# Development of Rock Embedded Drilled Shaft Resistance Factors in Korea based on Field Tests
## 4 Conclusions  
The load resistance factor design (LRFD) method can be used evaluate the resistance of a structure considering uncertainty based on a reliability analysis. In recent decades, LRFD has replaced allowable stress design (ASD) and working stress design (WSD) for the design of foundations worldwide, and is reflected in the design code for foundations in South Korea. However, the resistance factors used in Korea are based on those recommended by AASHTO in the U.S., which reflect characteristics of the bedrock in the U.S. that may differ from those of the bedrock in Korea. Based on such discrepancies in rock conditions, the direct application of the AASHTO-recommended resistance factors for design in Korea is inappropriate. Therefore, it was necessary to determine accurate resistance factors for drilled shafts considering the rock conditions in Korea based on reliable load test results.  
In this study, 13 sets of drilled shaft load test data were obtained using strain gauges, and a load transfer analysis was performed to determine the accurate shaft and base resistance values. For bi-directional load tests, the equivalent load-displacement curve was drawn to determine the total resistance. Calibrations of the elastic modulus of the drilled shaft concrete and of the equivalent load-displacement curve considering the axial load and elastic settlement were conducted to obtain more accurate resistance values. After determining accurate resistance values, a reliability analysis was performed to determine the target reliability index and the resistance factors using the advanced first-order second-moment (AFOSM) reliability method.  
For the AASHTO-recommended target reliability index of 3.0, the shaft resistance factors were found to be within 0.13-0.32 of the AASHTO-based values, the base resistance factors were within 0.19-0.29 of the AASHTO-based values, and the total resistance factors were within 0.28-0.42 of the AASHTO-based values for each bearing capacity equation evaluated. The resistance factors were accordingly determined to be 30-60% of the AASHTO-recommended values for the shaft resistance and 40-60% of the AASHTO-recommended values for the base resistance. These differences in resistance factors are the result of discrepancies in the condition of the rocks in which the drilled shafts were founded. The resistance factors suggested by AASHTO are determined using sedimentary and intact rock conditions prevalent in the US, while the bedrock in Korea tends to be weathered or soft granite and gneiss. This study has thus determined adjusted resistance factors to account for this difference in rock type and condition, providing improved design safety and load resistance accuracy when using drilled shafts in Korea. Also, suggested resistance factors were based on only 13 load test cases considering the regional characteristics in Korea. But, if additional load test data is collected, LRFDmethods can update and calibrate the resistance factors based on reliability analysis. So, the results in this study are expected to be helpful for future reference.  
[PERSON] organized the paperwork, performed reliability analysis and suggested the new resistance factors; [PERSON] supported the data collection and reliability analysis; [PERSON] initially suggested the idea of the study and helped the data interpretation; [PERSON] supported the reliability analysis and provided the guidance for resistance factor suggestion; all authors contributed to the writing of paper.  
This research was supported by a grant from (1) Residential Environment Research Program (19 RERP-B099826-05) funded by Ministry of Land, Infrastructure and Transport of Korean government and (2) R&D Program (PK1902A4) of the Korea Railroad Research Institute, Republic of Korea and (3) Review of Environmental Impact Assessment funded by Korea Environment Institute.  
The authors declare no conflict of interest.