Uluhlu lwe-NPD umbhobho wokutyhutyha umatshini ulungele kakhulu iimeko zejoloji kunye noxinzelelo oluphezulu lwamanzi aphantsi komhlaba kunye ne-coefficient ephezulu yokungena komhlaba. I-slag ekhutshiweyo ikhutshwe kwi-tunnel ngendlela yodaka ngokusebenzisa ipompo yodaka, ngoko ineempawu zokusebenza okuphezulu kunye nokusebenza okucocekileyo kwendawo yokusebenza.