Permeation grouting is used in civil engineering to reduce the premeability and to improve the stability of granular soils. In this process, a grout is injected into the soil via special sleeve pipes in order to fill the existing pores or cavities. The curing of the grout leads to both waterproofing and hardening of the grouted soil. Nevertheless, the problem of proofing the strength of the grouted soil arises especially in the case of granular soils, for which core samples are very difficult to obtain. The aim of the current work is to develop several testing methods for grouts and grouted soils, placing the focus on their strength properties. Therefore, the first part of the current work deals with the state of the art in grout testing and describes new methods for improving the quality assurance of cement based grouts. Subsequently, the objective is to develop new methods for determining the strength of grouted soil. ^This is achieved by developing a special method for estimating the strength of grouted soils based on simple correlations to the cement content. A large number of grouted soil speciments were prepared in the laboratory by varying the cement type, the water-cement ratio and the soil type, in order to investigate the strength properties of different grouted soils. Uniaxial compression and triaxial tests have then been performed to provide the strength of the grouted samples, whose cement content was examined afterwards. Based on the results of the tests fundamental correlations between the cement content and the strength, the dry density and Youngâ s modulus of the samples were elaborated. These are meant to improve understanding of the mechanical behaviour of grouted soil. In addition to laboratory investigations large-scale grouting tests were carried out on natural alluvial soils. ^With the invention of a special sampling method, high quality grouted soil specimens were obtained from the test sites. Moreover, during the large-scale tests, stresses, deformation and pore water pressure measurement have been undertaken for monitoring and understanding the soil behaviour during the grouting process. General conclusions were further drawn from the measurement data to improve the grouting process. Based on the results from the laboratory and in situ field tests, a relationship between the cement content and the strength of the grouted soils could be derived in order to provide a new method for estimating the strength of grouted soils.