낙동강 대심도 퇴적토의 하부 미개량 점토층이 항만구조물의 침하에 미치는 영향

다국어 초록 (Multilingual Abstract)

This study investigates the effect of ground improvement on the performance of port structures, with a particular focus on the lower clay layer situated beneath the intermediate sand stratum within the deep alluvial deposits of the Nakdong River. To this end, a comprehensive review was conducted on previous research concerning the depositional history and geotechnical characteristics of the deep alluvial sediments, as well as studies addressing the long-term settlement behavior of the underlying unimproved clay layer. Specifically, at the Busan North Port （Dongsam） International Cruise Terminal, the deep cement mixing （DCM） method applied as the ground improvement for the constructed block-type revetment was originally planned to improve the clay layer beneath the intermediate sand layer. However, in actual construction, the DCM columns were installed only within the intermediate sand layer leaving the underlying clay layer untreated. As a result, continuous displacement and structural damage occurred over time. The causes of the structural damage were investigated through site investigations including borehole drilling, elastic wave tomography and borehole imaging. Additionally, by analyzing the instrumentation data measured at the time of structural displacement along with settlement and structural deformation calculated through stress-strain numerical simulations, this study aimed to evaluate the effect of ground improvement of the lower clay layer beneath the intermediate sand stratum within the deep alluvial deposits of the Nakdong River on the performance of port structures. In the 80-meter extension section of the Busan North Port （Dongsam） International Cruise Terminal, both an intermediate sand layer and an underlying clay layer were identified. The N-values of the upper clay layer were found to be 0/30, those of the intermediate sand layer ranged from 4/30 to 33/30, and those of the lower clay layer ranged from 0/30 to 3/30. The thickness of the intermediate sand layer was identified to be approximately 7.2 to 8.7 meters, while the thickness of the lower clay layer was found to range from 3 to 6 meters. In the original design, ground improvement was planned to target the clay layer beneath the intermediate sand layer. However, during actual construction, the deep cement mixing （DCM） columns were installed only within the intermediate sand layer, leaving the underlying clay layer untreated. As a result, continuous displacement and structural damage occurred over time. The causes of the structural damage were identified through geotechnical investigations, including borehole drilling, elastic wave tomography, and borehole imaging. Settlement analysis indicated that the consolidation settlement of the lower clay layer beneath the intermediate sand continued over time, resulting in cracks in the capping concrete. However, despite the ground settlement, the measured settlement at the top of the capping concrete appeared to converge due to the interlocking effect of the WAROCK blocks. Later, the interlocking effect was lost as the WAROCK blocks became dislodged, causing a rapid settlement of the capping concrete. This settlement then converged with the calculated values from settlement and stress-strain numerical analyses. As the interlocking behavior of the blocks could not be simulated in the stress-strain analysis, the convergence of the measured and calculated values confirmed the reliability of the numerical analysis results. To ensure the stability of the port structures, additional ground improvement was planned for the target soil layer, and the jet grouting method was applied. The improvement depth covered the entire lower portion beneath the upper short column array of the deep cement mixing （DCM） method. After applying jet grouting, borehole investigations, elastic wave tomography, and uniaxial compressive strength tests on the improved soil were conducted to verify the quality of the treatment. This study investigated the effects of the lower clay layer beneath the intermediate sand stratum on port structures. It was verified through field instrumentation and stress-strain analyses that when a soft clay layer is present beneath the intermediate sand and remains unimproved, consolidation settlement in the lower clay layer over time can cause damage to port structures. By studying the correlation between the thickness of the clay layer in the middle sand layer located in the deep deposition section of the Nakdonggang River and the residual amount of subsidence to secure the stability of port structures and port background sites, the effect of the thickness of the lower clay layer on stability was studied, and based on this, reasonable maintenance guidelines for port structures and port background sites were proposed if the lower clay layer was not improved. The deep alluvial deposits of the Nakdong River extensively contain an intermediate sand layer and a lower clay layer beneath the upper clay layer; however, research on the lower clay layer remains insufficient. Therefore, further studies on the distribution, thickness, and physical and mechanical properties of the lower clay layer are considered necessary.

목차 (Table of Contents)

• 제 1 장 서 론 1
• 1.1 연구 배경 1
• 1.2 연구 목적 3
• 제 2 장 낙동강 대심도 퇴적토의 지반공학 특성 5
• 2.1 낙동강 조간대의 퇴적이력 특성 5
• 2.1.1 퇴적환경의 구분 5
• 2.1.2 낙동강 삼각주 퇴적특성 분석 사례 13
• 2.1.3 낙동강 조간대의 퇴적단위 종합 분석 결과 18
• 2.1.4 낙동강 지역의 퇴적특성 선행연구 성과에 대한 고찰 21
• 2.2 낙동강 하구지역 퇴적점토의 물리·역학적 특성 23
• 2.2.1 퇴적환경 및 물리적 특성변화에 따른 낙동강 하구 점토의 구분 23
• 2.2.2 낙동강 하구지역 퇴적점토의 물리 특성 및 압밀 특성의 상관관계 33
• 2.2.3 낙동강 하구지역 하부 점토지반의 물리·역학적 특성 분석 36
• 제 3 장 하부점토층 미개량에 따른 피해 및 원인 분석 39
• 3.1 하부점토층 미개량 사례 조사 39
• 3.1.1 항만배후부지내 연직배수재 지반개량 사례 40
• 3.1.2 항만구조물 하부 기초보강 지반개량 사례 48
• 3.2 하부점토층 미개량에 따른 피해현황 및 원인 분석 52
• 3.2.1 연구대상 항만구조물 설계 현황 52
• 3.2.2 연구대상 항만구조물 시공현황 59
• 3.2.3 현장 계측결과와 비교 분석을 통한 피해 원인분석 74
• 3.2.4 항만구조물의 피해 원인 분석 92
• 제 4 장 구조물 안정성 확보를 위한 추가 지반개량 방안 93
• 4.1 추가 개량공법 선정 93
• 4.1.1 미개량 점토층만 개량 시 해석결과 93
• 4.1.2 DCM 단주열 하부 개량 시 해석결과 95
• 4.1.3 최적 개량심도 결정 98
• 4.2 추가 개량 효과 검증 99
• 4.2.1 수치해석을 통한 보강 전·후 비교 분석 99
• 4.2.2 지반개량 품질 확인조사 결과 분석 105
• 제 5 장 하부 점토층의 두께에 따른 항만구조물 영향 108
• 5.1 하부 점토층 두께와 항만구조물 안정성의 상관관계 연구 108
• 5.2 하부 점토층 두께와 항만배후부지 안정성의 상관관계 연구 113
• 5.3 항만시설물의 합리적인 유지관리 방안 제안 117
• 5.3.1 항만구조물의 유지관리 방안 117
• 5.3.2 항만배후부지의 유지관리 방안 118
• 제 6 장 결론 123
• 참고문헌 125