Glenbrook Cables Project: Geotechnical Evaluation of Underground Conduit and Trenchless Crossings

Northeast Utilities is currently upgrading their electric power transmission capability in Connecticut and Massachusetts. GEI was engaged by Burns & McDonnell to provide geotechnical engineering services along a new 8.7-mile-long section of 115 kV conduit, referred to as the Glenbrook Cables Project (GCP).

The conduit will start at the Glenbrook Substation in Stamford, extend through Darien, and end at the Norwalk Substation in Norwalk.

The majority of the conduit will follow existing roadways, and will be constructed using open cut excavation techniques. The conduit will generally consist of a 39 inch square concrete duct bank, with 3 to 10 feet of cover over the top. Vaults will be placed along the alignment for access to the cables, spaced at 1,500- to 1,800-foot intervals.

Trenchless installation methods are being considered where the alignment crosses the Metro North Railroad (MNRR) in Darien, the Five Mile River in Norwalk, and the Norwalk River in Norwalk. GEI evaluated the feasibility of trenchless crossings at these locations as part of a separate work scope.

GEI conducted the following work for the project:

• Engaged a test boring subcontractor to obtain permits, clear utilities, drill borings, and sample the soil and rock
• Engaged a geothermal subcontractor to perform in situ geothermal testing in the test borings, and laboratory geothermal testing
• Engaged a geotechnical testing laboratory to perform laboratory tests on soil and rock samples.
• Provided full time monitoring of subsurface explorations, coordinated the subsurface exploration program, observed the explorations, described the soil and rock samples, and surveyed the test boring locations.
• Performed a site reconnaissance to observe site and bedrock features along the project alignment, and indications of possible shallow depths to bedrock.
• Evaluated the subsurface conditions, conducted geotechnical engineering evaluations, and prepared a report summarizing our recommendations.
• Prepared a separate geotechnical report that contains our evaluations and recommendations for the trenchless crossing