Table 4 Fitting equation for the strain energy density of rock under true triaxial compression.

σ₁

u^e

\(u_{1}^{e}\) = 0.53734u₁-0.006010

u^d

\(u_{1}^{d}\) = 0.46266u₁ + 0.00601

σ₂

u^e

\(u_{2}^{e}\) = 1.97983u₂-0.151430

u^d

\(u_{2}^{d}\) = -0.97983u₂ + 0.15143

σ₃

u^e

\(u_{3}^{e}\) = 2.17328u₃-0.11290

u^d

\(u_{3}^{d}\) = -1.16561u₃ + 0.11236

Entirety

u^e

u^e = 0.515200u + 0.00279

u^d

u^d = 0.48476u-0.0028