Geotechnical centrifuge facility

The centrifuge experiments for this research were conducted in the geotechnical centrifuge facility of the University of New Hampshire. The rotation radius of the centrifuge is about 1 m with 5 g- ton capacity. Figure 3-1 shows the centrifuge and its different parts. Figure 3-1(a) shows the front view of the centrifuge. This beam centrifuge includes the testing platform side (shake-table side) and the counterbalance side, as demonstrated in Figure 3-1(b). After a specimen is prepared, it is placed on the platform, shown in Figure 3-1(c). Then, weights with the same amount of mass as

the specimen are placed in the counter-balance side, shown in Figure 3-1(c). The centrifuge was renovated in 2014 with more detailed description available in Ghayoomi and Wadsworth (2014).

Figure 3-1. Geotechnical centrifuge facility at the University of New Hampshire: (a) a front view of the centrifuge; (b) a view from inside of the centrifuge; (c) a view of the shake table side of the centrifuge; (d) a view of the counter-balance side of the centrifuge.

Figure 3-2 illustrates some of the major components of the centrifuge. An electronic motor, shown in Figure 3-2(a), spins the arm of the centrifuge. In this centrifuge, energy for shaking the specimen with seismic motions is provided by a hydraulic pump, displayed in Figure 3-2(b). The pump increases the hydraulic fluid pressure to about 3000 psi (20.7 MPa). The pump sends the fluid to

slip ring, shown in Figure 3-2(d). The slip ring allows to send and receive fluids, such as oil and water, to the inside of the centrifuge. After the pressurized-oil passes the slip ring, it goes to two in-flight 1-gallon accumulators, demonstrated in Figure 3-2(e). When the in-flight accumulators are filled, the oil goes to a servo-valve, installed beneath the shake table. The servo-valve uses pressurized oil to move an actuator. The actuator moves the shake table to the desired location. The location of the shake table is measured by a Linear Position Sensor (LPS), shown in Figure 3-2(g). Data, measured by the LPS, is sent to the shake table controller to control the position of the shake table through a closed feedback loop.

As the servo-valve uses the pressurized oil to move the shake table, the oil pressure decreases. Thus, the servo-valve returns the low-pressure oil to the slip ring. The slip ring connects the low- pressure oil to a return one-gallon accumulator, demonstrated in Figure 3-2(e), to further decrease the oil pressure. Then, the oil with low pressure goes to the storage tank inside the hydraulic pump. Finally, the hydraulic pump boosts the oil pressure and resends it to the four-gallon accumulator.

Figure 3-2. Major components of the geotechnical centrifuge: (a) electric motor, located beneath the centrifuge; (b) hydraulic pump, located outside of the centrifuge; (c) 4-gallon accumulator, located outside of the centrifuge; (d) slip-ring; (e) two 1-gallon accumulators, located inside of the centrifuge, beneath the centrifuge

Figure 3-3 displays some other major components of the centrifuge. Variety of sensors, such as accelerometers, LVDTs (Linear Variable Differential Transformers), pore pressure sensors, and dielectric sensors are used in experiments performed for this dissertation. These sensors should be connected to a data acquisition system to measure different parameters. The accelerometers, LVDTs, and pore pressure sensors are connected to the Data Acquisition (DAQ) channels, shown in Figure 3-3(a). The channels are connected to the main data acquisition system, displayed in Figure 3-3(b). The centrifuge is equipped with an in-flight computer, illustrated at the top of Figure 3-3(c). The main data acquisition system is connected to the computer to record sensor measurements. When the centrifuge is being spun, the computer is remotely accessible with the Wi-Fi signal through a remote computer. Dielectric sensors should be connected to another data acquisition system, depicted in the bottom of Figure 3-3(c). The data acquisition system for the dielectric sensors is also connected to the in-flight computer. As mentioned, the movement of the shake table is automatically controlled by the shake table controller. The controller is secured in the metal box, as shown in Figure 3-3(d).

Figure 3-3. Other major components of the centrifuge: (a) ports for connecting sensors such as accelerometers, Linear Variable Differential Transformers (LVDTs), and pore pressure sensors to the main data acquisition system; (b) main data acquisition system, made by National Instrument; (c) in-flight computer, shown on the top of the photograph and data acquisition system of dielectric sensors.

The experiments, performed for this dissertation, require working with various fluids such as water, de-aired water, metolose, and compressed air. Figure 3-4 shows the primary equipment for this purpose. A control board, displayed in Figure 3-4(a), was built to facilitate working with fluids in the geotechnical centrifuge laboratory. The control board can be used for the following

2. Applying compressed air and vacuum to the large tank, shown in Figure 3-4(a), and the drainage tank, illustrated in Figure 3-4(a).

3. Sending water and de-aired water through the slip ring to the inside of the centrifuge The main application of the large tank is to store fluids such as water and metolose. A vacuum can be applied to the tank to de-air the fluids; furthermore, the tank can be pressured with compressed air to send the fluids to the centrifuge.

As it will be discussed in detail later, the pressure of the fluid, used to saturate a specimen should be kept below a certain value to prevent the sand boiling phenomenon, when specimens are being saturated by injecting fluids from the bottom of the soil layer. A small tank, shown in Figure 3-4(c), is used for this purpose. The small tank was connected to the large tank and was filled to an elevation less than an elevation causing the sand boiling condition in order to saturate specimens, Some of the experiments, conducted for this research, required fluid drainage from specimens during the centrifugation. The fluids were collected inside the tanks, mounted on the centrifuge arm, to keep the balance between the specimen mass and the counterbalance mass. Two of these tanks can be seen in Figure 3-1(c) on both sides of the in-flight shake table. After performing the experiments, a vacuum was applied to the large drainage tank, then fluids in the tanks were sucked to the large drainage tank. Figure 3-4(a) also displays a vacuum pump used in the centrifuge laboratory.

Figure 3-4. Major equipment for controlling fluids in the centrifuge laboratory: (a) a vacuum pump, a control board, and a large tank; (b) a large drainage tank; (c) a small tank.