Forensic Geotechnical Testing to Verify Proper Compaction

[KootK]

Are there geotechnical tests that can be performed to confirm whether or not soil was properly compacted fifteen years ago? If so, I'd very much like to hear about those tests.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Ron]

They are the same tests that should have been run back then.....a laboratory moisture-density relationship (Proctor) test and in-place density testing, preferably using a direct volumetric method such as a sand cone or drive sleeve.

Changes in compaction with time are minimal unless you have continuous vibration nearby, reactive materials or materials susceptible to shrink/swell.

Changes in moisture content do not change the compaction percentage since calcs are based on dry unit weight. Moisture content will affect reactive soils and soils susceptible to shrink/swell.

 

[KootK]

Thanks so much Ron. Last night I spoke to the guy that I consider to be the best geotechnical engineer in my area. Identical answer.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BigH]

How thick is the fill? Unless you are willing to carry out the various lab compaction tests and compare to in situ densities in test pits (on an "as you dig down" basis), how would you tell if the fill at 10 ft depth is well compacted?

There have been some who would suggest the use of dynamic cone penetration tests - it could tell you the uniformity of the compaction - even the miniature ones if the soil is not gravelly. (

http://researchspace.csir.co.za/dspace/bitstream/10204/3692/1/Paige-Green_2009.pdf)

- this is for pavement design, but again, it shows consistency. As an example - I have a few other references but don't have time at the present to look (shopping in Kuala Terengganu beacons!) Cheers

 

[oldestguy]

Depending on the situation (settlement problem, new loading, etc), why not just do the usual test borings to at least get a handle on the general situation? It's a common way to check out old fills that may or may not have been placed properly, or just dumped. Even Mother Nature doesn't always do a uniform job.

 

[KootK]

The fill is as deep as 30' in some places. I believe that we are wiling to carry out lab tests as we go. And we'll almost certainly do the usual test borings as well. I'm out of my depth with this and happy to accept whatever recommendations you guys have to offer. Thanks for what you've provided thus far.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[oldestguy]

I've seen a few cases like this, but it was known that no compaction was done, just dumped fill. Thickness to 30 feet in a few. In each of these we surcharged the fill and then built buildings with footing areas undercut and that backfill compacted, thickness of undercut established so pressure from footings above did not exceed the pressure from surcharge. Usually time for surcharge was short and a rolling surcharge worked. Buildings of many sorts, stores, theatre, etc. and no problems later.

 

[KootK]

Clever. How long a duration was the "short surcharge".

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[oldestguy]

Settlement platforms were set ahead of the windrow (long pile across the site). The pile was moved forward at 90 degrees to axis, by a bulldozer, crawling up and shoving over the top, always keeping the pile at least 8 feet high and 10 feet wide at top. As the pile reached the platform, readings there were observed for at least two days with full height there. Once the settlement stopped it was oK to move on. I don't recall any delays longer than a week. Soil in the fill was a variety of non-organic soil from clay to gravel. One area had disposed concrete slabs. A variety of sites from gravel pits to side hills, usually not pond areas.

 

[darthsoilsguy2]

soil borings. if the soils are gravels and sands they will likely have consolidated just from the overburden.... if the soils are silts and clays, you could push a shelby tube and collect an undisturbed sample and perhaps find some low densities. you can get the wet density from that tube, then moisture content once you cut it, then use the soil to run a proctor.

 

[BigH]

. . . but how deep is the fill? Surface testing is "easy" - depth considerations a bit more complex.

 

[fattdad]

what's the end game? Do you really care whether the original earth contractor met spec or are you more concerned about the engineering properties of this previously-placed fill? I don't want to take anything away from Ron's reply, 'cause that's my thought as well. I will add though, if it's a clay fill that's been compacted to some density (void ratio) and then it's sat for 15 years, technically there'd be some change in void ratio over time, aka secondary compression. If this is truely a forensic evaluation related to a legal dispute, you'd need to be prepared to address such change in void ratio over time. Bear in mind that secondary compression is defined as the change in void ratio in the absence of any change in effective stress. Some folks also attribute some similar change in void ratio for granular soils.

I'd jam a dilatometer in the ground and get a profile of the fill materials showing the various geotechnical parameters wiwth depth. I'd also do a boring or several, get tube samples and perform some lab testing.

I'm just not sure how you'd benefit by knowing whether this 15 year old fill exists at 94 or 99 percent compaction.

Do tell. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[KootK]

The end game is expert witness testimony. My simplified understanding of the compaction issue is this:

1) Most soil types will become more compacted with time.
2) If compaction is insufficient at present, that suggests that compaction was insufficient to begin with.

Is that logic flawed? I admit it: I'm a dirt-tard. If I understood correctly, this is what my local Geo-guru told me last week.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[darthsoilsguy2]

if you have access to existing conditions and grading plans, maybe there is a drainage feature that was filled over near the structure that you could drop a boring. if you hit the original subgrade elevation, a nice photo of an open split spoon with organics or fat clays at the bottom of the fill is a prize fish for the report.

 

[KootK]

It's a fairly steep site Darth. You may be on to something. Thanks for the tip.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.