Complex sentence: Because of abandoned coal mines beneath the construction site, real concrete had to be pumped underground before construction could begin; more concrete was said to be used in the filling of the old mines than was used in the mall itself.
Split into simpler sentences: Because of abandoned coal mines beneath the construction site, real concrete had to be pumped underground before construction could begin. More concrete was said to be used in the filling of the old mines than was used in the mall itself.