Compaction and Compressibility
Compaction and Compressibility
(OP)
Hi, how can I relate a specific grade of compaction to its compressibility? For example, if soils are compacted to 80% modified proctor, how can I relate it for settlement calculations? I guess I should start looking at the void ratio for the 80% compaction. Soils are silts with high water contents. We are stabilizing them with cement for roads subgrades, but looking at the Proctor results, soils may only be compacted to ~80%. I was worried about settlement issues due to low compaction degree. I think that other issue is that the subbase/base may not be able to be compacted properly if the subgrade gets compacted only to 80%.





RE: Compaction and Compressibility
I'm surprised that you can only achieve 80% of the modified Proctor. Why? Can you post the Proctor curve and a couple of in-place density test results? What method are you using for density tests? What is the character of the silt? Is it calcareous? Organic?
RE: Compaction and Compressibility
RE: Compaction and Compressibility
Also, double checking using this equation:
dry density = wet density / (1+W%), I got similar dry density values to what I got from the proctor curve. BTW, these silts without cement stabilization have very high water contents (about 100%).
My question is, what may be the implications when allowing only 80% compaction of the subgrade. I feel that it may be a problem when compacting the subbase to say 95% modified proctor. Perhaps also some settlement issues. But, I am still figuring out how to quantify/proof with numbers this issue...
RE: Compaction and Compressibility
So the main reason for not achieving say 95% DD is because the soils are too wet of optimum MC. Am i correct in saying that after cement stabilization the MC is at 75%? That seems very high.
We generally just undertake CBR testing for subgrades and dont specify a DD percentage. Regardless, what difference does the DD matter if its cement stabilized? The increase in strength through cement binding may not be readily picked up in a DD test but should increase strength dramatically with the right mix. I would assume that a cement stabilised subgrade would easily achieve a CBR of 25%. This should allow a standard pavement build up.
In the event that you are getting a low DD% or a low CBR say less than 3%, you may need to look at geogrids. Geogrids would be a good (but expensive) option as they can bridge over softer soils.
Or revisit the stabilisation method and develop a cement/lime combination that achieves a good CBR.
Interested to see what other more experienced engineers think.
RE: Compaction and Compressibility
RE: Compaction and Compressibility
If the material wasnt cement stabilised i would be concerned with 80% as you may not get good compation in your subbase due to cushioning effects from soft subgrade. However in relation to your quote below. I dont think the 80% is an issue as you are getting good CBR values. You should easily achieve 95% compaction in your subbase when compacting on a subgrade of CBR 3+.
ps - I would have thought you would have gotten better CBRs for cement stablilisation. Did you do some test cube mixes? Say 3,5,7, 10% of cement and crush them? Sorry bit off topic but just interested.
RE: Compaction and Compressibility
You need to develop the moisture-density relations for the silt. In that regard, you'd need to develop the Standard Proctor (or Modified Proctor) moisture-density relations. That'd be ASTM D-698 or D-1557, respectively. Either of these would be important if you are looking to compact the native soil without any cement/lime stabilization (i.e., prepare the site for subsequent fill placement). However, if you have plans to improve the subgrade AND also use cement/lime in the subsequent lifts of new fill, you'd need to develop the moisture-density relations using the cement/lime stabilized blend. There are separate ASTM standards for developing moisture-density relations on lime/cement stabilized soils. Use those!
To actually gauge the field compaction, you have to take the field density tests immediately after compaction (i.e., before the cement or lime is fully hydrated). If you are doing the tests after several hours (or the next day), you are not following protocol and you'll get incorrect results.
I trust that you will actually seek out opinions from the geotechnical community and am surprised that few geotechnical engineers are providing opinions. Sure you are now up to two, hopefully something helps. I must say to the OP 80 percent relative compaction is almost like not compacting at all. Under the worst of all possible conditions, you'd be hard pressed to get below 75 percent relative compaction. Then again, you may be talking about relative density, which is completely different. 80 percent relative compaction is akin to 0 percent relative density, so that doesn't even make sense for a silt!
f-d
ípapß gordo ainÆt no madre flaca!
RE: Compaction and Compressibility
Also - I am not sure that I have ever seen anything other than organic silt that would have a dry density in the order of 10 kN/m3 - this is the unit weight of water . . . so my question would be - what is the specific gravity of your silt? I've had volcanic residual clays with standard MDD of 13+ kN/m3.
As a bit of an aside, Dr. Jian Chu wrote an interesting paper for the 19th SE Asia Geotechnical Conference on "Innovation in Soil Improvement Methods". In it he describes, now that Indonesia is basically refusing to export sand to Singapore, the use of sludge or soft clays in land reclamation. To reduce the settlement, i.e., cause more settlement during construction times rather than normal consolidation after construction, they are looking at using horizontal drains (like PVD) between layers of the fills and then applying a vacuum to the drains - which causes quick consolidation of the fill. It is an interesting concept. I heartily recommend that my colleagues obtain a copy of the paper . . . a good read.
Getting back to the OP, he has not described if this is for a road, a land reclamation, or . . . but it might be a novel real test to use horizontally placed drains under vacuum to consolidate the fill quicker and reduce long term settlements.
RE: Compaction and Compressibility
RE: Compaction and Compressibility
More commonly, an unconfined compression test for cement stabilization would be used as the strength/stability correlation rather than a CBR. For lime stabilization, the CBR would be ok.
EireChCh.....why specify a CBR value if you don't also specify a compaction percentage? They go hand-in-glove. The CBR value is some stability value AT some compaction value. For instance, do you want a CBR of 10 at 100 percent compaction or a CBR of 10 at 95 percent compaction? These can be vastly different in their moisture sensitivity.
RE: Compaction and Compressibility
f-d
ípapß gordo ainÆt no madre flaca!
RE: Compaction and Compressibility
RE: Compaction and Compressibility
In my area (coastal plains soils, mostly poorly graded fine sands), we typically require stabilization to a CBR in the 20 to 30 range, depending on traffic loading. For unstabilized subgrade, we generally assume a CBR in the range of 10 or so.
Okiryu...are you sure about your specific gravity? It doesn't seem to fit with the zero air voids curve.
RE: Compaction and Compressibility
RE: Compaction and Compressibility
If you are not familiar, I have attached a graph from a TTRL publication showing compaction levels and CBR for a sandy clay - of course the moisture contents are out to lunch compared to yours but, for reference.
RE: Compaction and Compressibility
Also, the link for your attached TTRL publication did not open. Could you upload it again, please?
RE: Compaction and Compressibility
http://www.trl.co.uk/reports-publications/overseas...
http://www.transport-links.org/transport_links/fil... (Don't know why they blacked out some tables and figures)
RE: Compaction and Compressibility
RE: Compaction and Compressibility