Hot tub adjacent a retaining wall 1

[swBausch]

Friend asks me to provide guidance on "moving a hot tub".

Turns out the actual "moving" can be accomplished with rollers. About 20 feet over level ground.

Unfortunately, that "level ground" abuts a freshly created retaining wall about 3 foot high, the wall itself resting on hillside. The wall is created out of Home Depot pavers that have a setback lip on the underside. No facility for alignment rods or deadmen.

The spa is about 5,000 pounds when filled.


My gut response is anytime you want to park a spa on the edge of a hill that overlooks the next lot's roof, you might as well knock on the door and apologize in advance for destroying the neighbor's backyard.


The situation seems to be a catastrophic failure waiting to happen. If the retaining wall fails, the spa will be sitting on a 30 degree slope, slopping water over the low side. The bottom of this spa is a polyethelene sheet.

I also consider the spa adds the equivalent of four feet of soil; making the nearby retaining wall a candidate for engineering.


Any opinions on this?


The owner has ruled out any other remedy or location. Too close to a window, too far from the back door, too much money, etc.


My feeling is a spa should never be placed closer than 8-10 feet to a retaining wall of any height, or a retaining wall on the side of a hill.

 

[dicksewerrat]

Tell him he's on his own. You don't think it will be a safe location. Print out your comments and all the answers you get here. Maybe even send him and yourself registered letter with the comments. then tell him to hire a soils, geo tech or foundation engr.

 

[GeoPaveTraffic]

I assume that the spa will sit on top of the ground approximately level with the top of the retaining wall. If this is the case, then the spa does add to the effective height of the retaining wall. I'm not sure that it adds quiet 4 feet to the effective height, but it would increase the height. The amount of effect that the spa will have also depends on how close to the wall the spa is possitioned. If the close to the wall the spa is positioned.

That said, if the wall is stable without the spa and the spa is placed at least 3 feet from the back edge of the wall, then the effect of the spa on the wall should be insignificant. The better question may be "Is the slope without the wall and spa stable?"

 

[swBausch]

GeoPaveTraffic,


I agree with you about proximity to the wall, and whether the wall "squares off" the slope profile. If the wall was strictly a cosmetic attribute I wouldn't be too worried, but here the wall is being expected to keep the spa from sliding down the hill into the neighbor's deck. (Actually, there are some trees to stop it.)


The wall needs to retain the soil that the spa rests upon. The spa would be within a foot or so of the wall. It seems that the spa is also at the end of the wall; whether an adjacent wall is planned I cannot say.


The spa itself is 8 foot square and weighs 5000 pounds when filled. Works out to 78 pounds/sq ft.

I suggested a small deck supported on a dozen or so posts, but cost is a consideration.

Rainwater runoff from adjacent areas (100-150 sq ft) would also find its way to this location, since the retaining wall isn't high enough (got to keep the spa below the window sill.....).

 

[cvg]

I would refer your friend to a good geotechnical engineer and politely excuse yourself as the engineer of record...

 

[layfieldman]

I would also suggest refering to a geotechnical engineer. One option would be to rebuild the retaining wall as a reinforced wall with geogrid. This would involve excavating the portion directly behind the blocks probably to around 5' but that would need to be designed by someone with experience in reinforced segmental retaining wall design.

 

[Focht3]

You have gotten excellent advice!

[blue]layfieldman[/blue] has given an excellent - and inexpensive - option. That's the cheapest approach, but only under the supervision of a qualified geotechnical engineer, of course.

Unless the spa is really an above ground pool, it won't be the equivalent of four feet of soil. In fact, it will likely be equivalent to only a foot or so of soil. (Most spas are only about three feet deep, and one-third the volume occupied by the spa is insulation and open space. Two feet of water is roughly equivalent to one foot of properly compacted soil.) A two foot setback from a three foot high wall isn't too bad. I'd prefer four feet - to account for the wall's foundation - but at least it isn't putting any significant lateral pressure on the wall.

I have a hard time seeing a catastrophic failure. Property damage? Yes. Death? Not likely.

Put your comments in writing; you don't want to be responsible for your friend's foolishness. Send them via certified letter. Wash your hands of this quickly...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[roncity]

But consider if the spa leaks. Will the underlying soil become saturated and less stable?
And suppose during a party someone wants to see how many people can fit in a hot tub?

 

[Focht3]

Okay. You will superimpose a live load of something significantly less than 200 psf (for a crowd shoulder-to-shoulder) - which is about 1[½] feet of soil. It still isn't a big surcharge. And all those bodies will displace their own volume in water, so the realistic increase in weight is probably only about 50 or 60 psf - about [½] foot of soil. So the total surcharge is still less than two feet of soil. Significant for a three foot tall segmented block wall, but not a real big deal.

What is the likely mode of failure for a saturated soil? It's probably uneven settlement. But I still have a hard time seeing a catastrophic event, unless someone is standing in the hot tub when it shifts, resulting in a fall.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[MPENN]

Pehaps the greater problem is the clumbsy and somewhat tipsy guest that falls backwards out of the pool that is perched at the back of the wall and hits the old noggin at the base of the wall.