Soil Nail tests

[pileexpert]

Hi there,

I have a small question on soil nail test, plz help. (I would be more grateful if PEinc also comments)

for proof test FHWA says that we should calculate LBPt as greater of 3 m and 0.9*A*fy/Qall*FSproof then DTL=LBPt*Qall
for verification test is the same except FS=2.5-3 instead of 1.5
my question is if for example we have a 32mm bar with fy=4000 kg/cm2 and 9m long DTL for proof and verification would be differetnt
if we assume qall=20 kN/m
then DTL for proof=190 kN and for verification=10 kN
then if we do a proof test then 150% load=280 kN and for verification 200% (as FHWA suggests) =200 kN so verification test load is less than proof.
for most cases I mean for bar with more than about 7-8 m, qu dosnot have any effect on DTL, so can you help me with this how should I exactly choose the correct DTL for my tests?
Regards.

 

[PEinc]

pileexpert, which FHWA manual are you reading? FHWA'a Geotechnical Engineering Circular No. 7, March 20013, Pages 162 - 169, describes verification and proof tests. For verification tests, no recommended minimum unbonded or bonded lengths are indicated. For proof tests, the recommended, minimum, TEMPORARY, unbonded length is 1 meter or 3 feet. The recomended, minimum bonded length is 3 meters or 10 feet, but the test load should not exceed the "allowable bar tensile force" during the test. I assume that the "allowable bar tensile force" is 0.8 x GUTS of the bar. If a 1 meter bond length requires a test load higher than the maximum allowed test load for the bar, I would use a shorter bond length or a stronger bar. However, it is hard to use a stronger bar if you do not know which nail will be proof tested.

As I have mentioned before, I am strongly against proof testing of production nails. I have concerns about successfully installing and removing a temporary sheath for the unbonded length and have concerns about testing a nail with an uncased, ungrouted unbonded length. Testing a production nail with a temporary steel casing in the unbonded length is a good way to lose or ruin expensive steel casing pipe. If an uncased drill hole collapses during a production test, the nail is worthless and needs to be replaced. I prefer to do all nail testing on sacrificial nails with a bond length of 10 feet maximum. If the design ultimate bond stress is high (say for very dense soil or rock), I shorten the bond length so that I do not fail the bar. Sometimes, I will use a stronger bar for the test but care needs to be taken that the excavated face of wall or the shotcrete does not fail by punching shear. A large test load could require an unreasonably large area of cribbing beneath the test jack.

http://www.PeirceEngineering.com

 

[PEinc]

Also, FHWA's GEC No. 7 does not indicate a safety factor for the verification test but does use 2.0 in its example. For the proof test, FHWA shows a safety factor of 1.5. FHWA does not indicated a safety factor or test load at which the creep test should be run but uses 1.3 or 130% in its sample test load data sheet (Figure 8.30). I use a safety factor of 2.0 for my load and creep tests. My sacrificial test load is 2.0 x the allowable design bond stress x the bond length diameter x the bond length of 10 feet maximum. Therefore, my test percentage is equal to the maximum shown by FHWA; my creep test is run at a higher percentage than FHWA's example; I test at least the same number of nails that FHWA requires; all tests are on sacrificial nails; no production nails are damaged; I make my unbonded length long enough to reach the soil layer I want to test.

http://www.PeirceEngineering.com

 

[pileexpert]

Many thanks PEinc.
I agree with your commnets in regard to proof test, but the client has asked us to do some proof tests.
I am using the same manual you are using, but as you know in Appendix E it has introduced a formula to calculate DTL, and that's what I'm exactly confused, suppose we have 13m long nail with 32mm Dia, which is 1m is unbonded, could you please tell me what DTL we have to use for proof tests, Qu= 150 kpa, and safety factor to calculate Qall was assumed to be 2.0.
if we refer to Appendix E, the LBPTmax=0.9*8.04*4000/(1.5*10*3.14*100*1.5/2)=8.2m
so DTL=8.2 x Qall (23.55)=192 kN,
if we use 150% DTL then it would be 289 kN which is almost 0.9fy.
if we use the same procedure in appendix E for verification test, we reach the same value at max load, so what would be difference,
could you please tell me with the above parameter which DTL you use for proof test?
Thanks.

 

[PEinc]

You should not be testing a 12m long bond length or even an 8.2m bond length. The test load for these long bond lengths are too great and would require a much stronger bar and a lot of cribbing at the face of the wall. The bond length should be AT LEAST 1m. The bond length should be 3m minimum per 8.5.4 on page 166. Your post above says that LBPT is 8.2m MAXIMUM. You don't need to use the maximum. The maximum bond lenght is based on almost the ultimate capacity of the bar. Use less bond length, such as the minimum of 3m. Check that the test load calculated for a 3m bond does not exceed the capacity of the bar (as calculated by the FHWA equation for the proof test). If the test load associated with a 3m bond exceeds the bar capacity, increase the bar size or strength as described on page E-9, Section 1.11, just above Section C whic says, "Provide larger bar sizes, if required, to meet the 3-m (10-ft) minimum test bonded length requirement at no additional cost."

http://www.PeirceEngineering.com

 

[BigH]

Not butting in to a very informative thread, one might also look at the Hong Kong Design Manual on Soil Nails: See,

http://www.cedd.gov.hk/eng/publications/manuals/doc/eg7pdf_links.pdf

Good to have several references besides PEInc.!

 

[pileexpert]

PEinc,
ALL nails in the project have minimum 9.0 bonded length, so as I mentioned above Qall= 23.5 kN/m, so as you cited if we choose 3.0m bonded
then, DTL would be 70 kN if we choose a value between 3m to 8.2(which is the maximum) for example 5.0m then DTL is 117 kN and ...
so then what is the difference?, if we choose 3.0m the 150% DTL is 10.5. the nail maybe acceptable at this range but if we choose 5.0m and the nail might fail at 120 kN which is about 100% of DTL we selected, as I told before all nails have minimum 9.0m length, and so choosing a value beetween 3.0m and LBPT max leads to different levels of DTL, so my question is with these nail lengths how can I satisfy this sentence of Manual?
"Proof tests are used to ascertain that the contractor’s construction methods and/or soil conditions have not changed
and that the production soil nails can safely withstand design loads without excessive movement or long-term creep over the service life"

 

[PEinc]

You are supposed to be testing and proving the bond per unit of bond area. You do not need to test the entire maximum bond length. This is not a tieback anchor where the entire bond length is tested.
If you have to prove 10 psi bond, you have to stress a known bond length and diameter to the load required to give 10 psi without overstressing the bar. You can do this with a 10 foot (1m) bond length. You don't need to test a 30 foot bond length.

http://www.PeirceEngineering.com

 

[PEinc]

BigH, I clicked on the link but got a message that the site could not be found.

http://www.PeirceEngineering.com

 

[pileexpert]

Thank you all