土 (earth) is the indexing radical. 圾 appears almost exclusively in 垃圾 (lājī, rubbish) - the soil radical fits well, since rubbish is piled on the ground and eventually mixes back into the dirt. 垃 (lā, the partner half) also takes the same radical.
及 (jí) supplies the sound, unchanged. A pure phonetic borrowing with no semantic role. Same three-stroke phonetic drives a tidy family: 级 (level), 极 (extreme), 急 (urgent), 吸 (inhale), 汲 (draw water).
