Schematic illustration of pebble drift and its pile-up within a protoplanetary disk with a dead zone. The disk gas accretion is characterized by α-parameter a_acc, while the midplane diffusivity, being a dead zone (α_dead ≪ α_acc) in the inner region, is characterized by α_mid. During the inward drift of pebbles, the pebble scale height H_p decreases as until a KH instability prevents it from becoming smaller. A smaller H_p leads to an elevated local midplane concentration of pebbles within a thinner midplane layer. The elevated midplane pebble-to-gas ratio causes the back-reaction to be more effective in reducing the radial drift velocity of pebbles. Such a physical interplay with a sufficiently large pebble-to-gas mass flux results in a progressive accumulation of pebbles in a runaway fashion (i.e., the “no-drift” runaway pile-up).