Earthworks: Placing Controlled fill over Uncontrolled Fill 2

[mikealpha88]

Hello Fellow Geotech Engineers!

I have an interesting question. I came across a project where the site has a 1.2m to 3m layer of uncontrolled fill. As part of earthworks, another 1.4m of fill is to be placed on top of the existing ground level to raise the pad height.

Now, removing the uncontrolled fill of 3m would be an extremely expensive process so the project Geotech engineer suggested leaving the uncontrolled fill as is, removing the soft spots, having a briding geofabric layer between the uncontrolled fill and new fill and then filling the site with controlled fill in 300mm layers.

With this, I am concerned about the long-term performance of the building. The building is going to be an industrial shed with concrete tilt panels. The panels are designed to be supported on strip footings assuming controlled fill.

Does anyone have an experience with a similar scenario where the existing uncontrolled fill is left and then a new fill is placed on top?

Thanks for you help.

 

[BigH]

A few questions -
1. Where is the project located? I.e., in deep frost areas? In places like Florida or Texas, etc?
2. What is the nature of the uncontrolled fill? Is it cohesive or is it more or less cohesionless? Has anyone run heavy compactors on the uncontrolled fill as a proofroll to see how it behaves?
3. Is there any information for where the fill came? How it was placed? Thick layers or thin? just with dozers? Did loaded lorries travel over the fill during placement? Any records?
4. What are the loads to be placed on footings for the building?
5. How expensive would it be to remove the uncontrolled fill? Could it then be reused as controlled engineered fill?
6. Can the controlled fill be left in place for an extended period of time? Where is the water level? Could you put in augered sand drains at spacings of say 2 m and then use the controlled engineered fill as a preload - i.e., build out settlements before the building goes up as the load of the new fill will likely induce settlements in the uncontrolled fill.

Of course, it would be best to take out the uncontrolled fill and replace it with engineered fill. But the more you know about the uncontrolled fill the better is your understanding as to whether the fill needs to be removed, ground modified or left in place as is. How much risk is the owner willing to take? The geotech? The Engneer of Record? Unlike above grade structures, if the foundations and floor slab behaviour are an issue, remedial measures will far outweigh, likely the cost of removal.

One possible solution might be to dig trenches in the foundation footings and grade beams and then backfill with engineered fill. I do not know your column spacing but this might be acceptable - then your engineered fill placed on top. Intermediate trenches might considered as well - the trenches will "attract" more of the load being induced. I know of cases where granular material was placed atop piles and then the foundation placed on the granular material - a cushion so to speak.

Anyway - just some Sunday afternoon thoughts.

 

[mikealpha88]

Thanks for your reply BigH,

1. Where is the project located? I.e., in deep frost areas? In places like Florida or Texas, etc?

No it is a tropical area in Australia.


2. What is the nature of the uncontrolled fill? Is it cohesive or is it more or less cohesionless?

It is silty sand (non cohesive)


Has anyone run heavy compactors on the uncontrolled fill as a proofroll to see how it behaves?

Yes, pad foot roller was used to proof roll but due to extremely soft top soil they are not able to proof roll the site.

3. Is there any information for where the fill came? How it was placed? Thick layers or thin? just with dozers? Did loaded lorries travel over the fill during placement? Any records?

Unfortunately no records. The fill has dumped tyres, rubble and pipes.


4. What are the loads to be placed on footings for the building?

It is a warehouse building so minor loads. The main loads are on the perimeter from concrete panels. Maximum 50 kN/m on edges.

5. How expensive would it be to remove the uncontrolled fill? Could it then be reused as controlled engineered fill?

Extremely expensive probably 200-300K.

6. Can the controlled fill be left in place for an extended period of time?

Yes I would think so as it is supposed to support the building load.

Where is the water level?

Only at one place at 3m depth.

If we see the cost vs benefit of placing controlled fill over uncontrolled fill with a bridging layer of geofabric,It make sense to avoid removing the existing fill and placing the engineered fill on top. However, my main concern is what could go wrong in the worst case if the building is founded on top controlled fill with uncontrolled fill below.

 

[miningman]

So its a garbage dumo and you hope to avoid excavating the garbage ?? Good luck with that!!!!!

 

[mikealpha88]

Thanks Miningman, yes it is astonishing. This is the decision from the Geotech engineer and that's why as a structural engineer I am a bit concerned about the long term performance of the building if the controlled fill was placed on uncontrolled fill.

 

[BigH]

Topsoil? That I would remove although years ago in Canada, there was a paper on leaving topsoil in place. if it truly is a silty sand with a deep water level, I would think that a stripping of the top soil and then compaction with smooth drum vibratory roller should densify the silty sand fairly well. You could do quick SPTs (or Mackintosh probes) before compaction and then after compaction to confirm. BTW - what was the SPT values in the uncontrolled fill in original investigation.

 

[mikealpha88]

Thanks BigH.

SPT of 8 was recorded.

 

[JedClampett]

Not as expensive as fixing a building that's all cracked.

 

[GeoPaveTraffic]

As the engineer of record, I would not leave the uncontrolled fill in place. Is it expensive to remove it, yes; but so is removing the failed building and starting again.

The big problem with fills such as this are the unknowns. Everyone really has no idea what is 1 foot away from each of the site explorations. It could be the same as the exploration encountered, but it could also be a buried car or dead animal. As a result it is impossible to estimate how the structure will behave.

One option that could be considered is deep dyniamic compaction. Basically drop a big weight on a tight grid pattern. Did this once on an industrial site that was being reclaimed for a shopping center. It worked ok, but not great.

Mike Lambert

 

[TigerGuy]

I've done the same on what we called a rubble fill site, up to 6m thick. The fill included a variable mix of soil, concrete, asphalt, masonry, steel, etc. remnants. The site had been an unmonitored dump site for all kinds of construction debris (never saw any wood in the mess). The fill was amazingly well compacted. It was to be a single story office building, with minimal loads. Once the onsite storm system and other utilities were constructed, we had them place a 2m thick mattress of crushed aggregate for the building pad. 10 years later, there's been no complaints about the building performance.

 

[mikealpha88]

Thanks Mike and Tigerguy. On engineering drawings it is clearly showed to remove all the unsuitable material however, due to the cost the client is seeking a cheaper solution. Geotech engineer did pointed out the risks of leaving the uncontrolled fill. But the client is willing to take that risk to save money. As the loads are small it looks like a logical solution.

 

[EireChch]

Option 1 - Removal and replacement. Its called uncontrolled for a reason.

Option 2 - Pile the entire structure

Option 3 - Pile the structural elements and leave a floating slab on your new controlled fill. Highlight to client that they may need to grout the floating slab to re-level if it starts to settle excessively in the future. The Client is asking you (and the geotech) to take the risk here by saying the fill is "probably" ok, put the risk back on them.

Dynamic compaction wont work if you have fines of more than 10-15%, and closer to 10%. if its called silty SAND then I think it would be between 5-20% based on BS.

 

[mikealpha88]

Thanks for your reply.

Just wondering if anyone have any experience with placing geotextile bridging layer between existing and new fill?

 

[Ron]

I would wet the site as much as possible then proofroll with a heavy, rubber tire piece of equipment such as a large, loaded dump truck or loaded pan. Use multiple overlapping passes, then test the result.

Another approach is to attach a 36" diameter pipe to a vibratory hammer and insert on a 5-foot grid to compact the lower depths. Again, wetting the soil will help this process as well.

 

[BigH]

Just wondering last night while watching a replay of the Colts game . . . has anyone done test pits to determine the true nature of the uncontrolled fill? That might tell you a lot more than boreholes. And then, I agree as I noted before, proofrolling with a compactor . . .

 

[Milliontown]

Dynamic compaction was developed for situations like this, but it can be $$$. I know its been used on a lot of big landfill reclamation projects in California, including buildings. If that is out of the question, I agree with Ron and BigH, proof rolling with a compactor will help.

I don't have any more info on this, but I saw this image a few days ago online. This is a storage unit built on an old landfill. Pic. If storage units can do this, a warehouse might settle a little more.

 

[mikealpha88]

Thanks guys for your reply.

Any suggestions with this combigrid?

https://globalsynthetics.com.au/geogrids/combigrid-multi-functional-geogrid/

The Geotech engineer has recommended removing the soft spots from existing fill and then laying combigrid layer and then doing the controlled earthworks in 300 layers to 95% dry density.

I am just wondering with this method, most likely the pavement slab will be fine but what about edges supporting the concrete panels?