Trench Excavation near retaining wall

[dirtgirlhawaii]

I have a 5.0' retaining wall holding a pool shell. There is 4 feet of water inside the pool. I'm trying to evaluate an acceptable distance from the pool that a trench can be placed to replace a water line. I can easily evaluate the wall with the hydrostatic pressures vs. the lateral pressures, etc. for overturning, sliding, and bearing capacity.

However, lateral earth pressure theory for cantilevered walls has the lateral earth pressures on a vertical plane starting from the toe of the wall. I'm not seeing how the distance of the trench plays any role in the stability of the wall.

The trench will probably be only 2.5' to 3.0' feet below the pool deck. Can someone send me in the right direction? It makes sense if I evaluate the wall with an active or passive wedge of 45 - 45+phi/2. However if I'm evaluating at rest pressures, I need a total distance of 5.0 feet from the toe of the wall. I'm about six inches short of that based on current plans - how do I reduce the at rest earth pressures?

 

[Ron]

Create a "curtain" in the soil between the wall and the trench by injecting chemical grout. Not that expensive and should alleviate your concerns. Silicate grouts and expanding foam can be used. Provides more stability for the wall as well for the long term.

 

[BigH]

You do not indicate how large of a pool it is. If the trench is only 2.5 to 3 ft deep, why not excavate in short lengths - dig a short length, perhaps 5 to 6 ft; install your water pipe, backfill, move over and dig another 5 to 6 ft length (expose the end you had just laid, connect and install the pipe and backfill, the keep doing this. By doing in short lengths without opening up the whole length, the model shouldn't really notice that you have actually removed anything . . we have done this when digging out in front of a high tailings dam and we were putting in a stabilizing berm - but digging out some 5 to 6 m deep in front of it.

 

[Kuhuh]

Not knowing the exact dimensions of everything by why don't you just slope the dirt to the top of the pool at 1:1. This way you do not have to support the extra horizontal pressures of the soil and only focus on the pool's pressure and the soil in the critical slope. Grout curtain is a good idea but can be expensive and usually most would go with a slurry mix in this application and I would never rely on slurry for its compressive strength. Plus you would have to pour this curtain below the slip plane for it to be effective

 

[Riggly]

I see this post is a little way back, but I will still add my thoughts. I don't see much of a concern here. In fact I think intuitively you are seeing that when you stated that "I'm not seeing how the distance of the trench plays any role in the stability of the wall". I think its effect is minimal. You seem to want to know how to put the numbers on paper to apply your theory with a backfill that now has a trench in it. You are signicantly above the heel of the wall. If you were digging within the pool, near the toe of the wall, then the concern would escalate significantly. Is that pool a rectangular or circular pool? Is the vertical wall of the pool singly reinforced or doubly reinforce? If singly reinfoced I could see a minor concern, if doubly reinforce much less concern. Are you concern about the unbalance load due to the water inside of the pool? How about lowering the water by a foot or two? In that case you will have essentially a section of free-standing wall with no load on either sides. I think for the height of wall and the depth of trench, it would be over-kill to go through any rigorous analysis. I agree with BigH method - sections at a time.