Blow counts for SPT Value 4

[ONENGINEER]

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.

 

[EireChch]

In the UK it is typically presented as blows per 6 no. 75mm (3") readings - The first two 75mm readings i.e. 150mm (6")is you seating blows and then the sum of the blows for of the remaining 4no. 75mm (3") = total number blows per 300mm (12") is your SPT N 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.

 

[Mad Mike]

Check this carefully - I suspect the 4No readings relate to contractors who prefer to measure the rate per 3" penetration, which is common practice in my area. Thus, you would actually have 4 x 3" penetration, not 4 x 6" penetration as you've suggested.

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

 

[Bobby46]

For FHWA, the sum of the middle 2 readings is taken. The 1st reading is ignored because "it is a seating drive". The 4th reading is ignored "due to sampler side friction". I believe FHWA adopts the ASTM standard(more or less).

Check out FHWA Geotechnical Engineering Circular 5. They have some good info on subsurface investigations.

 

[Ron]

Originally, split spoons were 18 inches long in the sampling tube, thus the three 6-inch increments. Most research and correlation has been based on this process. Over the past 20-25 years, the 24-inch sampler has become common. My personal opinion (and I believe ASTM D1586 still requires this) is that the first 3 increments should still be used as with the original 18-inch sampler to provide better correlation with original research and studies.

Therefore, if I were interpreting your data, the N value would be 59.

 

[EireChch]

Whats the point of the 24" sampler if you are only concerned with its blows over the first 18" (ignoring seating, so the last 12" of that 18"). Is it to obtain more material in a split spoon sampler?

 

[Bobby46]

It is probably regional preference or requirements from local DOT/other agencies. ASTM allows samplers to be 18" to 30" long.

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

 

[fattdad]

Folks use 24-in samplers all the time in Virginia (USA). We ignore the first and last 6-in interval counts and sum the second and third counts to get N-value. Then we normalize it back to N60 in engineering evaluations, typically we see the automatic hammer.

The benefit of the 24-in spoon is you just get more sample for index testing.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

Back when I was close to the test boring phases, the equipment was pretty low capacity as to feet per day, so one method for advancing the hole at low cost and, hopefully, getting more details from down there. That was using 30" samplers and avoiding wash boring or augers in the first 15 feet. That's using the sampler for the "boring" method. It was continuous sampling also. Sure it may not meet ASTM, but the tests is so rough, we got by with it. How many here have had the spoons get pretty distorted and put the mechanics in the shop to work straightening them? Before I moved from DOT to a private small consulting firm, they lost their only one down the hole. Things surely are different now.

 

[Ron]

Agree with f-d. I use the same process. Yes, additional material can be used for index testing.

 

[geomane]

We use 24in spoons most of the time. The Navalue is the sum of the 2nd and 3rd increment. It provides more sample. The drillers also spend less time pulling the rod up and changing spoons.

 

[fattdad]

As I enter my fifth decade in geotechnical engineering and field geology, I've seen many attempted shortcuts in drilling. I now work for a state DOT agency where most projects include federal funds. I have helped edit the new FHWA GEC-5 and am active in the TRB Transportation Earthwork committee.

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!

 

[oldestguy]

A question for fatdad. How many members here ever actually work on the test boring crews? My opinion is none. However, new geotechs might like to know some of the tricks tricky drillers might do, even while you are watching them. This is especially likely where test boring footage is paid by the foot. When I had state and contract crews out there while with the WDOT, I spied on them now now and then, even if they were not paid by the foot. Short cuts can be done by anybody's crews, even state crews. How important procedures are may be discussed, but knowing what the effects the short cuts might have is my attitude as to those beginning days some 60 years ago before any ASTM standards or detailed specs. Even now the crude STP has its failings.

 

[MTNClimber]

fattdad - Why are you rejecting back to back samples? Too much theoretical friction on the second drive?

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.

 

[GeoPaveTraffic]

I've spent many a day on the back of rig logging borings. Worked with a lot of very good drillers and some that I only used once. The best driller I ever worked with only had one hand, could out drill almost anyone I ever saw.

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

 

[Ron]

OG...I too have spent quite a few hours on the back of a drill rig. I have also drilled with a tripod rig, where "wash and chop" had to be done manually with gravity and a pipe wrench to turn the drill rod and bit. Crude but effective.....Fun stuff!

 

[oldestguy]

Ron: Fun memories. One of my first experiences was part time work during grad school on a rig consisting of an auger rig on a single axle drive truck, capable of the SPT with cat head. Getting the stuck truck out was an experience. Driller had me drive it while he used the auger to push up and out. It worked great. When I told the boss how neat that was, he blistered: "I told those guys never to do that again because I had to pay for several ruined auger shafts".

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 %

 

[fattdad]

We work in a field, where if we chose to take shortcuts, we could likely justify them as there's already a lot of uncertainty. I just can't rationalize data integrity that way. I support the notion of maintaining data integrity in all steps. . . Then reflecting on the realm of uncertainty.

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!

 

[BigH]

Ron - me too! The tripod holes with chop and wash were "fun"!! Had a job in northern Ontario where we needed to put down a borehole to about 10 ft or so - you could see the location from the road - but there was a railroad track in the way. 4 hours to drive around to get to the hole - or we lug the equipment over to the hole (about 20 minutes) and put down the hole as OldestGuy did - continuous split spoon drives. We chose the latter method. Of course we had to pick up the hammer by hand to 30" and drop it - no tripod; no cathead - just me and driller's helper! Have done a number of jobs dropping the hammer manually.

 

[oldestguy]

BigH and Ron
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.