Lake Marl Compression Index

[MRM]

I am hoping that someone who's worked in the glaciated regions of the world or other areas containing soft lake marl can share some compression information with me.

I'd like your opinion of Cc and Cr for soft lake marl (WOH, moisture contents ranging from say 50% to 100%, relatively pure with small shells). I believe the marl on this site is very close to NC, even near the surface, based on my estimates from strength & overburden pressure correlations and so forth.

Although we encounter these soils quite frequently in the northern Lower Michigan peninsula, I have yet to perform consolidation testing on the material. On most sites, the depth is such that it can be removed, or a deep foundation is used to bypass it if it is deeper. These are typically the most economical options for most structures we've found. We normally recommend that the grade not be changed (raised) if the marl is left in place around the building/structure to avoid adjacent settlement.

I may have a potential project now that we could preload the marl to mitigate settlements arising from the extensive grading changes that are planned on a particular site. If the project comes into fruition, I'm planning to obtain samples and do some testing, but for now, I'm looking for preliminary info for rough planning purposes. I'm concerned about using my silty clay/clay relationships, which use moisture content, void ratio, etc. to estimate compressibility of the lake marl. I'm almost certain these relationships for lake deposited silty clays (which are actually a good first order compressibility indicator for these soil types, I've found), are not suited to the lake marls in question. I know that there are those that probably have a wealth of information on the subject! Also, I’m interested in what kinds of secondary compression values you’ve obtained on these soils, and any undrained strength information is welcome too.

For the lake marl I’ve described above, at this point, I would estimate a Cc of about 0.5 to 0.7 for a moisture content of 50%, close to saturated condition. Most of the marl to the depth it occurs has an undrained strength of less than 200 to 300 psf throughout a depth of about 15 feet. These numbers are based on my thumb and a pp where I could do it. I don’t have my notes with me and I’m going from memory here. When I get back to the office, I’ll correct my preliminary numbers if necessary.

Thanks, and I appreciate your comments and guidance.

 

[BigH]

What do you mean by "Lake Marl" - not a term I've used. I ran into "marl" in New Jersey but it was a greenish overconsolidated clay. You might be talking "loon shit" - that upper material that is very "soft" - or are you talking varved clay-like material? You give the NWC but how does it "stack up" to the liquid limit? What about sensitivity? I don't have my data with me overseas, here, but if I remember right, I would likely be using Compression ratio's of 0.25 or so as a preliminary estimate of the expected consolidation characteristics. I have found in West Bengal, even though the clay is very soft, it has a higher preconsolidation pressure than you would expect. In this case, I would use take the undrained shear strength times 0.25 or so to estimate the preconsolidation pressure. Again, the above is rough estimates to hopefully get you in the right ball-park for now. You will need, as you've indicated, good samples to be taken. Also suggest piezocone.

Check out Canadian Geotechnical Journals - especially say from 1970s and 1980s. They have a lot of material on soft Canadian clays that might be useful.

 

[MRM]

Hi BigH,
Thank you for your response. Yes, the LL and sensitivity-two important things I didn't mention. Typically (and including this site) the natural moisture content is somewhat greater than to sometimes about equal to the LL so the sensitivity is on the high side. This is also apparent when you work the soil. LL tends to be from 30 to 50, PL is usually between 15 and 25. It behaves like a silt in terms of dilantancy and the short time it takes to arrive at the PL.

BTW, the material I'm working with is indeed the "softer" of the soils you provided a description for! I typically use "Lake Marl" to describe this stuff and I reserve plain "Marl" for describing the O.C. stiff green clay you mentioned, although I haven't worked with the latter. It may be more of a colloquial term in this area more than anything though.

It looks like in terms of compression ratio, I'm thinking along the same lines as you are, or maybe a little higher. Do you have any comments on the collapse potential during loading on a soil like this?

 

[cphi]

MRM,

See ASCE 1983, Geological Environment and Soil Properties, page 269, estuarine deposits.

BigH,

lake marl defined in Terzaghi and Peck, 2nd ed. and T&P and Mesri 3rd ed. (page 5), but I believe MRM is applying the lake marl description to organic clays and organic silts of post glacial age.

 

[MRM]

I ran 3 consolidation tests on the Lake Marl (approx. 4' bgs) this weekend. You were right, BigH-the tests indicate a preconsolidation pressure of about 700psf. I'd say present eff. stress in place at that depth is about 230psf to 260psf with the current groundwater level. Compression ratio is about 0.28 on the big slope and 0.033 on the little slope. T90 (for a single drainage 1"+/- specimen) was just under 5 minutes in the NC range. T90 was less in the OC range. I've got the third test still running to get some secondary data. More on that later.

 

[BigH]

Not bad for a foggy brain - at times. MRM, one reference I do check out is Lambe and Whitman. They have a chart giving natural moisture contents vs compression ratios. Something to remember to get you in the ballpark.

 

[eric1037]

You may want to check out the Muskeg Engineering Handbook by Ivan C. McFarlin.

I have seen correlations based on water content. One in the Das Geotech book is Cc = 0.0115*w.c.

 

[MRM]

I've found that for this type of soil I've recently completed testing on, the Cc=0.0115*wc is a fairly good estimate. Of course I only ran three consolidation tests, each at approximately the same moisture content so I lack a good range of moisture contents to really come to a good conclusion on the relationship. For the moisture contents I did have though, the relationship seems to agree.

I plan to share some more results with everyone in the near future. I've been busier than a one-armed wall paper hanger lately... I've still got the third specimen aging at a constant pressure too-it's been about 2 weeks now.

 

[Focht3]

Thanks for sharing the results!

You may notice funny things happening with your long term tests; be aware that "critters" may be growing inside your samples, adversely affecting your test results. The test is usually fine for the first 2 or 3 weeks...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[MRM]

I never got back on those other results:

The secondary compression index (based on strain) was:
Calphae=0.013 (a pretty substantial value for Ca). I had a good idea of the results after about three weeks so I didn't get a chance to see if the slope changes as micro-organisms take over. I have a feeling it probably would.

Since that time I've completed another consolidation test on a different deposit of lake marl. This current lake marl is an older deposit judging by its depth (begins at +/-25 feet below grade, it is darker in color, has a more pronounced organic odor, and the shells it contains are much more brittle than the "new" deposit I began describing in this thread.

The current lake marl testing also indicates that the relationship Cc=0.0115*wc is a good estimate (much like the previous testing). Coefficient of consolidation Cv was an order of magnitude lower than the "new" lake marl. This was expected since the deeper lake marl had a lower void ratio being that it was consolidated under a higher stress. A loss by wash indicated approximately 50% by mass was sand size and 50% silt/clay size. The LBW was approximately the same for the previous lake marl. So far, the lake marls in this area appear to be relatively consistent in characteristics and how they were formed.

 

[BigH]

One of the reasons I like the compression ratio (Cc/(1+e0)) is that you don't need the e0 value - do a strain-logP' plot. This normalizes the compression values.
- and thanks for the information.