Mechanical behavior of New Mexico rock salt in triaxial compression up to 200°C

Abstract

Experiments were conducted to determine the ‘quasi‐static’ mechanical behavior of rock salt in support of the structural design of a waste isolation pilot plant. The salt studied was from the Salado formation in the Delaware Basin, southeastern New Mexico. Three groups of tests are discussed to identify the relative and site‐specific importance of deviator stress, confining pressure (mean stress), temperature, time (loading rate), and stress path. The three groups of experiments consist of (1) hydrostatic loading, (2) triaxial compression tests (σ1 > σ2 = σ3 = const), and (3) variable stress path tests including experiments at approximately constant σ1 and at constant mean stress. The range of parameters considered was 0 ≤p (hydrostatic pressure) ≤ 34.5 MPa, 0 ≤ σ 2 = σ3 ≤ 20.7 MPa, 0 ≤ (σ1 − σ3) ≤ 62 MPa, 23 ≤ T (temperature) ≤ 200°C, and 1.7 ≤ d(σ1 − σ3)/dt ≤ 3.5 kPa/s. All data were generated on 100‐mm‐diameter specimens. The rock salt exhibited nonlinear response under all loading coditions, a low initial elastic limit (σ1 − σ3) ≤ 0.7 MPa, and an apparent inseparability of permanent deformations into time‐independent and time‐dependent components. Pressure and temperature did not alter the elastic constants over the range 29.6 ≤ E ≤ 36.5 GPa and 0.17 ≤ v ≤ 0.26, depending on core lithology. Pressure and temperature affected the principal strain ratio, the ratio of volumetric to shear strain, rock salt ductility, and the ultimate stress. In particular, low pressure and temperature permitted pronounced dilatancy and loss in load bearing ability. Under such conditions the volumetric strains reached sizeable fractions of the shear strains, ∣∈/(∈1 − ∈3)∣ ≥ 0.2. Pressure remained important even at 200°C temperature because it influenced the rate of shearing at constant loading rates. Load path and stress history may be significant under deviatoric loading conditions and for large variations in pressure.