Bearing capacity of pile using N-SPT method at mixed soil

[AndreBC]

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.

 

[oldestguy]

Try this link to US Navy Soils manual. Section 7.2 page 200 is a start for your question discussion.

http://portal.tugraz.at/portal/page/portal/Files/i2210/files/eng_geol/NAVFAC_DM7_02.pdf#page=198

 

[AndreBC]

oldestguy:

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 ?

 

[moe333]

The soil you describe will most likely behave like a clay. I would use the clay procedure. You could also try both and see what is most conservative.

 

[oldestguy]

When sampling did anyone do at least some crude tests for unconfined compressive strength? Even without that one can estimate cohesion from the N values. If the samples are still available, at least do some pocket penetrometer readings. If you can get readings at all, that should answer your question. A competent drilling and testing firm would have all that data for you on the logs. I hope you know how many on the job drilling errors can be there with this N value business. Easily a 50 percent "error" can result from different pieces of equipment. For instance I once was inspecting as drilling job and found the split spoon tips were caved in leaving a small opening. That's about like driving a solid 2" diameter rod. Free fall is another question, as is the contact surfaces. Mr. Gow, the inventor, would turn over in his grave if he could view the many goofs that take place.

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.

 

[AndreBC]

moe333 :
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.

 

[oldestguy]

Likely some disturbance, but what do they look like? Still look like they came out of the sampler? If they are somewhat old, they may have dried some, but usually disturbance causes some weakening at least showing up for a while after sampling. There also may be some dampening effect from inertia in the rods, but I don't know of that being consistent in my experience.

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.

 

[AndreBC]

oldestguy:
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

 

[oldestguy]

While specifying a pin guided 140 pound drop weight fitted with a hard wood cushion block, might be followed, most drillers don't comply and use what ever they have. Mention that and all you will get is a blank stare. Split spoon sampler driving can vary also. I usually say drive it 18 inches and discard the first 6 inches count. But, let's say you sample every 5 feet. What goes on between sampling? Do they record drilling effort changes, material changes? That all depends on drilling technique. The hollow stem auger method leaves much to be desired, but competition-between drillers is a fact of life. No longer do we feel happy with 30 feet of drilling and sampling in a day, using the old fashioned wash and chop method. However, for building purposes, I like to spec a sample every 2-1/2 feet to depth of 20 feet, followed by 5 foot interval below that. Not so easy when all the rods and casing don't easily work out at 2.5 feet intervals.

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.

 

[AndreBC]

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.

 

[oldestguy]

I will tell a story. I stopped at a boring crew to tell them of the next job, a farther distance from the office. I sensed some disappointment at having to go there before heading home this time. So I went away, but turned back and came towards the job site from another direction and parked behind a building, leaving the driver's window open so my telescope could mount there. I watched for a while and noted some cheating going on as to drilling and sampling, getting more footage in a hurry. I then returned to the job and asked to see the log of the last boring. It sure looked legitimate, but was not. These were our employees, but not after that episode. There are many ways to cheat in that business.

 

[BigH]

At my first jobs (in Canada) with established geotechnical engineering firms, borehole logging was always left to starting out engineers and up to 5 years experience. We never let drillers log the boreholes - even though they were "on our payroll too". It was great experience - firstly in giving me the opportunity of identifying soils and substructures within those soils (varved clays, sands with laminated lenses of fine silt and clay, etc), and secondly, giving me experience in how drills worked, what the best drill for a particular stratigraphy would be (power auger, hollow stem auger, casing and wash boring) and got me outdoors in some pretty amazing places. AndreBC - if you've never been on a drill rig before, I would suggest that you seek that experience.

 

[oldestguy]

BigH:

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.

 

[AndreBC]

oldestguy:

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.