ECI – Engineers Construction

ECI’s Clients: Vermont Electric Power Company (VELCO) & Green Mountain Power (GMP)

Completion Date:         Summer 2010

Subcontractors: Maine Drilling & Blasting

The project was part of the Gorge Area Reinforcement (GAR) which will increase the power reliability to northern Chittenden County.   Power reliability is essential to economic vitality of the region.  ECI was the civil contractor for these two substations which included extensive earthwork and rock excavation, a pair of large retaining walls, over 250 individual foundations, miles of conduit and copper grounding.  The VELCO substation is a 115 kV three breaker ring bus with a 115/34.5 kV transformer.  The GMP substation is a 34.5 kV five breaker ring bus substation.  The two subs are connected by a concrete encased transmission line.

Sensitivity to the Environmental Surroundings

The project is located on a steep face of the upper bank of the Winooski River in South Burlington.  A variety of erosion protection, sediment control, and dust control measures were applied to protect the river.  Vibrations from blasting were monitored and limited to protect adjacent critical structures, such as the GMP Dam #18.   ECI took additional measures on the project to reuse excavated soil and rock into the final work.

Meeting the Challenge of a Difficult Job

A variety of issues made for a very challenging Project, including:

• The site is a narrow piece of land (800-ft-long by 240-ft-wide) located on a steep slope adjacent to the Winooski River.  The elevation change across the narrow dimension is 70 feet.
• A compressed construction schedule that required work throughout the winter months.
• Over 20,000 cy of rock excavation with blasting in close proximity to:
  • Under energized 115kV transmission lines serving Chittenden County, sometimes as the sole feed.   All blasts were heavily matted and designed to limit flyrock.
  • Within the BTV flight path.  All blasting was coordinated with BTV Air Traffic Control that restricted aircraft over the site during the blast.
  • Adjacent to the GMP Dam #18
  • As close as 8 feet from an AT&T transcontinental fiber line with a blasted face of 27 vertical feet
  • Adjacent to the New England Central Railroad and two of their major bridges over the Winooski River.
  • Near several neighboring commercial buildings.
  • Foundation design of the 200- and 750-ft-long Redi-Rock retaining walls evolved after starting the project because of unexpected changes to the rock profile.
  • The block retaining wall needed special field fitting to allow for the 1¼ inch of setback per block course, which over the 18 courses makes for significant field adjustments to the blocks.  This issue is further compounded at the 90 degree corners.
  • 1066 LF of reinforced electrical duct bank between the two substations.

Innovation in Construction Techniques, Materials, or Scheduling

A major component of the project was the Redi-Rock (precast block) retaining wall that consisted of over 2,700 block units with a face area of 15,600 square feet.  The wall was installed on a 700 cubic yard reinforced concrete footing that was anchored to bedrock with passive dowels.  The innovation was in the installation.  Most contractors use an excavator or loader to set block walls.  This standard process usually requires re-handling of the blocks from delivery to setting.  ECI used its TCC450 telescopic crawler crane to set the blocks.  The advantage was the crane could set blocks while an earthwork crew backfilled and installed geogrid without interrupting the other operation.  Furthermore, the crane allowed the blocks to be set directly from the delivery truck.  The result was that ECI was able to stage the work to accept and place up to 9 trailer loads of blocks per day – record delivery rate for Carroll Concrete, the block fabricator.

AGC of Vermont Award:

ECI won the 2010 Best Builder Award for Heavy Industrial for this project.