Plumbing – ABS cellular-core Pipes Safe

What you have is not solid. There's a foam or cellular interior to the pipe with two thin layers of sold ABS on the outside.

• It costs less and weighs less than solid pipe. Thus it's popular at home stores that compete on price.
• Not every jurisdiction allows use of cellular pipe. However it's common in the USA.
• You need to be careful about backfilling such pipe with sharp rocks, which can pierce the thin shell.
• When gluing, it's a good idea to coat the ends of the cut pipe, as the foam can and will slowly transport air and water (for the same reason, this pipe is never suitable for pressure applications).
• You can't use it for pressure applications: this is meant only as drain pipe.
• The pipe is fragile if dropped (take a section and stomp on it, chances are it will shatter into fragments, where a solid pipe would just hurt your foot).
• It's more flexible than solid pipe, which is good in some cases, but causes sagging in others.

Here's a picture from a pipe vendor, where the foam core has been made white to show the detail. The pipe I've seen in stores all has a black core that mimics solid pipe at first glance:

Look around you'll see various opinions on the stuff. Some plumbers say they love it because it brings them repeat business. The vendors all claim various advantages to the pipe.

It's definitely not good in freeze/thaw weather if the ends are exposed (as they might be for a drain waste vent, or the end of a drain pipe run).

Foam core is regulated by ASTM F-628 Foam Core, and ASTM D-2661 for solid pipe.

Here's video on bursting of such pipe: https://www.youtube.com/watch?v=tWoTGu5s1Ys

The caped pipe you could hear the toilet flush is the clean out . This the location a snake is put in if the pipes backup. Is the second pipe outside a bathroom or kitchen? If so it may be a second clean out especially if they are quite a distance apart and if it is a clean out it should be capped also. A second possibility for the uncapped pipe is a clean out for the drains to the storm water runoff from your gutters these don't require a cap but it is a good idea so material from the flower bed dosent fill it up.