reading of SPT tests
reading of SPT tests
(OP)
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.
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RE: reading of SPT tests
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!
RE: reading of SPT tests
RE: reading of SPT tests
RE: reading of SPT tests
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.
RE: reading of SPT tests
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!
RE: reading of SPT tests
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...
RE: reading of SPT tests
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.
RE: reading of SPT tests
RE: reading of SPT tests
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.
RE: reading of SPT tests
RE: reading of SPT tests
RE: reading of SPT tests
f-d
¡papá gordo ain't no madre flaca!
RE: reading of SPT tests
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.
RE: reading of SPT tests
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!
RE: reading of SPT tests
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?
RE: reading of SPT tests
To answer your question, I disagree with the geotech.
RE: reading of SPT tests
f-d
¡papá gordo ain't no madre flaca!
RE: reading of SPT tests