Step 7. Backfill Trench.
Fill the trench with granular soil. As you fill in the trench, compact the soil. (As a last resort, backfill may come from gravel in creek beds.)
Until the shelter is supported by backfill on both sides, it lacks strength. At this point, if a dump truck drops a full load on one side of the shelter it might collapse the shelter. If the backfill is delivered by dump truck, dump the load at the edge of the trench and fill the trench by hand using the shovel. As you build up the backfill support on both sides, the requirement for hand filling becomes less critical.
As you fill the trench, compact the soil. In order to achieve the earth arching effect, the soil must be compacted to the 95% Proctor level. Compacting can be done using a whacker tamper. A quick test of this level is to slam your heel into the compacted earth. If it barely leaves an imprint, you are at the 95% compacted level.