Can I get meaningful data from dried seafloor samples 2

[oceanman]

I am new to this forum, and am asking my first question. I am interested in learning as much as I can about the load-bearing and anchoring capacity of the sea floor in a certain area. At present, all I have are dried sea floor core samples. These were taken from about 400 meters depth off of San Diego's coast. At that depth, I want to determine the feasibility of anchoring or setting onto the sea floor a structure. The sea floor slope is about 10 degrees at that point. My background is not really in this field, but I did take a short course in sea floor engineering, so I at least know a few basics. (Please try to make your answer understandable by someone not totally versed in your field.) My short course taught me that samples should not be dried out for things such as shear strength, but at the present time that is all that I have. I have the original bulk density of the samples, so I could potentially rehydrate them to their original values. I thought that I would do a particle size analysis to at least characterize the soils to some extent. Should I try to determine the Atterberg index values, or will they be worthless? Any comments would be most welcomed. Also, any recommendations for a commercial lab that can do this sort of geotechnical analysis would be welcomed

 

[stresstracker]

Estimates of shear strength values on disturbed samples are nearly impossible to obtain. But if going back and obtain undisturbed samples, or perhaps in-situ testing are not possible, then there are a few things you could attempt with your dessicated samples and get, at best, a range of shear strength values.
1. perform a couple of grain size analyses.
2. determine the specific gravity of the soil.
3. determine liquid and plastic limits in a couple of samples (Atterberg limits).
For these three tests your dried samples are good, as long as they were not baked in a lab oven (for example, above 100 C).

The Atterberg limits don't mean much without knowing the natural or insitu moisture content of the samples. But if your values of bulk density are representative of insitu soil conditions (i.e. the sample were saturated at the time of "measuring' density) then you may be able to calculate an estimate of natural moisture content(nmc). Then you could compare this natural (insitu) moisture content with the liquid and plastic limits. The closer the nmc is to the plastic limit the stronger your soil conditions may be, an viceversa. You may be able to find some strength and consolidation correlations based on nmc and void ratio.
I realize this is a very crude estimation of strength, but perhaps better than nothing. Note that the determination of specific gravity is notoriously dependent on operator's errors. If the test is not done by an experienced operator, the results might well be misleading.
In the previous discussion I am assuming that there is no interparticle cemetation, and the the soil's structure is dominated by the fines content.

 

[oceanman]

Thank you for the rapid response. The particles are sort of stuck together and I have not yet tried to separate them, but I will do so soon. The samples were air dried, not oven dried, which I guess is good. They have started to crumble, so I assume they are not really cemented. It is my intension to take your advice and do these tests.

Also, I looked into getting new samples myself with a coring device, and now believe I may be able to obtain a sample, at least from the top layer 20 to 30 cm or so of the sea floor. Do you have a recommendation for how I can test the shear strength from a fresh sample from a 2" diameter core sample. In my class they talked about a vane shear measuring device, but I think my sample may be too small, but I am not sure. The sample would technically be disturbed, even if fresh, so the shear strength would have to be reported (I assume) as a remolded value. (Please excuse my ignorance on what is probably a trial issue, but I am very new to this field.) My preference would be to send the fresh sample to a commercial lab that does this type of testing on marine samples on a regular basis. If you know of a such a lab, sharing its identity with me would be appreciated.

 

[stresstracker]

If you could go back and do some sampling, I suggest using thin wall tubes (or Shelby tubes). Such samples would be considered undisturbed. Although, I'll agree with you, there will always be some disturbance. Also, if you go back for sampling, try to do insitu vane testing. Shear strength measurements using insitu vane tests are probably the best choice you've got.
If you use thin wall tubes, whether they are 50 or 70 mm ID, you can alway use a hand held vane tester to obtain undrained shear strength values. To minimize disturbance by handling and de-stressing of the samples, you ought to use the hand shear vane test as soon as you recover the sampler.
There are two types of hand vane devices. The Pilcon Edeco, and the Torvane.

The Pilcon Edeco type have two different blade sizes. The blade must be pushed into the sample (still in the tube) at least 100 mm from the tip. You start applying torque and fail the sample within 1 minute. You read the "undisturbed" undrained shear strength out of the scale. Let the blade in place, and after 5 minutes reapply torque, and see how much is the residual strength. This reading is the "remoulded" shear strength.

To use the Torvane you must extrude some of sample out of the tube (most likely you need to bring the sampler to the lab). Cut the extruded soil with a wire, and apply the Torvane on the "undisturbed" surface of the soil within the tube. The tip of the sample is througly disturbed, so you don't want to apply the Torvane at this point.

In my opinion, the Pilcon Edeco type provides better measurements.

I would be happy to test the samples for you. But, my company, we are consulting geotechnical engineer, is located in Ontario, Canada. I can tell you, Shelby tube samples do not travel well. You should be able to locate a geotechnical lab in your area, either a professional outfit or a university/college lab.

 

[Focht3]

I'm not real hot on using a hand vane for the only shear strength measurements, or even the primary strength evaluation technique. You can rig a miniature vane for field use, and run the tests in the Shelby tube before the samples are extruded. In my view, a combination of in situ vane readings and miniature van readings will give you a better overall picture of the soil strength profile.

What are you putting in 400 meters of water? What kind of geotechnical exploration budget are you working with?



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[BigH]

I agree with Focht3 on the use of miniature lab vanes in the end of the shelbys (thin walled tubes). This was standard practice in our firm on all samples obtained; and it could be done in the field before you even wax up the tube.

There was a lot of work done by various parties on correlations of Atterbergs and strength parameters. While all correlations are just that - for general guidance, still, it is not a bad way to go. Now if I remember correctly, look up a reference of Dwight A. Sangrey. He wrote a nice technical paper for the then Soil Mechanics and Foundation Journal of ASCE - I believe try years 1969 to 1972 - somewhere in there.

I think that Focht3 might be able to provide some insight on disturbance of samples brought up 400m from the ocean bottom - at least better than I can (first hand experience and all) - but I would believe that there might be some disturbance due to the huge pressure relief that you will put the sample to - from 400m x 10 kN/m3 - nearly 4000kN/m2 relief!! If I remember correctly, you might get a lot of expansion bubbles. There probably are "techniques" that can be used in these cases - try a geotechnical firm specializing in offshore investigations. Focht3?????

 

[BigH]

Sorry - forgot to add that you might need more than just the "surface sample" for anchoring purposes. How deep did your investigation go - just grab samples at the bottom, likely, from what I saw. Be careful; seabeds can vary in composition depending on the geological history of your site. Structures go deeper than a metre!

 

[oceanman]

Focht3, we are planning to put in a proprietary system to desalinate sea water, a portion of which needs to be placed on the sea floor. At our proposed site, we are on a ten percent grade which leads to the very deep ocean. This grade may cause some stability problems for anything we may eventually place on the bottom. So far, from just a visual examination of the six inches of bottom I accidently collected (long story), the bottom is "green mud". Clearly, I need to define the bottom better than that. I think that I can get proper core samples of about 60 to 90 cm length at low cost to our project. Longer cores will be costly to get. In the next few days, we will be doing some 3.5 KHz sonar work, which should give a pretty good indication of the deeper sediments and stratifications to see if the deeper soils are similar to those at the surface. The sonar will not provide us with actual soil parameters, but we should be able to identify changing layers. Right now we are on a shoestring budget of just several thousand dollars for this portion of the work. If things get going better, then our budget can be expanded. For now, I need to get our confidence level up before doing the very expensive testing we will ultimately need. We may also need to look at several different sites with cursory work prior to expanding our work on one site.

What we are proposing the put in may include some heavy pumps, motors and water pipes. There may be some lighter, floating equipment, which would be attached via anchor to the sea floor. We do not want the equipment to slide down the slope to the deep ocean and we do not want it to sink deeply into the mud. I hope this helps to clarify our situation a bit. I do appreciate all of the comments so far, and anymore that may come.

 

[BigH]

For a project of this scope and importance, it doesn't seem that you are getting the fees commensurate with your level of risk. You can be correct about ocean bottom slides - 10 degrees is not that flat in such areas (many cases). Be careful and why do the job for peanuts. Ensure you cover yourself adequately in the report.

 

[ashjun]

Hi,
I appreciate the points raised by bigH on the expansion bubbles being formed when the sample is raised to the sea level; which perhaps was altogether missed when the shelby tubes were suggested.

The point on atterberg linits deduced on the air dired samples is good; there are a few papers (I cannot recollect them now..but will try to get hold of them) which suggest that the sea bed samples may not be oven dried to deduce the atterberg limits.

I think the better option at present (and if you have a few sites to investigate) is to have a field vane shear apparatus prepared especially suited for your projects and as suggested by Focht3.
Regards

 

[Focht3]

Where is the site?

There is something really strange about trying to trim the end of a soil sample in a Shelby tube as it "grows" from the gas expansion. What an eerie experience! It is very hard to describe; my best analogy is that it's like trying to hold onto a greased piglet - it's damn hard to do, but it's kinda funny at the same time even if it is frustrating work. And that kind of condition is a real risk for soil samples from that depth range.

Your present budget is woefully inadequate for the kind of investigation that will be needed. And the 10% slope scares me, too. You need to be sure that your sonar work covers a very large area - so that any existing slope stability problems can be identified before you go too far with your design. How large an area? I'm not sure - it depends on where your site is, and whether the area is seismically active. In the absence of better information I'd suggest an area that is at least 4 km by 4 km, with the site at the center. This data will have to be reviewed by a marine geotechnical engineer / marine geologist team. It's better to do this on the sonar boat just in case the area of study needs to be expanded.

Are there any oil/gas exploration activities or subsea pipelines in the area? You might get some information on slope stability problems from the owners of those facilities. Not enough to design from, but enough to point you in the right direction -



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[oceanman]

Thanks for the comments. Without revealing the exact area, it is in the general area of 33 degrees north latitude, up to 4 miles offshore. There is no oil drilling at all in the area, and there likely will never be any. We did two days of sonar work this weekend with bottom penetrating 3.5 KHz equipment in a 500 meter by 500 meter square, and some furher away areas. We were able to see two layers of bottom sediments OK, and we have identified some built up sediment which could be vulnerable to seismic activity about a mile away from our site. I am not too sure if we really needs to do a much larger survey or not, as the immediate area was most homogeneous without any noticable hazards. While the data has not been fully processed as yet, we did get to see some preliminary results. It turns out that the slope is actually only about 4 percent in the immediate area of interest, not the 10 percent I had thought from looking at the charts. There appears to be about 10 meters of top sediment over something much denser. Without having (wet) physical samples as yet, it is hard to speculate what either layer is all about. Hopefully I can at least get a sample of top layer and do the suggested tests in the strings of this question. It is probably too difficult the get a sample from the next sediment layer, at least for now with the limited budget. Perhaps that will come later with the larger budget that this project will need to have in the future.

I appreciate all of the other comments provide to me by everyone.

 

[Focht3]

OOPS. I looked at 10% and thought 10&[ignore]deg[/ignore];. It's only about 5.7&[ignore]deg[/ignore]; - big difference. It's not that steep after all.

Well, 33&[ignore]deg[/ignore]; North latitude doesn't help me much. Frankly, a range in longitude would have been a helluva lot more useful. Oh, well - I might have been able to give some more specific comments.

I wasn't specific enough in my previous post; the larger survey could be done with side scan sonar as well. But you need to evaluate the surface expression of a much larger area than 500 meters square. You are looking for evidence of previous and/or incipient slope failures. Studying small areas is likely to be a waste of time - it is likely to give you a false sense of security. For example, the charts are based on observations of widely spaced points and should provide a reasonable estimate of the overall slope declination. (This assumes that the charts are current and accurate, of course.)

If the "local" seafloor only slopes at 2.3&[ignore]deg[/ignore]; (about 4%) then the site may be astride an 'ancient' slope failure ("slump&quot, since the overall slope is at about 5.7&[ignore]deg[/ignore]; (about 10%). This can be a real problem if you are near the mouth of a large river with an active sedimentation load (i.e Mississippi, Amazon, Ganges, etc.) that is actively depositing lots of underconsolidated clays on the seafloor. These very weak soils can create a series of nested slides with a "flat" area near the top of the next failure mass.



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[oceanman]

Focht3, thank you again. I hate to be too criptic on the location, but I just do not want to permanently publish it on the internet. However, you are welcome to the exact location which I have posted at the following URL:

http://f1.pg.briefcase.yahoo.com/bc/markmcgl/lst?.dir=/oceanman&.view=l

You do not have to open the file, as the name of the file is the location in the format of deg min sec. If this does not work, let me know. I hesitate to post my e-mail for fear of spam, but I have also listed my web site in my signature, which has an e mail address that will get to me. The file body has the same informaton, which you are welcome to open. I intend to remove this file once you tell me you have looked at it.

The site does not appear to be next to any sort of river, but there may well have been an ancient river in the area. I do have limited published side scan multibeam data from USGS, but it is not detailed enough to do any good. I guess that I will have to find a new service provider to help me get this data at the appropriate time. I do not doubt that the area has been surveyed before, but I do not really know who would have the data. I think that the area by shore has been the subject of a very detailed multibeam survey by Thales Geosystems, but they did not go out as far our location of interest. I sent an e-mail to Scripps to see if they had such data, but I have not heard back from them.

Thanks again for the help, and please post when you have looked at the file title.

Oceanman
Ocean's Tap Corporation

http://www.oceanstap.com