Evaluation of CLSM (Flowable Fill) for Trench Backfill
Creators: name: Meade, Bobby W.
name: Hunsucker, David Q.
name: Stone, Michael D.
Publisher: name: Kentucky Transportation Center, University of Kentucky
Year: 1993
Subjects: subject: Transportation Engineering

description: Controlled Low Strength Material (CLSM), commonly referred to as flowable fill, has been used for years as a trench backfill for utility repairs in Kentucky, but had not been previously used by the Kentucky Department of Highways (KDQH). In 1991, the KDOH awarded a contract for the reconstruction of Breckinridge Lane in Louisville where CLSM was used as trench backfill for all storm drainage. In 1992, the KDOH awarded a contract for replacement of cross drains for US 25 in Fayette County where CLSM was used as trench backfill. These two sites were monitored for field performance, cylinders were cast for laboratory testing, and a laboratory mix design study was conducted.  CLSM appears to be a very effective trench backfill when placed in a sufficiently flowable state. The two sites monitored included both concrete and steel pipe with the concrete pipe in a cradle bedding and the steel pipe completely encased in CLSM. There were few problems at either site after the CLSM mix had been adjusted to a flowable condition. The CLSM 28-day compressive strength, typically, ranged from 50 to 150 psi, with strengths as low as 36 psi and as high as 668 psi. In the interest of removability, CLSM strength should be low, probably less than 100 psi at 28 days.  A mathematical relationship developed by Brewer and Associates appears to reasonably evaluate the removability of CLSM when actual compressive strength is used. Since cylinders are not usually available for long term testing, a method for predicting ultimate compressive strength is included.  A cost comparison of CLSM and conventional backfill (manufactured limestone sand) indicates that when trench dimensions are the same and only direct costs (labor, materials, equipment, and associated costs) are included, CLSM costs approximately $9.50 per foot of pipe more than conventional backfill for a six-foot by six-foot trench. Other costs stemming from increased productivity, less inspection, less testing, reduced liability concerns, and reduced or no future remedial work associated with the use of flowable fill which were not quantified nor included in the cost comparison would tend to reduce the total cost difference between CLSM and conventional backfill. Because conventional backfill normally requires additional trench width for compaction equipment, a reduction of trench width on each side of a pipe from 21 inches down to 12 inches can be realized when using CLSM. The reduced trench width makes CLSM costs equivalent to conventional backfill, on a direct cost basis.  A mix design study involving the use of manufactured limestone sand (MLS) as compared to natural river sand for CLSM filler was conducted. Findings from this task indicate that CLSM with MLS filler is not suitable for typical highway construction due to increases in strength, bleed time, time required to develop acceptable bearing capacity, and shrinkage.  Both experimental sites are currently being monitored to assess long-term pavement settlement.