Design of in-situ soil mixing

In-situ soil mixing has become a widely and successfully used technique to improve soil properties, treat or encapsulate contaminated soil and to form elements and bodies with static effect. The design procedure for this method however is very different from that of other elements comprising only standardized and fully controllable components like piles, diaphragm walls and others due to the use of the existing soil as an essential component of the final product. Its physical and chemical properties govern the achievable strength, stiffness and permeability. Therefore the design process is done in several steps. First a suitable mixing process has to be selected in compliance with the scope of work and the existing soil, then the possible strength and stiffness has to be assessed and finally the geometry and its coverage by the single elements has to be fixed. The first step depends mainly on the available machinery and mixing tools of which innumerable types and variants exist. As long as a sufficient mixing of soil and binder is granted (e.g. expressed as a blade rotation number) the selection is an objective of the economic optimization. The last step is subject to conventional geotechnical design. Although it might be very complex it is well covered by the rules fixed in the known standard codes for design. The second of the above mentioned steps is the most difficult and requires a lot of engineering experience and appropriate statistical models. The same amount and type of binder in two different types of soil can result in very different strength and available laboratory tests for that usually cannot be fully transferred to field conditions. The necessity of a comprehensive continuous quality control to check the assumptions in reality in every project is obvious. The authors will present the current state of the art for the design of in-situ soil mixing in geotechnical practice and its representation in latest standardization attempts.