Soil creep in marine soil deposits

[Okiryu]

I have a site with very soft marine soil deposits (mainly silts) overlying and inclined bedrock. Soils just underneath the former seabed are normally consolidated, and soils on top of the bedrock are slightly overconsolidated (OCR=1.4). The thickest layer for these marine soil deposits is about 40 m. The site was land reclaimed. How can I check if soil creep is occuring/can occur? Soil creep is not "fully" understood, so if you also have good references to share with me, it will be appreciated.

 

[dirtsqueezer]

Do you still have vegetation? Creep is easily seen by the presence of a 'drunken forest', where the tree trunks are curved into an upward direction as opposed to trunk and tree being straight up and down.
I believe creep is caused by the anisotripic freezing of neighboring soil particles, one of these directions being lateral, causing the particles to 'move over', and then drop once they settle out from a freeze, usually in the case of a sloped surface. Other signs are tilted retaining walls or mass wasting at the base. I would imagine just an almanac ccould tell you about the temperature year-round so you can get a sense of how long a freeze had time to set in. Does that help?

 

[Okiryu]

Hi dirtsqueezer, my site doesn't have trees, it has only grass. I was checking on this, trying to educate myself and found that all the points you are talking about are the main considerations for checking creep action. I have not seen any large distortions in the ground surface (which is flat, but the bedrock is sloped, sloping at approximately 10% ). Although we have very cold periods here, we do not get the ground freeze. Also, considered that the marine deposits are very deep (more than 10 meters below the ground surface), I think that I do not have any freeze issues in these layers. Are any other considerations for thinking that creep is happening at my site? Some people are saying that we have creep in this site but I don't see any evidences of this could be happening, so just wanted to her other opinions... Thanks for your reply.

 

[fattdad]

after primary consolidation, soil undergoes compression in the absence of any change in effective stress or dissipation of excess pore pressure. In geotechnical engineering, we call this secondary compression. Many folks also call this, "Creep." Unfortunately, this word, "Creep" is not related to some geotechnical parameter. But, secondary compression is defined by C-sub-alpha.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

fattdad, I was also thinking on relate creep to secondary consolidation. My understanding is that secondary consolidation is more common in highly organic soils, however this is not the case in my site... what can be other considerations for deciding that soils can be prone to secondary consolidation?

 

[fattdad]

Just measure the slope of the time v. dial reading curve for given load increments and you'll know the value of C-sub-alpha. There are published references that correlate water content to C-sub-alpha.

just to be clear, I don't use the term secondary consolidation. To me, consolidation requires the development and relief of excess pore pressure. Compression (elastic and secondary) are independent of pore pressure.

If you have normally-consolidated soils where the liquid limit is at or close to the natural water content, I'd consider secondary compression and look at that accordingly.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

fattdad, do you normally consider that NC soils can have secondary compression? Although they are not organic soils? These are marine deposits and are NC silts, so I will take a look at this. I know that most of geotechnical books talk about this, but can you recommend a specific one? thanks.

 

[Okiryu]

f-d, one more question, if I do not have primary consolidation (no increase of stresses or additional stresses are very low: i.e no loading from fill and building supported by piles to bedrock with structural slabs, so I am not putting stresses into the soil), does secondary compression still continue to be an issue to consider?

So, in other terms, do I need primary consolidation to occur first in order to produce secondary compression?

 

[fattdad]

Even sand continues to have change in void ratio, even after the end of, "Immediate" compression. The magnitude is not that great, but over the intervening period from immediate compression to 30 years, it can represent another 50 percent from modulus-based compression.

So, yes, whether organic or not, my program at our state DOT would require the presentation of C-sub-alpha for fine-grained soils as determined from consolidation testing. Then we have a basis to know whether it's a problem or not a problem.

In a depositional environment such as the ocean bottom, yes, there is some continuing secondary compression. We don't (likely) quantify it, however. Intellectually though, these sediments are on some geologic journey to become shale.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Thanks again fattdad. This information is very useful. I will check in more detail this and post again here if I have any other comments or questions. Also, is any specific reason you don't quantify it? is it because it will take too much time for secondary compression settlements to occur?