Have you checked the example page of Spineboy? You can see the details of the Spineboy’s feet setup here, it is perfect for aligning feet on the top of ground:
Spineboy example: Legs
The foot tip angle must be properly animated, but the Spineboy’s feet basically does not go through the ground unless the front-foot-target bone and rear-foot-target bone are moved below the ground (below the X-axis line) even if you translate the hip bone in a large vertical direction. Here is a briefly modified version of Spineboy's walk animation to make it easier to understand the IK constraints' effect:
The bones highlighted in light blue are front-foot-target and rear-foot-target.
I hope this will help you.
