Bearing capacity of pile using N-SPT method at mixed soil
Bearing capacity of pile using N-SPT method at mixed soil
(OP)
Hi all,
I need some advices on pile design method. I only have N-SPT data, so I’m going to use N-SPT method. As far as I know, the equation for pile bearing capacity is governed by 2 conditions : drained (granular soil) and undrained (cohesive soils). It’s easy if the soil are simply cohesive (silty clay, clayey silt) or pure sand only.
But what if the soil composition more or less are fifty-fifty ? Let say it is a sandy clay with 55% clay and 45% sand.
Which equation should I use ? Should I just mixed the two of them ? (say 55% bearing capacity from “clay” equation and 45% bearing capacity from “sand” equation), or should I just used the smaller value (but it would be underestimating the bearing capacity) ?
Thanks in advanced.
I need some advices on pile design method. I only have N-SPT data, so I’m going to use N-SPT method. As far as I know, the equation for pile bearing capacity is governed by 2 conditions : drained (granular soil) and undrained (cohesive soils). It’s easy if the soil are simply cohesive (silty clay, clayey silt) or pure sand only.
But what if the soil composition more or less are fifty-fifty ? Let say it is a sandy clay with 55% clay and 45% sand.
Which equation should I use ? Should I just mixed the two of them ? (say 55% bearing capacity from “clay” equation and 45% bearing capacity from “sand” equation), or should I just used the smaller value (but it would be underestimating the bearing capacity) ?
Thanks in advanced.





RE: Bearing capacity of pile using N-SPT method at mixed soil
http://portal.tugraz.at/portal/page/portal/Files/i...
RE: Bearing capacity of pile using N-SPT method at mixed soil
Thank you oldestguy, I've read it. But it still doesn't answered my question.
I could use the Meyerhoff equation from page 200 US Navy manual for granular soil (sand). and it cannot be used for cohesive soil. So for cohesive soil then I could use equation like qp = 9 Su and fs = alpha* Su (Meyerhoff, Tomlinson, Reese, etc).
with Su can be derived from empirical correlation from N-SPT.
But what if the soil is consist of 55% clay and 45% sand ?
The soil is half clay and half sand, which equation should I use ?
RE: Bearing capacity of pile using N-SPT method at mixed soil
RE: Bearing capacity of pile using N-SPT method at mixed soil
If you have 30 percent clay and up, call it clay. Clay characteristics control.
I trust any computations will be only for rough estimating pile penetrations and capacities followed by actual on-site determinations as with a pile driving formula.
RE: Bearing capacity of pile using N-SPT method at mixed soil
Thanks for your suggestion. I think I'm agree with you, it behaves more like clay than sand.
Sorry that my question deeply involved judgment engineering, not technical.
oldestguy:
That's a great idea, thank you !
I think I will try pocket penetrometer on the sample. But since the samples were taken from
depth between 45 to 60 feet and N-SPT about 25 to 40, I think the samples were highly disturbed. Would that be okay ?
Yes I'm aware that the bias of N-SPT value is quite large, so I'm using N'(60) instead. The value of N'(60) is 20 to
30% lower than the original one. I intend to conduct a static loading test at unused pile as well, so I could sleep peacefully (I don't trust PDA test).
30% clay will control the behaviour ? I'll bear that in mind. Many thanks for sharing your experiences.
RE: Bearing capacity of pile using N-SPT method at mixed soil
The original Gow method involved a drop weight with a hardwood cushion block at the contact surface to the rods. It was inserted inside the weight and the weight was guided by a single rod down the center of the rods, called a pin guided weight. That would result is fewer blows as compared to steel on steel. As time went on people didn't know about the original and many different methods have been used. Hoisting was with rope on a cat-head, one wrap. Not many a driller followed that rule. Cat heads were dangerous and unfortunately at lest one person that I knew was killed that way.
RE: Bearing capacity of pile using N-SPT method at mixed soil
Found several fissures on some samples, 2 - 3 samples are not fully intact. I suspect the damage occur during sampling (unfortunately I wasn't there, the soil investigation was conduct by someone else, I just received their result).
Based on their report, yes they were using catheads.
What is the best tools for SPT according to your experiences ? I haven't got a lot of experiences in the field yet,
most of my works had put me behind the desk
RE: Bearing capacity of pile using N-SPT method at mixed soil
Numerous conditions affect N value. These generally are tied to ground water effects. Lower N value in saturated sands has been tied to various factors and one item not always mentioned is wash up into casing or hollow stems. Mud drilling can be more dependable. In weathered bedrock the material in the sampler can look like soil. However, if a construction contract has a payment item for rock excavation, you can be "bit" by finding an extra claim for rock when the sampling says soil. The stories can go on and on.
RE: Bearing capacity of pile using N-SPT method at mixed soil
Well now you make me feel doubtful of all field data that piling up at my desk. To think of it, mostly I don't know
how they were taken, which method they use, were they having some troubles when they do the test, etc.
Maybe the best thing that I should do is always watching them closely when they conduct a field test ?
These drilling and sampling business really is not as simple as it seemed then.
No wonder I often get a "strange" numbers like high OCR or else while I interpretating the results.
RE: Bearing capacity of pile using N-SPT method at mixed soil
RE: Bearing capacity of pile using N-SPT method at mixed soil
RE: Bearing capacity of pile using N-SPT method at mixed soil
I had a fun experience the first time out. It was a part time job while in grad school, 1955. It was auger borings only. Rig was similar to power company pole setting rig mounted on a 4 x 4 truck; me and the driller. When done, the last hole was in mud and the truck was stuck. Driller said, "You drive and I'll push". He used the drill to push at an angle to the rear as I drove. The drilling company was a separate company and close with my boss (the professor with a side business). So I remarked to the owner of the drilling company how neat it was to use the drill. He blew up. Stated that his guys have broken a few drill stems that way. Fun.
How about the time I had to show our drillers on the way you mount tire chains on a truck while it is stuck in the mud? A lot more than drill technique is involved.
RE: Bearing capacity of pile using N-SPT method at mixed soil
It's hard to get a good result if the team had not shared the same passion and determination to get the job done right..and to think that they were your own employees, I guess I won't get my expectations too high for my team then, since they're just an outsiders hired for the job. Sometimes the word "professional" held no meaning at all.
BigH:
Yes you're right..I kinda passed the stage of being a field engineer, went straight from college to designer's desk, I just visited the site to choose where they should drilled, tell them what kind of tests they should performed, and left without any comprehensive understanding of how they will carried out the tasks. I will spend some more time at the site in the future; thanks for your suggestion.