Proposed Shear Load-transfer Curves for Prebored and Precast Steel Piles

강관 매입말뚝의 주면 하중전이 곡선(t-z) 제안

Abstract

In this study, the load-transfer behavior along the shaft of the prebored and precast piles was investigated by pile loading tests. Special attention was given to quantifying the skin frictions developed between the pile-soil interfaces of the 14 instrumented test piles. Based on this detailed field tests, the load - settlement curves and axial load distributions of piles were obtained and the load-transfer curves (t-z curves) for the test piles were proposed. As such, it is found that the test results show two different load transfer behaviors; ductile and brittle behavior curves. The corresponding t-z curves are proposed based on the hyperbolic- and sawtooth-shape, respectively. By validating the accuracy of the proposed curves, it is also found that the prediction results based on the proposed load-transfer curve are in good agreement with the general trends observed by the field loading tests.

본 연구에서는 매입말뚝 주면부의 하중전이 거동을 확인하기 위하여 14본의 강관매입말뚝에 대한 정재하시험을 수행하였다. 정재하시험을 통하여 하중-침하 곡선 및 축하중 분포를 계측하였으며, 그 결과 말뚝-지반 사이에서 발생되는 주면마찰력을 정량화 하였다. 이를 토대로 강관매입말뚝의 일반적인 하중전이곡선 형태를 도출하였다. 본 연구 결과, 말뚝의 주면하중전이 거동은 연성과 취성파괴 두 가지 형태로 나타났다. 그 결과, 강관매입말뚝의 t-z 곡선을 쌍곡선(일수현상이 없는 경우)과 톱니 형태(일수현상이 있는 경우 또는 표준관입저항치, $N{\geq}50$인 풍화암 이상)로 각각 제안하였다. 그때의 최대주면마찰력은 쌍곡선의 경우 지반의 N치 또는 암반의 일축압축강도, 톱니 형태의 경우 주입된 시멘트풀에 의해 형성된 쏘일-시멘트층의 일축압축강도의 영향을 받는 것으로 나타났다.

Fig. 1. Shear load-transfer curve (Bilinear)

Fig. 2. Shear load-transfer curve of field load test (Brittle)

Fig. 3. Load transfer curves (Castelli, 1992)

Fig. 4. Grain size distribution curves of field load test

Fig. 5. Test pile embedment and soil condition

Fig. 6. A schematic representation of instrumented piles

Fig. 7. Load-settlement curves of field load test

Fig. 8. Shear load-distribution curve of field load test

Fig. 9. Comparison of shear load-transfer curve load test and proposed (Ductile)

Fig. 10. Comparison of shear load-transfer curve load test and proposed (Brittle)

Fig. 11. Soil profile with borehole and embedment for test piles

Fig. 12. Shear load-transfer curve (Baquelin, Vijayvergiya, Proposed)

Fig. 13. Comparison of load-settlement curves field load test(Baquelin, Vijayvergiya, Proposed)

Table 1. Physical properties of in-situ soil

Table 2. Physical properties of laboratory test

Table 3. Summary of field pile loading test

Table 4. Physical properties of piles (Ductile load transfer curve)

Table 5. Physical properties of weathered rock

Table 6. Comparison of shaft resistance (Measured – Equation)

Table 7. Ultimate unit skin resistance formula

Table 8. Empirical factors for shaft resistance 𝑎, β (Ductile)

Table 9. Physical properties of piles (Brittle load transfer curve)

Table 10. Empirical factors for shaft resistance 𝑎, β (Brittle)

Table 11. Summary of the proposed t-z curves