The Broad Street Quay Wall project has been hailed as the new gateway to a reimagined navy yard in Philadelphia.
It is an engineering feat years in the making. When completed, it will include two vehicle lanes in each direction, a two-way elevated bike lane and a scenic pedestrian plaza overlooking the Reserve Basin.
James J. Anderson Construction (JJA) is tackling the rectangular 214-metre-long jobsite. Located just inside the main gate of the navy yard, towering ships anchored in the square-shaped pocket of the Delaware River dwarf most of the construction equipment on site.
The navy yard operated as a United States naval base from 1876 to 1996 so it stands to reason that the underground conditions are as diverse as its history.
From an engineering standpoint, this project was necessary to abate the subsidence of Broad Street due to hydraulic erosion.
“The old seawall was cracked and leaking, allowing water underneath Broad Street,” said JJA’s Chief Engineer Bob Crawford. “This area undergoes a six-foot tide, so the water goes up and down in the Reserve Basin six feet twice each day. The tidal ebb and flow getting through the seawall was undermining the roadway to the extent that they had to shut down a portion of Broad Street.”
The joints in the existing cast-in-place concrete wall – located 6 metres apart – had widened over time. Every time the tide rolled out, small amounts of soil were removed. JJA has been at work on the project since June 2022.
“There was an existing concrete seawall sitting on a timber deck,” Crawford said. “Our overall scope was first to drive a line of NZ 38 sheet piles to create a new 700-foot steel seawall behind the existing concrete wall, then remove the old seawall and timber deck on which it sat, and drive 12-inch epoxy-coated piles in front to support a new concrete pedestrian walkway.”
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The new steel seawall had to be anchored to a concrete tieback sitting on batter piles.
The tiebacks sit 15 metres behind the seawall and 7.5 cm diameter galvanized tendons go from the steel seawall back to the concrete tieback.
“Once that system and the utilities are all in, we reconstruct the Broad Street roadway, new sidewalk, bike path and pedestrian plaza,” Crawford said.
To construct the concrete tieback anchor, JJA drove 158 pipe piles designed to be 24-metres long and driven on a 2V:1H batter.
During test pile installation, it was determined there were locations on the project where the tieback piles would need to be spliced and driven more than 30 metres to achieve design capacity due to changing soil conditions.
“None of the pipe piles on the project are seated in rock,” Crawford said.
The tension piles derive their capacity from skin friction with only a minor contribution from end bearing for the compression piles.
“We’ve seldom done friction piles so this was an experience to go through the test pile program and all the dynamic analysis iterations with Urban Engineers to determine that the production piles will attain the design capacity,” Crawford said.
JJA’s first Junttan rig
Since previous work was performed with a crane-mounted hammer, JJA was a first-time customer for Junttan USA. Junttan USA rented JJA a PMx28 pile driving rig with a HHK5S hydraulic impact hammer to drive the pipe piles , while the contractor simultaneously scheduled a rig from another vendor to vibrate in the sheet piles.
When that rig became unavailable due to an emergency project, Junttan USA’s Sales Engineer Matt Eastburn saw an opportunity to expand the role of its rig.
Help from Junttan Canada
Junttan USA had never rented a rig with a vibro package and leaned on its partners in Canada to supply the equipment to be installed on the PMx28. The cross-continental partnership paid off. “For the sheet piles, we had the rig equipped with a PTC variable moment hammer, so we had to mount the power pack on the back,” Eastburn said. “We took off the counterweight and essentially used the power pack in its place.”
Junttan USA had Junttan Canada ship the vibro and power pack to the Navy Yard where the PMx28 was waiting. JJA provided crane support on the assembly. The transition between vibratory and standard pile driving was seamless.
When the time came to drive the 40-cm and 46-cm-diameter piles, the vibro components were removed, and the counterweight and HHK5S hammer were re-installed. Aside from a small portion where a Philadelphia Water Department culvert pokes through the seawall, all pile driving was ultimately completed using the Junttan PMx28.
“We did all the engineering in house,” Eastburn said. “Junttan USA installed the vibro kit, which was supplied by Junttan Canada. They had retrofitted it to a PMx28 they had in stock.”
The most challenging part of the engineering was the stability analysis to ensure that the rig wasn’t going to tip over while holding the 2V:1H batter and the 3,630 kg pile and 10,660 kg hammer.
“With the elimination of a portion of the counterweight, we were able to lay the leader back with a full-length pile,” Eastburn said.
The key issue for JJA was finding an unconventional way to drive the 24-metre-long piles on an extreme batter that was not labour or material intensive. Although JJA has experience installing batter piles of lesser degrees, 26 degrees was a batter it had never encountered.
This angle was necessary due to the heavy loads such as large trucks and transports that will enter the navy yard. As the live loads will exert high lateral pressure on the new steel sheet pile seawall, they had to be counteracted by the batter piles in the tieback anchor.
“We’ve done batter piles before, but not quite this extreme,” Crawford said. “At this angle, we would have needed significant anchored false work to maintain the pile angle at such an extreme batter.”
As well, Crawford believes its conventional equipment would have faced other challenges in this application.
“We’ve had problems in the past with diesel hammers on significant batters. Diesel hammers tend to misfire when tilted. We don’t have that issue with the Junttan hydraulic hammer.”
Training supports production
Adapting to a new pile driving machine is no easy task for an operator. JJA’s operator embraced the challenge and the pile driving productivity reflected that.
The operator was accustomed to operating JJA’s standard setup – a crane with diesel hammer. Junttan USA sent in a tech and provided on-site training, which included an in-depth assessment of the rig’s components and how to operate it.
Since JJA had never used a Junttan rig in the past, production was a serious concern, especially considering the piles were to be driven with a 26-degree batter.
JJA was ultimately able to meet or exceed estimated production for the installation of the sheeting as well as the batter pipe piles.
Although Crawford acknowledges the PMx28’s ability to hoist and drive piles without falsework was the main factor for choosing Junttan, the operator’s ability was also factor.
“There’s a learning curve, of course. But once it has been overcome, production reached where it needs to be,” Crawford said.
The narrow site also made using a crane-mounted diesel hammer with leads and bottom falsework prohibitive.
“The crane would have to be positioned to control the top end of the leads where a majority of the pre-driving weight is located,” Crawford said.
“This would then require falsework at the bottom of the pile to maintain orientation and position. After completing one pile, we’d have to move the entire set up ahead every time. With the Junttan, it’s self-contained. It holds the pile and moves it ahead without expending time to disassemble and reset falsework.”
The sheet piles were driven straight and accurately as well also without the need for alignment falsework. At the completion of driving the steel sheet pile seawall, JJA had to connect a double channel waler to the back of the sheet pile to accept the tieback tendon connections. The adjustments needed to attain proper bearing for the waler attachment were all less than one inch.
Eastburn added having a single rig offered JJA easier setup, better site access, and less equipment to maintain.
“Our PMx28 setup is transported in three loads, one lowboy for the rig itself and two falloff loads on standard flatbeds for the hammer and counterweights. It requires a little bit of machine support to assemble, but once it’s set up, you can track back and forth to each pile on site,” Eastburn said.
JJA finished its pile driving work with the Junttan setup in July 2023, but continues moving toward the timely completion of the project in December.
