# The Seepage and Soil Plug Formation in Suction Caisons in Sand Using Visual Tests
## 2 Experimental Program  
The process of suction installation of the caisson foundations (shown in Figure 0(a)) with axisymmetric geometry is simplified to a plane problem in this paper. Due to the fact that the soil was replaced by the walls of the soil tank in tests, it is assumed that the forces of water adhesion to the walls are equal to that of the sand around the suction caissons. A soil tank (shown in Figure 1), as used in tests, has dimensions of \(20\times 0.5\) cm in its plan view, and a depth of 28 cm. Transparent fiberglass was used to make the soil tank, with a thickness of 1.0 cm. The two pumping outlets in the soil tank were connected to the suction loading system by a thin pipe and a drainpipe, respectively. Figure 0(b) shows the diagram of the test device, with a constant suction \(S\) in the caisson caused by the suction loading system creating the different head \(H\). The value of the applied constant suction can be expressed as \(S=\gamma_{\mathrm{w}}H\), where \(\gamma_{\mathrm{w}}\) is the unit weight of water (10 kN/m\({}^{3}\)). The suction caisson with an external diameter \(D\) of 12 cm had a height of 17.5 cm \(L\) and a thickness \(T\) of 0.5 cm. In order to create an enclosed compartment between the soil tank, the suction caisson, and the soil, UV glue was used to stick the soil tank and the suction caisson together. The suction caisson model was located at a distance 7.5 cm from the bottom of the soil tank. The siliceous sand shown in Figure 2 was used in tests because it is commercially available and shows a deep contrast with the carmine stain. The void ratio \(e\) of the soil was determined according to the standard soil testing methods, and can be written as \(e=\rho_{\mathrm{w}}G_{\mathrm{s}}/\rho_{\mathrm{d}}\)-1, where \(\rho_{\mathrm{d}}\) is the dry density of the sand and \(G_{\mathrm{s}}\) is the specific gravity of the sand. Table 1 shows the properties of the siliceous sand in tests. The siliceous sand particles had an average radius of 0.748 mm, and they obeyed uniform distribution,  
Figure 1: Test devices. with a ratio of maximum to minimum radii of 2.0. The pluviation method and the compaction method were used to prepare uniform sand specimens in layers in the soil tank. The permeability coefficient of the siliceous sand was obtained by using an empirical equation, \(k=2{d_{10}}^{2}e^{2}\)[21], where \(d_{10}\) is the effective size of the sand. Due to the uniform particle size of the sand, it is suggested that \(d_{10}\) is equal to the average particle size (0.718 mm). Before carrying out every test, the sand specimen was left to stand for 24 h.  
The seepage field around the suction caisson model was visualized by the tracer titration system, which consisted of an infusion system and carmine stain. The infusion system was made of a medical syringe with a thin pipe (radius of 2 mm), shown in Figure 1b. Three holes were drilled in the soil tank at points A, B, and C, connecting to the infusion system with the thin pipes. At points A, B, and C, the carmine stain was released with pinpoint accuracy by the infusion system. To reduce the effect of turbulence, the carmine stain was injected into the soil used in tests as slowly as possible. The suction caisson model had a buried depth \(h\) (5, 10, and 15 cm) before each test, to simulate the process of suction installation. The influence of the temperatures of the environment and the water were not considered in the tests. A video camera was used to collect images of the experimental phenomena of seepage and the soil plug in the suction caisson. At the end of the trial, the height of the soil plug did not seem to change. The testing programs are listed in Table 2 in tests. The pre-test results show that seepage failure took place in the sand when the suction in the caisson with buried depths of 5, 10, and 15 cm were greater than 2.0, 3.5, and 4.0 kPa, respectively.