Unshrinkable Fill for Utility Cut Restorations


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Title: Unshrinkable Fill for Utility Cut Restorations

Author(s): J. Emery and T Johnston

Publication: Special Publication

Volume: 93


Appears on pages(s): 187-212

Keywords: aggregates; backfilling; compacting; costs; impact; pavements; performance; repairs; specifications; strength

Date: 9/1/1986

Utility cut restoration problems (settlements, voids, cracks and protracted maintenance) have a significant negative impact on pavement serviceability. A recent Metropolitan Toronto study quantified this impact and identified unshrinkable fill (very weak concrete) as the preferred solution. In addition to repair costs, road user costs and nuisance effects, substandard utility cut restorations increase the Metro maintenance surfacing costs by some $3 million annually. Pavement impairment was found to be a function of the large number of utility cuts (about 4000 per year) and significant non-compliance with specifications covering granu-lar backfill quality and compaction. While tighter granular spe-cifications enforcement was considered, the characteristics of controlled density backfill were assessed to provide a materials approach to improved restorations. The desired controlled density backfill characteristics were: reasonable cost; flowability; pavement structural support; excavatability; wide availability; standard materials without need for inspection; satisfactory long-term performance; and demonstrated use. Based on long-term Winnipeg experience and utility comments, unshrinkable fill (25 kg cement per m3, conventional concrete aggregates, 160 to 200 mm slump, usually air entrained, maximum 28 days strength of 0.4 MPa) was subjected to extensive field trials and cost analyses. It was found that unshrinkable backfill meets the technical requirements and is generally cost effective compared to properly compacted and inspected granular backfill. Unshrinkable fill from qualified suppliers is now required for all Metro utility cut restorations.