60" ID Sanitary Manholes. Our manhole includes integral base, risers, top, steps, o-rings, pipe to manhole connectors as specified, and precast flow channel. A transition slab or reducer cone is used when possible to facilitate use of 48" ID cones, and when not possible, our flat top slab is used. Doghouse holes can be formed when it is necessary to drop the manhole over existing pipe. An extended base is also available.