Complex sentence: In order to prepare the adequate construction area for the new stadium that was to be 12 meters lower than the previous one, over 350,000 cubic metres of soil and 15,000 cubic metres of stone had to be excavated.
Split into simpler sentences: In order to prepare an adequate foundation for the construction of the new stadium, it was to be started 12 meters lower than the previous one. Over 350,000 cubic metres of soil and 15,000 cubic metres of stone had to be excavated.