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
The GEC design was based on using an array of 27-feet deep, 30-inch diameter columns spaced at 6 feet on center in both directions. Kleinfelder's analysis indicated the rail bed would settle 4 to 6 inches if supported by conventional stone columns. The settlement of the GECs would be less for the following reasons:The geotextile material acts as a separator, keeping the bay mud from contaminating and weakening the sand and gravel within the wrapped columns.The geotextile sock ensures that the confined column laterally displacement of the bay mud, thus increasing compaction, density, shear strength, and the in situ stress conditions of the mud.Upon being loaded, the sand and gravel in the columns will settle, until confined by the geotextile “hoop stress” resistance.
In order to confirm the actual time settlement curve of the column array, a load test was designed to simulate 1.6 times the weight of loads anticipated for the spur line. The load was applied with a surcharge fill located at the critical axis of the original bearing capacity failure. Condon Johnson, an Oakland-based contractor, was selected to install the GEC support system, which they promptly renamed “rocks in socks.”
The project proved GECs could be employed as an economical ground improvement or embankment support system for railroad and highway applications. The construction method can be implemented with conventional sheet piling installation equipment and a steel casing modified with a trap door at the tip. Railway construction can proceed almost immediately after the installation of the columns.
GECs have the benefits of vertical drainage provided by stone columns, but with the capabilities of being installed in very soft week soils without excessive bulging and resultant settlement. The method produces no waste and can advance at a rate of 40 or more columns per day. The elimination of bulging of the rock column controls the rock quantity, making construction costs more predictable. A comparative cost analysis indicates the GEC method can be 30% less expensive than alternatives.
—Short is senior geotechnical engineer with Kleinfelder Inc.Constructing a GEC system
To begin the construction phase on the Burlington Northern Santa Fe Railway grade project near Oakland, Calif., the existing rails and ties were removed intact and set aside to allow the repair to proceed. The geotextile-encased columns, or GECs, were installed through the fill using a 30-inch diameter casing pipe, fabricated with a jaw-type trap door at the bottom. Crews forced the casing into the ground with a vibratory sheet piling installation rig. The geotextile socks were manually placed inside the casing, firmly strapped to the top of the pipe and filled with recycled, well-graded sand and gravel made from crushed concrete. After the contractor filled each sock with gravel, it was unstrapped and the casing pipe was vibrated out of the ground, compacting the gravel in the process. The process was quick—the contractor installed 40 columns per day.
To test the system, 10 feet of fill was placed in a 20-foot square test area directly over the rail bed at the axis of the failure. The test fill was constructed by stacking two lifts of K-rail sections to form a square box. Crews filled the box with granular backfill, with a dead weight of 1300 pounds per square foot. Two settlement markers were placed over the columns—one over the bay mud, the other over a rock column. Settlement measurements were taken during the 10-day construction period. Estimates placed settling of the unconfined stone columns at 4 to 6 inches. The test fill settled 1.6 inches after the initial compression of the loose fabric and rock near the surface. After 24 hours, however, the rate of settlement slowed to a negligible rate.
Two inclinometers (instruments used to gauge the angle of an object with respect to vertical) were installed about 2 feet from opposite edges of the test fill area, on either side of the rail centerline. Technicians read the inclinometers prior to, during, and after the fill placement. The inclinometers measured approximately 2 inches of bulge under the test fill at a depth of 16 to 17 feet.
The GECs were then capped with a geogrid, a polymer-coated fiber net used to reinforce soil, plus 24 inches of compacted aggregate base and 6 inches of ballast. The ties and rails were reinstalled at the original grade and the spur line project was completed. Rail service restarted several days after completion with the delivery of plate steel.