Loose soil discovered lying in the a foul sewer is most often a sign that a rat has discovered a way out of the system. Bear in mind a sewer should have two ‘ends’: one is at the sewage treatment plant, the other end should terminate at a bath, sink, shower, etc. Neither of these locations would drop soil into the pipe so one has to immediately ask how such material got there.
If it had been washed up-stream because of a localised flooding of the pipework then the soil would be compacted and, most likely, on the concrete sides of the chamber (the ‘benching’) – but loose soil in a pipe suggests there’s a fault that allows earth to get into the branch. Our next step would be to examine the network with a CCTV camera and see what’s going on.
As an aside, this particular sewer has a secondary issue: there’s standing water in the pipe. That could suggest a problem downstream – or it may just be a badly-laid pipe, something that happens more and more because modern plastic materials allow for more lax tolerances in trench-work.