중진지역에 적합한 액상화 평가 생략기준 및 지진규모 보정계수에 관한 연구 A Study on Magnitude Scaling Factors and Screening Limits of Liquefaction Potential Assessment in Moderate Earthquake Regions원문보기
기존의 액상화 평가법은 대부분 미국, 일본, 그리고 유럽과 같이 지진 발생빈도가 높고 그로 인한 액상화 피해가 빈번한 국가에서 주도적으로 연구되어왔다. 이런 지역적 특성을 토대로 개발된 액상화 평가방법들은 높은 지진규모(M=7.5)에 바탕을 두고 있다. 국내의 경우, 1997년 실제적인 내진 연구가 시작된 이래 액상화 평가의 구체적 규정은 항만시설의 내진설계 표준서(1999)에 언급된 바 있으나 이는 문헌연구를 통해 제시된 것으로 실제적이지 못하다. 그러므로, 국내 적합한 설계기준을 작성하기 위해서는 지진피해자료의 부족을 국내 지반을 대상으로 한 동적실내시험을 통하는 것이 바람직하며, 일반적인 정현하중 진동시험 보다 실제 지진하중 재하 시험이 훨씬 효과적일 수 있다. 본 연구에서는 실제 지진파 고유의 특성을 적용한 진동삼축 시험을 통하여 상대밀도와 세립분함유량의 변화에 따른 액상화 저항강도를 산정하였다. 실험결과를 국내의 대표적인 항만지역의 지진응답 해석 결과와 비교 분석하고 중진지역에 적합한 액상화 평가의 생략기준을 제시하였다. 또한 실제 지진하중 삼축실험 결과를 이용하여 국내 여건에 적합한 지진규모 보정계수를 제안하였다.
기존의 액상화 평가법은 대부분 미국, 일본, 그리고 유럽과 같이 지진 발생빈도가 높고 그로 인한 액상화 피해가 빈번한 국가에서 주도적으로 연구되어왔다. 이런 지역적 특성을 토대로 개발된 액상화 평가방법들은 높은 지진규모(M=7.5)에 바탕을 두고 있다. 국내의 경우, 1997년 실제적인 내진 연구가 시작된 이래 액상화 평가의 구체적 규정은 항만시설의 내진설계 표준서(1999)에 언급된 바 있으나 이는 문헌연구를 통해 제시된 것으로 실제적이지 못하다. 그러므로, 국내 적합한 설계기준을 작성하기 위해서는 지진피해자료의 부족을 국내 지반을 대상으로 한 동적실내시험을 통하는 것이 바람직하며, 일반적인 정현하중 진동시험 보다 실제 지진하중 재하 시험이 훨씬 효과적일 수 있다. 본 연구에서는 실제 지진파 고유의 특성을 적용한 진동삼축 시험을 통하여 상대밀도와 세립분함유량의 변화에 따른 액상화 저항강도를 산정하였다. 실험결과를 국내의 대표적인 항만지역의 지진응답 해석 결과와 비교 분석하고 중진지역에 적합한 액상화 평가의 생략기준을 제시하였다. 또한 실제 지진하중 삼축실험 결과를 이용하여 국내 여건에 적합한 지진규모 보정계수를 제안하였다.
Conventional methods for the assessment of liquefaction potential were primarily for areas of severe earthquake zones (M=7.5) such as North America and Japan. Detailed earthquake related researches in Korea started in 1997, including development of the seismic design standards for port and harbour s...
Conventional methods for the assessment of liquefaction potential were primarily for areas of severe earthquake zones (M=7.5) such as North America and Japan. Detailed earthquake related researches in Korea started in 1997, including development of the seismic design standards for port and harbour structures, which was later completed in 1999. Because most contents in the guidelines were quoted through literature reviews from North America and Japan, which are located in strong earthquake region, those are not proper in Korea, a moderate earthquake region. This requires further improvement of the present guidelines. Considering earthquake hazard data in Korea, use of laboratory tests based on irregular earthquake motion appears to be effective to reflect the dynamic characteristics of soil more realistically than those using simplified regular loading. In this study, cyclic triaxial tests using irregular earthquake motions are performed with different earthquake magnitudes, relative densities, and fines contents. Assessment of liquefaction potential in moderate earthquake regions is discussed based on various laboratory test results. Effects of these components on dynamic behavior of soils are discussed as well. From the test results, screening limits and magnitude scaling factors to determine the soil liquefaction resistance strength in seismic design were re-investigated and proposed using normalized maximum stress ratios under real irregular earthquake motions.
Conventional methods for the assessment of liquefaction potential were primarily for areas of severe earthquake zones (M=7.5) such as North America and Japan. Detailed earthquake related researches in Korea started in 1997, including development of the seismic design standards for port and harbour structures, which was later completed in 1999. Because most contents in the guidelines were quoted through literature reviews from North America and Japan, which are located in strong earthquake region, those are not proper in Korea, a moderate earthquake region. This requires further improvement of the present guidelines. Considering earthquake hazard data in Korea, use of laboratory tests based on irregular earthquake motion appears to be effective to reflect the dynamic characteristics of soil more realistically than those using simplified regular loading. In this study, cyclic triaxial tests using irregular earthquake motions are performed with different earthquake magnitudes, relative densities, and fines contents. Assessment of liquefaction potential in moderate earthquake regions is discussed based on various laboratory test results. Effects of these components on dynamic behavior of soils are discussed as well. From the test results, screening limits and magnitude scaling factors to determine the soil liquefaction resistance strength in seismic design were re-investigated and proposed using normalized maximum stress ratios under real irregular earthquake motions.
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가설 설정
(2) Proposed MSFs were lower than those recommended from the NCEER workshop. But, proposed MSF obtained in this study is 1.
제안 방법
In order to compare liquefaction resistances between irregular earthquake and equivalent sinusoidal cyclic loadings, additional tests were performed. Jumunjin sand samples with no fines were tested under confining pressure of 100 kPa and relative density of 60%.
Effects of fines content and relative density were also analyzed based on cyclic triaxial test results under real irregular earthquake motions. In particular, a series of cyclic tests under various soil conditions were carried out expecting to be used for future modification of the Korean seismic design standards in two aspects: criteria for screening limits and MSFs of liquefaction analysis. The final goal of this paper is to propose MSFs and to evaluate criteria for the earthquake screening limits under which detailed liquefoction analysis could be omitted in a design process.
If a denser sample is to be prepared, compaction energy is applied by hitting the side of the mold stepwise during the process of sample placement. In this study, densification of the sample was accomplished by carefully and symmetrically tapping the sides of the sample mold immediately after soil deposition. Because the mass of sand and silt used in sample preparation could be accurately estimated, it was possible to obtain a relative density that was reasonably close to a target value by measuring the height of the sample as it densifies.
Because MSFs and screening limits currently available are primarily for earthquakes with large magnitudes and clean sands, uncertainties still exist for moderate earthquake areas and silty sand deposits. In this study, laboratoiy cyclic tests using both regular sinusoidal and irregular earthquake loadings were performed and compared. Effects of fines content and relative density were also analyzed based on cyclic triaxial test results under real irregular earthquake motions.
The maximum possible frequency allowed in the system is 70 Hz. Measurements in the tests include axial deformations, volume changes, deviatoric stresses, and pore water pressures with time. Triaxial tests under sinusoidal loading were also carried out for the comparison of results between irregular earthquake and regular sinusoidal loadings.
Soils used in cyclic triaxial tests were the Jumunjin sand, a representative silica sand in Korea, with four different fines contents equal to 0, 10, 20, and 30%. Tests with clean sand samples (i.
Soils used in cyclic triaxial tests were the Jumunjin sand, a representative silica sand in Korea, with four different fines contents equal to 0, 10, 20, and 30%. Tests with clean sand samples (i.e., fines content of 0%) were used for investigation of MSF while criteria for liquefaction screening limits were analyzed using both clean and silty sand samples of fines contents equal to 0, 10, 20, and 30%. The silty sands were prepared by mixing regular Jumunjin sands and silt-sized crushed Jumunjin sands at desired weight ratios.
In particular, a series of cyclic tests under various soil conditions were carried out expecting to be used for future modification of the Korean seismic design standards in two aspects: criteria for screening limits and MSFs of liquefaction analysis. The final goal of this paper is to propose MSFs and to evaluate criteria for the earthquake screening limits under which detailed liquefoction analysis could be omitted in a design process.
To revise the present Korean seismic standards for the liquefaction potential assessment, cyclic triaxial tests using irregular earthquake motions, different earthquake magnitudes, relative densities, and fines contents were performed. The following primary conclusions are obtained as a result of this study:
Measurements in the tests include axial deformations, volume changes, deviatoric stresses, and pore water pressures with time. Triaxial tests under sinusoidal loading were also carried out for the comparison of results between irregular earthquake and regular sinusoidal loadings. For these tests with sinusoidal loadings, Jumunjin sands with no fines were used.
이론/모형
5 for earthquake-related design without detailed specification of MSFs. In this study, MSFs were re-investigated based on laboratory cyclic triaxial test results given in Fig. 8 using method derived by Seed and Idriss (1982). To propose MSFs in this study, the amplitudes of the applied loading, as quantified by maximum stress ratio, were determined as a function of the earthquake magnitudes required causing liquefaction in Fig.
Basic property tests were performed for each test soil, and results are shown in Table 1. The maximum and minimum index densities (i.e., minimum and maximum index void ratios) of soils with different fines contents were determined by the procedure specified in ASTM-4253 vibratoiy table and ASTM-4254 method B, respectively.
6 were selected as input motions for considering moderate earthquake magnitudes in Korea. The seismic demand can be determined for a soil profile at any desired depth based on results of maximum acceleration and stresses using the SHAKE91 analysis. In the SHAKE91 analysis, all the earthquake input motions with peak acceleration level of 0.
성능/효과
(3) Triaxial Tests were carried at conditions of different relative densities (60, 70, and 75%) and fines contents (0, 10, 20, and 30%). In the test results for both clean and silty sands, effects of the relative density appear to be significant representing higher liquefaction resistances with increasing relative densities. It is also observed that the liquefaction resistance of silty sands increases with increasing fines contents.
후속연구
It is also found that soils of a relative density equal to 70% with a fines content of 20% represents a similar condition mentioned above. Although additional investigations are needed, results obtained in this study can be used for further development of the liquefaction screening limits in Korea.
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