Help with a retaining wall

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1. Apr 6, 2016

Joe Mc

My name is Joe. I am 49 and have retired in the last year. I was not planning on this, and have had a hard time getting traction around the house. There are projects that I have had in mind and have dreamed about for years and now, I am finally able to get to them. The hobbies I once did that required the construction of assemblies consisting of small parts, I can no longer due. My hands shake to much..so only BIG projects. The main one that I have on deck for this spring is a retaining pond on the downhill / backside of the property. I will give more details in the forthcoming section.

Semper Fidelis ..

2. Apr 6, 2016

Joe Mc

The dimensions of the projected retaining pond are 50' X 20' X 4'. The maximum capacity of this project is set to be 4K gallons, 32K pounds of water. The brain cells that once held the power to solve such hydrodynamic questions, have been covered with over 30 years of ballistics tables and trig problems, these being very useful while adjusting the firing ark of the supporting artillery onto the "bad guys" ...

I was figuring a Retaining Wall that was 5' wide at the top and slope down and out to finish with a base of 8'. The slope of both sides of this retaining wall will be 105* +/-.
I have designed my Wall, with a nod to those Roman Engineers who at Cesar's request, built a bridge wide enough for a Legion to march in formation, across the Rhine River. Using logs of 6" dia for both the face and rear of the structure with a core of crushed aggregate, clay and fill. My question is, taking all the details and converting them into an equation to determine both the static load and the ascending load per foot of rise, will the retaining wall I have planned, be at least 1.5 times what is needed to safely create my DIY Backyard Retention Pond?

Semper Fidelis ..

3. Apr 6, 2016

Joe Mc

I have suffered some small measure of recall... The Trapezoid I has a volume of 1300. The breakthrough .. 1300/27 = 48.15 cuyrds * 1.5 = 72 tns .... So now the question is, will a retaining wall with a sloped face that is 4' tall, and a mass of 72 tons +/-, be sufficient to safely hold back a body of water that has a (V) of 4K gls / 32K pds? The logs that form the face and rear of the wall will be logs of at least 6" dia and 8' in length. This 4' of structure below grade will also need to be factored in. Is my math correct?

4. Apr 6, 2016

Tom.G

Having written a production program for retaining walls about 40 years ago, I strongly suggest you find a structural engineer for that wall. After all these years, of course, I don't recall any details, but it was certainly non-trivial! The equations CAN be solved directly but they are cubic equations for overturn stability. I finally used an iterative approach.

I hear there are some programs available on the web, but I haven't looked at them. You could try a few to get a ballpark idea of what's needed. They will probably each give a different answer.

Unless you are really out in the back country, the local Dept. of Building and Safety is going to require calculations that are at least certified by a licensed engineer.

A couple things to consider: What happens when the logs start to rot? What happens when a heavy rain fills the interior space of the wall? Remember that clay is 'expansive', i.e. when it gets wet it expands and shrinks when it dries; probably not a good material to dump inside an almost enclosed space. Look at a cracked dry lake bed to get an idea of the volume change involved. Is the below grade soil, where the posts are sunk, also clay or even partially expansive?

Sorry to sound so negative. This is one of those things that 'looks easy on the first pass.' That's how I got sucked in way back when; and I was working with a structural engineer thruout.

5. Apr 6, 2016

Joe Mc

Dear Tom G.,
I will have to complete a lot more research and have ALL the figures reviewed and verified prior to completion. I do thank you for your candor and level of concern, this speaks to your integrity and personal measure of professionalism. With the below grade strata consisting of a level of clay that sits on top of bed rock, I would think that as water cannot penetrate it and therefore not be an issue of concern as far a frost heaving, or undermining the logs ( they will be debarked and sealed with a permanent oil based sealer ).I would think that the undisturbed internal base would be a reasonably sound and long lasting foundation on which to build? If you say no however, I will most certainly take your warning to heart and make sure that I complete my due diligence.

6. Apr 6, 2016

Tom.G

My concern with the below grade clay is that a heavy rain will seep down to it, and especially with the bedrock base, the water won't readily drain. The clay both expands and softens. When it finally dries it will shrink, leaving room for the posts to sink until they hit the bedrock. Of course the posts will have an overturn moment just to aggravate the situation. The sinking may or may not be a problem but with the added overturn force you may need a foundation.

Here is a somewhat related story you may find interesting.
Many years ago I lived in a 3 story apartment building over partially buried parking. On one side, the building was supported by the block wall of the parking structure. The other side was supported by concrete columns so the cars could get to their parking slots. Well, someone didn't do their homework. As the building aged, it slowly tilted; several inches off vertical. It turns out we were on an ancient flood plane, and the column footings slowly sank deeper into the ground. No real excuse for it either, this was in a city in the Los Angeles area.

7. Apr 7, 2016

256bits

Having some trouble visualizing the wall. What are the logs for again? Why not just dump the aggregate and have that as the wall?

8. Apr 7, 2016

9. Apr 8, 2016

256bits

Good reference. Thanks.
I could have used that when someone was asking about the retaining wall next to his house a few years back.

10. Apr 8, 2016

Nidum

It is only the pressure generated by 4 ft of water which is loading the wall .

Load on wall is mean pressure times wetted area .

Retaining wall calculations are often based on load per unit length of wall rather than load on the entire length of wall .

11. Apr 8, 2016

Tom.G

And the soil, rock, and rain water all the way down to the bottom of the wall, for its full length?

Effectively anything solid above the soil angle of repose from the bottom of the wall, plus any fluid (rain, irrigation, leaks from the pond) that accumulates. Remember that clay soil acts as a fluid when it is wet.

Or am I missing something?