# Mechanical Properties of Cement-Treated Soil Mixed with Cellulose Nanofibre
## 1 Introduction  
In recent years, it has become possible to produce cellulose nanofibre (CNF), which is a material composed of ultrafine fibres of wood cellulose fibrillated to nano-order level [1, 2, 3]. CNF is expected to be widely used because its strength is five times and its mass is one-fifth that of steel, and its thermal expansion is one-fifth that of glass. The characteristics of CNFs, such as high specific surface area, edibility, light weight, high strength, low thermal expansion, biodegradability, and biocompatibility, have been utilised to develop various applications. For example, transparent resins such as acrylics and epoxy resins are reinforced without much loss of their transparency (transparency reinforcement). The characteristics of light-weight and high-strength fibres are used to produce moulded products that are as strong as steel but one-fifth their mass by injecting phenolic resin into nanofibre sheets and then laminating and curing them. In addition, the thixotropy (the property that the fluidity increases when an external force is applied) of CNF solutions has already been used to commercialise ball-point pen haks, which prevent dripping and ease writing. In the field of concrete, the addition of CNFs to improve the strength of concrete has been studied [4, 5, 6]. This is attributed to its ability to accelerate the cement hydration reaction as well as to the high strength of CNF itself. The use of CNF as a stabiliser to prevent the separation of concrete compositions and bleeding before solidification, and as an additive to increase the flowability required in the pressure injection method using thixotropic properties, are being investigated and have been partly implemented. In the field of geotechnical engineering, very little progress has been made in the development of techniques for using CNFs. The authors have focused on the use of CNF as an additive in cement treatment for soft ground, where cement is added to solidify the ground, for example, by the deep mixing method [7] to improve the ground and to construct geotechnical structures. This is because cement treatment is a relatively inexpensive and highly reliable ground improvement method. Although it is widely used, there are still some problems such as brittleness, low tensile strength, strength variability, and low short-term strength. If more cement is added to solve these problems, the environmental impact will increase. If the long-term strength of the treated soil is too high, the treated soil could make rehabilitation of the geotechnical structure difficult. The possibility of reducing these problems using the CNF effect can be considered as follows: (1) The high strength of fibres increases the strength of the treated soil and reduces the risk of brittle or tensile failure. (2) As in concrete, CNFs accelerate the cement hydration reaction. (3) Owing to its water absorption and thickening properties, shear strength can be expected to develop immediately after stirring. (4) Thixotropy improves uniformity when stirring and mixing cement, and it ensures fluidity when pumping the treated soil in the slurry state before solidification. (5) Dispersibility increases uniformity during the stirring and mixing process. Previous studies have shown that fibrous materials of a few millimetres in length can be used for mixing with the treated soil [8; 9; 10; 11; 12; 13; 14; 15]. The mixture of long fibres can have the effect described in (1) above, although not many other effects can be expected. When using existing ground improvement machines, such as deep mixing machines, it is difficult to stir long fibres into the treated soil, which presents a number of challenges for on-site use. Carbon nanotubes (CNTs) are similar to CNF, and research has been conducted on mixing them into the treated soil [16; 17]. In these studies, the addition of these materials was found to increase the initial strength of the treated soil, even at the nano level. However, CNTs are poorly dispersible and require the addition of dispersants.