Can I get meaningful data from dried seafloor samples
Can I get meaningful data from dried seafloor samples
(OP)
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





RE: Can I get meaningful data from dried seafloor samples
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.
RE: Can I get meaningful data from dried seafloor samples
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.
RE: Can I get meaningful data from dried seafloor samples
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.
RE: Can I get meaningful data from dried seafloor samples
What are you putting in 400 meters of water? What kind of geotechnical exploration budget are you working with?
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Can I get meaningful data from dried seafloor samples
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?????
RE: Can I get meaningful data from dried seafloor samples
RE: Can I get meaningful data from dried seafloor samples
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.
RE: Can I get meaningful data from dried seafloor samples
RE: Can I get meaningful data from dried seafloor samples
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
RE: Can I get meaningful data from dried seafloor samples
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 -
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Can I get meaningful data from dried seafloor samples
I appreciate all of the other comments provide to me by everyone.
RE: Can I get meaningful data from dried seafloor samples
Well, 33° 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° (about 4%) then the site may be astride an 'ancient' slope failure ("slump"), since the overall slope is at about 5.7° (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.
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Can I get meaningful data from dried seafloor samples
http://f1.pg.briefcase.yahoo.com/bc/markmcgl/lst?.dir=/...
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
www.oceanstap.com