## 4 Long-Term Properties of the Interior of Cement-Treated Soil
### Unconfined Compressive Strength and Needle Penetration Resistance  
The distributions of the unconfined compressive strength of the treated soil in the depth direction obtained from the five surveys are shown in Figure 9, along with the mean variation over time. For the 2002 survey, only the results for the samples collected at sites P23 and H24 are shown. Figure 9a shows that although the results varied, they were generally distributed in the range of 100-500 kN/m\({}^{2}\), with most of the data exceeding the design strength of 120 kN/m\({}^{2}\). The average value of the variation was also higher than the average value of 157 kN/m\({}^{2}\) estimated at the time of the design. The figure shows the regression line of the unconfined compressive strength in relation to the depth. According to the regression line, the strength tended to decrease with an increase in depth. The above-mentioned distribution of water content showed that the water content tended to be slightly higher at deeper depths, suggesting that some water may have been taken in by the submerged casting. However, considering the design strength, the difference in strength in the depth direction was not a problem. Figure 9b, which shows the change in strength over time, indicates that there was no long-term decrease in strength, although small and large values were observed depending on the year of investigation. The range of standard deviations (1\(\sigma\)) from the mean value is shown by error bars, and it can be seen that the mean value minus the standard deviation was larger than the design strength for all the survey years. From these results, it can be concluded that overall, the strength properties of the treated soil were almost stable.  
In the 2006 and 2012 surveys, needle penetration tests were conducted on cuttings from the interiors of the treated soil samples. As representative examples, Figure 10 shows the results of the 2012 survey, which was conducted ten years after construction. These results  
Figure 8: Distributions of water content and wet density in the depth direction.  
Figure 9: Depth distribution and ageing of unconfined compressive strength.  
demonstrate the relationship between the penetration depth and penetration resistance. For the uniformly treated soil, the relationship is proportional, as shown in the figure, and this gradient represents the stiffness and strength of the treated soil. If the strength had been reduced by deterioration, the gradient would be smaller. Conversely, in areas where the strength was higher, the gradient would be larger. No changes in these gradients were observed in the results presented in the examples, indicating that the strength of the treated soil was uniform. According to the Japanese Geotechnical Society Standards [21], a gradient up to a penetration of 10 mm is referred to as the needle penetration gradient. The relationship between this value and the unconfined compressive strength of a specimen in the vicinity of the needle penetration test is shown in Figure 11. This figure also shows the correlation between the two obtained in previous studies [24]. The plotted data show that the correlations did not deviate significantly from those obtained in a previous study and were typical of cement-treated soils.