Dirt jumps with wood I am looking to build a dirt jump track and after realizing how much dirt is really required for a proper jump I am exploring using a wood frame to pile dirt on top of to reduce the need for dirt. I live in San Diego with little rain, so it seems this is as good a climate as there is to use wood without being overly concerned about the wood going bad or rotting or decomposing. My plan was to build a bridge more or less with about 18-24 inch &quot;hand rail/walls.&quot; I would then fill the dirt in over the bridge up to the top of the wall (so the dirt would be about 18-24 inches thick) so there is a table top with a take off and landing I can sculpt. I was going to use pressure treated lumber when possible for structural integrity/longevity, set the posts of the bridge in concrete, and use sealed plywood where necessary. It seems this would be very solid.
If it lasts 3 years, I would be happy. I suspect it will last longer than that in San Diego. Questions:

Has anyone seen this/have any feedback?

Is 18-24 inches enough dirt to properly compact and hold? I would think so as I have seen small 24 inch tall jumps and they don't fall apart.

Any thoughts on pressure treated lumber lasting a long time in contact with dirt that will need to be lightly watered somewhat frequently? I've seen posts saying pressure treated wood should last decades in contact with the ground.


Thoughts?
Thanks,
MB
Properly treated timber will last decades in the ground. Wood treatments are generally rated for there preservation ability. Different treatments are used for protection from insect and moisture damage, and the timber is treated to suit the local conditions. Most countries have a set of standards - e.g. American Wood Protection Association for USA.
Some timber will last several years in ground with no preservative, and a lower level preservative treatment might last long enough for your purpose
As far as longevity - the problem really comes down to safety - who will know when the timber has degraded, who will check, and what damage will result form a collapse?.   Also you need to consider the construction methods - building something to last a few days is very different to building something to last a few years - do you have the wood working skills to be sure you will built the structure strong enough to last years of jumps.
While you could build a structure in timber, cost factors into it. You not only need timber, you need the fasteners and time and tools to build the structure. Dirt is cheap, and for jumps versatile and easy to change profile - most likely it will be much cheaper to get a truck to drop off the dirt you need to build the jumps,  but do cost out having the dirt taken way when you no longer want the jumps.  It your that way inclined, its also a decent an excuse to hire an excavator and have some fun...