Repairing the line
San Diego-based engineering firm Kleinfelder used geotextile-encased columns (GECs) to repair an uplifted section of railway in Oakland, Calif. Each GEC measured 30 inches in diameter and 27 feet deep, and they were spaced 6 feet apart on center. Source: Richard Short
As an officer of the Deep Foundations Institute, I have had the opportunity to travel the globe and meet engineers from around the world. We have found common ground in our engineering challenges and shared countless ideas and experiences, to the mutual benefit of our respective clients.
During a trip to Europe, I encountered a demonstration project that struck me as having huge potential for public works projects in the United States. In the course of a tour of European highway projects, we viewed a geotextile-encased column (GEC) system for supporting embankments under roads and railways. The system was used to stabilize soft soils under the right of way for a high-speed rail system in Germany.
Cost is one advantage of the system; strength and settlement control are others. After seeing the demonstration overseas, I was anxious to see the method put to the test in this country.
Kleinfelder, a San Diego-based consulting and engineering firm, finally got the chance to test the system on a project in Oakland, Calif., near the shoreline of San Francisco Bay in June 2004. A section of the Burlington Northern Santa Fe Railway had uplifted after concrete debris was dumped adjacent to the railway. The assignment: determine the best way to get the railway back on grade, and the costs involved.
A GEC consists of a 24- to 30-inch-diameter column of sand or gravel, sunk about 15 to 40 feet deep and encased in a sock made of geotextile fabric. The columns can be used to support railroads or highways constructed over highly compressible soils. The technique requires less rock than other unconfined methods, such as stone columns, and greater load capacity is achieved through the confining hoop stress of the fabric. Vibratory equipment used to install sheet piling also can be used to install the pilot casing.
For the Oakland project, Kleinfelder evaluated several commonly used methods: deep mixed columns (DMC), auger-cast piles (ACP), and a concrete-pile-supported slab. Results showed these more common methods would cost substantially more than the “rocks in socks” method. The approximate unit cost for 30-inch diameter DMCs is $30 per foot; for 18-inch diameter ACPs, $25 per foot; and for driven piles, $28 per foot. That compares to about $18 per foot for 30-inch diameter GECs. Additionally, the GEC system provides an additional time savings of about one month, thanks to the ability to load the columns immediately after installation, rather than waiting for a preload or curing period before putting them to use.
In Oakland, the rail spur line of the Burlington Northern Santa Fe Railway lies on a vacant lot adjacent to Highway 880. The site is underlain by fill, which overlies 20 feet of soft clay. The property on either side of the spur line was used for a recycling operation that included stockpiling and crushing of concrete debris. A bearing failure occurred when crews piled the debris higher than it had been before, lifting the adjacent rail bed up 6 feet vertically and rotating the tracks 2 feet, rendering the line unusable.
The first step involved investigating the site to determine subsurface soil conditions along the spur right of way. Crews bored to depths of 35 to 50 feet. These borings indicated the soil profile consisted of 1 ½ to 7 feet of variable clay and sand fill, underlain by 20 to 25 feet of soft fat clay known as “bay mud.” The mud is underlain by stiff clay and sand layers. A stabilized groundwater level was encountered at a depth of 4 feet.
Kleinfelder was tasked with designing a system for the spur right of way that would safely support the weight of a train, and provide for acceptable post repair settlement of less than 3 inches.
Next, the firm evaluated the original bearing capacity prior to failure, given the soil profile determined from the field investigation. This bearing capacity analysis indicated that the failure took place when the stockpile exceeded a height of approximately 10 feet, or a vertical pressure of 1300 pounds per square foot. The failure had pushed the bay mud up to the level of the rail bed. Findings in hand, the company set about to design a system that would transfer the railway loads to the deeper stiff clays below the bay mud, and increase the factor of safety for the line.