Micropile and Soil Nail Wall Underpinning 2

[MTNClimber]

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?

 

[EireChch]

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

 

[MTNClimber]

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?

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

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.

 

[MTNClimber]

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.

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[MTNClimber]

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.

 

[MTNClimber]

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.

 

[PEinc]

MTNClimber, here is a detail from a recent project of mine. I used Chance brackets with micropiles. If you batter the micropiles slightly, you can use beveled washers at the brackets. Turning the lower anchor nut for the core steel will preload the micropile.

www.PeirceEngineering.com

 

[MTNClimber]

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.

 

[PEinc]

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-builders-483351237

http://www.PeirceEngineering.com

 

[MTNClimber]

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.

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[MTNClimber]

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.

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[MTNClimber]

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.

 

[PEinc]

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

http://www.PeirceEngineering.com

 

[MTNClimber]

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.

 

[PEinc]

jpeirce

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