Where Urban Layout and Geotechnical Investigation Meet: ADT’s Role in NYC’s Green Infrastructure Plan

Water is essential in every environment. But when it comes to stormwater, there is a lot of concern surrounding the risk of pollution within urban environments, and disruption of the infrastructure designed to move water. As the rain moves across impervious surfaces like roads, sidewalks and parking lots, it picks up sediment, debris, nutrients, bacteria, and dangerous contaminants. Without a comprehensive program to proactively manage stormwater, these pollutants make their way into natural waterways, posing harm to human health and the environment.

In 2010, the city of New York released the NYC Green Infrastructure (GI) Plan - an alternative approach to improving water quality by integrating “green infrastructure,” (namely rain gardens, green roofs, and right-of-way bioswales) within the city’s existing urban layout. This program represents a multi-agency effort led by the Department of Environmental Protection (DEP). Within the GI Plan, DEP and agency partners design, construct and maintain a variety of source-control GI assets, including green roofs, rain gardens and bioswales. The goal of the program is to collect at least 1 inch of stormwater runoff from the street gutter, sidewalk and roadway and infiltrate it into the surrounding permeable subsoil.

Background

New York City is largely serviced by a combined sewer system wherein stormwater and wastewater are carried through the same pipes. Although wastewater treatment plants (WWTPs) are designed to treat and disinfect twice the dry weather flow, the system is at risk of exceeding its capacity during heavy storm events.

Streets and sidewalks - two top contributors to stormwater runoff - make up approximately 27% of land. Therein lies a significant opportunity to proactively manage stormwater using source controls. Source controls are systems designed to detain or retain storm water at the source rather than at the end of pipe. The plan required the design and installation of approximately 5,000 green infrastructure assets, namely source controls known as bioswales in the right-of-way.

Project Scope and Investigation

Geotechnical drilling and environmental sampling are required to determine soil characteristics, permeability rates, and depths to groundwater table and bedrock when encountered prior to the construction of each GI asset. The data gathered during this investigation is used to determine suitability for the bioswale installation.

REQUIREMENTS

The investigation requires the drilling of a minimum of one soil boring and administering of at least one percolation test for each potential bioswale location throughout the five boroughs.

• For permeable pavement in the right-of-way, borings and percolation tests are required at every street segment near the downstream end.
• For areas with permeable pavement planned for both sides of a street, the borings and percolation tests are to be staggered on alternate sides of the street.
• The specifications call for use of a 4 inch inner diameter hollow-stem auger or a 4 inch inner diameter drilled casing for soil borings. A rotary tricone roller bit with clean water as drilling fluid is also acceptable under certain conditions.
• Mud drilling is strictly prohibited.
• Each soil boring location requires a standard penetration test conducted continuously in accordance with ASTM D1586 (i.e. a 24in-long, 2in-outside diameter split-barrel sampler driven by blows from a 140lb/64kg hammer falling freely from a height of 30in) to the depth of 20ft bgs.

AQUIFER DRILLING & TESTING

In 2014, Aquifer Drilling & Testing (ADT), a Cascade subsidiary, was awarded a significant portion of the geotechnical drilling within the NYC GI plan. Soon thereafter they were faced with the express challenge of accommodating the intense resource commitment. The expansive scope of services and lengthy project timeframe for the NYC GI plan presented two significant challenges:

1. Long-term commitment of fleet assets and personnel

Based on initial estimates, the company expected to commit 8-14 crews to the project over a minimum of three years.

2. Access limitations in an urban work environment

Working throughout the five boroughs of New York City means planning for and dealing with obstructions such as parked vehicles, as well as very small work sites, often plots no larger than 5ft (1.5m) wide by 15ft long.

With a workforce of 66 field personnel, and a local fleet of 45 rigs and support equipment, the team assessed the opportunity to acquire additional resources, as well as take advantage of the expansive resources available throughout Cascade Environmental.

RESOURCES:

• Each ADT crew on the GI project consists of one lead driller, one driller assistant and one drill rig.
• ADT quickly assembled a base of 14 dedicated local crews.
• Four new direct push drill rigs were purchased and dedicated to the project.
• Additional personnel and equipment are mobilized to accommodate peak periods.

ACCESS LIMITATIONS

The urban landscape within New york City presents unique access challenges for geotechnical drilling and subsurface investigation activities - namely obstructions by parked cars, trees and narrow sidewalks.

Meeting the required drilling and testing specifications while navigating the small work sites proved to be quite a challenge at first. To overcome this, ADT modified the Geoprobe 6620DT and 7822DT direct push drill rigs with auto hammers and mud swivels, allowing the skilled crews to drill with 4 inch casings and providing easier, faster mobility around each investigation site. These modified direct push  enabled ADT to meet the demanding schedule and location requirements of the project.

With some skillful ingenuity, ADT is now playing a significant role in the Plan’s bioswale site-investigation activities, consistently meeting the arduous demands of the NYC GI Plan implementation.

Results

According to the city’s most recent Green Infrastructure Performance Metrics Report, 4,470 assets have been constructed, were in construction or were in design as of February 2016. Approximately 90% of these are right-of-way bioswales. The city expects to continue adding green infrastructure assets through to 2020. ADT successfully completed over 4,000 locations and expects the work to continue throughout 2017.