# Mechanical Properties of Cement-Treated Soil Mixed with Cellulose Nanofibre
## 5 Effect of CNF on Permeability  
### Test Method  
In addition to the strength of the treated soil, its permeability was also investigated. Previous studies showed that the permeability of clayey soils decreases when they are mixed with cement [28; 29; 30]. The treated soil is occasionally used as an impervious material, and permeability is one of the most important aspects of the treated soil. In addition, a high permeability indicates that the calcium ions in the treated soil can easily diffuse to the outside, and therefore, permeability is an important parameter for evaluating the deterioration of the treated soil. It is known that treated soils start degrading from the exposed surface [31; 32; 33; 34; 35; 36], which is one of the points that must be understood when using cement treatment in the field. This section discusses how the permeability changes with the addition of CNFs.  
The specimens prepared using Method 2 were cured for more than 200 days. The unconfined compression test results of the specimens are shown in Figure 11c. As part of the specimen was used for the permeability test, the unconfined compression tests in this study did not use specimens of the size specified in JIS A 1216 and JGS 0511 [21; 22]. It should be noted that because the compression tests were conducted on small prismatic specimens (15 mm wide \(\times\) 15 mm deep \(\times\) 30 mm high), the test results could contain errors. All other methods were implemented in accordance with the standards. The unconfined compression test results showed that the strength of the specimens with CNF was lower than that without CNF. This was in agreement with the trend of the results described in Section 3, where it was found that the addition of CNF reduced the long-term strength.  
The remainder of the treated soil in the unconfined compression test was subjected to a permeability test. As there is no standard method for permeability testing of treated soil, the tests were conducted using the method utilised in the past study [37]. Figure 15 shows a conceptual diagram of the test apparatus. In this test method, a cylindrical specimen with a diameter of 50 mm and height of 30 mm was permeated by percolating water above the specimen, applying confining pressure from the side through a membrane. The lateral confining pressure prevented the upper pressurised water from leaking at the side of the specimen. The water pressure above the specimen was 400 kN/m\({}^{2}\), and the average hydraulic gradient inside the specimen increased to 1361. This was because the permeability of the treated clayey soil was low, and the hydraulic gradient had to be increased to increase the drainage rate. The drainage water that passed through the specimen was collected to determine the permeability per unit time. The pH, calcium ion concentration, and sodium ion concentration were measured periodically in the collected effluent. The permeability period depended on the specimen: 114 days for 'no CNF', 114 days for TEMPO-CNF, and 49 and 97 days for ACC-CNF-S, respectively. The reason for the long-term measurements was to check whether the addition of CNF would change the condition of the effluent, as it is known that continued permeability degrades the treated soil.  
Figure 15: Schematic view of test apparatus.