The following creatures, in order, are a sandgroper, a mole cricket and a mole, all creatures that are well adapted for life underground:
All of them converged into a similar bodyplan, with powerful, broad and shovel-like forelimbs for shoving dirt, and with claws or claw-like structures for better penetration into rock and dirt.
And here is the giant ground sloth, thought to have carved out tunnels into solid rock in South America, something nearly unheard of outside of creatures just melting rock to make their tunnels:
The reason I'm showing these is because I was designing an arthropod-like creature which I intended to spend most of its time digging below the earth, feeding on plants and other creatures it ambushes by springing from below. The creature is meant to be pseudo-realistic, in that while it's muscles and claws are stronger and more durable than what would be expected for a normal animal, they're meant to be otherwise ordinary creatures following the natural laws.
As for my problem: I'd like for them to be at least as large as grizzly bears, and for them to be ideally able to dig through loose soil at speeds close to 20 kilometers per hour, while also being able to make tunnels through Rock at speeds at least greater than a couple meters per generation (like the sloths likely did).
The problem with this is that I'm not sure what they body should look like in order to accommodate something even remotely close to this type of movement without straight up hadwaving all away. By basic principles, it must have short, powerful forelimbs and a rounded body shape,but as for its forelimbs I'm at a loss. Searching into machines for inspiration was also mostly fruitless, as drills rely on mechanisms troublesome for an animal to have, and the current most used piece of technology for tunneling is similar in its nigh impossible design to adapt to a fleshy creature.
The only seemingly good example in a machine that I can see as translatable would be the bucketwheel excavator, a behemoth that can scoop out earth almost constantly because of its bucket wheel, causing there to be constant clawing at the rock and soil. Based on this, the first solution I considered was something with 8 limbs and a bodyplan and proportions similar to those of a mole cricket's, but with 4 digging forelimbs instead of 2, in an attempt to mimic a bucketwheel excavator's ability to dig relatively fast by adding in more limbs into the work. I'm however struggling to understand whether such a bodyplan could actually allow it to dig better or faster, especially since no animals as big as a bear are live digging under the earth, and those that do, even creatures that had more than 4 limbs, seemed to only dedicate one paid for digging, looking roughly like the following sketch to better clarify what I mean.
With that said and after this apologetically large amount of images, would such a creature actually reap meaningful benefits from an extra pair of burrowing limbs to move faster through dirt and rock? As of now, I can't quite tell if simply adding extra limbs could benefit its digging speed, or whether it would even be worth it, as I lack good natural examples.
