The SHARON® process, developed in Europe, is a state-of-the-art system of removing high concentrations of nitrogen from wastewater streams. Construction of the first SHARON denitrification facility in the United States, at the NYC Department of Environmental Protection's Ward's Island Water Pollution Control Plant, required micropile installation to support both the facility itself and an associated Heat Exchanger building. General Contractor, Silverite Construction of Long Island, NY, retained Moretrench to design, install, and load test the 605 rock-socketed micropiles needed for this complex project.
The SHARON site footprint was less than one acre and was situated over an area of filled land connecting the once separate Ward's and Randall's Islands. Subsurface conditions were characterized by loose to soft soils, boulders and cobbles, and a variable rock surface ranging from 28 to 41 ft below planned pile cutoff. An aggressive construction schedule, dictating just a 3-month timeframe for micropile installation, added to the challenges already posed by size constraints and difficult subsurface conditions. To further complicate matters, 56 of the micropiles were constructed in lead-contaminated
soils under HAZMAT conditions.
Innovative Approach - Offsets Site Challenges
The SHARON Facility structure, only 194 feet by 130 feet in plan dimension, required 576 of the 605 total micropiles. The specified design pile capacity of 60 tons in compression and 6 tons in tension was complicated by a 5-ton lateral load requirement, critical for seismic performance, and the particularly weak near-surface conditions.
To meet project specifications, Moretrench's design for the SHARON Facility micropiles consisted of 9.625-inch diameter casing, with a No. 14 Grade 75 reinforcing bar and 5,000 psi neat cement grout. Pile spacing ranged from 4.5 feet to 8.5 feet. Design rock socket length was
5.5 feet. Design was complicated by the lateral load
requirements and the particularly weak near-surface conditions. Welded casing joints were specified for the project, but because of the requirement for N80 grade steel and its physiochemical properties, the field welding process would have introduced significant QA-QC issues. The owner's Engineer, Metcalf and Eddy of New York, NY, agreed to waive the requirement for welded casing joints if the first joint were situated IS feet below the pile cut-off.
Similar issues existed for the Heat Exchanger structure where ultimately 7.625-inch diameter piles were constructed with No. 18 reinforcing bars and a design rock socket length of 7 feet.
Given the site footprint and tight pile spacing, making it infeasible to use more than two to three drill rigs and support crews, Moretrench opted to utilize two high production drilling rigs with long-stroke capabilities.
One of the rigs was capable of installing up to a 45-foot long single piece of high strength pipe casing, but the potential variation in the rock surface was problematic. Pile drilling took place near cutoff, approximately el. +I0, but the rock surface was known to vaty from el. - I8 to el. -3I ,making the minimum casing length 28 to 41 feet. and pushing the limits of the single-stroke drilling rig.
Concentric overburden drilling systems were used to advance the casing thmugh the fills and boulders, and into the rock mass to construct the sockets. The piles were grouted 1n batches using a high production colloidal shear mixer and 30-ton cement silo.
The General Contractor's 3-month timeframe for pile completion had not included conducting a planned total of 30 compression, tension, and lateral load tests, an unusually high number given the size of the site and pile total. However, Moretrench's selection of high production drilling rigs and construction techniques ensured a pile production rate such that Silverite was able to follow closely behind and construct the reinforced concrete mat foundation in quadrants.
Extensive Testing Verifies - Pile Design & Construction
Metcalf and Eddy used compression, tension, and lateral load testing both before and after production pile installation to assess Moretrench's micropile design and to serve as quality control for the foundation.
Moretrench personnel conducted I0 compression, 8 tension, and 9 lateral load tests on production piles scattered throughout the site. The compression and lateral load tests provided insight into the consistencies in the soil and rock behavior. All of the compression load tests showed very similar axial behaviors with similar total vertical settlements and almost complete rebound indicating elastic responses. The lateral load tests, conducted under fixed and free-head conditions, exhibited much more variability and pointed towards the widely varying subsurface conditions within the upper I 0 to 20 feet across the site.
Proactive Work Sequence - Ensures On-track Production Schedule
The proactive approach taken by Moretrench in resolving schedule, site constraints and difficult subsurface conditions is characteristic of the company's commitment to providing every client with superior engineering and constr-uction service.
In this instance, the construction techniques used allowed micropile installation, including grouting and pile cutoffs, to be completed in 20 production weeks. While this was longer than the originally planned 3-month micropile schedule, adroit scheduling and sequencing by Moretrench was instrumental in allowing the general contractor to maintain its intended rate of foundation construction.