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A buried pipe was installed using controlled low strength material (CLSM) as backfill as part of a study of installation procedures for buried pipe. A laboratory evaluation of mix designs was undertaken prior to the field test to determine the suitability of the component materials and establish a mix with acceptable flow characteristics and a 28 day compressive strength of less than 1000 kPa (145 psi). The mix selected developed an average compressive strength of 334 kPa (48 psi) at 7 days and 600 kPa (87 psi) at 28 days, and Young's moduli of 120 MPa (17 ksi) at 7 days and 190 MPa (28 ksi) at 28 days. The field installation was in a trench with a width equal to the pipe outside diameter plus 600 mm. One length each of 900 mm nominal inside diameter concrete, corrugated steel and corrugated HDPE pipe was elevated on bags of gravel to allow the CLSM to flow under the pipe. The plastic and metal pipes were weighted with bags of gravel to minimize flotation. The CLSM was placed to the springline in two lifts, two hours apart. The metal pipe floated slightly while the second lift was being placed. Backfilling above the CLSM with a native clay commenced about 16 hours after installation of the CLSM. The CLSM provided excellent support for the pipe in especially hard to reach areas underneath the pipe. Pipe deflections were small and similar to those for the same pipe and high quality crushed stone backfill carefully placed and well compacted. Re-excavation of the installation about three weeks later showed that the CLSM backfill provided excellent support; however, excavation was difficult, even at that early age. A lowering of the target strength for the CLSM mix design to between 200 kPa and 400 kPa (30 psi and 60 psi) at 28 days is recommended for providing more excavatable backfill.
CLSM, buried pipe, field installation, fly ash, compressive strength, Young's modulus, backfill, trench, flowable fill
Senior Engineer, University of Massachusetts, Amherst, MA
Principal, Simpson Gumpertz and Heger, Inc., Arlington, MA
Professor of Civil Engineering, University of Massachusetts, Amherst, MA