Calculating a potential lateral load on a retaining wall.

[67kato]

I am in the process of designing a retaining wall to replace a failing wood rr tie wall. There is an existing inground pool within 8 ft and behind the proposed wall. The top of the new wall and the pool are at the same elevation. The bottom of the new wall is approximately 7 ft down (which includes keying it into the original grade by 1 ft). The backfill will most likely be 1" stone. I am planning on specifying a Redi-Rock segmental gravity wall for simplicity and ease of construction.

There was a time when I could remember how to calc these passive/active loads but it is escaping me. Any help would be appreciated.

Thanks,
John

 

[Lion06]

you need a friction angle (phi), and that comes from testing. It is possible some foundations books or geotech books give approximations for your seemingly standard stone fill. Once you have the friction angle, it is a pretty simple calc found in any geotech book or foundation book.

 

[67kato]

Thanks. I thought it wasn't that invloved.

 

[minorchord2000]

I think you should check if the pool structure will add surcharge loading to your retaining wall. The pool depth and distance from the wall are critical values.

 

[DarthSoilsGuy]

i would think there would not be a surcharge b/c water weighs less than most in-place soils, even drainage aggregate. i would think there would be some benefit because the bearing would be lowered to the pool slab base and less of the cone of influence would be attacking the wall than if soils.

major problem comes if you
1.need geogrid the wall
and then
2.the pool prevents lengths required.
or
2. the pool is close to the required length and requires the protection of pool from during construction undermining or other damage.

Redi-rock makes design charts for different phi angles of soil and it's on their site. i would go ahead and assume the worse soils on their chart for your schematic design to see if you may end up in the pool.

on another note,
our office typically provides the wall elevations and grading plans. The contractor then goes to the supplier at bid time and the wall itself is a design/build submittal inside the overall site design we provide. Problem being that wall designs are material specific block/grid/soil and the many different contractors bidding will want to bid and use their suppliers. it's not a major issue except when there are site constraints (your pool) that affect the design. a site constraint we have often is a stormwater pipe that collects from area drains we like to put near the top-of-wall.

 

[minorchord2000]

Darth Soils Guy:

Yes water weighs less than soil but its equivalent fluid pressure is much higher. Water exerts the same pressure down as it does laterally, unlike soil. There is no friction factor for water, hence the lateral loading on a retaining wall subjected to surcharge from a swimming pool is a real issue, and should not be ignored.

 

[DarthSoilsGuy]

Minorchord,

i had the pool walls retaining outward lateral loads in my mind when i wrote it. you're right, not a safe assumption. thanks for clearing me up on that.
dsg

in addition to my original reply, i should have mentioned that the lowest phi angle that redi-rock lists (28 degrees) in their reference designs could still be better than the on-site soils at your site. (To redi-rock's credit, many manufacturer's literature out there won't give standard design info for phi's less than 30, and even 35 min for some.)

 

[67kato]

I am designing the wall using the gravity style redi-rock so the geogrid will not be necessary.
How should I model the load from the pool? The pool depth is 8 ft and there is a 6 ft space from the rear of the wall to the pool. The bottom of the retaining wall and the pool are almost at the same elevation. Will using an aggregate backfill help of hurt me in between the pool and the wall? I realize that some aggregate would be necessary regardless for drainage against the wall.

Thanks!!!

John

 

[MSEMan]

How exactly do you propose to get the new wall in without disturbing the pool and it's deck features etc.? Why not drill through the existing wall and soil nail it (hollow core nails if the soils are collapsing)? Alternatively place concrete counterforts in an base on the outside of the existing wall?

 

[67kato]

MSEMan,

For safety, I plan on draining the pool and the owner is replacing the deck (which is rotted too). The existing wall is a mix of rr ties (rotted out) and a collapsing stone wall. It is on if not over the property line. The neighbor below is pressuring my client to replace the wall before it collapses onto her driveway and car. I am planning on relocating the wall 2 feet in from the property line. With the water out of the pool, the new wall will be installed and the void between the two (if the excavation reaches the pool) will be filled with a coarse aggregate (1" stone). I have found out that the vinyl lined pool is constructed using an x-brace system that supports the top 3 feet of the pool (attached to metal walls). The rest of it slopes to the deep end and is constructed from a weak concrete. I will look to secure these x brace points as they are conveying concentrated loads to the soil, however, I feel that their depth will make them less of a load factor than say an above ground pool. Any thoughts?

 

[DRC1]

If the pool is 6 feet back and the wall is 7 ft above grde, the influence of the pool on the wall will be noticable, but will not drive the design of the wall. If the pool walls are self supporting, then little lateral load will be transfered to the retaining walls. As darthsoilsguy sugeested, you could simly design the slope as if there were soil there. The one concern I would have is very good drainage. If the pool develops a leak, the soil could quickly become satuated and may overload the wall.
Aggegate generally has a higher strength than soil and therefore will help you. It will also provide good drainage, just e sure you use filter fabric to keep the fines out and that you give the aggegate a place to drain to.
Most of these propritary wall systems have enginers available to help with the design of your project and can suggest details to make the system work better.

 

[67kato]

DRC1,

Thanks for the thoughts. I plan on providing perforated drain pipes behind the wall and just below it. I may even run some out towards the pool to be safe. Filter fabric is in the given.