Governors Island Park and Public Space Project Geotechnical Instrumentation

Challenge

In 2002, the northern-most 22 acres of Governor’s Island in New York City had been declared a National Monument while the 150-acre southern portion was sold to the City of New York. The City’s development plans include this Governors Island Park and Public Space, including construction of natural features and hills using fill soils barged to the island. The earthworks contractor employed by the city needed to be able to track multiple geotechnical data points to ensure quality of work and safety.

Solution

GZA provided geotechnical instrumentation, consisting of settlement plates and vibrating wire piezometers, to measure pore water pressure dissipation. Prior to earthwork activities, GZA oversaw installation of 20 settlement plates, approximately one plate for every 5,000 square feet of fill, which were monitored prior to construction to establish a baseline elevation. As the contractor added fill, additional riser pipes were added to each location to allow for continuous settlement monitoring throughout the fill placement work. Once the fill was placed, the settlement monitoring continued over a period of up to 16 months.

Each riser pipe attached to the settlement plate was fitted with a prism to allow for an Automatic Total Station (AMTS) to measure the coordinates and document differential movement. The prisms were monitored multiple times throughout the day and the location was remotely transmitted via wireless transmission or collected in a data collection center, then posted to the project website. The total measured settlement was plotted daily and available on a project-specific website for the team of engineers, owner and contractor.

Vibrating wire piezometers were installed at two locations in the southeast portion of the first phase of work. Each piezometer had three sensors installed at 30, 60 and 90 foot depth below the ground surface. The groundwater level was measured in terms of pore water pressure with each instrument prior to the placement of fill to establish the baseline. As fill was placed during the earthwork the pore water measurements were collected remotely over time and plotted. This data was also plotted and posted similarly to the riser measurements.

Benefit

The contractor and client were able to gather effective data that informed the project as it unfolded. Remote access via website saved time and made necessary data more widely accessible to project participants. GZA’s instrumentation work won a Platinum Award from the New York chapter of the American Council of Engineering Companies (ACEC) in 2016.