Math question for any engineers out here

[trcothorn]

I’m in the process of building a parking pad in this spot. I ran a string line to measure my retaining wall height. If I have the line 3 feet above the ground and check the line for level, it is still just barely sloped back. I do not want to build a wall taller than 3 ft due to increase chance of the wall leaning or giving way. So I went inside the trailer with my 4ft bubble level and measured, if I rise the back of the bubble level 9.5 inches it shows level.

So my question is, for every 4ft there is a 9.5 inch drop. My trailer is 30ft. So do I divide 30 by 4, and times that by 9.5 to figure how to get level? Doing that tells me I need a 71 inch wall to get level. Am I doing this all wrong or over thinking it? Really would appreciate input on this my brain is hurting.

[schlep1967]

For some reason I'm not seeing the pics so I'm taking a shot in the dark here.
You can build your 3 foot wall at the back end of the trailer and then slope up a little steeper to just behind where the trailer tires are and then level out from there. That will give you a better shot at having something to put stabilizers down on and not have a straight drop directly behind your axles.

[GDS-3950BH]

dedmiston wrote:
I'm no engineer, but I assume you're going to trench down and pour footings for the retaining wall?

As for the final slope, it's good to have some pitch to it for drainage, and then you can use blocks for final leveling if you need to.

No footings are required if the OP does the correct type of retaining wall. A Versalock (or similar system) wall can bear on a compacted gravel footer, and uses a tieback mesh (Geogrid) in layers for stability. You could go 50+ ft high. They spec you can go 8' high without the geogrid but I would do at least a layer every 32".

9.50 inches fall over 4' equals 71.25 fall over 30' (5'- 11 1/4"). The OP gets it correct and see PastorCharlies post above. Some of the other posts look like an engineering report for building a nuclear reactor, and read like War and Peace, but I would expect nothing less here. It's not that complicated but neither is towing a trailer or airing up a tire LOL.

[Lynnmor]

trcothorn wrote:
The drawings really help put everything in perspective thank you. I decided to make the pad shorter, closer to just behind the rear axle, instead of the full length of the trailer. Thanks for all of the suggestions

That will reduce materials quite a bit, but do add a curb to prevent the trailer from rolling back. Keep in mind that heavy winds can push or turn a trailer if there is nothing to secure it.

[trcothorn]

The drawings really help put everything in perspective thank you. I decided to make the pad shorter, closer to just behind the rear axle, instead of the full length of the trailer. Thanks for all of the suggestions

[Sjm9911]

trcothorn wrote:
This is the set up I’m going for. Gravel base for drainage, deadman tee anchors for support. All that.

The one shown dosen't look like it has enough deedman built in, if any. Make sure you add more. And go down a corse or two. Rebar is old school and works but they have new screws that tighten up the ties real good. Expensive, but worth it. And you will be adding stone every so often, no getting around that as it sinks in, but as long as the frame is sturdy. Have fun!

[d3500ram]

Just a thought experiment...

If you wanted no more than new wall= 36" (max.) at a location just behind the rear tires, then the slope would be cut by more than half. Perhaps a couple lally columns could be added at the rear of the trailer, but this would only support downward force unless a diagonal brace of sorts were added to reduce shear, braces would be needed in two directions:

[d3500ram]

trcothorn wrote:
....
So my question is, for every 4ft there is a 9.5 inch drop. My trailer is 30ft. So do I divide 30 by 4, and times that by 9.5 to figure how to get level? Doing that tells me I need a 71 inch wall to get level. Am I doing this all wrong or over thinking it? Really would appreciate input on this my brain is hurting.

Based on the numbers provided, your arithmetic is correct.
The resultant slope is a little over 11^ which is almost 20%;

[Grit_dog]

trcothorn wrote:
I ran a string line to measure my retaining wall height. If I have the line 3 feet above the ground and check the line for level, it is still just barely sloped back. I do not want to build a wall taller than 3 ft due to increase chance of the wall leaning or giving way. So I went inside the trailer with my 4ft bubble level and measured, if I rise the back of the bubble level 9.5 inches it shows level.

So my question is, for every 4ft there is a 9.5 inch drop. My trailer is 30ft. So do I divide 30 by 4, and times that by 9.5 to figure how to get level? Doing that tells me I need a 71 inch wall to get level. Am I doing this all wrong or over thinking it? Really would appreciate input on this my brain is hurting.

Your calcs are correct, however a string run 30+' from ground level at the front and 3' above existing grade at the back was not "barely" sloped back. Your trailer floor is at LESS of a slope than your grade because it is tongue low. Therefore, your actual grade is a bit steeper than what you measured in the trailer, if you measured that correctly.
Pictures are deceiving. Your pic "looks" like a 3-4' tall wall at the back of the wheels will give you a pretty level pad. But start over as I suggested and run a level line from existing grade wherever you want the start of the pad back to where you want the back of the pad and start from there.

[JimBollman]

I had a sloped area I needed to bring up level that was going to need about an 8' retaining wall. After some thinking I decided I didn't care if the area behind and beside the flat area was usable so I had about twice as much fill brought in and let the back and sides just taper off. I used shale which locally was very cheap, topped off with crushed stone and it worked fine. Been about 3 years now and everything is still stable.

PS: I did put a low wall around 12" at the bottom edge so the bottom edge would not keep spreading out. It is just pressure treated straked in place because it doesn't have a lot of weight pushing on it. would be optional but I had another flat area at the lower level that I didn't want to spill into.

[wnjj]

rhagfo wrote:
Nice, but you considered the larger concrete retaining wall stacking blocks? Build a good base packed crush stone, keep it level.

I was going to suggest those too. I built two walls from the 6” ones to widen my driveway and reshape the yard years ago. They never decay and can be stacked up to 4 1/2’ according to Home Depot’s website. Price may be a factor.

Blocks.

[rhagfo]

trcothorn wrote:
This is the set up I’m going for. Gravel base for drainage, deadman tee anchors for support. All that.

Nice, but you considered the larger concrete retaining wall stacking blocks? Build a good base packed crush stone, keep it level.

[wnjj]

1. The pad doesn’t have to be completely level since the tongue jack can drop beyond level.
2. As others have pointed out, the rear overhang need not be over the pad.

If you don’t want the tail hanging out in space, consider a step down at a wall behind the tires and a second wall further back. The second “pad” that is formed could be sloped enough that it can hold fill up behind the first one, reinforcing it and/or reducing its height.

[Grit_dog]

I Blubeamed a dwg based on your pic, but its a pain in the ___ to post.
Presuming you have the trailer where you want it, stake a string at ground level at the back of the truck.

Pull string along trailer to about 1' behind the back wheel, level the string (top of pad) however, Bubble level, water level, etc. This is the height of the pad that will barely fit your trailer wheels on it and minimize the length of pad and minimize the work and materials.

If you're using RxR ties, you almost need to make the top of the wall/pad level. If you use keystone blocks, you could slope the pad down 4-6% (2-3 deg which you could run the fridge on, even if you don't lower the tongue down) pretty easily, reducing the height of the wall/pad by 1' or maybe a bit more at the tall end.

The biggest challenge is going to be tiering the wall from the ground up.
Even if you slope the top of the pad down towards the rear, use a level line on each side to measure grade to dig in the ties or blocks.
Have fun.

[SDcampowneroper]

When we owned and developed our camp I built dozens of retainer walls, of native stone, treated achitectural 5x6 treated timbers and yes, RR Ties. 100s of them
In our cool relatively dry climate # 2 creosote ties will last many, many years. .
For aesthetics, ease and engineering, treated 5x6x16' timbers are more pleasing to the eye, require less pinning ( joinery) which more than compensates for the cost of the new material.
RR Ties ruin saw chains, 1/2" installers drill bits used to drill for rebar. To drive rebar pins we rented a 40# electric jackhammer for which I made an adaptor, 1" x2"long steel shaft drilled 1/2" x 3/4" deep on both ends that I could place on the rebar and set the jackhammer driver point. In the end cost, use new treated timber. Sharpening the chains and bits will use up savings on ties. OK if you are handy like us, it was just a few minutes to resharpen in our shop. I could justify cheeper RR ties - barely.

Long ago years before we bought a camp the best design for a campsite is level side to side, 1' / 10' slope to rear of site. You must have drainage.
Any towable is more stable in a slight tongue low, As do not want to raise the front, but raising the rear 2-3" is just fine, Bs would never notice the 1", Cs are made high inthe rear on their truck chassis so 1"/ 10" high in the rear is level for them.
Ever noticed Cs nearly always have blocks under the front wheels to level?
RR Ties will work just fine for your project. Eventual cost, material, labor, is yours to judge.

[PartyOf_Five]

I'd think about the rear overhang and what (rear) approach angle the trailer can tolerate as you back up. On that steep of a grade, the pad height ie. Retaining wall height will certainly be much higher than the pic you added later.