Rammed Aggregate Piers® Support Rail System

By Chris Ourand | September 28, 2010

It's today's version of the classic word problem/trick question: Two trains of differing lengths leave the Midwest for New Jersey loaded with ethanol. Which one gets unloaded first?

If the year was 2004, it would be the short train because the railway offloading facility in Sewaren, New Jersey, could only handle trains with 23 tank cars. Today, the terminal accommodates 100-car trains where 2 million gallons of the fuel are off-loaded daily. The increased capacity is going a long way toward meeting demand in the New York metropolitan market because of the commodity's increasing importance as a partial substitute and additive for gasoline in the face of high crude oil costs.

However, the terminal's expansion didn't come without its own costs and construction challenges.

Terminal owner Motiva Enterprises, LLC started the project in 2004 with the addition of two tracks that could accommodate a total of 40 tank cars. Furthermore, with the 28,000 gallons of liquid in each car weighing more than 200,000 pounds — plus the cars themselves and the thousands of square yards of concrete slab on which the tracks sit — there is a lot of weight to support. The trouble there was that the team of geotechnical engineers from French & Parrello found that the ground was comprised of silty soil with layers of peat that were prone to settlement. These conditions could lead to uneven settlement of the rails or worse, a derailment and spill of a highly volatile substance.

Reinforcing the soil fell to design-build contractor GeoStructures, a specialist in ground improvement that recommended installation of the Rammed Aggregate Pier® (RAP) system. Also known as the Impact® Pier system, RAPs were well suited because their fast installation fit the condensed construction schedule.

"More trains were scheduled to arrive within weeks and the 24-hour-a-day facility needed to accommodate them," says Michael Cowell, P.E., president of GeoStructures.

Impact Pier elements are built by pushing into the loose ground a mandrel and tamper foot that has a sacrificial cap to prevent soil from entering the mandrel. At the designated depth, open-graded aggregate is poured through the mandrel, which is then lifted so that the sacrificial cap is left in place at the bottom. The tamper foot is raised and dropped to ram the stone downward and outward, creating a dense pier. Particularly appropriate in soft soils or where drilling and excavation may cause cave-ins, Impact Pier elements allow soil to remain in place, which has the financial and environmental benefit of not requiring soil excavation and disposal.

"Impact Pier elements carry most of the load, much like the springs in a bed," adds Cowell. "They raise bearing pressures up to 10,000 pounds psf to provide reliable settlement control."

Because of the potential for ethanol or petroleum spills and soil contamination, and because of the need for additional stiffness in the sandy soil, GeoStructures took the extra step of using a patented technique for "grouting" the RAPs. By injecting cement grout into the 192 elements as they were constructed, it could span through the peat layers where bulging might have occurred and reduced the permeability of the elements so they would not form a conduit if any liquids penetrated the soil. For some projects, the environmental benefit of keeping soil in place and not trucking it away also offers enhancement points for LEED credits, The Leadership in Energy and Environmental Design that is part of the Green Building Rating System.

As the terminal's expansion continued, Motiva Enterprises focused on how it could continue to eliminate bottlenecks and improve the transit and turnaround times. Ethanol tank cars are sometimes coupled with other freight cars, and they had to be uncoupled outside the terminal before they can enter and unload. To avoid this congestion the company planned to handle ethanol-only trains of tank cars at the terminal, so it made room by tearing down a building that housed a former oil processing plant.

To complicate the pier installation, the former building was supported by piles and pile caps that did not match the footprint of the new terminal and rail yard, so they did not offer the support necessary. More Impact Piers were needed, but the existing piles made for a tricky installation process. Slight adjustments were made where the 1,218 RAPs were installed, which kept the project on schedule. The piers reinforced the ground enough to support an extra 60 tank cars and four more tracks, along with the concrete containment systems. Motiva, a refining and marketing joint venture between Shell Oil and Saudi Aramco, also built an underground pipeline from the terminal to the storage tanks, which eliminated the disruption of closing down a city street when trains arrived and had to reach the terminal.

"There are many factors involved when determining how to stabilize the ground," concludes Cowell. "Certainly there is the budget and timeline, but the site's geology and minimizing environmental risks are essential so there is no settlement, no harm to the environment, but also so there is no costly over-engineering."

Editor's note: Chris Ourand ( www.chrisocom.com ) is a Maryland-based writer who frequently writes about construction and related industries.