## 1 Introduction  
Soil is the loose particulate natural material that covers the Earth's surface. Soil is a multiphase material containing an aqueous, gaseous and solid phase, each composed of inorganic and organic components. The interactions and relative proportions between the different components of the soil, and the arrangements, size and shape of the solid particles determine the soil's physical and chemical properties [1, 2], and ultimately its geomechanical behavior (soil's response in terms of strength and deformability to external actions). In many cases, the soil does not meet the safety and stability requirements for construction, and ground improvement techniques are required [3, 4, 5, 6]. This is the case of soft soils, characterized by exhibiting low strength and high deformability. One of the most successful ground improvement techniques applied to soft soils is chemical stabilization with additives [7, 8, 9, 10]. This ground improvement technique consists of in situ mixing additives to the soil, aiming to increase the soil's strength and decrease the soil's deformability. (There may be other objectives such as reducing permeability or immobilization of pollutants in the soil). The traditional additives most used in chemical stabilization of soft soils are Portland cement and lime, applied alone or in combination in percentages ranging from 5% to 20% \(w/w\) (additive/soil) [11; 12; 13]. These additives have high costs and high environmental impacts associated to their production, which encourages the development of new additives. Industrial byproducts (e.g., slag, fly ash, rice ash), pozzolanic materials (e.g., fly ash, natural pozzolana, silica fume), biobased products (e.g., polymers, enzymes) and nanomaterials (e.g., carbon nanotubes, carbon nanofibers, nano-ashes, nanoclays) are some examples of promising additives that may be used as a total or partial replacement of the traditional additives [11; 13; 14; 15; 16; 17; 18; 19; 20]. Additives consisting of extremely fine particles (nanomaterials) are particularly attractive for use as replacement of part of cementitious additives, resulting in environmental, technical and economic advantages [21; 22; 23]. Due to extraordinary properties of carbon nanotubes (CNTs) (fine structure, ultrahigh specific surface, very high strength and moduli of elasticity, elastic and ductile behavior [24; 25; 26; 27; 28]), they have a great potential to be used as an additive in chemical stabilization of soils, replacing part of the cementitious main additive. However, due to CNT morphology and very high aspect ratio, CNTs have a natural tendency to aggregate, resulting in the loss of their beneficial properties [29; 30]. Different strategies have been proposed to minimize this problem, including mechanical methods (e.g., ultrasonic energy applied to disperse CNTs in suspension) and chemical methods (e.g., functionalization of CNT surface by the addition of surfactants/polymers to the system), among others. The introduction of surfactants has a double advantage since it allows the dispersion of the CNTs and other additive particles, while at the same time minimizes ultrasound energy requirements. The application of CNTs to geotechnics is still at the laboratory development and proof-of-concept stage, with few studies published thus far, being possible to conclude the following: (i) The introduction of CNTs in a content of 0.2% up to 1% of soil's dry weight is able to increase slightly the specific gravity, dry density and pH [31]; increase the plasticity index; increase the compression and swelling indices and reduce the hydraulic conductivity of the soil [32]. (ii) Mixing a clayey sand with CNTs applied in a content of 0.05-3% by weight of the soil promotes an increase of the compressive strength of the composite material up to 120% when compared with the original clayey soil, and increases the cohesion while decreasing the friction angle [33].