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Title: Flowable Fill As Pipe Support Material
Author: Richard A. Oneil, Harry E. Stewart, Daphne C. Dzurko
Source: American Gas Association 2000
Year Published: 2000
Abstract: material consisting of flyash, sand, cement and water, was found to be an excellent backfill support material for pipe under traffic load. Full-scale field-test results on 6-in. (152-mm)- diameter cast iron pipe showed that flowable fill provided better support than well-compacted sand. Cast iron gas main, recently removed from active gas service, was cleaned, reassembled, instrumented with strain gages, buried in a typical size utility trench and then subjected to heavy static and dynamic loads. Compacted sand was used initially as a support material to determine baseline data with static and dynamic truck loadings. The sand then was excavated, flowable fill installed and the same static and dynamic loads applied. The cast iron pipe had less strain and deflection when supported by flowable fill than sand, thus showing that a flowable fill mix can be superior to granular backfill. Flowable fill also is easier to install as it is self-leveling and self-compacting. The flowable fill mixture provided adequate pipe support and was ready for traffic loads when the curing strength reached 10 psi (70 kPa) unconfined compressive strength (UCS). Paving operations also can be performed when the Proctor penetrometer resistance test reaches 650 psi (4.5 MPa) or approximately 10 psi (70 kPa) UCS. This strength can be reached overnight or in a couple of days depending on the design of the mix. Curing time and strength versus the ability to re-excavate is the trade-off that the user needs to consider. The use of flowable fill as a support and backfill material should be applicable to: 1) all types of underground pipe and 2) all odd-shaped excavations (such as keyhole pits) that are difficult to compact properly using conventional compaction equipment. Background Members of the New York Gas Group (Niagara Mohawk, KeySpan Energy, Rochester Gas and Electric, Consolidated Edison, Central Hudson, Orange and Rockland, Long Island Lighting, Central Hudson, and New York State Electric & Gas) wanted to evaluate the performance of flowable fill in utility operations. One specific application involved the use of flowable fill in situations where cast iron was undermined by its own operators or by a third party. By current NY State Public Service Commission requirements, gas utilities must remove any 8 in. (203 mm) or smaller diameter cast iron that becomes exposed for a length greater than 3 ft (0.9 m). This section must be replaced with steel or plastic. The utilities wanted to determine if flowable fill could provide adequate support and thus safely retain the cast iron. The utilities, led by Niagara Mohawk and united in the consortium by the NY Gas Group, contracted with Cornell University, the School of Civil and Environmental Engineering to perform studies on undermined cast iron. Methodology and Instrumentation The general methodology was to load and stress full-scale cast iron pipe in a test trench (Figure 1) and then measure strain. The cast iron pipe was first supported by compacted sand, then undermined three different times at widths of 3, 6 and 10 ft (0.9, 1.8, and 3.0 m). With each undermining, the compacted sand was excavated and replaced by flowable fill. The cast iron pipe in each flowable fill section was then loaded 00-OP-44 with a heavy truck, and strains measured the same way as with compacted sand. The flowable fill was loaded at carefully controlled cure dates to evaluate the progressive strength. A comparison between strains measured when the pipe was supported in compacted sand and when supported in flowable fill indicated which material provided better support.