# Development of Rock Embedded Drilled Shaft Resistance Factors in Korea based on Field Tests
## 1 Introduction  
The load resistance factor design (LRFD) method can evaluate the resistance of a structure considering uncertainty based on the results of a reliability analysis. Using a reliability analysis, the probability of failure can be quantitatively evaluated, and the design of consistent and reliable geotechnical structures can be made possible. In recent decades, LRFD has replaced the allowable stress design (ASD) and working stress design (WSD) methods worldwide for the design of foundations [1; 2]. The LRFD method was introduced in South Korea about a decade ago, and has been reflected in the design code for foundations in the country since 2012. The new Korean Highway Bridge Design Code (KHBDC) (limit state design) was published in 2012 and took effect for field applications starting in 2015. The resistance factors for foundations suggested by KHBDC [3], however, are based on American Association of State Highway and Transportation Officials (AASHTO) [4] in the U.S.; indeed, almost all the resistance factors were simply borrowed from AASHTO. Resistance factors for drilled shaft in rock from AASHTO were suggested as 0.50-0.55 for shaft resistance factors, and 0.50 for base resistances. The resistance factors in AASHTO, however, reflect the characteristics of the bedrock found in the U.S., which may differ from the characteristics of the bedrock found in Korea. Notably, the resistance factors have already been calibrated for a number of states in U.S. based on area-specific bedrock types [5; 6; 7]. Also, the representative bedrock types in Korea are granite and gneiss classified as moderate or hard rock by strength criterion, they exhibit significant discontinuity and weathering. As such, these rocks are classified as weathered or soft rock by the weathering degree criterion [8]. This classification constitutes a discrepancy between the bedrock conditions in the U.S. and Korea, and as such, the direct application of the AASHTO resistance factors to Korean domestic design is inappropriate. Considering these discrepancies, a great deal of research into resistance factors has been conducted in Korea. In 2008, the Korea Institute of Construction Technology [9] suggested total resistance factors for driven steel pipe piles (0.29-0.38 for target reliability index of 2.33) and drilled shafts (0.20-0.70 for target reliability index of 3.0). [PERSON] [10] proposed total, shaft, and base resistance factors for drilled shafts using a numerical analysis. From [PERSON]'s research, resistance factors for target reliability index of 3.0 were determined as 0.55-0.80 for total resistance, 0.20 for shaft resistance, and 0.20-0.70 for base resistance. [PERSON] et al. [11] suggested the base and shaft resistance factors for axially loaded driven piles in sands based on the Imperial College pile (ICP) design method. The calculated resistance factors (target reliability index of 3.0) were 0.61-0.86 for base resistance and 0.55-0.96 for shaft resistance. [PERSON] [12; 13] updated the total resistance factors for driven steel pipe piles based on Bayesian theory to incorporate the results of proof tests. The updated resistance factors were determined as 0.19-0.71 for target reliability index of 3.0. Recently, there were some researches to suggest the resistance factors for deep foundations using load test data in domestic region [14; 15] and researches for suggestion of resistance factors and comparison according to design methods for shallow foundations [16; 17]. Most of this previous research, however, has been limited to suggesting the total resistance factors, and the validity of the shaft and base resistance factors suggested by [PERSON] are somewhat doubtful as they are derived from the results of a numerical analysis. Therefore, it remains necessary to determine accurate resistance factors for drilled shafts considering the discrepancy between the rock conditions in the U.S. and Korea using reliable load test results.