A standard manhole base features multiple incoming and outgoing pipe connections converging at a central flow channel. The precast or cast-in-place concrete base features a smooth finish channel carried to three-quarters the vertical height of the pipe, with a one-to-twelve slope minimum typically applied to the shelf areas. Pipes extend into the manhole with a maximum two-inch projection beyond the interior wall, grouted smooth at the penetration. General notes specify commercial grade concrete, compliance with ASTM C-478 for precast sections, and properly radiused smooth channels directing flow to the outlet pipe.
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