PURPOSE: To make it possible to minimize sulfate ions dissolved in water and hence improve water quality maintenance by adding a phosphoric acid group compound, providing a specified blending requirement, injecting into the ground and solidifying the ground.

CONSTITUTION: As a grout which is injected into the ground so as to solidify the ground, an assistant agent is added to an acid reaction agent, aqueous glass and phosphoric acid group compound or this group so as to turn the grout from acid to weak alkali under control. In this case, there is provided a specified requirement which must be satisfied between the composition of aqueous glass, sulfuric acid in the acid reaction agent and phosphorous in the phosphoric acid. More specifically, the following requirement is set: one liter of a grout blending liquid must contain 3.5>SiO2, mole concentration >0.7 and 1.4> (H2SO4 mole concentration/Na2O mole concentration >0.9 and 50> (H2SO4 mole concentration/P mole concentration >3. As an assistant agent, there is available alkali metal or hydroxide in alkali earth metal or salts. This construction makes it possible to enhance water quality maintenance technique, and what is more minimize the effect upon underground structures.