Solutions to mitigate settlement (Total & Differential) caused by deep fills.

[Adeclue]

I am working on a roadway/bridge project that has a extremely variable subsurface profile across the site that will be seeing anywhere from 30-50 feet of fill.
The general subsurface profile looks like this
0-10 feet. High-plastic clays (LL-60-75) some rock
10-25to40 feet (variable thickness) gravely high-plastic clay(LL - 58-82) within this zone there are rock ledges, floating bolders; overall very rough drilling during our Geotechnical investigation.
25to40-45to65 feet. HIGH-plastic clays (LL- 100+)

The thickness of the 2nd and 3rd soil profiles are extremly variable in composition and thickness(changing between borings 20-feet apart).

Over this profile, the plan is to place 30-50 feet of fill. And we have estimated between 6-10 inches of settlement could happen, and this could take ~20 months(where the 3rd soil profile is 6ft or less), in other areas could be up to 2 years for full settlement.
This fill profile look something like this from one end of site to the other:
At grade to ~50feet of fill at bridge abutdment-
Bridge-35 feet of fill between bridge abudtments(~300 feet)-Bridge-50 feet of fill at bridge abutment tapering off to grade.

We have solved down drag on piles with pre-boring. But client does not want to wait 20 months to begin construction after fill placement, so trying to come up with some mitigation options.

Due to rocky nature of soil and the main settlement concerns are really deep wick drains really aren't a viable option.

Same for stone columns/aggregate piers. As well as, I don't have a good feel on how these would work in this subsurface profile.

We have provided light weight fill as an option and our client said that wasn't an option.

What we have come up with, that seems to work to a degree is:
Undercut under walls and some pavements to depth of 7 feet and replace with compacted aggregate. This zone will then taper to a 5ft then 3ft remove and replace for a specified distance. Goal with this is to reduce settlement at bridge abutment to where after 8 months only around 2" of settlement remains, as well as evening up the settlment(reduce differential). Then the roadway remediation is to help even up settlement so it doesn't effect the walls.
In theory this appears to work, has anyone had experience doing this or have other options we havnt thought of?

 

[geotechguy1]

It seems like all of the options that would actually help have been ruled out and what you are left with is the 'Put some gravel down and pray' option. I hear upon opening a new type of bridge in England in the 1800's, the designer (Telford or Stephenson perhaps) was found on his knees nearby praying.

 

[DanielSgeo]

Sounds like you need to manage client expectations. I think you’ve thought about most cost-practical options. You could consider pile supported embankment but that’s probably difficult given the boulder clay. How sharp is your pencil on consolidation parameters and stress history? Are you being too conservative on your consol and t90 estimates?

 

[TigerGuy]

How much of the settlement will occur as you are still placing fill? A fill like you describe doesn't happen in a day, so some of the settlement will be taking place during grading operations.

How much coarse material vs fines does each layer have? A soil with 40 percent gravel and LL=80 will behave quite differently from a soil with 10 percent gravel and the same LL. Can you share your consolidation data from the lab?


Have you discussed with the specialty installers for stone columns/aggregate piers? They usually have a trick or two they don't share with the rest of the world, but can save time and money on a project. Plus, if you uses these, your drainage distance for consolidation settlement is reduced, accelerating the rate of settlement.

Can you surcharge the area with extra fill while the bridge construction takes place? The extra weight will help accelerate the settlement.

 

[BNGeo]

I agree with the above responses. At some point you just have to tell the client that the options have been presented, and the client has to make the choice based on the risks you’ve informed them of. I have had a project that aggregate piers were used for the combined benefits of drainage and stiffness improvement. In that project, by the time all the fill had been placed, the post-fill placement settlements were negligible. To the point of a previous poster, it’s possible that the duration of time it takes to place all the fill allows for much of the settlement to occur.

 

[ABTSGeoScience]

How much of the settlement is attributed to the fill mass itself, and how much to the natural subgrade? If the natural subgrade areas are accessible to construction equipment and there is money in the budget, you could try dynamic compaction. You would definitely eliminate the settlement in the natural soils anyway. I worked on a project that was a new rail line that had to go over a 30 year old landfill. We dynamically compacted the landfill using a 20 ton weight dropped from about 60 feet. Each time the weight hit it sank about 10 feet, but after about a week of that the landfill area was almost completely compacted.