time due to the weight of the embankment. the settlement "stops" and the thing is put in use. but once the vibrations of the roadway (i assume motorway is the u.s. equivalent to highway) are transmitted down, the saturated sands may be induced to densify under the low frequency of the traffic. psuedo liquefaction to some degree i suppose...

i guess i'm kind of asking the question now but it seems relevant to jomando's situation. any opinions about my thoughts? (i might be way off based on what little i know about the actual conditions)

 

[fattdad]

I worked on a project where we had to reclaim a portion of a wastewater treatment lagoon. The base of the lagoon was thick with sludge and there was at least 8 ft of "water" above the sludge. The contractor just end-dumped fill and created a splitter berm, outlining the area for land reclaimation. You just can't believe how effective end-dumping this fill worked (it was a fairly clean sand). When the berm was in place and about 2 ft higher than the "water" level, they then dewatered the reclaimation area, removed the sludge and reclaimed the land for new construction.

From that experience, I'd figure the bridge soils in the lower depth of your embankment are fairly stable. If the peat is removed and you have good frictional granular soils placed and you are then able to develop a subgrade for subsequent compaction, you are doing well. I'd follow that with a series of borings (or dilatometer soundings) to gauge the soil strength/modulus value and then evaluate the long-term performance. I'd vote that it's all going to work out just fine for an embankment. Where you have to be most careful is on the edges where an embankment slope failure could develop.

f-d

¡papá gordo ain’t no madre flaca!