1 INTRODUCTION
The finite element method has been used for several years to calculate the creep closure of Strategic Petroleum Reserve (SPR) storage cavities in rock salt. The two-dimensional axisymmetric finite element calculations performed thus far have required many simplifying assumptions to reduce the three-dimensional cavern geometries to two dimensions (Preece and Foley, 1984). Centrifuge testing has been employed to qualify the finite element program on a well-controlled laboratory experiment which bears resemblance to an oil-filled cavern in rock salt. Centrifuge testing produces an acceleration induced geostatic stress field around the cavity similar to that around an actual cavity in rock salt. The three centrifuge experiments reported here were single cavity tests which were performed at three different accelerations and employed the creeping material plasticine. Plasticine was used because it gives measurable cavity closure in a reasonable centrifuge test time. These preliminary experiments had two purposes: 1) explore the feasibility of creep testing on the centrifuge, and 2) provide an experiment with well-controlled boundary conditions and material properties for qualifying a two-dimensional axisymmetric finite element computer program with creep capability. The single cavity centrifuge experiments and the attending two- dimensional finite element analyses are presented in this paper.
2 MATERIAL PROPERTIES OF PLASTICINE
Plasticine has been used by several investigators to simulate the behavior of creeping materials like salt, e.g., see Ramberg (1981) and McClay (1976). McClay characterizes the creep behavior of plasticine using a power law formulation. However, he notes that these material constants differ according to manufacturer, grade and color. For this series of experiments, several varieties of plasticine were obtained. In premixed colors, we used ivory, green, gray and black. Red and blue were produced by mixing Tempera pigment with ivory plasticine.
2.2 Material Tests
The density of the plasticine was measured using the standard water displacement technique. The density for the ivory, green, gray and black plastlcine was found to be 1.71 gm/cm3 with a standard deviation of 0.004 gmfcm3. With the red or blue pigment added to the ivory, the density increased to 1.75 gmfcm3 with a standard deviation of 0.009 gm/cm3. The plasticine was tested in uni-axial compression at several constant strain rates. The test specimens were 25 4 mm (1 in) in diameter and 50.8 mm (2 in) in length. The tests were conducted at strain rates varying from 1E-3fs to 3E-6fs. The temperature of the sample was controlled to 25°C. A typical set of stress-strain plots, for green plastlcine, is shown in Figure 1.
Figure 1. Typical Stress-Strain Curves for Green Plasticlne. (available in full paper)
