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Construction in the City – First Do No Harm

Monitoring equipment around the light rail expansion constantly records data.

The advice of ancient Greek physician Hippocrates rings true when it comes to heavy construction in a densely populated city: prioritize the protection of the valuable, existing assets and reducing the impact on nearby residents and businesses. Terracon is doing our part to achieve that noble goal as we assist in the construction of the METRO Transit Light Rail connecting Minneapolis and St. Paul to the airport and well-known Mall of America. With more than 80 million total rides in 2018, this expansion of the light rail line is an essential improvement, but in a precarious, constructed environment. While the impacts of COVID-19 are unknown, ridership is projected to rise in the future as the METRO Light Rail grows for the third time since it opened in 2004 as part of the Southwest Light Rail Transit expansion.

Wiring monitoring instruments.

This latest expansion, scheduled for 2025, stretches approximately 13.5 miles from Eden Prairie, Minnesota, to downtown Minneapolis. Our Minneapolis/St. Paul office is helping bring the Southwest expansion to life by providing noise and vibration monitoring and geotechnical monitoring services.

This marks the first time Terracon has contributed to a METRO Light Rail project in Minnesota.  Monitoring services are scheduled to continue during construction activities. The data produced – every 30 seconds, in some cases – will be used to safely help make the expansion a reality.

Gathering Data Around the Clock

The Southwest expansion alignment is threaded through developed property, making construction challenging. Common practices could damage or adversely affect existing structures near the new rail line. Earthwork and pile driving activities require constant vibration and noise monitoring to help protect the structures, freight line, and health and safety of people living and working near the light rail the extension.

Terracon initially performed predictive modeling calculations at more than 90 locations along the alignment to estimate noise and vibration levels expected from the planned construction activities. The model guided the actual field monitoring requirements, and, in some cases, was able to show that noise and vibration would be minimal.

But the key to confirm success of such efforts is detailed instrumentation. Our professionals have already installed 50 instrumentation units to continuously monitor noise and vibration. This instrumentation is proactive. All the noise and vibration monitoring systems are tracked with web-based software systems, sending their information to the cloud for real-time awareness by the owner, contractor, stakeholders, and other parties.

Instrumentation Helps Mitigate Risks

Much like a fire detector is meant to make the occupant immediately aware of such danger, this instrumentation discovers any exceedance of project specifications triggers and alerts the contractor and Terracon team in real time so work can be modified to mitigate the potential cause(s). The approach to vibration is particularly important for this project, because some of the structures along the alignment are historical landmarks.

In addition to noise and vibration monitoring, a variety of geotechnical equipment, including ground monitoring points, utility monitoring points, crack monitors, tilt meters, inclinometers, vibrating wire piezometers, rail track monitoring systems, and Automated Total Stations (AMTS) are being used to monitor structures along the Kenilworth Corridor tunnel.

Light rail construction was nearby many existing structures.

In this corridor, plans call for the rail line to go through a cut-and-cover tunnel approximately 55 feet deep. Construction for the tunnel is as close as two feet to existing structures and within seven feet from the centerline of an active freight rail line.

Our team has worked closely with the client to develop proper parameters for the alarms based on the work being performed. This has helped the client gain a better understanding of potential concerns, and when they should be addressed.

The Southwest Transit Light Rail expansion will be a great benefit to the growth of the Minneapolis-St. Paul area. Terracon’s team of industrial hygiene professionals and geotechnical engineers have combined to provide both health and engineering solutions. We are the sentinels to the ambitious construction effort, and we are taking the advice of Hippocrates to “first, do no harm” very seriously.

Luke Hanson, P.E., is a senior staff engineer in Terracon’s Minneapolis office and is dedicated full time to serving the METRO Light Rail project. He has six years of experience in geotechnical engineering and began his Terracon career in Terracon’s DC Metro West office.

Rabi Yadava, Ph.D., P.E., is a senior associate in Terracon’s DC North office and provides professional services in field of geodesign, structural engineering, geotechnical instrumentation, and noise and vibration monitoring for large projects nationwide. He has 30 years of experience in engineering design and construction support.