Questions From the Field

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Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.

If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.

Brittany writes: When conducting an infiltration or exfiltration test in accordance with ASTM C969, “Standard Practice for Infiltration and Exfiltration Acceptance Testing of Installed Precast Concrete Pipe Sewer Lines,” is the precast concrete structure supposed to be backfilled prior to conducting the test?

NPCA Technical Services engineers answered: A structure must be backfilled to conduct the infiltration test in accordance with ASTM C969. The exfiltration test can be conducted on a structure that is installed and not backfilled or installed and backfilled. However, it is advantageous in many scenarios to conduct the exfiltration test before backfilling. If troubleshooting is required, it could be challenging to locate the source of the issue and resolve the issue if the structure is backfilled and has limited access to, or visibility of, the structure’s exterior.

Kevin writes: I want to determine the minimum manhole size needed to have four 15-inch PVC pipes in the same plane all at 90-degree angles from each other. I found the NPCA Manhole Sizing Recommendations Guide at www.precast.org, and the equations accommodate up to three pipes. What would the angle equations be for the fourth pipe, and what would be the allowable angle ranges when using four pipes?

NPCA Technical Services engineers answered: To accommodate a fourth pipe, you not only would need to add two allowable angle range equations for the fourth pipe, but you also would need to modify the allowable angle range equations for the third pipe. See the table below. The Manhole Sizing Recommendations Guide has been updated to accommodate a fourth pipe and is available at www.precast.org.