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BACKGROUND The existing 10-inch sanitary sewer line alignment on Vineyard Avenue was removed and replaced in 2003. A few years after construction, multiple sinkholes were observed along this sewer trench due to settlement from trench backfill. The 15-foot depth of the trench contributed to the problem of settlement of the backfill. The original sinkholes were repaired by open trench in 2006. Additional sinkholes were repaired through compaction grouting in 2008 and 2012. Sinkholes continued to develop on the remaining unrepaired areas along the sewer line alignment. On December 18, 2018, City Council awarded the contract to the lowest responsible bidder, The Pressure Grout Company, in the amount of $155,700 to stabilize the remaining segments of the sewer trench. The project consisted of stabilizing the unstable sewer trench not completed previously along Vineyard Avenue, between Sauterne Way and Clara Lane, by a compaction grouting method. The compaction grouting not only repaired the ongoing roadway settlement and sink holes along the existing sewer trench, but also provides a viable long-term solution to avoid potential future settlements. The work was done as part of the preparation work in advance of the scheduled slurry seal placement this fall. DISCUSSION Approximately 1 ,074 linear feet of sewer trench was stabilized by the injection of 3,506 cubic feet of a stiff, mortar-like grout under pressure into the loose trench backfilled material between 3 inches and 16 inches underneath the roadway surface. The project has been completed to the City's satisfaction and staff recommends the acceptance of the work performed by The Pressure Grout Company. Budget The final construction contract cost, including contract change orders, was $130,464. The following change orders resulted in a net decrease of $25,236 to the contract amount: 1 . An increase of $4,450 for extra work performed by Contractor to drill through the unforeseen layer of slurry/concrete beneath the roadway surface at 30 various injection points. 2. A decrease of $29,686 in reduced contract quantities of actual grout installed. Only 3,506 cubic feet of grout was utilized to stabilize the trench as compared to the estimated volume of 5,300 cubic feet of grout. The reduction was due to the elimination of an injection area adjacent to the existing utilities, including a PG&E 230 KV electrical line, and a reduction in depth and lower compaction pressure to avoid damages to existing pipelines within the compaction zone. Page 2 of 3