Dear JedClampett Sir, I have such kind of soil 20 m below general ground, which contain about 65% of Natural Moisture Content, arround 60% Liquid limit, Plasticity index 15, Field SPT N value 8 with SPT hammer efficieny of 45%, soil contents about 5-8% organic matter, undrained shear strength is about 50-70 Kpa... is this Super Soft Clay...whats kind of phenomena happen during earthquake? is this soil behaves like Water or i mean Liquefy?
First I suggest you talk to the geotechnical engineer for the project about the beavior of the soil, not just during an earthquake, but during the life of the project.
Second, generally speaking clays don't liquefy. This doesn't mean that there will not be displacement due to a seismic event. Again, the geotechnical engineer for the project can help with that analysis.
Lastly, 50 to 70 kPa is not super soft, not even soft. This equates to about 1,000 to 1,500 psf which we consider stiff clay. Th fact that your moisture content is above the liquid limit, but your strength is pretty good indicates that the material is sensitive to disturbance. Again, you need to discuss your project and the impact of this material with your project geotechnical engineer.
according to A. Fakher, Lecturer, Department of Civil Engineering, Tehran University, Iran, Clay having a very high water content behaves neither like a liquid nor like a solid, it has very little shear strength but some times Shear strength may varie from <0.1 KPa to 1000 of KPa, which is case sensative and can be termed a super soft clay. clay is neither a liquid nor a solid; it is a material with characteristics bordering between the two. It may be defined as a soil with no practical bearing capacity, often displaying a fluid-like consistency and behaviour.
Similarly, you should be clear that there were many cases, Flow liquefaction occured in Clay having Plasticity Index greater than 20, having Liquid Limit > 50.
I think, You may not gone through recent study over liquefaction, please once search liquefaction on clayey soil and reply me.
I don't know the term "super soft clay", but the properties you list classifies it as a silt. You might want to read about the Turnagain landslide near Anchorage during the 1964 earthquake. The formation called the "Bootlegger Cove Clay" is a sentitive silt with water contents above the liquid limit. During the earthquake, slides formed in a cliff overlooking Cook Inlet and progressed many hundreds of feet back into the plain. Each slide block continued to move into the inlet on a layer of greatly-reduced shear strength while new failures formed behind it.
I don't recall the index properties of the Bootlegger Cove Clay, but your soil sounds similar. Be careful!
I think there is a terminology problem:
Terminology:
"Sand-like" (or cohesionless) refers to soils that behave like
sands in monotonic and cyclic undrained loading. Onset of
strength loss and large strains is "liquefaction."
"Clay-like" (or cohesive) refers to soils that behave like clays
in monotonic and cyclic undrained loading. Onset of strength
loss and large strains is "cyclic softening."
As I indicated, "generally" speaking clays don't liquify. Strain softening, yes, but that is not liquifaction.
Again, the strengths are are reporting are not soft. Clay with strengths that you report would be fine supporting fairly light structures under most conditions, therefore it does have a practical bearing capacity.
Could your clay loose strength due to remolding in an earthquake due to sentivity, maybe. But an online form is not the place to be getting advice if you have that kind of problem. You need people experienced in the local geology and who understand the seismic risk.
Just a point - what is the sensitivity of the clay? The term seems to be a "made up term" linked to a particular professor or region. We have many clays in Canada where the natural moisture content exceeds the liquid limit - typically, they are sensitive - go past the peak strength and you get a sharp and quick drop-off in strength. Important for soil mass movements (landslides, etc - see Leda Clay).
Dear friends thank you for your kind suggestions, now problem my PGA is arround 300 gal, and soil is classified as group D-E for site amplification factor for zero-period spectral acceleration (LRFD Article 3.10.3.2), so please give me some suggestion is this clay softaining during earthquake?
Low plasticity silts and clays may experience pore-water pressure increases, softening, and strength loss during earthquake shaking similar to cohesionless soils. Fine-grained soils with a plasticity index (PI) less than 12 and water content (wc) to liquid limit (LL) ratio greater than 0.85 are considered potentially liquefiable and require liquefaction analysis. While PI is regularly
investigated for pavement subgrades, it has rarely been considered in the past for structure soil borings. However, in order to investigate liquefaction susceptibility of fine-grained soils, the plasticity of such soils should be examined when conducting structure soil borings. Drillers should inspect and describe the plasticity of fine-grained soil samples. Low plasticity finegrained
soils, particularly loams and silty loams, should be retained for the Atterberg Limit testing with the results indicated on the soil boring log.
What happen if my soil is clayey silt? is that liquefy now? if my PGH is arround 300 gal?
To answer your question, you would need to 1) have the PI data for the soils, and 2) perform a liquefaction analysis on the soils.
You seem to have all the information and answers, so why are you posting the question here? We are not going to run the analysis for you.
As has been pointed out, strength loss and movement of the clay or any soil are possible due to a seismic evenet whether the material liquifies or not.
Actually I’m worried only about soil having high moisture content than that of Liquid Limit can loss strength during earthquake and behaves like a liquid of not...Whether i have to consider higher level of safety factor or not?
From my experience, if the natural water content is greater than the liquid limit, it's either a sensitive clay or an underconsolidated clay (silt?). Considering that we don't know the site location it's hard to make any conclusions other than it's not likely to be underconsolidated if it's overlain by a dense sand. I'd say you are dealing with a sensitive clay (silt?). Depending on the strains during an earthquake, who knows what'll happen. Please updated us when you complete your analyses.
Have you done any crosshole shear-wave testing to derive the low-strain shear modulus?
I have six different borehole, i have some physical tests, i have NMC, LL, PL, PI, Unconfined compression data, Triaxial data, Consolidation data, Seive Data with SPT value, but dont have any dynamic parameter like shear wave...due to limitation of technology and cost available on my country i have no any other option for deatail field works and further analysis...neither we have any software nor standards or books. We are still following old books and mannuals i.e. from late 80's.
So i need modern methodology, codal provison and findings from you guys.
We are still using cast insitu bored pile, no driven pile nor precast pile within my country.
Even bored pile is uncased....
So whats ur suggestion for me, to resist 2000 KN of load with single cast insitu bore pile for Road Flyover....
Soil Parameter,
Layer 1 SPT Nfield-6, from ground to 11 m, silty sand, Phi=31
Layer 2 SPT Nfield-50 average, 11m to 20 m, Coarse sand, Phi 34
Layer 3 SPT Nfield-6 average, 20 m to 100 m, clayey silt, c=50Kpa, LL=60, NMC=70, PI=15, cc=0.4, clay=9%, silt=85%, organic matter=6%
For liquefaction analysis PGA=300 gal,
If any one have any idea please advice me depth of required pile, if 1500 mm dia pile will preferred?
you see, I can't help you with the liquifaction problem. It seems to me you could evaluate bearing in the N=50 layer and then determine how much settlement would occur in the underlying clay. I use English units, so your 2000KN load is about 250 tons. May need a couple of piles.
I have always been under the impression that below 40 ft (13 or so meters), liquifaction is not an issue. Too much confining pressure.
Thank you, very much...ok i will conduct bearing capacity analysis, please let me explain little about possibility of clay softening in above case...how can i check it with out any dynamic wave test?
yup i can evaluate pile settlement, based on canadian practice and Indian code of practices...juss im unkown about clay softening calculation..or evaluation..
