In situ soil mixing

The use of in situ soil mixing (SM) to improve the engineering and environmental properties of soft or contaminated ground has increased widely since its genesis. Use has increased especially in Japan, Scandinavia and the United States, as well as Southeast Asia, China, Poland, France, Germany and UK, and to some extent in other countries. This indicates growing international interest and acceptance of this relatively new and quickly developing technology. In situ soil mixing is used in diverse marine and land applications, mainly

for soil stabilisation and column-type reinforcement of soft soils, construction of excavation-support walls with inserted steel sections and as gravity composite structures, mitigation of liquefaction, environmental remediation and for in-place installation of cut-off barriers. In this method of ground improvement, soils are mixed in situ with different stabilising binders, which chemically react with the soil and/or the groundwater. The stabilised soil material that is produced generally has a higher strength, lower permeability and lower compressibility than the native soil. The improvement becomes possible by cation exchange at the surface of clay minerals, bonding of soil particles and/or filling of voids by chemical reaction products. The most important binders are cements and limes. However, blast furnace slag, gypsum, ashes as well as other secondary products and compound materials are also used. For environmental treatment, binders are replaced with chemical oxidation agents or other reactive materials to render pollutants harmless. SM technology can be subdivided into two general methods: the Deep

Mixing Method (DMM) and the Shallow Mixing Method (SMM). Both DMM and SMM include a variety of proprietary systems. The more frequently used and better developed DMM is applied for

stabilisation of the soil to a minimum depth of 3m (a limit depth proposed by CEN TC 288, 2002) and is currently limited to treatment depth of about 50m. The binders are injected into the soil in dry or slurry form through

hollow rotating mixing shafts tipped with various cutting tools. The mixing shafts are also equipped with discontinuous auger flights, mixing blades or paddles to increase the efficiency of the mixing process. In some methods, the mechanical mixing is enhanced by simultaneously injecting fluid grout at high velocity through nozzles in the mixing or cutting tools. The complementary SMM has been specially developed to reduce the

costs of improving loose or soft superficial soils overlying substantial areas, including land disposed dredged sediments and wet organic soils a few metres thick. It is also a suitable method for in situ remediation of contaminated soils and sludges. In such applications, the soils have to be thoroughly mixed in situ with an appropriate amount of wet or dry binders to ensure stabilisation of the entire volume of treated soil. Therefore, this type of soil mixing is often referred to as ‘mass stabilisation’. Mass stabilisation can be achieved by installing vertical overlapping columns with up and down movements of rotating mixing tools, as in the case of DMM, and is most cost-effective when using large diameter mixing augers or multiple shaft arrangements. With this kind of equipment it is generally possible to stabilise very weak soils to a maximum depth of about 12m. More recently, however, another method of mass stabilisation has been

implemented, and the mixing process can now be carried out repeatedly in vertical and horizontal directions through the soil mass using various cutting and mixing arrangements that are different from the tools originally developed for DMM. The depth of treatment for this relatively new system is generally limited to about 5m. Consequently, in the classification scheme used in this chapter the SMM includes both systems of mass stabilisation. It is important to note that the differentiation between SMM and DMM is not solely attributed to the available depth of treatment criterion because, in principle, soil mixing at shallow depth can also be performed with DMM. In situ soil mixing is a versatile ground improvement method. It can be

used to stabilise a wide range of soils, including soft clays, silts and finegrained sands. Stabilisation of organic soils such as gyttja (sedimentary organic soil), peat and sludges is also possible, but is more difficult and requires carefully tailored binders and execution procedures. However, the engineering properties of the stabilised soil will not only depend on the characteristics of the binder. They will also depend, to a large extent, on the inherent characteristics of each soil and the way it has been deposited, as well as on mixing and curing conditions at a particular worksite. Therefore, a thorough understanding of chemical reactions with the above factors is necessary in order to ensure successful application of this ground improvement technology. In this chapter, the current status of in situ soil mixing is outlined, taking

into account recent execution and design practice, international literature and experience. General application areas are identified and discussed, and a few case histories selected from recent international projects are included

for illustration. The focus is on civil engineering applications of DMM and, to a lesser extent, of SMM. Some specialised soil mixing issues in relation to environmental projects, such as mass treatment of subsurface hazardous wastes by various processes including solidification, stabilisation and chemical treatments, reactive barriers, etc. are only touched upon, therefore the cited literature should be referred to for more information. Furthermore, overly extensive descriptions of the complicated chemical processes occurring in the stabilised soil when mixed with various binders have been excluded from the contents. This choice, however, should not undermine the importance of this aspect of soil mixing. It may rather reflect the fact that in spite of considerable knowledge about basic reaction mechanisms, identified and described for instance by Babasaki et al. (1996) for soils stabilised with lime or cement, it is still not possible to predict the strength of in situ mixed soil with a reasonable level of accuracy. As a consequence of this fundamental deficiency, which we are challenged to overcome, it is believed that the development of SM will be continued along a somewhat erratic experimental path, and will be to a large extent dependent on accumulated experiences. Therefore, the scope of this chapter instead concentrates on the characteristics of equipment in current use, execution procedures with reference to selected operational methods, applications, merits and the limitations of the technology. Design aspects as well as quality control and quality assurance issues of DMM are also considered. The design approach outlined herein follows the practice established in Japan, the USA and Europe, assuming that the treated soil is practically an impermeable material. The approach used with respect to deep mixing (DM) columns stabilised with unslaked lime or lime and cement, which may act as vertical drains, has been covered in Chapter 8.