Even with a totally plastic material (which is a stretch, since there are certainly granular elements even in clay soil) the material will deform until it fills

Even with a totally plastic material (which is a stretch, since there are certainly granular elements even in clay soil) the material will deform until it fills the gaps around the arch shape. At that point the material itself is forming an arch, and even clay has compressive strength since it has nowhere else to go. While it may increase the load on the steel, it's still a long way from having to support the full weight of the material above.