Backfilling Trench Excavations: Final Report

In the summer of 1969, seventeen dummy trenches were excavated and backfilled using the original A-6 day loam soil. The backfill was compacted to densities from 95.2 to 103.5 percent of AASHO T-99 maximum density. Surface settlements and settlements at depths of 0.7, 2.0, 4.0 and 6.0 ft were measured over a period of 21 months. The settlements were mostly in the 0.04- to 0.08-ft range. Only four settled less than 1/2 in. Seventy percent of the settlement occurred during the first fall and spring after construction. Observed settlement was dominantly in the upper 2 feet of the 6-ft backfill. There was essentially no movement at the bottom of the trench. It is concluded that settlements of approximately 1/2 to 1 in. must be anticipated for trenches with plastic soils even when compacted in relatively thin layers to relatively high densities.