If we have a section of sewer line that is one foot below grade (due to avoiding having to blast ledge), can that sewer line freeze? In a prolonged cold period can the waste water and effluent freeze to block the line even if we have legal slope to the city main sewer?

It can freeze if there is something leaking that creates a steady, small flow of water through the sewer.

A shallow set sewer line is nothing more than a covered trench, and will likely freeze with the right temps and prolong freezing.
But sometimes you have to ask yourself,
"Do I feel lucky today?"
Best Wishes

It can only freeze if the water stays in it long enought to drop to the freezing point. Water flowing through it, combined with the heat common in most municipal sewer systems, will not normally "stick around" long enough for this to happen. But water from a slow leak could reach the freezing point before it exited the shallow portion. Once it started to freeze, then ensuing water would build up behind that dam and also freeze and from then on it would be a continuous process until the pipe froze solid, or a new high volume source of water began to flow and start to thaw it. Whether it could completely thaw the ice buildup would depend on a lot of variables that would be specific the that exact time when it occurred.

We are actually building a stone retaining wall about two to three feet high along the section of pipe that is shallow. We will put at least two feet of dirt on top of this so we have three feet or so of dirt surrounding the pipe. This was all approved by the chief engineer for the DPW in our town, so formally we have the town approval. We will plant a garden of annuals on this terrace above the sewer pipe.

There was one section of ledge near the attachment to the town sewer that caused this problem. This was about three feet away from a main gas line serving our neighborhood.

I had done searches in this forum and on the internet in general for information on freezing in sewer lines, and surprisingly there is very little.

The town's approval and $4.95 may get you a Starbuck's Latté, but it will not guarantee a sewer that will not freeze.

If the town normally requires sewer pipe to be four feet deep, then I presume building a terrace that surrounds the sewer pipe by four feet of dirt would be the equivalent. This would mean the frost line under normal conditions does not reach the sewer pipe (six inch PVC). Thus even standing water would not normally freeze. What else can be done? There is nothing we can do now about the location of the sewer line. It is easier and much cheaper to build a terrace with a stone wall that surrounds the sewer line than it is to try to blast through ledge.

the idea is that sometimes you cannot do anything about a shallow sewer. Just don't let any faucets or toilets leak and not fix them, which is a good idea any time of the year, when it gets cold.

Sewer lines will freeze Ive seen enough to know when to insulate them well.

Last one I was on occured on New Year's Eve
at a very busy night spot, we had snow and very cold weather weeks before and the manager made sure the parking lot was well plowed prior to the sewer blocking-up. We got them going after about two hours avoiding them closing the doors on there busiest night of the year.

I would and always insulate with 2, 1" thick picses of polystrene with the joints overlaping, box around the pipe and fasten the joints together, or just buy the insulated sewer pipe, it's great stuff but cost a little more, always do a neat job and protect piping and insulation when back- filling.

I suppose you know that insulation does not prevent freezing, it just slows the process. So if nothing happens to change the dynamics the sewer is still going to freeze even with insulation.

Yes it can and yes they do, thaw a few every year here. Changed towns and cos, and a youngster found the goofiest cure, but it works! this im sure of, because a regular freaser stayed open till the fan quit! Put a 4" heating duct type booster fan (low CFM)on the main stack vent, blowing out. With local code offical OK. Fan only lasted 3 years tho.....

I suppose you know every part of the sewer and ground has some heat in it and the insulation does protect it from freezing.

A couple of years ago we had to dig up a water service 8' below a parking lot, ground was froze all the way down to the insulation, under it where the water pipe was, was perfect .

Frozen ground does not have any heat in it, (in fact it can be colder than the atmosphere during a short warm spell),and the heat in the sewer has to be adequate to overcome the cold generated by the surrounding ground.