Soil improvement using calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) has been an approved method in road construction for 60 years. It is used to prevent settlement of cohesive soil materials and to build sustainable roadbeds. During the last few years soil improvement by using calcium oxide or calcium hydroxide has also become a common technology in building construction, where sustainable layers of improved soil materials are created underneath foundations and floors. Generally, soil improvement is getting more and more importance in civil engineering as an alternative to soil replacement, the latter being always associated with high costs for transport and raw materials. In this thesis the reactions of calcium oxide or calcium hydroxide with the soil materials are characterized. The tests that are run on the soil material during the preparation to soil improvement works, are described as well as the tests (in field and in laboratory) during the construction works and the procedures of the soil improvement works themselves. The accomplishment of the works in practice is compared to the current directives. The most important factors for successful soil improvement of cohesive soils are shown on three building projects in Upper Austria. The geotechnical test are completed by means of petrographic and clay mineralogical analyses.