Project Location: Town of Mount Holly in Rutland County on Vermont Route 155 over Mill River. Bridge 7 is located approximately 5.7 miles northeast of the intersection with VT-100, at approximately mile marker 3.398.

The Mount Holly Bridge 7 project includes the replacement of the existing multi-plate steel pipe arch culvert. The existing structure has a clear width of 15’ – 4” and a clear height of 9’ – 3”, providing a waterway opening of 109 square feet. The structure currently has a rating of 4, poor and has cracking along the bolt line. This culvert was installed in 1969 and is 104 feet in length. There are signs that the outlet end of the culvert is deforming and settling due to perforations in the invert, leading to scour and undermining beneath the structure.

VTrans evaluated alternatives for rehabilitation or replacement of the culvert in an engineering study completed in April, 2017. This study assessed the proposed design criteria for the structure, right-of-way, rare/threatened/endangered species impacts, hydraulics, archaeological resources and wetland impacts. Several alternatives were considered, including: no action, rehabilitation, replacement using trenchless technology and replacement using an open cut. Finally, complete replacement was considered using a short term road closure with a detour, temporary bridge, or constructing the new culvert in phases while maintaining traffic through the work zone.

Given the age of the structure, site constraints and current conditions, the engineering study recommended replacement of the culvert with an open bottom concrete arch or frame while maintaining traffic on an official signed detour. Closure duration will be approximately four weeks in which the detour will add approximately 8.1 miles to the thru route. There is also a local bypass route, adding 1.2 miles of travel for thru traffic.

The existing roadway meets most of the geometric provisions of the Vermont State Standards. The horizontal alignment is the only known roadway deficiency within the project area. As this is not a high crash location, no changes to the roadway are proposed for this project.