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Micropile and Soil Nail Wall Underpinning
2

Micropile and Soil Nail Wall Underpinning

Micropile and Soil Nail Wall Underpinning

(OP)
This underpinning project came across my desk for review. I was hoping to get some opinions on the load transfer and resulting deflection of the pile to see if I'm overthinking this. The pile will not be preloaded so the load transfer will cause some amount of settlement of the existing building, which needs to be reviewed and accepted by the building owner's engineer.

Would the anticipated settlement be PL/AE plus lateral deflection of the pile from the batter? Does the theoretical deflection even come close to the actual settlement?

RE: Micropile and Soil Nail Wall Underpinning

PL/AE is elastic shortening of the micro pile itself. There will be settlement of the soil its self.

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Are you sure? Since, theoretically, the pile is using skin friction only, there shouldn’t be any movement at the tip of the pile. If zero load makes it to the tip of the pile, how are you causing soil settlement?

RE: Micropile and Soil Nail Wall Underpinning

Making the connections between the micropiles and the building foundation may be difficult. Also, how will you design the soil nail wall for the lateral load in the micropiles? You may be better off with vertical or near vertical micropiles or helical piers with brackets to the wall footing. If the loads are low enough, helical piers should be cheaper and faster than micropiles. Rather than PL/AE, you probably should use LPile or AllPile to calculate pile settlement.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

Quote (MTNClimber)

1) Does the theoretical deflection even come close to the actual settlement?

2) Since, theoretically, the pile is using skin friction only, there shouldn’t be any movement at the tip of the pile.

1) Actual settlement is usually less than theoretical deflection (when calcs are performed by traditional methods). This is because soil friction load transfer is typically not distributed evenly along pile length. Load supported by friction is greatest near the top of the pile. Load transfer to soil decreases with pile depth.

2) The pile tip can settle, depending on soil properties.
Consider a friction pile group. Recommended pile spacing is often specified as 3 pile diameters, or more... so that interaction between piling is minimized. Otherwise, the entire pile group's soil block can settle more than expected.
Same thing (settlement of soil around, but not touching an individual pile) can happen... just on a smaller scale.

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Thanks for the input everyone.

SRE - FHWA recommends elastic deflection to be calculated as PL/AE for the unbonded zone and P(0.5L)/AE for the bonded zone to account for the decreasing load on the pile with depth… which makes sense to me. I’m just not sure how realistic it is when translating that to building settlement.

I understand the group effect on settlement but I don’t think we’ll have it on this job due to the wide pile spacing and the cohesionless on-site soils.

PEinc - I agree, using a software to see how the pile reacts would be a good idea too. Vertical piles would be ideal but I’ve seen this type of underpinning a couple of times before so I know it works. I just haven’t had the opportunity to provide comment on the design.

Thanks again. Any further comment is more then welcomed.

RE: Micropile and Soil Nail Wall Underpinning

Quote (MTMclimber)

Vertical piles would be ideal but I’ve seen this type of underpinning a couple of times before so I know it works.
The micropile or helical piles should be as vertical as possible to minimize eccentricity and lateral loading to the soil nail wall. I would use a bracket to attach the pile to the wall footing. Take a look at A. B. Chance's underpinning bracket detail.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
PEinc - I understand what you're saying. We've used the brackets on a couple of projects before.

To be clear, this is not our design. We have been asked to review the shoring contractor's design. I've seen this micropile and soil nail wall combination perform adequately on several projects, so I can't make them change their approach since there's proof that it works. I have concerns about the stiffness of the micropile and how the building will react if the pile is not adequately reinforced. You are 100% correct in wanting to know how that pile will affect the soil nail wall. That concern has already been voiced to the contractor. They will be getting back to us on that item.

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Now that I'm thinking more about the bracket method, we could pre-load the pile using the brackets which would reduce the anticipated settlement if done carefully. Might be a good option if the neighbor's engineer doesn't like the anticipated amount of settlement. It also resolves the issue with lateral load on the soil nail wall.

Thanks PEinc.

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Thanks PEinc. It looks like these piles are transitioning to a permanent pile system so there will have to be a discussion on adjusting the current design. I'll try to persuade the team into to heading in this direction.

Any experience with preloading the bracketed piles with a bottle jack and a load cell? It may be overkill with 25 ton piles.

RE: Micropile and Soil Nail Wall Underpinning

MTNClimber, in 2000 I did a helical pile job in northern NJ where each pile had a jack so that a portion of a house could be raised and leveled. The contractor was Heinbokel Builders, LLC. Call them or talk to someone else like Ram Jack. Jacking may not be needed if the pile loads are not great. Turning the nuts may be effective and cheaper. I would not waste money using load cells.
https://www.mapquest.com/us/new-jersey/heinbokel-b...

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Looks like they are sticking to their guns by saying they can't use brackets because they're meant for helicals, even though I've pointed out that we've used them before with micrpiles.

We questioned their pile loads as they seemed to be low for the column loads provided by the structural. The underpinning/shoring designer responded that the pile would only pick up half of the column load, assuming the other half of the footing would resist the column load via bearing pressure. They included the bearing pressure as a surcharge load in the soil nail wall analysis, but there is a strain capability issue here. A stiff pile would have to deflect a fair amount for the footing to engage the soil. They did not include that in the analysis; my gut says this is odd reasoning. Has anyone seen this justification before? Please note this is now a permanent system and is not to be considered temporary.


RE: Micropile and Soil Nail Wall Underpinning

I would never pick up just the front edge of a column footing with just bracketed piles - helicals or micropiles. The bracket should be located at the face of the wall or column, with the footing trimmed as needed. Easier for a wall than a column. And, tell the designer that the bracket does not know whether it is supported by a helical or a micropile.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Sorry the sketch in my last post was confusing. From my first post, you can see the proposed arrangement. The pile goes through the foundation wall and the near center of the footing, so it would technically pick up the load at/near foundation's center. I agree when using brackets, the footing is typically saw-cut back to the foundation wall where the bracket is attached. I'm still working on resolving my issues with their analysis regarding their proposed arrangement.

The other issue is the column load at the building corner is 100 kips, where they propose one 50 kip pile under the column with the next closest pile 4 feet away. I don't think the piles will equally share the load, meaning the pile under the column will become overstressed. Their "workaround" (poorly explained above) claims half of the load will go into the pile, and the other half will go into the soil from the footing. It seems ridiculous to me and to other engineers I talked to, but I figured I'd see if anyone else has seen this reasoning studied in a white paper or some text we haven't seen before.

RE: Micropile and Soil Nail Wall Underpinning

Sounds to me like the project needs a real underpinning contractor. What you describe sounds to me like there is a drilling contractor dabbling in underpinning when all he knows is drilling (i.e., a one-trick pony). This looks like a project needing concrete underpinning piers for the columns. You may be able to use bracketed helical piers for the wall between columns but not for the columns. Also, it may be difficult to make a 100 kip connection between a drilled and grouted micropile and a relatively thin column footing.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)

Quote (PEinc)

Also, it may be difficult to make a 100 kip connection between a drilled and grouted micropile and a relatively thin column footing.

That's probably why they are refusing to increase the pile capacity.

I'm going to respond by telling them approaching the shoring in this direction is considered high risk. It's clean sand that can easily slough as they cut for the soil nail wall. The piles should be able to hold up the entire building assuming the footings no longer provide support in the instance that the soil under the footing sloughs into the excavation.

RE: Micropile and Soil Nail Wall Underpinning

Send me an email with the contractor's name and where they are from?

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
What does your email start with? I can assume how it ends based on your website.

The contractor performs pit underpinning and a variety of other services, not just micropiles and soil nails. Honestly, it's the engineer that they hired that is being so cavalier. I don't know if the contractor understands that their engineer is pushing the limits here with this approach, but everyone will after I submit my response.

I've been running into this with older engineers close to retirement. Some of them develop this attitude of having one foot out the door, so they don't care if they're being unconservative. We get asked to design for contractors, but I just don't know if I can compete with other engineers that live on the razor's edge.

RE: Micropile and Soil Nail Wall Underpinning

jpeirce

Lots of contractors do pit underpinning - the wrong way! Most get lucky; some don't.

RE: Micropile and Soil Nail Wall Underpinning

(OP)
Just figured I would give an update on this. The design engineer was able to provide me with details for 4 other similar underpinning projects that were successfully completed without issues. With proof that this method works in real life but not quite on paper, I notified the stake holders of the potential risks and they decided to move forward with this soil nail/battered micro pile underpinning approach.

Construction of the system took about a month, including soils nails for the other 3 sides of the basement excavation. It's been about a month since the pile contractor demobed. Automatic movement monitoring data has shown negligible movement (within the error of the system).

My guess is that the micropiles are barely seeing a load, if any, and the soil under the footing is still doing the heavy lifting. I would like to instrument the piles on the next job just to see how much they are seeing.

Thanks for all the input (especially John).

RE: Micropile and Soil Nail Wall Underpinning

Quote (MTNClimber)

With proof that this method works in real life but not quite on paper
In my opinion, it HAS to work on paper also! If it does not work and does not work on paper, you are screwed!

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

MTN - you're talking about it not working on paper, did you try to look at using Plaxis or something more advanced to "make it work"?

RE: Micropile and Soil Nail Wall Underpinning

(OP)
The issue is there isn’t an accurate way to determine how much load is going into the piles. For the piles to engage, the soil underneath the existing foundation would have to start to compress. But if the soil is stiff enough to support the building load even after they excavate up to the footing, how much load is transferring to the pile? My guess is near 0% on this job.

RE: Micropile and Soil Nail Wall Underpinning

MTNClimber, the May/June 2023 issue of Foundation Drilling magazine from ADSC has a story by Nicholson Construction where they did this same type of underpinning project for Colgate University's Olin Hall. The story really doesn't give much info such as building loads on the column footings or performance data (vertical or lateral movements). The story does describe the soils as "dense to very dense granular materials with varying percentages of fines and very stiff clays. Groundwater was not encountered in the borings within the excavation depths or encountered during construction." Nicholson also noted that "The initial SOE proposed adjacent to Olin Hall was a secant pile wall, consisting of 24-inch diameter drilled elements.....The university had concerns that the thickness of the secant wall would encroach on the new below-grade space, limiting the useful area of the basement." Therefore, in my opinion, the owner and CM accepted a riskier, lower cost solution that also provided more basement area. There was no mention of how much of the column load the micropiles were designed to support.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
PEinc,

Thanks for sharing. It looks relatively similar. We had medium-dense outwash sand and gravel, without groundwater. We did not pre-stress any of the nails like they did, which would have been a safer approach. The building we were underpinning did not have its wall torn down like they did for Olin Hall, which made it even more important that the building didn't move.

While I'm here, they installed the foundation wall for the new building and filled the gap between the soil nail wall and the new foundation wall. The automated movement monitoring system has been de-commissioned. Still no complaints from the tenants regarding cracking or other related issues. All-in-all, it seems like it's an effective system to underpinning. I think this approach needs a little more attention from the research community to help rectify the hanging questions regarding actual load transfer to the piles because it still doesn't make sense on paper.

RE: Micropile and Soil Nail Wall Underpinning

I am not too sure of the effectiveness of pre-stressing soil nails which are installed without any unbonded length. If nails have unbonded lengths, they are tieback anchors, not soil nails. So, if you pre-stress a fully bonded soil nail, you probably are pulling only on the front section of the nail where it probably will fail its way through the soil. Therefore, how effective is that "failed" portion of the nail? For the same reason, I do not believe in testing production nails as FHWA calls for. If the entire test nail length is grouted, you will fail the bond in the upper length of nail. If you use an unbonded length, it isn't a nail. If you grout only the lower portion of test nail so you can test it (and grout the remaining length after the test), you can have serious risk of collapsing the ungrouted portion of the drill hole during the test. Then you have a problem. If, during the test, you try to leave some casing in the ungrouted portion of the nail, you may unintentionally grout some of the casing which will also affect the test. I prefer to install additional, sacrificial test "nails" with an unbonded length which will serve to verify the ultimate grout to soil bond strength. I test these just like I would test tieback anchors.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
That's a good point regarding pre-stressing. I haven't specified it on any of our soil nail projects but I have seen it specified by others. I'll have to keep that in mind going forward.

For testing, the newer standard (FHWA-NHI-14-007, 2015) talks a bit about issues with load testing. I have to battle a lot of contractors and their engineers on the testing procedure. They want to use a piece of PVC taped to the reinforcing bar to create their unbonded zone, which was the standard when I started. Unfortunately, research (Cadden 2010) has shown this method can create artificially high bond stresses since the grout from the bonded zone can transfer the load to the grout in the unbonded zone. For sacrificial load tests, I make them wash out grout using the tremie to create the unbonded zone. The next day you can check the unbonded length by probing with rebar to find the top of the hardened grout and adjust your loading schedule accordingly. Usually, we are within 0"-2" of our target bonded length.

This washout method is an issue for testing production nails for permanent walls since the hole along the unbonded zone collapses. In these cases, we opt for only testing sacrificial nails for permanent walls.

RE: Micropile and Soil Nail Wall Underpinning

Quote (MTMClimber)

Unfortunately, research (Cadden 2010) has shown this method can create artificially high bond stresses since the grout from the bonded zone can transfer the load to the grout in the unbonded zone.
This sounds nice but........Why would people worry about this when they don't ever worry about it when testing much higher capacity, more critical, tieback anchors with unbonded lengths?

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
That's usually the response from the other side of the table. PTI has conflicting information regarding what they recommend. Upfront PTI says this:




And later on they say this:



Not sure what they're trying to say.

But regarding soil nails, I'm holding the team to the standards provided in the latest soil nail design manual. If it says not to use the PVC bond breaker as it causes erroneous results, I'm sure as sh*t not going to let it happen, even if it's inconvenient when we talk about tie-backs. I don't make the standards, I'm just here to abide by them.

RE: Micropile and Soil Nail Wall Underpinning

I would like to hear of others' experiences and problems encountered when testing partially grouted, uncased, soil nails per FHWA.

www.PeirceEngineering.com

RE: Micropile and Soil Nail Wall Underpinning

(OP)
I would too. Feel free to dig through some of the supporting documentation too: https://rosap.ntl.bts.gov/view/dot/42934

It's focused on HBSN but still applies to typical soil nails. They refer to the artificially high bond strengths as the "Doughnut Effect".

RE: Micropile and Soil Nail Wall Underpinning

(OP)
PEinc,

GTR recently designed an active soil nail underpinning system, without the use of piles, that was featured in the July/August DFI magazine. Looks like they had success with the system. See the article on page 79.

RE: Micropile and Soil Nail Wall Underpinning

MTNClimber I just lost a long response I had written. So, I will give you my short version.
Soil nailing is not underpinning.
Sometimes, it might work as an alternate to underpinning.
I don't do designs that MIGHT work.
This DFI project had shallow, very competent soil over high bedrock.
With a good contractor and the admittedly conservative design soil nailing worked - this time.
I have designed and built soil nail walls SI ce the 1980's. Never heard of active soil nails.
Soil nails with unbounded lengths are not nails; they are tieback anchors.
I don't believe that Snail can analyze nails with unbonded lengths.

www.PeirceEngineering.com

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