Soil Nail Testing Procedures 2

[stevey]

Hello, all. We have a small soil nail project here in Portland, and have questions about testing procedures. Our wall is fairly low, approximately 5 ft, and extends under a couple of existing freeway bridges. Our purpose is to widen a frontage road under these bridges. One row of soil nails has been specified by our engineer, and the fill material has been determined (by test pits) to be a sandy silt material with interspersed cobbles. Directly under the bridges is concrete block slope paving, with about 18" of clean sand fill, then the previously described native material.

We tested per FHWA specs last week and evidently got 2 failing tests of the two that we did. The contractor elected to continue the drilling of the remaining anchors (74 total) and do post grouting of the nails in an attempt to get a passing test. We reluctantly agreed, but informed them that they were performing the drilling work at their own risk, as we have not any passing tests. They proposed testing 4 of the drilled production anchors to verify the post grouting worked. As of yet, they have not tested any anchors, and now want to shotcrete the face, and test afterward.

I know virtually nothing about soil nails (and this is not even my project), but it seems to me the contractor is jumping the gun. How can he test against a shotcreted wall? And what happens if the production anchors fail? Is the contractor trying to razzle-dazzle us?

Any help would be appreciated.

Cheers.

Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering

 

[PEinc]

How did the nails fail? By pullout or by creep? Were the tested nails production nails or sacrificial nails? Is there a 5 foot high, unshotcreted, but nailed, cut standing exposed? Were the already-installed nails installed with regrout tubes? If not, how can the contractor regrout the nails?

For my soil nail projects, I specify sacrificial test nails which are purposely made short with an unbonded length. That way, I can test them to the assumed ultimate bond stress or to failure. Testing a long, production nail to about twice the design load or allowable bond could possibly overstress the nail tendon and/or the shotcrete facing.

Some designers and/or soil nail wall contractors believe in shotcreting first, then installing the nails. Others believe the opposite. I prefer to excavate, install nails, shotcrete immediately around the nails, and then install the nail bearing plates against the fresh concrete to assure full bearing. Nails hold up the excavated face; shotcrete does not. I prefer to install the support (nails) before the shotcrete.

I have no problem with testing against the shotcrete because my test loads are relatively low. However, I do have the contractor use sufficient cribbing beneath the test jack to prevent damage to the shotcrete and punching shear failure of the jack into the dirt face.

If both of the 2 nails failed their tests, the ultimate and allowable soil-grout bond stresses are incorrect. Perhaps the designer's design values (unit weight, phi angle) for the soil are also incorrect. This should be checked.

If the contractor tests production nails and they fail, he needs to know at what load they failed and then use 50% of that load as his allowable load, then install supplementary soil nails. For nails not yet installed, the contractor has the option of using longer nails, larger diameter drill holes, closer nail spacing, or regrouting the nails.

Are the freeway bridges on piles? Has the designer accounted for any bridge surcharge loads in the soil nail wall design? How was the stability of the excavated cut before the soil nails were installed? Did the designer check this condition?

 

[stevey]

PEinc -Thanks for the timely response.

The initial tests were sacrificial per our specs - this is a small production job, so per FHWA guidlines, only 2 tests were required. From what I understand, the anchor tests were considered failures because the contractor had to adjust the jacking force on one of the anchors during testing, and somehow the jacking timbers rotated under load under the 2nd anchor (I'm getting this from the geotech that witnessed the tests). Evidently, both anchors were considered passing in creep as the total creep movement was within the spec limit of 2mm per log cycle. However, the creep rate for at least the first test was not constant and didn't drop off (again, I'm getting this from the geotech). I'm still not sure whether the nails failed in pullout - I'm getting conflicting accounts on this.

Yes, the grade is actually 5 ft. high and is so far unshotcreted and standing exposed. All of the production anchors, except for the first two (installed before the revised plan was developed) have been installed with regrout tubes. The anchors themselves are spaced at 5 ft O.C. and are set about 3.5 ft above existing grade.

Our designer used fairly low values for both phi and gamma (28 degrees and 120 pcf, respectively), and adhesion was estimated at 53 kN/m2 (sorry about the units change) which is at the low end of the FHWA guidelines for this type of soil (sandy silt). All values were reviewed by our geotech prior to beginning our design, and we have not received any directions (as of yet) that these need to be revised. The soil values were derived from test pits dug adjacent to the highway bridges, because the existing slope paving placed directly under the spans prohibited closer initial testing. No test borings were made. The slopes under the bridges were at roughly 1.75H:1V, and there were no known stability problems, even in the unpaved areas between the bridges. Although we have experienced a little bit of ravelling of the slope behind the cut, this is fairly minor, and the contractor is not experiencing difficulty with this aspect of the job.

The bridges themselves are on H-piling located about 25 ft beyond the face of our excavation. Our nails are specified at 20 ft length at a downward angle of 15 degrees. Since the bridges were on piling, we did not consider additional surcharge forces on the wall other than normal slope surcharge for the backslope.

Incidently, we had originally considered different earth retention systems (ULTRA BLOCK and Keystone w/grids)for the soil bank but these were vetoed by the various interested parties for different reasons - such as being too unattractive, or in the case of the grids, requiring a deeper excavation that put the temporary cut bank too close to the bridge piling (the bridges are actually owned by the state highway department).



Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering

 

[PEinc]

If the anchors were "passing in creep", they must have held the test load. So, pullout must not have been a failure mode. I'm not exactly sure what you mean by "...the anchor tests were considered failures because the contractor had to adjust the jacking force on one of the anchors during testing, and somehow the jacking timbers rotated under load under the second anchor." It sounds like the contractor is inexperienced in anchor installation and testing. Is this true? Did you have any qualification requirements for the soil nail wall contractor?

Did the contractor use centralizers on the nails? Was the creep movement measured on the end of the bar with respect to the independent, fixed reference (correct method) or was it measured on the end of the bar with respect to the body of the test jack (incorrect method)?

I assume that the 53 kN/M2 (7.7 psi) bond stress is the allowable bond. If so it is a fairly average value. If it is your ultimate bond value, it is probably conservative for a straight shaft, gravity grouted anchor.

With the bridge being on piles and no soil borings, I can only assume that the soils are relatively loose in the anchor zone. Did the contractor have to case the drill holes to prevent them from collapsing? The soil near the excavated face is probably more dense and compact from original construction of the slope and slope paving. 20 foot nails are pretty long for a 5 feet high wall even with a sloped surface. It sounds like the wall is conservatively designed. Therefore, I would suspect the cause of the test failures lies with either an inexperienced contractor or loose soils back along the nail length (or test nail bond zone). If the contractor knows what he's doing (but it doesn't sound like he is), then the problem may be differing soil conditions and regrouting may be the better solution. You may also have to pay the contractor for the regrouting.

 

[stevey]

Thanks again for the reply.

My involvement in this particular project is fairly indirect - my original objective was to get a free education in the soil nailing and testing procedure as I had never had the opportunity to witness this in the field before.

Our drilling contractor is experienced in anchor setting and testing, and has an engineering staff that has provided some input to us. We have also been getting input from our geotech. I've forwarded these threads to our geotech and engineer and have asked them to review and comment directly to the forum, and hopefully they will be able to provide more accurate descriptions of the testing.

From what I could observe in the field, the contractor was using centralizers on the nails, and these were spaced closely enough to be effective. Likewise, a fixed reference platform was used for measuring the apparent movement of the nail. Our geotech was onsite to observe the tests, and evidently had no problems with the procedure.

The contractor did have to case the holes, and he reported finding some cobbles in the upper reaches of the holes. It was taking him about 40 minutes to complete each hole. This is only speculation on my part, but it seems to me that the two highway bridges were built on fill, which was probably surcharged prior to placing the piling for the abutments.

I look forward to the posts from my colleagues who can hopefully fill in some of the sketchy details I have so far provided. Again, thanks to you and to the forum (this is a great site!).

Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering

 

[PEinc]

"Experienced in anchor setting and testing" is a relative term. Everyone thinks he's an expert. The real determination of experience can be more easily made by looking at the number of anchor jobs completed, the size of the jobs, the type(s) of drilling and grouting they do, the capacity of the anchors they install, and for whom they have worked (their clients). Some contractors dabble in specialty work. Some really are experts.

 

[ESSSOE]

HI,

WE are designing a 40 foot high soldier-pile and lagging anchored wall (pile spc 5 feet and 6 feet) in very soft gray varved clay in upstate NY. Can you even use timber lagging to support the clay? At what pile spacing?. Excavation will be open for about two years. We dont want the clay to fail and flow!!. There are some references that support the use of timber lagging and some that dont at various pile spacings. Any experience out there?

Thanks.

 

[PEinc]

Are you working for the project owner or for the contractor? What is "very soft clay"? Soldier beams spacings of 5 and 6 feet are very close and therefore probably uneconomical. If the soil really is soft, perhaps you should consider a tiedback steel sheet pile wall. This will give you the strength you need without the concern for timber lagging. Into what type of soil will the tiebacks be anchored?

Because this is a temporary sheeting wall, you probably should not be designing it. You should obtain and provide pertinent information (soil borings, soil properties, wall height, location, special surcharge loadings, easements, etc.) and then issue a performance specification. Let the contractor get design-build proposals from specialty contractors who are very experienced in tiedback wall design AND construction. It will be hard for you to design the wall and tieback anchors if you do not know what type of anchor the contractor intends to use, how he intends to drill and grout the anchors, or the anchor capacity the contractor can obtain in the bond zone soils.

I guarantee that if you design this wall for the owner so that contractors can bid on the wall, the contractor who eventually builds the wall will want to redesign the wall. If so, the owner has wasted a good amount of money on an unused design. If the contractor does not want to at least partially redesign the wall, he probably isn't qualified to build it.

Also, if the soils are soft clays, don't forget about checking global stability also.