Crude SPT inAfghanistan

[la433bridgeman]

Have a theory for testing soils in Afghanistan where modern methods and equipment are rare finds. I'm trying to devise a crude substitute method for SPT in order that we can field test compaction, in lieu of proctor/modified proctor. Follow if you will....

SPT is based on 140 lbs @ 30" drop, or 350 ft-lbs. Most hammers are 60%-70%. Found values correlating to 60%.

VL Very Loose
L Loose
M Medium
D Dense
VD Very Dense

From Skempton 1986
VL L M D VD
Dr (%)------------0 15 35 65 85 100
SPT [N60] 0 3 8 25 42 58

Of course, we don't have SPT rigs here, so I propose:

1) 20 LB sledgehammer
2) person shoulder height 60"
3) drop from 36" above shoulder height
4) 2.5" std pipe 36" above ground
5) flat plate on pipe edge
6) 90% energy efficiency
7) reduce drive distance
8) no boring samples are taken

Energy = F*d
F, one drop, = 60%(350)(12/[N60])
E = 0.9*20*(60+36-36)/12 = 90 lbft

VL L M D VD
Dr (%)------------0 15 35 65 85 100
SPT (N60) 0 3 8 25 42 58
F (lbs)-----------0 630 1680 5250 8820 12180
E (lbft, prop'd)--90 90 90 90 90 90
'd' (in/blow)-----0 1.71 0.64 0.21 0.12 0.089
N per 1"----------0 0.85 1.56 4.76 8.33 11.2
N per 2"----------0 1.17 3.13 9.52 16.7 22.5
N per 4"----------0 2.34 6.25 19.0 33.3 44.9
Rounded N/4-------0 3 7 19 34 45

In lieu of having "expensive" lab tests done, this is my proposal. [Remember, this is AFG we're talking about.]

After each layer is compacted, the Contractor while being observed by COR & CI perform these tests in random spots, will set up the pipe and drive the pipe 2" into the ground. Then the he would drive the pipe another 2" into the ground and record the number of blows. Then he would drive the pipe another 2" into the ground and record the number of blows.

The last two blow counts are what we would measure (N per 4"). The goal would be that the Contractor keeps making passes until each layer of fill achieves an "N" of 34.

I know some poor soul will suffer swinging a hammer "exactly" the same way every time, but what do you think?

v/r,

Jacob Parker, PE
TF Mountain Warrior
FOB Fenty
Afganistan

 

[dgillette]

Necessity is a mother.

It's certainly worth a try, but I would experiment with it before you actually use it for construction QC. The energy transfer ratio could be quite a bit less than 90%, and without the higher confining pressure you would get in a drill hole ten feet down, the blowcount-density correlation could be quite different. (In sand of a given RD, the blowcount is very sensitive to confining pressure.)

Suggest you start with a test fill, with moisture controlled as well as you can by guess and by golly. (If it's clayey, W-opt is usually pretty close to the plastic limit, so tell the inspector to try for just dry enough to roll worms.) Compact it very thoroughly with the most appropriate equipment, and try your test. Then, try what you think is poor compaction, two passes or whatever, and then see what you get, etc. until you have an idea of what blowcounts correspond to L, M, and VD.

Unfortunately, the penetration resistance for many materials would be sensitive to both density and water content, so you might get the same number of blows for 92 percent at W-opt minus 4 and 97 percent at W-opt plus 1. (Numbers out of my, uh, hat, not real data.)

What kind of fill material? I'll assume that it's not clean sand or sand and gravel, since you are talking about Proctor and not RD. Do you have a decent supply of water?

If you have access to a welder, consider making a post-driver like the ones they use for T-posts for fences, instead of using the sledge hammer, which might save someone's fingers when the sledge hammer misses. Make it from a piece of steel pipe larger in diameter than your "drill rods" with two handles and a cap welded on one end. Weld on some weights too, if you think the weight of the pipe isn't enough. You'll wind up with awesome biceps from working this post driver. Paint a stripe on the rod, so you take the hammer the same height each time, and make the "drill rod" long enough that you don't have to bend over to work the driver.

Good luck!
DRG

 

[Ron]

A simple way to do this is to get or make a "Sowers Penetrometer". It is a dynamic cone penetrometer, hand operated with a 15lb hammer dropped on the rod and collar. All portable...all hand-held. Correlates to actual "N" values. Have used many times with success for what you are doing.

 

[la433bridgeman]

Making a sowers penetrometer is more likely than buying one. It's amazaing what cannot be obtained here. Simple things are hard to get.

I am talking about Relative Density here, not proctor/mod proctor. We have always specified Proctor/Mod Proctor here in Afghanistan, but getting the tests done and reported truthfully and accurately has been the challenge. Looking for a more viable way to test compaction and/or density of the soil without having to have outside labratory test. Heck, it is a challenge to get them to understand the importance of using water when compacting, much less testing the materials.

Hate to say it, but it's kind of like starting from scratch hundreds of years ago. Very frustrating at times.

And don't get me started on trying to explain the importance of clean water/aggregate fo concrete...when they're mixing it in the ground.

Fun times.

Thanks for the help. I'll see what devices are locally available.

 

[Ron]

la433....attached is a guide for the Sowers Penetrometer. You can probably build one from this...

Good luck.

 

[molerat2210]

Try the proctor penetration method (ASTM D 1558). I have an old one we got from an army auction. It was probably created for project similar to yours where you don't have access to much equipment.

The fate or the war effort could depend on the density of the subgrade. Geotechnical squad leader calls out "Cover me while I go measure the density of that subgrade".

PS:There is a correlation of any size weight, rod diameter and drop height to SPT contained in Foundation Engineering Handbook, Fang.

 

[BigH]

To my way of thinking, why not carry out test fills and measure the "settlement" after each pass (or two). Determine the most suitable number of passes to get it so that additional "settlement" (i.e., change of height due to compaction) is minimal with additional passes. If your material is "clayey" or "sandy" - need to estimate (likely by experience) that the moisture content is at or near optimum - if clayey soils you would get more peaked curves - in sandy soils more "flat" curves and the moisture content would not be that critical so long as it wasn't dry or exuding water. If clayey - too many passes would, depending on fill depth, induce residual pore pressures.

As Ron has indicated, the Sowers dynamic penetrometer (or the TRRL miniature penetrometer) can be used for subgrade testing.

I remember a mentor of mine who, during the Korean War was asked to help determine if a clayey field was suitable for tanks to cross . . . ended up using correlation using the depth of a .45 calibre bullet shot into the ground. Interesting.