Agenda - Public Works Committee - 02/15/2011

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Public Works Committee Date: 02/15/2011 By: Tim Himmer Engineering/Public Works Item #: 5. 3. Information Title: Consider Change Order for City Improvement Project #10-32; 176th Avenue Culvert Replacement Background: In the summer of 2010, staff noticed that a section of the roadway along 176th Avenue NW, between Okapi and Marmoset Streets, had been settling and eroding over time. Upon further investigation it was discovered that the culvert under that roadway, which serves as the drainage way for County Ditch #51, had deteriorated and was showing signs of failure. The roadway subgrade from above was falling through holes in the culvert causing roadway failures and it appeared as if the pipe had been settling. It was decided to address the problem prior to the end of the year to avoid further damage. Staff solicited quotes from 3 contractors to perform a culvert replacement project, which work entailed: • Mobilization, traffic control, and erosion control • Ditch by-pass construction and dewatering • Removal and replacement of a 42" culvert • Muck excavation • Installation of flared end sections and rip -rap placement for armoring • Removal and reinstallation of the bituminous roadway Council authorized award of contract to Dryden Excavating, Inc. on November 9, 2010, and work commenced immediately thereafter as it was late in the season and we were hopeful that it would be possible to complete the roadway paving before the end of the construction season. Notification: Observations: Staff conducted soil borings at each end of the culvert and, based upon the results, it was assumed that there would need to be a 6' subgrade correction under the length of the pipe, resulting in approximately 100 cubic yards (CY) of muck excavation and granular soil import. When the work was being conducted it was discovered that additional removal and import would be required, as the muck under the roadway extended to an average depth of 15' (see attached photos). It was the recommendation of our geotechnical consultant to remove all substandard material from underneath the culvert and roadway to avoid a similar settling issue in the future. This additional excavation and import resulted in the need for approximately 800 CY of removal and import, causing a contract overage in the amount of $20,395.56. Staff investigated the option of reusing some of the soils within the excavation backfill but the material was too wet and it wasn't worth the effort to attempt to dry the soils during that time of year, or risk the integrity of the road. Staff was able to, however, reuse some of the non-structural soils outside of the right-of-way for restoration activities in an effort to lessen the financial impact of additional muck export. In analyzing this project staff could have performed additional borings along the length of the culvert, but that would have included more cost on the front end (approximately $750 per boring) and may not have shown the entire soil impacts as the borings would have had to take place on either side of the pipe and not capture what was actually under it. It was decided to forego additional borings, with the idea that if additional poor soils were discovered that money would be spent on addressing the issue rather than the front end cost of the investigation. Funding Source: