I should’ve probably added that the only thing I would do with a 36 inch bucket is dig trenches for a septic field. So I’d only go about 3 feet deep with it. But digging 36 inches wide at one time should really help keep the trench straight and speed up the dig.
String both edges of each trench, and mark where the string was with inverted survey marking paint. Then use the smaller bucket.
Who designed your system? I don't install leach fields, but observe quite a number being installed. ALL are engineered systems. Flat ground (I think it's less than 1% grade) most often is the plastic culverts split, they are laid level round outside up. Rows are used & if needed, three rows, each level, but not necessarily level with each other. Effluent is piped. It might be gravity from septic tank, or pumped from a pump tank. The leach field is covered with dirt of any variety.
Organics from effluent form a muck on what was once grassy ground under these half culverts, there bacteria eats organic mater, evaporation eliminates the water component. You mow the grass 6" above the top of this assembly.
Another system uses a build up of any soil to make a level area. It is built up so no portion of the leach area is below the original grade. A layer of "septic sand" goes on. Then more soil of any sort. Trenches are dug in the hauled in soil, they are filled with 1-3/4" drainage stone. 2" effluent pipe is laid on the stone. It gets a series of holes drilled in the top.
The engineer comes, clean water is pumped into the effluent lines. He measures the height of each little fountain. He may want bigger holes. Once he approves, more crushed stone is added to cover the effluent pipe, then landscape fabric. Fabric is covered with topsoil & grass seed/hay.
My point is I haven't seen a new engineered leach system with any component of it below the original grade. Exception being fine sand with excellent drainage beneath. It must be shown that at no time of year is any soil to a depth of 3' below is saturated with ground water.
