Trench Paving AC subsidence

[jffr602]

Contractor installed 15” VCP sewer line approx 12’ below finished grade. Pipe was shaded with sand, the trench was backfilled with native material (lean clay and sand) to 95%, patched with cold mix AC and reopened to traffic. The area sustained 2 significant rainfalls and daily traffic without showing any effects. Recently the trench was excavated down to sub grade, compacted to 100% and the first 4” lift of AC (1.5”) was placed. The second lift of 1.75” AC (.5”) was to be placed the following day. That night we received 1.5” of rainfall. The trench is adjacent to the gutter line and runoff from the area does travel over this spot. The next day several 6’ diameter sinkholes appeared ranging in depth from .5” to 1.5’. Excavating these areas revealed saturated material of 20-25% moisture (14% optimum) all the way down to 8’. Much of this saturated material was outside the original trench. My concern is the Contractor disturbed a waterline or storm drain although no fines were found on the nearby surfaces. I don’t know where all this material could have gone to and I am worried this area may fail again after the warranty period. The base course AC is fairly open graded and was left 1.5” below the surrounding surfaces but is it possible for that quantity of water could penetrate so far into this material?

 

[cvg]

how about damage to the 15" sewer? could the material have gone into the sewer?

If this (backfill) material is clay and compacted to 100%, how did the water seep 8 feet down? Infiltration rates through clay should be quite low. Was it really 100% compaction? Was it already too wet to compact (ie: it was already at 20% moisture)? Was there lenses of sand mixed in with the clay? Are you sure the voids weren't already there before the rain and the extra moisture simply caused it to settle?

 

[jffr602]

The 100% was limited to the top 1' of subgrade. The rest of the fill was 95% and was at optimum when compacted. When the subgrade was prepped the top 1' was scarified and recompacted. Moisture was not measured below subgrade but it did not pump during construction. I looked in the manholes and did not see any dirt but perhaps we should investigate that more. I agree that the moisture should not seep to that depth. Thanks

 

[RWF7437]

Consider TV inspection of the 15" line.