# Effect of Micropiles on Clean Sand Liquefaction Risk Based on CPT and SPT
## 3 Discussion and Results
### Comparative Analyses of Factor of Safety Against Liquefaction in Clean Sands  
In this section, improvements of the factor of safety against liquefaction in soil layers containing only SP clean sands is analyzed following CPT-based and SPT-based approaches. Figure 10 highlights the results of the factor of safety before and after soil remediation using micropiles at various depths in the SP soil based on the CPT method as a stacked column chart. Similar depth values belong to different borehole numbers in site I. The Juang method predicts the most improvement rate for the factor of safety, whereas the Boulanger and Idriss method reports the most conservative factor of safety assessment. These two methods, along with the Eurocode specifications, showed similar trends in enhancing the factor of safety based on the CPT results after jet grouting, as is illustrated by the dashed lines in the Figure 11 (left). A maximum factor of safety value of 5 after jet grouting was recorded for SP soil layers at depths of 6, 8, 9, and 12.5 m according to the Juang and Eurocode criteria, and for a 3.5 m depth based on the Boulanger and Idriss approach. The average percentage change of the factor of safety in clean sand following the Boulanger and Idriss approach was 77%, after jet grouting. This value increased to 110 and 178% in accordance with the Eurocode and Juang methods, respectively.  
Figure 9: Relationships between \(\mathrm{q_{CINCS}}\) values (**left**) and \(\left(N_{1}\right)_{60\mathrm{cs}}\) values (**right**) before and after jet grouting using micropiles for coarse-grained clean sands (SP) and fine-grained soil SP-SC, SP-SM, SC, SM.  
Figure 12 reports the factor of safety values based on SPT results prior to and after jet grouting by micropiles at different depths of SP layers in site II. The same depth values in the chart are related to different borehole numbers in site II. Three SPT-based approaches to assess the potential of liquefaction were used in the analysis procedure. Boulanger and Idriss, Eurocode, and NCEER methods showed equal factors of safety of 5 at depths of 6 m (BH-II-3), 15 m, and 17 m (BH-II-1), along with 13.5 and 17.5 m (BH-II-2), after soil remediation. The dramatic increase of the factor of safety to 5 was also reported at other depths. However, the results may vary based on the methods used, as shown in the Figure 12. The trend lines of average values for the factor of safety using the three SPT-based methods are shown separately in Figure 11 (right). In this Figure, all three methods are comparable and indicate an increase of the factor of safety after soil remediation. The average percentage changes based on the  
Figure 11: Comparison of the factor of safety values based on two-point moving averages, according to three CPT-based method (**left**) along with three SPT-based approaches (**right**) for assessing the improvement of the clean sand soil against the liquefaction, after jet grouting by micropiles.  
Figure 10: Improvement of the factor of safety against liquefaction triggering based on CPT, before and after jet grouting using micropiles in soil layers containing only poorly graded sand (clean sand).  
Boulanger and Idriss and NCEER methods are 187 and 169%, respectively. Eurocode conservatively predicts 110% of improvement after the jet grouting.