Rehab Of A 100-Year Old Brick Storm Sewer

By Datta Shirodkar, Boyer Inc. and Michael Spero, P.E., Danby LLC | July 2010 Vol. 65 No. 7

The PVC liner panels, besides being water-tight and corrosion-resistant, are essentially the “form” for the high-strength grout. It is the cementitious grout that provides the structural rehabilitation of the pipe. The current state of stress of the existing pipe was determined through a finite element analysis and the minimum grout thickness was calculated using the Flexural Stress Cracking Design Method (Composite Material Design). The grout consists of Portland cement (57 percent), Fly Ash (43 percent), and water and super-plasticizer admixture. Although the design is based on a grout compressive strength of 6,000 psi, typically the tested 28-day strength is 7,000 – 9,000 psi. The grout will normally set-up in four to 5 hours (firm to the touch) and develop a compressive strength of more than 1,000 psi in 24 hours.

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The cement and fly ash are premeasured and pre-mixed and delivered to the job site in 1,000 pound “super sacks.” The grout components (full sacks) are introduced into the Colloidal Mixer which uses a high shear pump and baffles to recirculate and thoroughly mix the constituents, which is done for eight to 10 minutes. The grout mix is then transferred to the agitator/holding tank. Water and then the cement/fly ash blend is added to the colloidal mixer for the next batch. The grout mix remains in the agitator/holding tank for up to 10 minutes where it is mechanically agitated to maintain the mixture in suspension. The grout is pumped through a progressive cavity pump into the pipe to the injection point. The grout is very fluid (18 – 25 second standard flow cone).

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The liner is grouted in eight lifts of about12- inches each. Grout and vent holes are drilled in the liner about every 20 feet. The grout hose discharges into the grout hole and the mixture flows by gravity into the annular space between the liner panels and the host pipe. When grout starts flowing out of the vent hole, the hole is plugged and the hose is move to the next grout hole. The next grout lift is placed a minimum of 12 hours after the previous lift is completed.

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That is the sequence of operations if the work is progressing according to the work plan. In anticipation of the project, Boyer located a local supplier of cement products in the DC area, which was part of a large national supply company. Boyer obtained samples of the cement and fly ash and tested these materials to ensure the materials met specifications.

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