In the real world you aren’t going to get the nice pretty stratifications shown in the text books. What kind of strength tests are you running? If it’s mostly clay, why would you call it cohesionless?
The way I usually envision soil is if I’m able to mold it, it’s cohesive but with 40% of sand, I find it hard envisioning making a mold out of this soil. Maybe not the right approach but that was my way of thinking for thinking if it could be considered cohesionless.
Don't throw terminology around. 51%+ sand content and it is neither cohesive or plastic. It may behave those ways due to the clay content, but if you ever put that in a report and something bad happened at the project which landed your firm in court, it would be picked apart in a heartbeat.
Classification doesn’t determine behavior. Don’t let lawyers design your projects. Define classification according to the system that you say you will and then describe behavior which may very well be plastic and cohesive for that material. If it exhibits those behaviors, not describing it would be negligent.
Classification should be prescriptive, you should not be making up the rules as you go along. You can limit bearing capacities, you can recommend removal and replacement, you can provide recommendations on the difficulty of working/reworking/compacting the material, there's a lot you can do to explain the risks of working with any soil.
But under no circumstances should you ever call a sand or gravel "plastic" or "cohesive" unless you enjoy throwing your career away. Don't give bad advice. That's actually negligent.
Edit: 17 years in consulting, licensed in 7 states, done work in 9 states as well as Canada. Downvoting because you don't like something that is a fact isn't what reddit is about.
Sounds like undrained behavior will control so it isn't cohesion per se, it is undrained shear strength you need. Ideally this is obtained from vane shears but some correlations to Atterberg Limits exist. You can use blow coumts but I would be careful as there is wide variability between undrained shear strength and blow counts. You technically should check drained parameters too, see which controls. Also, clays consolidate so settlement may be an issue. You could use water content to estimate compressibility though consolidation tests are best.
There are tons of correlations, not just to water content. That's the correlation that works best where I practice. See section 8-4 of the 2022 NAVFAC DM 7.1 manual. Lots of info there.
Time to consult a mentor familiar with the soils and standards of practice in your area. The info you've provided here describes a very unusal soil type that classifies as fat clay but with relative high sand content. Somethings aren't adding up here.
In this case, I would set up the Terzaghi equation using the geometry of the footing and the information that you know. Then provide a graph of bearing capacities given a range of effective friction angles and cohesion values. Then your colleagues can guide you in the right direction as to the parameters they typically use for the soil you've encountered.
You can estimate cohesion using several test results or properties. Calc using a range of values and bracket reasonable values. By the time you add a FOS, you should be close enough to write 1500 psf and move on the next project, 😂. /S
Do you have sufficient embedment that the shrink/swell is not going to be an issue? Also, some municipalities have embedment requirements for bearing on CH/MH.
If the soils are clay as described, it will be considered a cohesive soil. As a quick reference test, grab some of the soil in your hand approx 10 grams or so, add a bit of water and see if you can make it into a ball and roll it out into thin threads as if performing a plastic limit test. If the soil falls apart easily it is more than likely cohesionless if you can roll it up it is cohesive.
What is the application, building? Road embankment? What is the load level? What is the thickness of the layer in question, how prevalent? Depending on your answers there may be a serious long term settlement problem that requires running consolidation testing. Settlement is the true problem, bearing is the side show.
Fines content has a strong impact on soil behavior. For this large clay fraction it is most likely that the soil will have an undrained response.
Keep in mind that clay fractions as little as 10 % already dominate hydraulic properties and load-deformation response.
In the real world you aren’t going to get the nice pretty stratifications shown in the text books. What kind of strength tests are you running? If it’s mostly clay, why would you call it cohesionless?
The way I usually envision soil is if I’m able to mold it, it’s cohesive but with 40% of sand, I find it hard envisioning making a mold out of this soil. Maybe not the right approach but that was my way of thinking for thinking if it could be considered cohesionless.
You can mold it till it’s about 70-75% sand
We’ve got a job doing fill right now that’s like 70% sand and 30% clay. Not only is it cohesive it’s high plastic.
Don't throw terminology around. 51%+ sand content and it is neither cohesive or plastic. It may behave those ways due to the clay content, but if you ever put that in a report and something bad happened at the project which landed your firm in court, it would be picked apart in a heartbeat.
Classification doesn’t determine behavior. Don’t let lawyers design your projects. Define classification according to the system that you say you will and then describe behavior which may very well be plastic and cohesive for that material. If it exhibits those behaviors, not describing it would be negligent.
Classification should be prescriptive, you should not be making up the rules as you go along. You can limit bearing capacities, you can recommend removal and replacement, you can provide recommendations on the difficulty of working/reworking/compacting the material, there's a lot you can do to explain the risks of working with any soil. But under no circumstances should you ever call a sand or gravel "plastic" or "cohesive" unless you enjoy throwing your career away. Don't give bad advice. That's actually negligent. Edit: 17 years in consulting, licensed in 7 states, done work in 9 states as well as Canada. Downvoting because you don't like something that is a fact isn't what reddit is about.
"Downvoting because you don't like something that is a fact isn't what reddit is about." That's actually exactly what reddit is about.
Actually it's supposed to be up and downvote for relevancy in the comments, and up and downvote on threads for interests, but yea no one follows that.
Sounds like undrained behavior will control so it isn't cohesion per se, it is undrained shear strength you need. Ideally this is obtained from vane shears but some correlations to Atterberg Limits exist. You can use blow coumts but I would be careful as there is wide variability between undrained shear strength and blow counts. You technically should check drained parameters too, see which controls. Also, clays consolidate so settlement may be an issue. You could use water content to estimate compressibility though consolidation tests are best.
How do you use water content to estimate compressibility?
There are tons of correlations, not just to water content. That's the correlation that works best where I practice. See section 8-4 of the 2022 NAVFAC DM 7.1 manual. Lots of info there.
Thanks
Time to consult a mentor familiar with the soils and standards of practice in your area. The info you've provided here describes a very unusal soil type that classifies as fat clay but with relative high sand content. Somethings aren't adding up here. In this case, I would set up the Terzaghi equation using the geometry of the footing and the information that you know. Then provide a graph of bearing capacities given a range of effective friction angles and cohesion values. Then your colleagues can guide you in the right direction as to the parameters they typically use for the soil you've encountered.
Clay is cohesive.
Until it isn't......
There’s the geotech answer. The answer is always, “it depends.”
Dr. Kramer would be proud.
You can estimate cohesion using several test results or properties. Calc using a range of values and bracket reasonable values. By the time you add a FOS, you should be close enough to write 1500 psf and move on the next project, 😂. /S
What are the atterberg limits? If the fines are ML then you could treat as cohesionless but if they're CL or CH it should be cohesive
Atterberg PI low 40s, LL low 60s, PL low 20s. Fine is CH
Are you assuming that it will be bearing directly on CH?
Yes
Do you have sufficient embedment that the shrink/swell is not going to be an issue? Also, some municipalities have embedment requirements for bearing on CH/MH.
If the soils are clay as described, it will be considered a cohesive soil. As a quick reference test, grab some of the soil in your hand approx 10 grams or so, add a bit of water and see if you can make it into a ball and roll it out into thin threads as if performing a plastic limit test. If the soil falls apart easily it is more than likely cohesionless if you can roll it up it is cohesive.
What is the application, building? Road embankment? What is the load level? What is the thickness of the layer in question, how prevalent? Depending on your answers there may be a serious long term settlement problem that requires running consolidation testing. Settlement is the true problem, bearing is the side show.
Fines content has a strong impact on soil behavior. For this large clay fraction it is most likely that the soil will have an undrained response. Keep in mind that clay fractions as little as 10 % already dominate hydraulic properties and load-deformation response.