Lake Marl Compression Index
Lake Marl Compression Index
(OP)
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.
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.





RE: Lake Marl Compression Index
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.
RE: Lake Marl Compression Index
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?
RE: Lake Marl Compression Index
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.
RE: Lake Marl Compression Index
RE: Lake Marl Compression Index
RE: Lake Marl Compression Index
I have seen correlations based on water content. One in the Das Geotech book is Cc = 0.0115*w.c.
RE: Lake Marl Compression Index
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.
RE: Lake Marl Compression Index
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?"
RE: Lake Marl Compression Index
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.
RE: Lake Marl Compression Index