Method for water tightening of water bearing zones and stabilization of sand in underground constructions, by precipitation of at least one mineral, by introducing into the construction, at the least one aqueous solution of salts comprising Ca2+ ions and urea, and an urease. The urease may be plant based, and made by grounding the plant wherefrom the urease is based, adding water, and soaking at occasional stirring between 2 and 20 h at room temperature. Then the achieved solution is filtrated, and the filtrate is lyophilized. The urease may also be biotechnologically produced by bacteria in an aqueous solution, where after the achieved solution is filtrated, and the filtrate is lyophilized.