I'm used to big silage operations, so I'm not much help. However:
The object of silage is to get 40-65% moisture plant material with enough starch into storage as quickly as possible, pack it tightly, and seal it off from any oxygen at all. Too little water & the good bugs don't grow well; too much water & they drown - this is important to remember on the edges of your silo as well.
The better you are at doing that, the better the silage will be. The less spoilage there will be. There is _always_ some spoilage (mold), but you want as little as possible. Any oxygen at all will cause mold, you need to eliminate as much O2 as possible. The O2 bugs cause mold, the bugs that live without O2 cause pickling juice. Unfortunately the O2 bugs are easier to grow....
'A hole in the ground' would be like a bunker silo. They have the highest spoilage, as the top is exposed, it's harder to pack the silage because the pile is shallow. Most are concrete walls, with just dirt you will have a lot of liquid loss, and oxygen entering, so the sides would spoil. The top would spoil - Bunker silos they drive big 4wd tractors over many times to pack tight (air gaps in the silage hold O2, and cause spoilage) and cover with plastic, but still the top foot or 3 will not be good silage - it dries out & gets exposed to O2.....
Then, legumes & grass do not have much starch, so the good bugs you want to grow don't have much feed to produce their pickling juice. Certainly lots of silage is made from alfalfa, but it's more difficult to do. Often some innoculant helps on these crops.
Another problem I see is that you will be making a 'harvest' about every week or 2, and trying to add to the pile? This is not the best way to produce silage. The whole 'silo' should be filled & left alone for 3 weeks. Contantly adding to it will introduce O2, disurb the good bugs, and create multiple lines of poor 'top' silage in your bunker.
Now, as I say, what you want to try might work well, I don't really know. But you really will need to learn & practice to understand how a silo works.... Good luck with it.
An important safety warning: A well funtioning silo will produce gases that are heavier than air, and displace all oxygen. As well as probably producing some gases that make your lungs want to foam (farmer's lung). If your 'hole in the ground' is actually a deeper pit, be very careful of this, especially in those first 3 weeks. Many, many farmers have needed hospital visits or coroner visits from entering a silo too soon and running out of oxygen. I'll guess it's not too serious on such a small scale, but I wouldn't feel right in not mentioning this. If you do get the good bugs to work properly, they try real hard to keep oxygen away from themselves, and this has real negative affects on humans.
--->Paul