Standard Penetration Test & Allowable Bearing Capacity Of Pile 2

[mikealpha88]

Hi Frieds,

I am working on a project where we designed a pile based on medium dense silty sand below the water table. The pile is 600mm dia and 5m deep and supporting an ultimate load of 250kN. Based on Terzaghi's equation I took the angle of internal friction of 32 with drained analyses of the pile and it was working fine.

However, onsite due to the water table and collapsing sand, a Standard penetration test was ordered to determine the penetrations. only two readings were taken one at 5m with 3,4,4 and another at 12m with 6,8,10. The soil was classified at 5m as silty clay (instead of medium dense silty sand as per previous logs) and at 12m as sandy clay/clayey sand. I am not sure if the SPT values are corrected due to overburden or not and if the water was added to the borehole to balance the pore water pressure when taking this reading below the water table. I am assuming that these readings are straight out of the SPT.

I checked the piles for both silty sand and silty clay using Meyerof's formula (1976) and Skempton's formula respectively.

With Meyerof's formula (40ND/B x Ap)+(2N*pi*dia*depth) I am getting Qult of 993kN and with S.F of 2.5 the Qall = 397kN

With Skeptons formula Su=50 & phi' = 21 I am getting Qult of 744 kN and with S.F of 2.5 the Qall = 297 kN

Can I request your opinions if the above two methods are too generous or too conservative or just about right?

Thanks in advance!

 

[HTURKAK]

Is this bored or driven pile? If driven , what is the material ?

You should consider the load test of the pile and see the situation..

 

[mikealpha88]

It's a bored pile.

Are you talking about pile load test? That is a very expensive and not useful for such small Load.

 

[EireChch]

How long is the pile? 5m you say?. If 5m then you are only concerned with the make up of the material to a depth of 3-5D below (cant remember exactly but check this) the pile tip. So you need data to maybe 6.8-8m depth.

You're reporting N values incorrectly, just report the blow counts for the last 2 intervals of 0.15m of movement. So 4+4 is an SPT N of 8 at 5m. This is a loose silty SAND, if your onsite observations are correct. Phi of 32 is too high in my opinion. I would use 28 since you have very little (and possibly dodgy data).

But the question is what is material from 5m to 8m. Is this granular or cohesive. Confirming this and using a layered approach (if its granular over cohesive ) is the most accurate method.

The 744kN for cohesive, is this your drained or undrained capacity. Noting that you are using Su and Phi'.

Also, I wouldn't put too much faith in direct SPT to pile capacity. They are not the norm in my experience.

Also - it is a lightly loaded pile at 250kN, however since you seem to be designing with very limited information, a CAPWAP capacity could be a cheaper option that I would recommend.

 

[mikealpha88]

Thanks for your reply.

Yes that is correct SPT is 8 at 5m depth. I think the operator was not experienced with SPT and may have not corrected the values for overburden. There is ground water at 4m depth and I think the operator has not used water in the borehole to normalise the pore pressure giving may be 20-30% less blows. I would have done the SPT at 4m 5.5m and 7m. However this guy did it at 5m and 12 m.

Phi of 32 was based on medium dense silty sand which was nominated in the bore logs of another soil tests.

The SPT sample taken has come up as silty clay so cohesive I think. And as the soil is below watertable I am considering this as untrained with phi = 0 and Su=100kpa. However as it is not confirmed if it is granuler material or cohesive material I am taking a middle approach with Su=50kpa and phi' = 21.

The ultimate load is 250 kN and dynamic load testing is very expensive so will have to come up with solution from theory only.

Thanks!

 

[geotechguy1]

The trouble is, you have dodgy data and these formulaes are all imprecise at the best of times. Adding to the dodginess of the data I don't think you actually have enough data to assess the shaft and toe well.

Personally I'm inclined to be very conservative in this situation. I would probably take N = 8 (and thus Su of around 50 kPa) given how far the higher N value is from the toe and the lack of information in between. As a quick example using the methods described in the CFEM I get Qtoe = 120 kN and Qshaft = 220 kN (both ultimate, and for the shaft neglecting the top 1.5m - I don't know where you but in many areas it's common to ignore the top 1 to 1.5m due to frost effects or shrink swell effects). But, my approach in this case is conservative because it seems that you don't have enough data.

Just curious if you are working for a structural firm and are basing this on a report from a separate geotechnical consulting firm? If so, I would ask the geotech consulting firm to provide you with end bearing and shaft values that you can use in kPa and then do you calculation based on that to minimize your own liability. If it's a local firm they may have their own knowledge of load tests done in similar soils in the local area which could give them more confidence in providing reasonable values.

 

[mikealpha88]

Thanks for your reply.

The easiest option would be to go wider than deeper.

There is no geotech report just SPT values at 5m and 12m in an email.

 

[geotechguy1]

I don't know where you practice and what the standard of care is, but in both regions I've practiced in a structural engineer would be sticking their neck out pretty far by interpreting SPT values into pile capacities without a geotech interpreting the investigation data into skin friction / end bearing values. As in, the insurance of the structural company might not actually cover this situation if things went south.

 

[BigH]

My first thought - who the 'heck' was logging the boreholes - the original one and even the second set. Did the geotechnical investigation firm(s) actually have an engineer log the hole or did they just let the driller log the borehole. The latter is fraught with danger from experience. In Canada both Golder and Geocon (Canadian geotechnical firm) required that all borings be logged by an engineer from the organization (1970s-1990s, then I lost track of practice as I have been out of country since). How can one describe as silty sand the other as silty clay / sandy clay-clayey sand. I'd be scratching my head on this.

I would tell the firms to get out there and do a proper investigation - of course, this will be too late . . . but why stick out your neck when the geo firms are not doing, in my opinion based on what the OP has stated, their due diligence. Depending on the job - be conservative.

My second thought is why one would specify a bored pile in a silty sand (if as the first investigation noted) when it is an almost given that the sand will collapse? - which apparently has been noted.

 

[mikealpha88]

Thanks everyone for your reply.

Confirmed with geotech that it is silty clay, they did not add water in borehole to stabilise pore water pressure and the values are not corrected and straight out of SPT.

I think the reason it may have logged as silty sand can be due to the water table. Below water table it is very difficult to differentiate between silty sand and silty clay unless you take the undisturbed sample.

The reason bored piers were recommended was due to all other boreholes had silty clay. I recommended using liners to avoid collapsing soil however it was ignored by the contractor which led to further investigation.


I think the conservative approach is way to go with wider and slightly deeper bored piers with bentobite slurry and CFG auger.

 

[dik]

A sieve analysis (hydrometer if necessary) should easily determine if silty-sand or silty-clay, I would think?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BigH]

mikealpha88

"I think the reason it may have logged as silty sand can be due to the water table. Below water table it is very difficult to differentiate between silty sand and silty clay unless you take the undisturbed sample."

Sorry - but that is, to be polite, incorrect. I have logged many projects having encountered silty clay and also silty sand and the two are no where near "similar". In my view, if, in the field, one cannot distinguish silty sand from silty clay, one should not be logging boreholes.