Soldier pile and lagging next to adjacent building

[milkshakelake]

I need to retain about 16'-5" of soil close to a house. Diagram below. I specified a tangent wall to prevent soil loss, but I'm getting hammered by the owner and GC that it's too expensive. Is it possible to use a raker/waler supported soldier pile wall in this case? I'm mainly concerned about soil loss, but a friend told me to use louvers to space the lagging boards and pack the gaps with filter fabric. Does that work to prevent soil loss? I'm still worried about installing the boards in the first place, which exposes the soil face momentarily.

My other concern is about the influence line of adjacent building foundation loads. I know it's actually a bulb per Boussinesq theory, but anyway, this is a simplification. It lands squarely in the middle of the soldier piles. The general advice I heard is that it's only okay if it lands at the bottom of the soldier pile. Is this something I need to be concerned about? Or if I calculate the soldier pile stress properly, it should still work out?

 

[MTSOE]

In my experience, it is best to avoid lagging within the influence of adjacent buildings and construct a tanget or secant wall as you previously suggested to your client (however, based on your sketch, it doesn't look like you have clearance for a tangent or secant wall). This is not for structural reasons but rather liability reasons. There's no structural reason why you can't have the boussinesq distribution occur at the mid span of your pile. Just size the steel, embedment, and bracing for the load. As you state, the problem with this system is that you must temporarily have the lagging bays unsupported during board installation. What is the soil like? Do you think the soil will have a good enough stand up time such that the lagging boards can be installed and backfilled before it collapses? If it does, are you confident that the contractor understands the significance of properly backfilling behind the lagging boards to prevent settlement of the adjacent house? Louvers aren't used for lagging in my area, so I won't comment on that but I understand the logic.

Another thing I'd like to bring your attention to is where you have the lagging in your sketch. It appears to me that you have it positioned behind the back flange of the pile so the foundation can be built as close to the property line as possible. I would recommend against this. It is likely that the contractor will want to open multiple lagging bays at a time. If they chip away the concrete from behind the back of the pile flanges, the soil loses the pile as a support and the span across which it must arch increases. Bigger spans are more likely to collapse. The soil will be able to vertically arch between the base of excavation and the previous lift of lagging below, but i would much rather have the soil arching vertically and horizontally during lagging, especially if I'm supporting someone else's house.

Also, I know you didn't ask this but do you actually need a waler? Your sketch shows rakers every 6ft. I assume your pile spacing is also 6ft, no? If you have a raker on every pile, do you really need a waler? Also I don't think you could construct your permanent foundation wall with that waler in the way. You could do form block outs around the rakers but the waler is anotger story.

I hope this helps.

 

[milkshakelake]

Thank you! You brought up a lot of good concerns.

What is the soil like? Do you think the soil will have a good enough stand up time such that the lagging boards can be installed and backfilled before it collapses?

The soil is average, maybe slightly above average. It's mostly medium fine sand with traces of clay and pebbles, with an average 1' blow count of 24. In my limited experience, this should be fine for a vertical cut while installing lagging boards. Though I'm kind of worried about the surcharge from the house and the weight of the soil itself pushing the soil out.

If it does, are you confident that the contractor understands the significance of properly backfilling behind the lagging boards to prevent settlement of the adjacent house?

I didn't think of that, and I'll mention that in the notes and bring it up in a meeting. Thanks for that. Although the backfilled soil won't be compacted, since I don't see a way they can do it, it's still better than nothing.

About the placement of lagging boards, it's right up to the property line, so I can't place them on the outer flange. Also, they are round piles (kind of like a round HSS pile with grout). I think I can mitigate the soil arching issue by using soldier piles at 3' or 4' instead of 6'.

About the walers and rakers, I'm going to detail them later. I just showed it schematically. I'll probably use a "stub" and plates with blockout to push the waler out of the foundation wall, mitigating waterproofing/rebar issues, and use rakers at 12' on center.