I've heard this said before, but I see very little evidence that this is true. The soils below the stadium are largely unsaturated alluvial-fan deposits consisting of interbedded sands, gravels, and silts. They don't appear to be soft, collapsible, or compressible and the liquefaction risk (east of Canyon Road) is classified as Very Low (according to the UGS and Utah County Liquefaction Hazard Maps). The native soils in the area should support structural loads between 4,000 to 6,000 psf. Given this, I don't see any reason to believe that fairly conventional foundation systems couldn't be utilized to support properly designed and engineered additions to the east or west stands.