Can we Design tunnels without a circular shape?.
Examples - Triangle, Square, Rectangle, Kite, Rhombus etc?.
It is intuitively obvious that one could design such channel/tunnels but it is NOT intuitively obvious as to why it would be necessary or desirable to do so. Over the century round circular tunnels have been the norm because it has the greatest strength and uniform pressure associated with the outside crushing forces that surround the tunnel channel.
I have seen earthen tunnels that are elliptical in shape and other nearly round designs but I have never seen square, triangle or rectangular piped tunnels but I have seen earthen rectangular tunnels used in underground roadway designs. The following tunnel digging machine is interesting to consider.
which shows tunnel digging machine that can do many shapes such as round, ablong and square. If you go to page 3 of that link it shows how it digs tunnel at 90 degrees.
I don't know about kite and rhombus shapes. That's for another day of research.
But, again, such shapes can likely be made but to what purpose. Underground tunnels for vehicular traffic may be with flat bottom, flat vertical sides and rounded or circular roof.
Hope this will provide some idea for your thought.