Have you ever wondered how high you can safely stack earth filled tires as a structural retaining wall?
We designed a simple retaining wall with no banter, on a flat grade. The wall was designed as a gravity wall only with soil compacted in each tire.
4.2 feet or 1.28 meters is the maximum height!
Even if you fill the wall with compacted gravel the maximum allowable height is increased to 4.4 feet or 1.3 meters. This barely qualifies as a retaining wall under the Ontario Building Code.
This design was fairly simple and included no extra reinforcing. The structural analysis is below and only included failure and rotation, the actual allowable height could be less. This design also assumes soil properties that will vary between sites.
The factor of safety should also be increased since there isn’t a design standard for this type of construction and a lot of unknowns exist.
Every site will be different but this type of construction is possible with some careful design. If you plan on using a back filled tire wall as a structural retaining wall you should consult an engineer. Some possible solutions would be:
- Add a banter to the wall – this would mean the wall would lean back into the slope.
- Partially fill voids with rebar and concrete. This wall add flexural resistance as well as increase the weight of the gravity retaining wall.
- Use multiple layers of tires. Adding three or four courses may be able to increase the height.
- Provide geotextile tie backs at each course into the slope.
Engineers are great at coming up with solutions to any problem. These are some general solutions but conditions will vary from site to site. This was also a very quick calculation, there are several more failure modes in retaining walls that would need to be checked. These calculations are for demonstrative purposes.