During installation of suction caisson in permeable soils such as sands, seepage flow is induced by suction pressure. The seepage flow plays a predominant role in installation of the caisson. When the suction pressure is applied to caisson top, water seeps from outside the caisson into the interior of the caisson. The sand around the caisson tip is disturbed by seepage flow and the sand inside the caisson is loosened by upward seepage flow. Accordingly, the seepage flow facilitates caisson installation by reducing the internal wall friction as well as the tip resistance. However, the seepage flow also has a negative influence on the installation procedure. Critical hydraulic gradient is developed in the sand inside the caisson due to upward seepage flow during suction installation. At this state, the sand heave and piping failure occurs.
The sand heaving prevents the further penetration of the caisson and the caisson cannot reach the target depth. This may affect the mobilization of the bearing capacity for supporting the structure and settlement of the foundation after installation. In addition, the soil disturbance by seepage flow cause change of permeability, which is an important parameter for the estimating the suction pressure. Therefore, the soil disturbance by seepage flow should be considered while designing the suction caisson, especially for prediction of suction pressure during installation and performance evaluation of the caisson after the installation.
In this study, three methods were used to evaluate the soil disturbance due to suction installation: two-dimensional model tests, parametric study, and a centrifuge test. First, the soil displacement pattern during suction caisson installation in sand was investigated by two-dimensional model tests and the mechanism of sand heave formation was analyzed from the test results. For this purpose, testing apparatus for observing the soil movements during the caisson penetration in plane-strain condition was developed and an image-based deformation measurement system using particle image velocimetry (PIV) was used. Next, the effects of installation variables on soil disturbance was evaluated by parametric study. The wall thickness, pumping rate, and caisson weight are considered as the installation variables. Finally, a centrifuge model test was performed to simulate suction caisson installation in sand and the soil disturbance during suction installation was evaluated using shear wave velocity (Vs) measured inside the caisson during the centrifuge test.
From the two-dimensional test result, the mechanism of sand heave formation was analyzed. The most of sand heave was formed by the expansion in the volume of sand inside the caisson. There is almost no sand inflow from outside the caisson, but the sand displaced by the penetration of the caisson mostly flow into the caisson interior. The results obtained from parametric study show that the use of thinner-walled caisson and additional weight has beneficial effects on installation of suction caissons. This is because the suction pressure required for penetration of the caisson and sand heave formed during suction installation are reduced in these conditions. The centrifuge test result show that the Vs of sand inside the caisson decreased 20% compared with that before suction installation. This finding provides evidence that the soil density inside the caisson was reduced due to suction installation. In addition, the variation in permeability during suction installation were estimated based on the amount of sand heave obtained from the tests. It is assumed that the sand inside the caisson is disturbed homogeneously and there is no sand inflow from outside. However, under this assumption the results are sometimes unreasonable. Nevertheless, this approach of estimating variation in permeability provide basis for predicting accurately the variation in soil property during suction installation.
석션 케이슨은 해상 구조물 지지를 위한 기초 및 앵커로 사용되는 구조물로 비교적 간편하고 빠른 설치가 가능하여 기존 말뚝 형태의 기초를 대체하여 널리 활용되고 있다. 석션 케이슨의 설치는 케이슨 내부의 물을 펌프를 이용하여 외부로 배출시킴으로써 발생하는 케이슨 내외부의 압력차에 의해 이루어진다. 투수성이 큰 모래지반에서는 케이슨 내부의 물이 배출 될 때 케이슨 외부로부터 압력이 낮아지는 케이슨 내부 방향으로 침투수류가 형성되며, 상향 침투수류의 영향을 받는 케이슨 내부의 모래지반은 유효응력이 감소되어 지반이 연약화(Loosening) 되고 융기 (Heaving) 되는 현상이 발생한다.
석션 설치에 의해 케이슨 내부에 형성되는 Heaving은 케이슨의 추가적인 관입을 막아서 케이슨이 원하는 심도까지 설치되는 것을 방해하며, 케이슨의 설치 후 기초로서의 성능 발현에 영향을 줄 수 있다. 그러나 석션 설치 중 Heaving의 형성은 다양한 인자에 영향을 받으며 아직까지 Heaving의 형성 메커니즘에 대한 연구가 많이 이루어지지 않은 상태이다. 따라서 모래지반에서 석션 설치 중 지반교란에 의해 발생하는 문제를 방지하기 위하여 석션 케이슨의 설치가 케이슨 내부지반의 교란에 미치는 영향에 대한 평가가 필요하다.
본 연구에서는 1g모형실험과 원심모형실험상에 석션 케이슨 설치를 모사할 수 있는 시스템을 구축하고, 이를 활용하여 석션 케이슨 설치 시 케이슨 주변 지반의 거동을 분석하였다. 먼저 석션 케이슨의 설치를 모사하고 시각화 할 수 있는 2차원 실험 시스템을 이용하여 석션 케이슨 설치 시 케이슨 주변 지반의 변형패턴을 파악하고 그 결과를 통해 케이슨 내부 지반의 Heaving 형성 메커니즘을 분석하였다. 그리고 전단면으로 제작된 모형 케이슨을 이용한 3차원 설치 실험을 통해 석션압과 지반교란에 영향을 미치는 인자들에 대한 매개변수 연구를 수행하였다. 마지막으로 원심모형실험을 수행하여 1g 실험을 통해 얻은 결과의 타당성을 검토하고, 추가적으로 벤더 엘리먼트를 활용하여 석션 설치에 의한 케이슨 내부지반의 물성변화를 평가하였다.
