Blow counts for SPT Value
Blow counts for SPT Value
(OP)
Text books describe SPT testing by penetration readings for three 6" intervals and counting the blow counts for the last 12 inches. i.e. ignoring the reading for the first 6".
Some drillers and software have readings for four 6" penetrations, a total of 24 inches. E.g. blow counts of 12-23-36-40. In the later case what would be the SPT value? 59 or 76 or 61.
Some drillers and software have readings for four 6" penetrations, a total of 24 inches. E.g. blow counts of 12-23-36-40. In the later case what would be the SPT value? 59 or 76 or 61.
RE: Blow counts for SPT Value
The SPT cone is driven for a total length of 450mm (18")
See below example and an extract from our standard report.
The drillers / software are driving too far IMO with a 600mm (24") drive. If i was to interpret those values it would be:
12 : seating blows taken to drive 150 (6")
23 & 36: blows taken to drive cone 300mm (12") therefore, your SPT value is 59.
I would ignore the 40 value.
RE: Blow counts for SPT Value
Whole point of the standard test is that its taken over 12", with the first 6" being disregarded; in fact, standard Raymond spoon samplers used to conduct the test aren't able to penetrate 24" to the best of my knowledge.
All the best,
Mike
RE: Blow counts for SPT Value
Check out FHWA Geotechnical Engineering Circular 5. They have some good info on subsurface investigations.
RE: Blow counts for SPT Value
Therefore, if I were interpreting your data, the N value would be 59.
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
Maybe the larger samplers are used to get more material for index testing?
I mostly see 24" samplers. I have never seen a 30" sampler
RE: Blow counts for SPT Value
The benefit of the 24-in spoon is you just get more sample for index testing.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
Any driller attempting to drive back-to-back spoons will be rejected from any job I'm working on, that's for certain! Any driller attempting to use the spoon as the plug when auger drilling will also be rejected. Sure, these things are done. I just don't quite see the value of publishing all the ways we've seen shortcuts in geotechnical engineering.
Now if somebody wants to make a thread about stupid stuff, that may be fun? I just don't quite see the value of looking back at the bad work that folks used to get away with.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
When I lived in New England, that was standard on continuous sampling jobs. They would drive say 0-2', pull the sampler and detach the spoon, hand over the first spoon, attach a second spoon to the rod, drop it back down the hole before it collapsed and drive 2'-4'. The second sampler had a coupler that matched the outer diameter of the A-rod so it could be driven without being obstructed. After the 2'-4' sample, they would then rollerbit down to 4' and start the process all over again. I rarely saw N values have a pattern of lower, higher, lower, higher, ect.
RE: Blow counts for SPT Value
As for 24-inch spoons, I also use the 2nd and 3rd 6-inch increments for the SPT.
As for short cuts, I agree with fattdad, no way I'm letting drillers drive back to back spoons without augering the boring out.
The best shortcut I ever saw, that I liked, was pushing a Shelby tube and then following up with a spoon. In that case, I see no reason to auger out the boring, so time is saved.
Mike Lambert
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
As to precision of the SPT, here is a quote from ASTM D 1586 A darn rough test by any criteria.
9.1.1.1 Variations in N
-values of 100 % or more have been
observed when using different standard penetration test appa-
ratus and drillers for adjacent boreholes in the same soil
formation. Current opinion, based on field experience, indi-
cates that when using the same apparatus and driller,
N -values in the same soil can be reproduced with a coefficient of
variation of about 10 %
RE: Blow counts for SPT Value
Starting out as a field geologist, I've spent literally months on field assignments. Field camps, field cooks, helicopters, the whole deal. I've seen so many ways that drillers get out of control. So, my state DOT requires that there's a field geologist or engineer with every drill rig.
We are also comforted by ASTM Standards. So, if the standard says to do or not do something, that'd be our expectation also.
We chose an interesting career!
f-d
ípapß gordo ainÆt no madre flaca!
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
I started out in grad school with part time job with B.K.Hough (Prof with side business). He came from US Corps of Engineers. He insisted all crews have a pin guided weight. The 140 lb weight was fitted with a 7/8" dia. rod, threaded in. At that place a cavity was bored out and a hard wood cushion block was fitted in there. Then top of "A" rods had a drive head. Weight was guided in the A rod. Rod had a mark at 30 i8nches. Hough claimed the weight would take about half as many drops as the weight guided by a piece of pipe commonly used. As years went on I adopted that type of weight. Had the crews at WDOT equipped that way also. Don't know if they use it now.
RE: Blow counts for SPT Value
He made hole; however. Had a few helpers too!
Life's a bit different outside the USA! That said, when I did work in Seattle (I-95, c. 1979), we had no choice but to drill at that spot! So, we packed in a tripod and did a wash bore. Dude also had a donut for that boring.
Now anybody packed in a Winkie!
f-d
ípapß gordo ainÆt no madre flaca!
RE: Blow counts for SPT Value
Heck of an adventure for an Iowa farm boy!
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
RE: Blow counts for SPT Value
SPT-N values are computed as usual: the sum of the 2nd and 3rd blowcounts.
RE: Blow counts for SPT Value