# Effect of Micropiles on Clean Sand Liquefaction Risk Based on CPT and SPT
## 3 Discussion and Results
### Correlations of CPT and SPT Values Before and After Jet Grouting  
Figure 9 highlights correlations of q\({}_{\text{C1 NCS}}\) and \(\left(N_{1}\right)_{60\text{cs}}\) indices before and after jet grouting for SP clean sands and soil layers containing SP-SC, SP-SM, SC, and SM. For each correlation process, the soil is categorized if it includes SP layers. The results are based on a total of 10 boreholes, five for each of the CPT and SPT tests. The combined results are presented in a single analysis for each test and based on the soil profile. The R-squared coefficient of determination was used to analyze the relationships between the improved \(\left(N_{1}\right)_{60\text{cs}}\), and among the enhanced q\({}_{\text{C1 NCS}}\) indices. In general, a correlation coefficient closer to 1 shows a stronger relationship between the measured values. Previous studies preferred q\({}_{\text{C1 NCS}}\) as a more consistent index over the sleeve friction index f\({}_{\text{S}}\) to analyze the outcome of the CPT [51].  
Figure 8: SPT-based liquefaction analysis of clean sand for \(\sigma^{\prime}_{\
u 0}=100\) KPa and \(CRR_{M=7.5}\), following NCEER recommendations for: (**a**) Liquefaction assessment before jet grouting, (**b**) liquefaction evaluation after jet grouting using micropiles and according to Eurocode 8, (**c**) liquefaction assessment before jet grouting, (**d**) liquefaction evaluation after jet grouting using micropiles.  
The left graph in Figure 9 aims to show the effect of jet grouting to improve the \(\mathrm{q_{CINCS}}\) index in the CPT. This graph includes a total of 50 data pairs, before and after jet grouting: 32 pairs as coarse-grained clean sand SP, and the remaining 18 pairs as fine-grained soils. The \(\mathrm{q_{CINCS}}\) index before the soil remediation in the SP layers ranged between 71.4 and 170.5. The lower and upper values of the \(\mathrm{q_{CINCS}}\) after SP soil stabilization were measured as 117.8 and 232.8, respectively. The clean sand soil had relatively higher R-squared values compared to the fine-grained soils. The effects of jet grouting by micropiles to increase the \(\mathrm{q_{CINCS}}\) values were noteworthy for both types of soils. The figure indicates a moderate linear relationship between \(\mathrm{q_{CINCS}}\) values before and after jet grouting for the clean sand with \(R^{2}=0.497\). There was a strong linear correlation between \(\mathrm{q_{CINCS}}\) values for the fine-grained soil, \(R^{2}=0.786\), based on the CPT.  
The correlation between \(\left(N_{1}\right)_{60\mathrm{cs}}\) values before and after jet grouting is presented in the right graph in Figure 9. Thirty-one pairs of data belong to the SP soil, and the other 19 data pairs are related to the fine-grained soil in the project. The \(\left(N_{1}\right)_{60\mathrm{cs}}\) values for the SP soil before the jet grouting measured between 19 and 35. The \(\left(N_{1}\right)_{60\mathrm{cs}}\) improved values ranged from 24 to 41. The R-squared values showed comparable and strong correlations between \(\left(N_{1}\right)_{60\mathrm{cs}}\) for both coarse-grained and fine-grained soils. The \(R^{2}=0.847\) indicated a stronger correlation between the \(\left(N_{1}\right)_{60\mathrm{cs}}\) values in the SP compared to the soil containing clay and silt with \(R^{2}=0.796\).  
The soil profiles at both sites I and II mainly contained clean sand SP layers, and both CPT and SPT results showed positive linear relationships between \(\mathrm{q_{CINCS}}\) and among \(\left(N_{1}\right)_{60\mathrm{cs}}\) indices before and after the jet grouting method using micropiles. Such an improvement in soil properties emphasizes the effectiveness of this method in soil including clean sands with relatively thin layers of fine-grained soil.