My good OP, pit traps are a part of my fortress defence as well. I will share what I know:
Eight Z levels of free fall paired hard rock surface will not kill every goblin that plummets down your welcome chasm. Sad but true as Metallica says. Upright spikes won't always do it either, I'm afraid. This is why use my dwarven plumbing to drown the rascals should they accidentally live (i.e. crawl around with broken bodies for an annoyingly long time). Water preserves the goblinite compared to magma.
The only issue I've had with my spiked pit trap/drowning pit was the time four crippled trolls swam around in the pit. They floated while the goblins and humans in the pit died, and when I drained the pit in frustration, one troll got sucked through my fortifications and into my plumbing (unscathed, I was disappointed) and down and around the waterways of my fortress. I had a little fun! removing him and his comrades. Oh, and a horse was with the trolls too.
But I digress. Here are my observations:
8 z level drop on 10 wooden menacing spikes won't always kill a goblin or troll or human (or horse).
The logs report the bodies striking the ground with reports like "Right upper arm was shattered" and "Left upper leg bone was jammed right through lower body"
Next, logs report spinning menacing spikes doing brutal damage to random places (once each)
Vomiting or death may occur
Sometimes dead goblins get stuck on the spike trap, viewable with [t]. Gore-anteed amusement!
Upright Spike/Spear traps work for this purpose without any triggers. Simply make a trap of spikes and gravity will do the rest.
I've done this setup with 2x8 floor hatches linked to two pressure pads at the far end as well as a retracting bridge. The work basically the same with a few differences
- The floor hatches are basically instantaneous: the lead goblin will set off the trap, and the little goblin followers go AAAAAaaaaaaaaaah! immediately.
- the bridge takes a second to activate, which can mean 2 or more goblins can get past the trap, but since it is delayed, it works excellently for catching those goblins when they inevitably break and run.
Hope that helps! Have fun splatting goblins