The Bauer BG 18 H rotary drilling rig works in low overhead configuration to install drilled shafts near the Crum Creek Viaduct west abutment with limited overhead clearance.
A Bauer BG 18 H rotary drilling rig sits wedged between an earthen embankment, a shotcrete-covered bridge abutment, a maze of rusty steel trestles and the underside of a historic railroad viaduct in Swarthmore, Pa., on a glacial January morning. The rig, working in a low overhead configuration, was supported by a Bauer BG 20 H, both of which prime contractor Walsh Construction of Chicago rented from the nearby Aldan, Pa., office of Equipment Corporation of America (ECA).
This high-profile $89.9-million Southeastern Pennsylvania Transportation Authority (SEPTA) project requires the complete replacement of the 925-ft. (282 m) long, 100-ft. (30.5 m) high Crum Creek Viaduct. The structure Walsh is replacing, built in 1895, underwent repairs in 1983 but was in dire need of replacement to safely carry SEPTA's passengers on the Media/Elwyn commuter rail line. The original bridge on this site was a pre-Civil War era, five-span timber arch truss bridge on masonry piers. It was owned by the Philadelphia, Baltimore & Washington Railroad (PW&B RR) and later acquired by the Pennsylvania Railroad.
Walsh began construction on the substructure in summer 2015 and is working aggressively toward an etched in stone deadline of summer 2017. Despite the variety of construction processes involved, drilling the foundations for new footings, piers and abutments was the key to the project.
No Ordinary Site Conditions, No Ordinary Drilling Rigs
Walsh and ECA began discussing the project in March 2015 at The International Foundations Congress & Equipment Exposition (IFCEE). ECA's Aldan Branch Manager Tim Dutton and Engineering Sales Manager Gordian Ulrich walked the site in April with Walsh Superintendent Richie Vance to determine which drilling rigs would work best on this challenging site.
It was ultimately determined that Walsh would rent the BG 18 H and BG 20 H Premium Line drilling rigs because they were light enough to navigate rough terrain with adequate hydraulic output to core through hard rock of up to 25,000 psi. The low headroom capability of the BG 18 H was determined to be optimal for drilling in work areas with height restrictions.
ECA delivered both drilling rigs in June 2015. The BG 20 H, part of the Aldan location's existing rental fleet, worked on site through Christmas. ECA coordinated manufacturing and delivery of the BG 18 H with Bauer Maschinen and imported the rig from Schrobenhausen, Germany. Its ability to work in both low overhead and standard configuration kept the BG 18 H on site until February 2016. ECA brought in an operator from Bauer to assist with training, but all onsite service and reconfiguration of the BG 18 H was handled by its own technicians in Aldan.
These Premium Line rigs offer some additional benefits over Bauer's Value Line that were ideally suited for the Crum Creek Viaduct project. The main difference between the two is that the Premium Line features a crowd cable system with a winch as opposed to the crowd cylinder system on the Value Line. The Premium Line rigs, as a result, deliver more crowd force. They also comply with the latest Tier IV emission standards, and are heavier machines with greater hydraulic output.
Foundation Work Sets the Pace
Walsh is tasked with building a new viaduct beneath the original structure before demolition can commence. The keystone of the project is foundation work for the substructure, which includes two abutments and four sets of piers. Walsh used the BG 18 H and BG 20 H Drilling Rigs to install eight 20- to 58-ft. (6 to 17.6 m) deep, 36-in. (91.4 cm) diameter drilled shafts at each pier and 12 at each abutment.
Vance indicated in January 2016 that drilling was a week and a half behind schedule primarily due to unexpectedly hard rock and environmental permitting delays, but he was confident that Walsh would be back on track shortly. Despite the unforeseen challenges, Vance was satisfied with the drilling production rate.
“Drilling is make or break,” he said, as a SEPTA railcar rattles across the viaduct behind him. “Depending on how that goes, it's almost how the whole job goes.”
Soil Conditions? It's Complicated
Project Geologist Bill Bradfield of Schnabel Engineering's West Chester, Pa. office describes the Crum Creek site as “interesting.”
“We ran the full gamut of drilling conditions within the span of this bridge,” he said, noting that metamorphic rock in this region can be highly variable over short distances.
The site contained primarily Schist, but production was hampered when crews hit Amphibolite. This extremely hard metamorphic rock was prevalent near the western abutment and the Crum Creek channel.
Bradfield recalled watching production drastically improving from as little as 2 ft. (.6 m) per day with an auger to 1 ft. (.3 m) an hour using the Bauer roller bit core barrel, which is recognized as a respectable rate of production in hard rock. Walsh initially tried a cluster drill but had no luck keeping the tool straight. The switch to roller bit core barrels also quieted drilling chatter and reduced the strain on the drilling rigs.
Roller Bit Core Barrel Keeps Production Rolling
Dutton confirmed Bradfield's observations, noting that the production increase was mainly due to the switch from conventional tooling to the Bauer roller bit core barrel.
“With conventional tools they were getting two to three feet per day,” he recalled, pointing to an extracted Amphibolite core. “When the roller bit core barrel was running in optimal conditions they were getting a foot an hour in some really hard rock.”
Conventional drilling tools could not stand alone on this site, according to Dutton.
“This is the beast here that did a lot of the hard rock drilling,” he said, pointing to a Bauer roller bit core barrel. Walsh initially considered down-the-hole and hammer drills, but ultimately decided the core barrel was the right tool for the job.
Walsh purchased 10 tools from ECA, including primarily 36-in. augers, core barrels, drilling buckets, and roller bit core barrels. There were a few 42-in. (106 cm) tools for overburden areas where the rock was not immediately below the surface.
Site Access, Low Overhead, Steep Slopes & Vibration Monitoring
The topography surrounding the viaduct is complicated by steep inclines, wetlands, the meandering Crum Creek and a narrow, snake-like access road, all encapsulated within a densely wooded area that happens to be a designated arboretum. This challenging terrain dictated not only the selection of rigs, but also the site preparation.
One of Walsh's main challenges, however, was drilling foundations in hard rock beneath the viaduct with limited headroom near the east and west abutments. Watching the BG 18 H drilling below the farthest western span of the viaduct, makes it clear why its unique low headroom capabilities were essential. The rattling tip of the mast is within mere inches of the steel girders. The BG 18 H worked in standard configuration from late June through early October and was then reconfigured to low headroom.
Vance said the height restricted areas were more severe than expected because the viaduct elevations Walsh was initially given did not account for drilling nuances. He indicated that maneuvers such as putting the drilling rig in crowd force, or pulling up a full bucket can require up to 8 in. (20 cm) of additional overhead space.
One of SEPTA's key concerns was ensuring that its commuter trains could continue to pass safely across the existing viaduct during construction. Every pier and tower on the viaduct has a sensor to monitor for vibration. There are also four inclinometers in each slope to measure movement.
Custom Casing Drive Adapter Reduces Waste & Cost
ECA designed and fabricated a custom casing drive adapter for Walsh to facilitate overburden drilling. Dutton watched as the operator of the BG 18 lowered the mast, inserted the adapter into J-shaped notches in the casing, and began to seamlessly drill it into the soil from the cab of the rig.
“We fabricated that at the Aldan shop,” he said, as the casing descends into the soil. “You need something to transfer the torque against, so Walsh cut those j-shaped notches in the top of the casing.”
Vance said this innovation reduced costs and eliminated wasted material.
“We pulled a lot of the casings out as we drilled shafts so we saved a lot.”
Bridging the Gap Between Utility & Aesthetics
Walsh has demonstrated considerable environmental sensitivity throughout the project because of active involvement by neighboring Swarthmore College. Although SEPTA had the right-of-way available for the viaduct, it negotiated with the college to use a necessary adjacent piece of land instead of using eminent domain. In addition to obtaining permitting from multiple agencies, the contractor will have to rebuild wetlands, replace up to 6,000 trees and shrubs, and reconstruct a Stonehenge-esque circular formation of Wissahickon Schist slabs dubbed “Crumhenge” by locals.
SEPTA will shut down service for roughly 10 weeks during summer 2016 to allow Walsh to install new girders, bridge deck and catenary towers. Rail cars will travel over the new viaduct by Labor Day 2016. Walsh will conclude the project by summer 2017 with demolition of the existing structure, removal of the access road and restoration of the site to original condition.
As SEPTA passengers, college students and local residents admire the towering modern viaduct that has replaced the rusty, outdated structure that once straddled this area of natural beauty, the critical foundation work performed by two drilling rigs with custom tooling will remain unknown. The folks at Walsh, ECA and Bauer, however, will relish their role in providing safe passage for mass transit passengers and improving the aesthetics of a critical piece of infrastructure.
(This story also can be found on Construction Equipment Guide's Web site at www.constructionequipmentguide.com.)