Potomac Yard Storm Sewer, USA: Before work could begin on a new retail center in Alexandria, Virginia, the developers had to improve the site's storm water drainage system. Their improvement plan included the construction of a 72-inch (1.8 m) collector pipe to carry storm water from the site to the Potomac River nearby.
Construction of the pipe was complicated by the presence of a railway embankment between the site and the river, so the developers had no choice but to tunnel under the embankment. This was a serious undertaking because the embankment carried tracks for two busy railroads, the Washington Metropolitan Area Transit Authority (WMATA) and the CSX railroad. Rail traffic could not be disrupted and rail safety could not be compromised by the tunneling operations.
Before tunneling began, engineers drilled seven small-diameter horizontal shafts, spaced 6 feet (1.8 m) apart, through the embankment, and then used the shafts to inject grout into the embankment material. The grout formed a stiff shelf that helped support the embankment and minimize ground loss. A tunnel boring machine was used to excavate the tunnel. As the machine advanced, it jacked precast concrete ring segments into place to form the collector pipe. Some ground loss was inevitable during this operation, so compensation grouting was required to fill the voids and eliminate settlement.
As might be expected, the grouting and tunneling operations were of great concern to the railroads. WMATA determined that settlement or heave greater than 1/8 inch (3 mm) was unacceptable. Thus it was imperative that the trackbed be monitored during all phases of the construction. Crescent Resources, the developer, retained Facility Engineering Associates (FEA) to design, implement, and manage a real time monitoring program for this purpose.
FEA worked closely with all concerned parties to create a satisfactory instrumentation program. The monitoring system consisted of 18 EL beam sensors with 6-foot gauge lengths, a CR10X data logger, and a computer running MultiMon software. The EL beam sensors were installed along the alignment of each track and also along the planned alignment of the storm drain and tunnel. The beams were linked by common anchor points and referenced to a temporary bench mark so that they could provide a profile of settlement and heave. The data logger was programmed to read the sensors every 60 seconds. An on-site computer retrieved readings from the logger to make them available to MultiMon, which processed the readings and updated the computer display, all in near-real time. The display provided a graphic representation of the site and the location of the sensors. Sensor readings were shown in color-coded boxes that changed from green to blue to red if a reading exceeded certain threshold values.
As part of the agreement between the owner and WMATA, grouting and tunneling were undertaken only on weekends when trains were fewer and could run at slower speeds. Monitoring was started one week before construction began. This provided baseline readings of embankment behavior during normal train operations. The first phase of grouting was started in December, 2001 and took several weekends to complete. During these operations, heave in the track bed remained within acceptable limits. Tunneling was completed in mid January, 2002. During tunneling, there were periods when settlement exceeded 0.5 inches (12.7 mm). With close coordination between the grouting team and FEA technicians, the track bed was raised to compensate for the loss of material during that time. The real time monitoring program proved itself on several occasions as the tunnel-boring machine inched forward.
The 72-inch storm drain was completed on schedule and is now in service. The track monitoring program designed and implemented by FEA was a first in the United States for real time monitoring of track behavior on a high-speed rail system.
Thanks to Steve Pennington and others at Facility Engineering Associates
for this story and the site photographs. FEA can be reached at 703-591-4855