I do something similar in New Zealand but for a different reason. They are burying soil which gets to wet and bringing up freer draining soil, where I am burying sandy free draining soil and bringing up silt/clay soils which can hold more than twice as much moisture. I have been using excavators of about 45 tonne. We have been able to grow 40 percent more pasture. We are bringing up ancient soils that were buried after volcanic eruptions. Nutrient levels coming up are higher than top soil except for phosphate which is cheaper to apply.