# Mechanical Properties of Cement-Treated Soil Mixed with Cellulose Nanofibre
## 4 Effect of CNF on Strength Variability  
### Dispersion of Unconfined Compressive Strength  
CNF has dispersive properties that make substances in liquids homogeneous, and this property has been considered for use in a variety of applications. CNF is also expected to improve the dispersibility of cement when it is mixed with the soil, and its thixotropy property increases the uniformity of cement. The uniform mixing of cement with soil could reduce the variation in strength. This section presents the test results regarding the effect of CNF mixing on the strength variability. It is important to consider variability, as a reduction in the strength variability can reduce the design strength. Figure 10 shows the relationship between the frequency distribution of intensity occurrences and the design strength. During design, the percent defective is determined at the beginning, and the mean strength (design strength) is determined so that it is satisfied. As shown in the figure, the smaller the variance in strength, the lower the mean strength for the same percent defect. The advantages of reducing the strength and dispersion are significant. This reduces the risk that large dispersals will result in treated soil of extremely high strength as well as high cost of cement and high environmental impact of cement production. As mentioned before, large-strength treated soil can hinder the rehabilitation of geotechnical structures.  
Unconfined compression tests were conducted at 28 days of age on a total of 60 specimens prepared by Method 2 with a short mixing time of 1 min. The axial compressive stress-strain relationships obtained from the tests are shown in Figure 11a,b. Figure 11c shows the results of the unconfined compression test of the treated soil used for the permeability test (preparation method: Method 2, mixing time: 5 min), which is discussed later in Section 5. Most of the peak strengths (unconfined compressive strengths) of the specimens without CNF were below  
Figure 10: Conceptual diagram of frequency distribution of strength and design strength.  
50 kN/m\({}^{2}\), which was quite low. By contrast, the peak strengths of the specimens with CNF were relatively higher, with unconfined compressive strengths ranging from 100 to 350 kN/m\({}^{2}\). Comparing the mean values, the unconfined compressive strength was 43 kN/m\({}^{2}\) (without CNF) and 219 kN/m\({}^{2}\) (with CNF); the strength ratio was significantly larger: 5.1 times. As the strength ratio of the well-mixed specimens with and without CNF at 28 days of age was 1.1-1.3 times, as mentioned in Section 3, the large strength ratio of 5.1 indicates that the addition of CNF caused the cement to mix evenly with the soil. The failure strains of the specimens with CNF were relatively low owing to the higher strength of the specimens. The average failure strains were 3.9% (without CNFs) and 1.4% (with CNFs).  
Figure 12 shows photographs of the sheared surfaces generated by the unconfined compression tests. The sheared surface of the specimen without CNF showed that the solidified portions were scattered within the poorly solidified soil. The unconfined compressive strength was extremely low, which was probably because the solidified portions were not continuous. On the other hand, the sheared surface of the specimen with CNF appeared to be homogeneous, suggesting that the cement spread over the whole specimen, and that this condition led to the development of a relatively high unconfined compressive strength.