Controlled Low Strength Material (CLSM), as a structural material for bedding vitrified clay pipe, is a mix consisting of Portland cement, fine aggregate, coarse aggregate, water, entrained air, and chemical admixtures to accelerate cure time. CLSM is gaining popularity as a bedding material for vitrified clay pipe (VCP) due to simplification of pipe installation, reduction in labor and inspection costs, and the benefit of a 2.8 load factor. Computation of predicted backfill load on rigid pipe is performed utilizing the Marston Equation originally developed by Professor Anson Marston in 1913 and 1930. The variables of the Marston Equation consist of the depth of cover, the unit weight and type of the backfill soil and the width of the trench measured at the top of pipe. This design theory is still used today for rigid pipe design even though bedding system standards were yet to be adopted when it was developed. At that time, rigid pipe were generally laid on a hand-shaped trench bottom and imported bedding was rarely used. Professor Marston theorized that the material at the sides of rigid pipe was so loose compared to the rigidity of the pipe that the support of any backfill load by the sidefill would be negligible resulting in a conservative design. The presumed inability of the sidefills to carry a significant share of the backfill load is not applicable when CLSM bedding is used since it neither settles nor compacts or shrinks significantly. CLSM will support a large portion of the backfill load, which would otherwise be carried by the pipe. Thus, utilizing the Marston Equation in its conventional form has proven to be inappropriate on CLSM projects. The growing and successful use of CLSM as a bedding material led the clay pipe industry to conduct research on actual loads received by the pipe both in field installations and laboratory environments. This research demonstrates a load transfer mechanism in which the load on the pipe can be reduced by a ratio of the outside diameter of the pipe to the width of the trench at the top of the pipe (B_c/B_d) resulting in the Modified Marston equation.
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