Test Fill for compressibility characteristics 1

[Hcubed]

I am considering a "test" preload to determine compressibility characteristics at a site that is underlain by highly variable fills (including wood waste and sawdust) and soft native ground.

Space is somewhat limited, so I'm considering stacking ecology blocks and monitoring them with a survey level over time. The site will be developed with a few lightly loaded buildings and we'd like to avoid piling. Very little fill will be imported, which is why I'm considering the eco blocks rather than fill.

My current idea is to use about a 10' by 10' footprint by 5 blocks high (12 feet or so).

Has anyone done a test like this or can you offer suggestions?

Thanks

 

[Focht3]

Your test area and load should approximate the footprint and weight of the proposed improvements. As a minimum, you will need to install settlement plates at each corner and the center of the loaded area. Depending on the site conditions, it will take months to decades to get a meaningful result. You will need to monitor the settlement plates daily for at least a week, then decide on spreading out the readings based on your initial results. The readings should be taken no more than 30 days apart, and more frequently if the test setup's performance indicates.

Piles may still be cheaper - and may be the only way to assure a successful outcome. I'd suggest a more detailed geotechnical analysis.

Also be aware that the sawdust will rot, and will attract termites as well -



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[gandersen]

Hcubed:

I agree with Focht3's assessment that that time necessary to get meaningfull data could be prolonged by the consolidation of higher plasticity soils. Another concern that I have is the presence of organic material in the fill. You could run a settlement test but if the time frame is short, the biodegredation of the organics will not show up and continued settlements will occur over time that could not be predicted by a short-term settlement test.

Additionally, if the fill is highly variable as you state and you do not test the entire area, the results may not be representative of the worst case and hence any differential settlements that you estimate from the results may be significantly underpredicted.

Good luck

 

[ashjun]

Hi Hcubed,
I beg to differ (correct me Focht3 and gandersen).

By pre-loading I presume you want to cause primary consolidation of the ground? Of what I was taught in my school, preloading is not an option in soils containing organic matter and peaty soils. It simply doesnt work there.

 

[Focht3]

I concur with the comments by [blue]ganderson[/blue] and [blue]ashjun[/blue]. Biodegradation of organics, variability of the fill, and difficulties in preloading the fill materials ("trash&quot you described are all very serious concerns. You should proceed with extreme care.

Years ago, I had an associate who wanted to put a part of a chemical plant on an old dump site without remediating it. Soft clays, wood and metal debris (including 55 gallon drums), shallow (fluctuating) groundwater, on the 'cut' bank of a river near an environmentally sensitive estuarine area - you get the picture. He thought that a mat would be "appropriate" for the conditions. It wasn't - fortunately "management" readily saw the problem with his approach - and I didn't have to quit my job over it.

Yet I understand that in many third world situations, bad housing is better than no housing. Proceed with extreme caution - recognizing the large risks involved.

Let us know what you do -



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[geodan]

Hcubed

You stated you wish to "determine compressibility characteristics at a site that is underlain by highly variable fills (including wood waste and sawdust) and soft native ground."

Soil of itself is generally anisotropic and nonhomogenous. When you introduce 'highly variable' fill including wood waste and sawdust (anything else?) to the bearing soil, you are giving a whole new meaning to the idea of non-uniform.

I'm not certain where you are planning construction, but in our area the structural engineer will refuse to design any buildings or appurtenances without a soils report (required by local code) from a geotechnical engineer addressing soil strength parameters including consolidation/settlement analysis. And when this scenario (old fill) is encountered, the findings are presented in the report and highly variable organic fill is always deemed unsuitable for construction. Finding a soil engineering company in our area which would recommend site development on conditions you describe could be quite a task.

The reason is simple enough. There is simply no way to definitively predict long term settlement or strength of "highly variable" fill soil. Unless you have excavated every cubic yard of fill soil and visually inspected it, there is no way to say for certain what exists below the ground in areas between your test borings.

Again maybe the differnce in local regulations, but if our firm (geotechnical) was to prepare reports which essentially allowed construction on the type of fill soil you have described, we would be searching for a new insurance company.