Strength of Stabilised Soil - A Laboratory Study on Clays and Organic Soils Stabilised with different Types of Binder

Abstract: Stabilisation of soft soils by deep mixing with binders is the most frequently used method of ground improvement in Sweden today, and is increasingly being used internationally. The most common binders employed are cement and lime, but a variety of other binders may also be used for stabilisation of soils. For further development of the deep mixing method, there is a need for more extensive research on the undrained and drained strength properties and behaviour of soils stabilised with various types of binders. The overall objective of the study presented in this thesis was to improve the understanding of some of the important aspects of the strength behaviour of stabilised soils. For this purpose, a series of laboratory tests was performed on four soils stabilised with different types of binders. The soils included two types of clay and two types of organic soil. Cement, lime, blastfurnace slag and fly ash in different combinations, alone and together with a number of admixtures, were used as binders. The laboratory testing was focused on the strength properties. However, a number of other basic properties of importance for the understanding of the strength behaviour of stabilised soils were also investigated, such as the density, water content, degree of saturation, permeability and the compression properties. Unconfined compression tests and triaxial tests were used to investigate the strength of the various samples stabilised in the laboratory. The unconfined compressive tests were performed mainly in order to study the effects of different binders on the increase in strength with time after stabilisation. The triaxial tests, conducted both as drained and undrained tests, were performed mainly to study the strength behaviour under various drainage and stress conditions. The influence of different testing procedures, such as different back pressure and rate of strain, was also studied. Both active and passive undrained triaxial tests were performed in order to investigate the strength anisotropy of the stabilised soils. The results provided illustrative examples of the effects of different binders on the increase in strength of stabilised soils, which may be linked to the chemical reactions taking place after mixing. In addition to the composition of the soil and the binder, a number of other factors also affect the strength of stabilised soil. The results show that the drainage conditions and the stresses acting on stabilised soil may have a considerable influence on the measured strength. A stress dependence of the undrained strength as well as the drained strength is evident from the test results. A quasi-preconsolidation pressure, which is governed not only by the stresses applied to the stabilised soil, but also by the cementation taking place during curing, is shown to influence the strength behaviour. An improved strength model is proposed that describes the strength behaviour in the same stress plane commonly used for natural unstabilised soils. It was concluded that although the type of binder may strongly affect the rate of strength increase and the final strength, the general strength behaviour is the same for soils stabilised by the most common binders. The strength and deformation properties of stabilised soils are similar to those of cemented and overconsolidated natural soils. The same set of parameters used to describe the strength of natural soils can also be used for stabilised soils.