Abstract: A high water content soft soil, for example from the sea or river bed, may be improved in strength by incorporating thereinto an additive composed of ingredient A including a mixture of 5-45% by weight of gypsum and 95-55% by weight of a water-granulated iron blast furnace slag having an particle size within the range of 100-1 .mu.m and ingredient B including a Portland cement. The ratio by weight of ingredient A to ingredient B is within the range of 75/25-55/45. Ingredient A is incorporated into the soft soil before incorporation of ingredient B.

Abstract: A water-saturated soft soil is efficiently improved in its strength by incorporating therein an additive composed of ingredients A and B; the ingredient A comprises gypsum and the ingredient B comprises a mixture of 40-55% by weight of a Portland cement and 60-45% by weight of a water-granulated iron blast furnace slag having a particle size almost equal to or smaller than that of the Portland cement. The ratio by weight of the ingredient A to the ingredient B ranges from 10/90 to 30/70 and the total amount of the ingredients A and B used for the soft soil is sufficient to strengthen the soft soil.

Abstract: A water-saturated soft soil is efficiently improved in its strength by incorporating thereinto an additive composed of ingredients A and B; the ingredient A comprises gypsum and the ingredient B comprises a mixture of 40-55% by weight of a Portland cement and 60-45% by weight of a water-granulated iron blast furnace slag having a particle size almost equal to or smaller than that of the Portland cement. The ratio by weight of the ingredient A to the ingredient B ranges from 10/90 to 30/70 and the total amount of the ingredients A and B used for the soft soil is sufficient to strengthen the soft soil.