Helical Tie-Back for Residential Foundation Wall

[PEYooperWall]

Hello,

First time posting, looking forward to some insight. I am working on a deep foundation plan for a 2-story residential structure located near a waterbody (approximately 200 ft+ inland). I am utilizing a grade beam, helical pier system due to a relatively thick and soft organic soils layer 5' below existing grade (layer of organics is about 12' thick with N-Values of 2 or less).

Part of the foundation wall is 9 FT in height (retaining 8'-7" of soil). I am designing a triple helix square shaft tie-back to provide stability against the soil pressure (lateral restraint near top of wall), and utilizing a double helix 3.5" dia. helical at the footing to take up the vertical loads, with a 5" reinforced slab at the bottom of the wall providing sliding resistance. The helical tie-backs will likely be installed using a PVC sleeve in the wall, with a backer plate on the interior side of the reinforced concrete foundation wall as a connection. The reason for the tie-back is to prevent creating a typical cantilevered retaining wall (very expensive in my analysis, and not very safe/effective with soft soil) and to mitigate potential movement at the top of the wall that would then push against the wood floor framing.

My question is: Is there a feasible way to prevent water infiltration through the PVC sleeves? I am thinking of a sort of neoprene gasket to mount the plate over and tighten the threaded portion of the helical to create a seal. Yet, the area around the shaft itself is still an infiltration point. I have also considered using epoxy around the shaft in the PVC.

I have already considered embedding the plate in the wall, but I have calculated a 9.5 kip factored shear load on the 12" thick concrete wall, and my analysis on the shear with 8" embedment & steel reinforcement is unsatisfactory. Any recommendations or comments would be much appreciated, or if more information is required.

 

[XR250]

Most residential structures use the floor framing to brace the top of the foundation wall.
What is your lateral soil pressure?

 

[PEYooperWall]

The Geotech Report recommended analyzing it as a cantilevered foundation wall with Sand Back Fill using an EFP of 90 PCF Active and 120 PCF Passive in an undrained condition. The lateral load at the tie-back elevation is 1.51 KLF. The Owner supplied plans by a home designer with very basic sections and no sheathing/nail pattern.

My S.O.W. only involves the foundation design, but it seems like I would have to design the floor diaphragm as well if I am using it as part of the resisting system. I might add it is quite a large residence around 4000 SFT.

 

[XR250]

That's probably too much load for a floor system to effectively manage.

 

[PEinc]

Did you calculate how much consolidation of the 12' thick soft organic soil you will get due to the weight of the added fill? Will you need to support the slab-on-grade and the interior bearing walls so they don't settle? Consider timber piles supporting the walls and slab? If you tie back the perimeter walls with helical anchors, they may be damaged from bending (combined stress) when the soil inside the perimeter walls settles.

http://www.PeirceEngineering.com

 

[XR250]

It might be cheaper and more reliable to remove 12' of fill than support the entire house on helicals.
I imagine the owners can afford it if they are forcing this lot to work.
Could also use a better backfill at that point to relieve some pressure on the basement wall.
Helicals can settle if not installed properly and it is tough to fix that later on interior ones.

 

[PEYooperWall]

The slab & interior walls are supported by helical piers as well. The Owner has reached his decision for the deep foundation based on the Geotech recommendations (which explicitly determined a deep foundation to be more cost-effective). Based on that, I did not calculate consolidation as the subbase of the slab is not meant to be in a bearing state.

There is a fairly dense sand layer below the organics, with N-Values of 12+ which is where the helical will be embedded based on the torque of the installation equipment w/ a FOS of 2.0.

I also agree with you XR250, it seems to be digging and benching back a 12' hole in the ground might be expensive, but not as expensive as helical piers peppered throughout the footprint. It also allows for a longer service life, from what I understand organic soils can be corrosive and it may be that in 50+ years(galv. coating) that this house may be having very expensive issues. Which at that point is out of my control.

 

[XR250]

I wonder what metrics the Geotech used to come to the conclusion of the "cost effectiveness" of a deep foundation?

Even if it is 50k more to dig it out, I would still go that route to avoid problems.

 

[PEYooperWall]

The ground water level between the two borings (about 50' apart) is reported at 13.5' below grade, and 3.5' below grade (Dense sand layer is 14'-18' below grade). They reference this ground water level and the depth of unsuitable soils as being impractical for removal of unsuitable soils. I understand the water issue, pumping it is an option, but it could make quite the mess excavating swampy wet soil.

Long story short, I am designing a system within the parameters I have been given, and its making the foundation very expensive. If I can keep water out, and the Owner can swallow the pill on the cost, I believe it will be a success.

If I can avoid helicals, which I am going to discuss with them today, then that would also be a success. It just depends on who's quote is more affordable, the helical contractor or the excavator.

 

[XR250]

Make sure they factor in the cost of engineering and upgrades to the foundation to be supported sparsely
Regardless, the diff. in foundation cost will likely get lost in the noise compared to the other construction costs.