ultra fine particlesultrasound deviceunbalanced hoistingunbiased estimatorunconfined aquiferunconformityunder drillingunder flowunder grindingunder loadingunderchargeundercurrent circuit breakerunderground degasification holesunderhand stoping methodundersaturatedundersizeunderthrust faultuniaxial compressive strengthuniaxial stress fielduniform flowuniform reliability principleuniversal crusheruniversal kriginguniverseunstable gravimetersunstable groundunsteady flowup thrust faultupcast shaftupstream damutility factorutilizationutilization factor