You might try cotton balls dipped in white glue that is cut with water. It will pack into a smaller item you can shape like a wad of turf. (This does take a while to dry!) Stack them up and let them dry and then paint with brown and greens. And remember that many of them were built into the side of a bank or hill to provide good insulation and solid earth packed walls. Other possibilities: Strips of leather Brown paper towels rolled and glued with the white glue/water mix. A foam backed placemat I would use small strips or pieces of artificial grass or some sort of carpet. Its easy to visualize the soil and grass layers.