reading of SPT tests 1

[oneintheeye]

if a drillers log shows spt values which seem low for the ground description how would you handle this? Design on figures or on description. Soil is dense gravels and is a piling situation.

 

[fattdad]

Do you have any confidence in the driller?

What are the N-values for the gravel?

What is your basis for classifying the gravel as "dense" if not for the N-value?

Where is the water table?

As a geologist first and an engineer second, I'd also look at the geologic setting.

Not sure I'm helping, but these are the things I'd consider. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[oneintheeye]

dense gravel is from the drillers description. Water table is high (1.5m from ground level. gravel is deep. Water was added during drilling which (it is commented on) can reduce SPT values.

 

[msucog]

what's the n-value? auto hammer or rope & cathead? how deep is the blow counts you are concerned about? is it old fill, alluvium, residuum, etc?

 

[BigH]

If in gravel, the N values would be suspect anyway unless you are using a Burmeister spoon (or California spoon? - see previous posts on spoon size). With the standard 51 mm OD spoon, you can get a piece of gravel in the tip and it is then like a cone - or you just plain get interference of interlocked gravels as you are driving through them. The driller might be saying the gravel is dense due to the difficulty that he is experiencing in advancing the casing/auger (whatever) which is being affected by the gravel size. Best to use a Becker-hammer for gravels anyway.
A second point is - why is the driller logging the borehole? Didn't you have an experienced geotech technician or junior engineer on the job logging the boreholes for you? If not, that, in my opinion, is a remiss. Drillers are not engineers (and many drillers are good if not better than some engineers in the field) and he is not versed as to the ramifications of missed details and that. I know of one job where the drillers said the material was sand (and it was) but the excavation during the laying of a large sewer pipe at depth blew out on them - why? The driller failed to note the occasional thin laminations of clay/silty clay (sorry fattdad!) and fine silt that were present in the sand.

 

[fattdad]

And a big "Right On" to BigH! Fugitive data collection is critical in logging a bore-hole - data that if you don't capture it during drilling it's lost forever.

Pertaining to adding water: This implies that the hole was drilled using hollow-stem augers (nothing wrong with that). However, drillers add water when there is a quick condition at the bottom of the hole and the sandy material runs up the inside of the auger. This happens when the augers are below the water table and also happens when they don't support the augers at the ground surface with the "auger fork" (or whatever you call that thing). For the former, you may really have loose wet soils. For the latter, the driller may have been able to take steps to minimize heave up the augers. It's just too hard to tell. . .

We always have a geologist on the drilling rig - something I spent my first 6 years after college doing myself (a.k.a., character building - ha).

f-d

¡papá gordo ain’t no madre flaca!

 

[Dirtguy4587]

Adding water itself won't necessarily reduce the SPT values. However, if (as fattdad suggests) there is a quick/artesian condition at the site, the upward flow of water from the bottom of the borehole can cause reduced SPT values - the sand/gravel at the bottom is actually liquefying as the water is flowing upwards.

To be honest, if something doesn't seem to jive in a field investigation I typically either go see for myself (if I can), or do more drilling to confirm...

 

[DRC1]

A quick condition in gravel, esp dense gravel is unlikely. What are thr blow counts(the sum of the values for 6-12 &12-18 in. penetration)? I don't understand why it would be called as dense with low blow counts. If he were encountering drilling difficulties, that would probably be due to cobbles or boulders, which should be noted on the log. It sounds like more exploration is needed. I know everyone will say there is not enough time or money and we will deal with that in the field, but, when the contractor comes in with a big extra and needs a time extension, an extra boring or two will look cheap.

I agree with Big H and add that we would not leave the job until the borings were reviewed to see if there were any questions like this and if more borings were needed to resolve the questions.

 

[civilperson]

I would trust the blow counts as more reliabile than the description if the blow counts seem low. I would trust the description of the soil if the blow counts seemed high, (easy to not drop the correct height or put an upward force on the cathead rope during impact).

 

[Stego1]

Why not back up the boreholes with a day's worth of dynamic cone testing? You can get quite a few down in a day and the results would reinforce (or otherwise) the SPT data. It's not too expensive either.

The problem with NOT engaging an experienced geotech/engineering geologist to log the boreholes is that liability for poor data and subsequent problems ends up on your doorstep (or in your pocket), even more so if there is no supervision whatsoever. If you employ a site investigation contractor to oversee the boring any repercussions due to poor logging, poor drilling technique or overlooked detail would be their responsibility if it came to claims.

THe legal side is probably a bit more complicated than that but it's a simple way to explain to overenthusiastic young geotechs who're trying to save the client a bit of money.

 

[Stego1]

I meant static cone penetration testing in the post above! Can you edit posts once posted?

 

[oneintheeye]

to clear up. The SI was appointed by the client, we have no direct control over what or how they produce the report. The SI was passed to the piling contractor for the pile design. The geotechs water was added to assist drilling. The piling contractor used the spt's to design on piling method. drilled to this depth then hammer through gravel. This might now not be possible as the geotechs say the spt are lower due to added water. I haven't been to site as it is up in North england and we are south east (for those who dont know, a long drive). I dont think at this stage i will gain anything from this. The piling contractor will claim additional costs for now having to drill through gravel with a rotary rig. This will effect client not us directly. This has been brought up in meetings and telecons. I dont need to pass any official advise as this is outside my expertise and scope but would like to know who is correct and incorrect. Should the piling contator be aware of this effect? Is this statement by the geotech correct?

 

[fattdad]

Do a test pile program and do retaps the next day. You may be surprised just how much the pile capacity (based on driving resistance) increases in the sand/gravel layer. Run a static load test and prove your results.

f-d

¡papá gordo ain’t no madre flaca!

 

[oneintheeye]

sorry fattdad im not trying to prove pile capacities here.

The piles will go through the gravel to a deeper layer due to high settlements assosiated with a clay layer below the gravels.

The issue is the additional cost to the piling contractor for the extra work in getting through the gravel.

The question is 'if water was added to assist drilling is it comman for the SPT results to be considered lower than will actually be the case'. This is what the geotech is saying that the piling contractor should recognise this.

 

[fattdad]

(likely repeating something already stated. . .)

In an otherwise stable boring, there is no need to add water to a hollow stem auger boring. So if water was added, the sand may been disturbed by the drilling (i.e., greater N-value than reported), or such. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[muuddfun]

We usually add water to the borings to suppress a quick condition when below the water table and you get higher SPT than otherwise. So if there was no groundwater and you added water to the boring and you are in gravel I don't see it making much difference to the SPT, but I am not really familiar with the conditions and that can make all the difference.

What I don't understand is how a gravel with a lower SPT will hinder the Piling Contractor more than he already planned for when he did the design?

 

[DRC1]

Usually drilling mud is added when drilling below the water table. Head should be slightly above natural water table to maintain positive pressue. Should not effect SPT results. Improper technique can effect results dramitacally. Gravels would not be expected to show effects of excess porewater pressure. In short as a foundation contractor, I would not know the blow counts were wrong unless some one told me. If it is so obvoious to the geotech he should have noted it on logs. (note that testing proceedure and report requirements are dictated by ASTM). If I could look at the boring logs and know that not only the blow counts are wrong, but what the right values should be, why even take borings? Defective borings are the engineer's problem. Finally it sounds to me like the problem is more that the material is cobbles instead of gravel, or the gravel is very tight.
To answer your question, I disagree with the geotech.

 

[fattdad]

For the benefit of this thread, was the field exploration performed using rotary drilling or auger drilling? For the latter, I'm o.k. with my posts. For the former, I'm with DCR1 the use of drilling fluids will maintain hole stability and have no affect on N-value.

f-d

¡papá gordo ain’t no madre flaca!

 

[oneintheeye]

it was percussion drilling. Water table is approx 2m below ground level.